Swimming pool chemistry.

School Science Lessons
2024-11-14
(topic18a)

Swimming pool chemistry
Contents
18.1.0 Chemicals used in swimming pools
18.2.0 Chlorine used in swimming pools
18.3.0 pH of swimming pools
18.4.0 Management of swimming pools
18.5.0 Tests for swimming pools

18.1.0 Chemicals used in swimming pools
18.1.1 Algaecides, control of algae in swimming pools
18.1.2 Biguanides sanitizers
18.1.3 Bromine products, HBrO
18.1.4 Calcium hypochlorite, Ca(OCl)2
18.1.5 Chloramines in swimming pools
18.1.6 Cyanuric acid, conditioner, stabilizer
18.1.7 Dichlor
18.1.8 Inorganic hypochlorites
18.1.9 Stains in swimming pools
18.1.10 Lithium hypochlorite
18.1.11 ORP (Oxidation-Reduction Potential)
18.1.12 Oxidizing agents
18.1.13 Oxidizers
18.1.14 Phenol red
18.1.15 QAC (quaternary ammonium compounds
18.1.16 Redox potential
18.1.17 Ozonator
18.1.18 Sequestering agent, chelating agent
18.1.19 Sodium bicarbonate
18.1.20 Sodium hypochlorite
18.1.21 Sodium carbonate
18.1.22 Sodium dichlor
18.1.23 Sodium monopersulfate
18.1.24 Sodium persulfate
18.1.25 Sodium sulfite
18.1.26 Sodium thiosulfate
18.1.27 Trichlor
18.1.28 Zeolite

18.2.0 Chlorine used in swimming pools
18.2.1 Available chlorine
18.2.2 Chlorination of swimming pools
18.2.3 Chlorine and water
18.2.4 Chlorine demand
18.2.5 Chlorine enhancer
18.2.6 Chlorine gas in swimming pools
18.2.7 Chlorine generator
18.2.8 Chlorine levels in swimming pools
18.2.9 Chlorine lock
18.2.10 Chlorine lost by ultraviolet radiation
18.2.11 Chlorine neutralizer
18.2.12 Chlorinated isocyanurates
18.2.13 Chlorinating concentrates
18.2.14 Chlorinating tablets, chlorinating concentrate
18.2.15 Chlorinator
18.2.16 Combined chlorine
18.2.17 Cost of chlorinating
18.2.18 Dissolve chlorine in swimming pool water by electrolysis
18.2.19 Green hair and faded hair from swimming pools
18.2.20 Methyl orange test for chlorine in swimming pools
18.2.21 Stabilized chlorine
18.2.22 Superchlorination, shock treatment, breakpoint chlorination
18.2.23 Test kit for chlorine levels in swimming pools
18.2.24 Tests for free chlorine in water
18.2.25 Total chlorine in swimming pools

18.3.0 pH of swimming pools
18.3.1 Acid demand of swimming pools
18.3.2 Alkalinity, total alkalinity and buffer capacity
18.3.3 Adjusting the pH of swimming pool water
18.3.4 Hypochlorous acid, HOCl
18.3.5 Swimming pool water and pH

18.4.0 Management of swimming pools
18.4.1 Swimming pool chemistry
18.4.2 Contaminants in swimming pools
18.4.3 Corrosion in swimming pools
18.4.4 Defoamers
18.4.5 Disinfectants
18.4.6 Make-up water, fill water
18.4.7 Filters
18.4.8 Ionizers for swimming pools
18.4.9 Langelier saturation index
18.4.10 Liquid acid for swimming pools
18.4.11 Magnesium hardness
18.4.13 Marbelite
18.4.14 Nitrogen pollution
18.4.15 Non-chlorine shock, potassium peroxymonosulfate
18.4.16 Organic waste
18.4.17 Sanitizers
18.4.18 Scale
18.4.19 Scum
18.4.20 Sediments
18.4.21 Stabilized and unstabilized swimming pools
18.4.22 Starting to use a filled swimming pool

18.5.0 Tests for swimming pools
18.5.1 Parts per million
18.5.2 DPD test for swimming pools, diethyl-paraphenylene diamine, C10H16N2
18.5.4 OTO test for swimming pools, orthotolidine
18.5.6 Total dissolved solids (TDS) water hardness, scale
18.5.7 Suspended solids
18.5.8 Turbidity
18.5.9 Water hardness

18.1.1 Algaecides, control of algae in swimming pools
If a swimming pool has an algae problem first vacuum and backwash, then breakpoint chlorinate.
Use an Algaecide as a last resort.
Algae is the most common fouler of swimming pools, because it is propagated by air-borne spores.
When conditions favour their growth, e.g. heavy rain, intense sunlight, and presence of nitrogenous materials, they can cause black and green spots on swimming pool walls and form an algae bloom in the water, resulting in a sharp rise in pH, as the algae consume carbon dioxide in the swimming pool water.
If algae bloom is present, use superchlorination followed by an Algaecide to control it and prevent its reoccurrence.
Algaecides should be used as a backup to a routine sanitation program.
Maintain a free chlorine residual in the swimming pool by sanitizing with stabilized swimming pool chlorinating concentrates and adding Algaecide according to the directions on the label.
Algae do not cause disease, but may provide an ideal substrate for bacteria.
Algaecides as liquids or granules include copper and silver compounds, Quat compounds, chlorine enhancer and herbicides.
They may be obtained formulated for a specific type of algae, e.g. green algae, mustard algae, blue-green algae, black algae (black spot), the hardest to treat, and be a bactericide.
Pink algae or red algae-like organisms may be a bacteria.
Chelated copper Algaecides that contain an ingredient to prevent the copper from staining the swimming pool surfaces or producing coloured water, but do not affect the ability of copper to kill algae.
Copper may also be used in the equipment and plumbing in swimming pools.
Copper (II) sulfate was one of the original copper Algaecides as with aluminium sulfate it also provides a flocculent.
However, it may harm some aquatic creatures in natural swimming pools and stain solids in swimming pools.
Small concentrations of copper or silver ions produced by electrolysis can be effective.
Materials similar to cationic surfactants are also used as Algaecides.
In addition to getting rid of algae, Algaecides extend the effectiveness of chlorine residual.
While chlorine is an Algaecide, add additional quantities as a backup, a maintenance dose.

18.1.2 Biguanides sanitizers
Biguanides is the name for a certain class of sanitizers using the polymer PHMB, the only non-halogen sanitizer available for swimming pool use.
Soft Swim and Baquacil are manufacturers of this sanitizer.
Biguanides are NOT compatible with the swimming pool Wizard.

18.1.3 Bromine products, HBrO
Bromine is a halogen family used as a sanitizer or disinfectant to destroy bacteria and algae in swimming pools and spas.
It is resistant to heat and rapid pH fluctuations and is available as a tablet or as sodium bromide "bromide", a granular salt.
BCDMH (bromochlorodimethylhydantoin) is a bromine sanitizer.
When BCDMH dissolves in water, it produces hypobromous acid, HBrO, the active form of bromine in swimming pool water.
Bromamines are by-products formed when bromine reacts with nitrogenous compounds including swimmer waste.
Unlike chloramines, which are strong smelling and have low sanitizing properties, bromamines are active disinfectants and do not smell, although high levels are harmful to health.
A brominator is a mechanical or electrical device for dispensing bromine at a controlled rate from a floater filled with bromine sanitizer tablets.
Sodium bromide is used to raise the bromine levels in a swimming pool before using bromine tablets.
Residual bromine is the amount of free available bromine remaining in the swimming pool water after the bromine demand has been satisfied.

18.1.4 Calcium hypochlorite, Ca(OCl)2
Calcium hypochlorite, in bleaching powder (30% chlorine), chlorinated lime (OXD 2208)
Calcium hypochlorite, Ca(OCl)2, 65% available chlorine, white granular powder or compressed into pucks, pH 11.8, also contains 5 to 8% of insoluble material, which can cause cloudy water.
A by-product of this reaction is the calcium ion (Ca2+), a major component of water hardness, and a contributor to scaling tendencies in the swimming pool.
Ca(OCl)2 + H2O, Ca2+ + 2OCl- + H2O
calcium hypochlorite + water + calcium ion + hypochlorite ion + water.

18.1.5 Chloramines in swimming pools
Chloramine NH2Cl, dichloramine NHCl2, trichloramine, NCl3
Organic chloramines, RNHCl, are quite stable and useful sources of chlorine for bleaching, disinfection and oxidation when necessary to kill bacteria.
However, chloramines can be eliminated from swimming pool water only by using activated carbon filters.
In pure form it is an oily liquid that is highly reactive and explosive.
Some chemists who first discovered it or first used it were badly injured.
It may be formed in swimming pools when disinfecting.
If the pH of swimming pools is < 6.8, urea forms chloramines.
Chloramines are mildly toxic to bacteria, but react with urea from swimmers' urine to irritate the eyes and mucous membranes.
Nitrogen trichloride, not chlorine gas, causes the "swimming pool smell".
Nitrogen trichloride gas is very volatile and will escape from solution in to the air to give a characteristic noxious chlorine smell.
It is toxic, it is also very corrosive and is responsible for stress corrosion cracking of the fabric of buildings and ventilation systems.
chloramines may cause asthma in competitive swimmers and swimming pool instructors.
The reactions of chlorine with ammonia and ammonia compounds formed from organic waste to form chloramines are often written as follows:
(1.) 4NH3 + 3Cl2 --> NCl3 + 3NH4Cl
ammonia + chlorine --> nitrogen trichloride (trichloramine) + ammonium chloride
In hot water ammonia forms
(2.) NCl3 + H2O --> NH3 + 3HOCl
nitrogen trichloride (trichloramine) + hot water --> ammonia + hypochlorous acid [chloric (I) acid]
There is no chemical that changes colour when someone urinates in a swimming pool.
There are dyes that could cloud, change colour, or produce a colour in response to urine, but these chemicals would also be activated by other compounds, producing embarrassing false positives.
Also, ammonia and ammonia compounds react with HOCl to form chloramines.
(3.) Formation of monochloramine
NH3 + HOCl --> NH2Cl + H2O
ammonia + hypochlorous acid <--> chloramine + water
(4) Formation of dichloramine:
NH2Cl + HOCl --> NHCl2 + H2O
chloramine + hypochlorous acid <--> dichloramine + water
Chloramine may react with each other,
(5) Reactions of monochloramine with dichloramine
NH2Cl + NH2Cl --> N2 (g) + 3HCl
dichloramine + dichloramine, nitrogen + hydrochloric acid
or, in the presence of excess HOCl, produce nitrogen trichloride
Total equation: 2NH3 + 3HOCl --> N2 (g) + 3HCI + 3H2O
(6) Further addition of chlorine forms nitrogen trichloride, the "smell of chlorine", that leaves "agitated" water and may cause eye irritation:
NH2Cl + HOCl --> NCl3 + H2O
dichloramine + hypochlorous acid, nitrogen trichloride + water.

18.1.6 Cyanuric acid, conditioner, stabilizer
See diagram 18.1.14: Cyanuric acid (CNOH)3.
See diagram 16.13.8: Dichlorocyanuric acid, "dichlor".
(Also: isocyanuric acid, 1,3,5-Triazine-2,4,6-triol and dichlorisocyanuric acid, dichloroisocyanurate)
C3H3N3O3 + 2 Cl2, C3HCl2N3O3 + 2HCl
cyanuric acid + chlorine, dichlorcyanuric acid + hydrochloric acid
C3H3N3O3 + 2OCl-, C3HCl2N3O3 + 2HO-
cyanuric acid + hypochlorite ion, dichlorcyanuric acid + hydroxide ion
Stabilizer: also called cyanuric acid or conditioner.
A granular chemical added to the swimming pool water that provides a shield to chlorine for protection from UV radiation.
Too much can result in chlorine lock.
Cyanuric acid can stabilize chlorine in swimming pools by protecting chlorine in the water against the effects of the UV rays from the sun.
Cyanuric acid is made by heating urea or some of its derivatives.
It is a selective herbicide, very toxic to barley and radishes.
It reacts with chlorine to give dichloro(iso)cyanuric acid in a chemical equilibrium.
Stabilize swimming pool chlorine by using cyanuric acid at a minimum concentration of 30 parts per million, ppm.
(1 ppm = 1 mg per litre.)
In the stabilization process, a portion of the chlorine residual is temporarily bonded to the cyanuric acid molecule and protected from the destructive effects of sunlight.
The nature of this bond is such that the chlorine demand is imposed upon the system and continues to be released as long as a demand exists.
As OCl- or HOCl is used up in the swimming pool more OCl- is released from the dichloro(iso)cyanuric acid to re-establish the equilibrium to maintain a constant amount of chlorine in the swimming pool.
Other industrial chemicals are trichlorocyanuric acid and sodium dichlorocyanuric acid.
The amount of cyanuric acid is kept constant in the swimming pool between 30 and 80 mg / L by the initial addition of any of the three compounds then only hypochlorite should be added to keep the level of free chlorine at 2 mg / L.
The equilibrium in the reaction stabilizes the concentration of OCl-.
If excess cyanuric acid is used, the chlorocyanuric acids do not absorb the ultraviolet light from the sun.
To find the level of cyanuric acid in the swimming pool a reaction with melamine forms a salt that precipitates and scatters light.
The turbidity is proportional to the amount of cyanuric acid.
The turbidity is measured by the depth of solution required in a standard Nessler tube to just obliterate an object at the bottom of the tube.
Anionic surfactants are measured by a similar salt formation chlorine using methylene blue.
The salts formed are sufficiently lyophilic to transfer into organic solvents and to be estimated by the depth of colour in the organic solvent.
C3H3N3O3 + C3H6N6 --> C3H3N3O3.C3H6N6
cyanuric acid + melamine --> melamine cyanurate (melamine cyanuric acid precipitate)
The test can be done with a small test-tube and a calibrated stick with a black spot on the end.
The divisions on the stick where depth of spot seen is calibrated with cyanuric acid concentration in ppm, follow Beer's law, Concentration = Ke-a Depth.
During a swimming pool season, the conditioner may decrease from leakage, swimmer activity causing splash out or drag out, and maintenance operations, e.g. vacuuming and filter back washing.
Because of the slightly acidic pH of conditioner, addition of a small quantity of PH plus or acid may be necessary to maintain pH at the desired level.
If the conditioner concentration is too high, it can be lowered by dilution with fresh top up water.
The CYA stabilizer reduces the free chlorine, which is what does the sanitizing, and hence more must be added to maintain free chlorine and an ORP within the range 650 mV to 750 mV.
Swimming pools not exposed to the sun, e.g. indoor swimming pools should not be treated with CYA.
Some Australian State governments have banned the use of CYA in swimming pools.

18.1.7 Dichlor
See diagram 16.13.8: Trichloroisocyanuric acid, TCCA
Trichloroisocyanuric acid, TCCA, C3Cl3N3O3, white crystalline powder, disinfectant, bleaching agent.
"Dichlor" is the common name for dichloroisocyanuric acid, C3HCl2N3O3, or the active ingredient in dichlor cleansing powder sodium dichloroisocyanurate C3Cl2N3NaO3.
Dichlor is a quick dissolving chlorine compound made up of chlorine and cyanuric acid (stabilizer) and has a pH of 6.9.
Shock treatment with dichlor is not recommended as it may result in over stabilization and chlorine lock.
If dichlor is used, a monthly check of the cyanuric acid level is recommended, to prevent over stabilization and chlorine lock.

18.1.8 Inorganic hypochlorites
Hypochlorites including calcium hypochlorite, sodium hypochlorite and lithium hypochlorite are used as a disinfectant, sanitizer, bactericide, Algaecide and oxidizer in swimming pool water.
Calcium hypochlorite is also used as a disinfectant in drinking water.

18.1.9 Stains in swimming pools
A stain is a discoloration or a coloured deposit on the walls or bottom of a swimming pool.
Stains are usually the result of metals such as iron, copper or manganese in the swimming pool water.
The stains may be green, grey, brown or black.
They may discolour the swimming pool water without affecting the clarity.
Sometimes a sequestering agent, chelating agent or stain remover may remove them.
If that does not work, the easiest way to remove the stains is to drain and acid wash the swimming pool.
However, some professional pool stain removers claim if you drain and acid wash a pool to remove black spot algae, it will return because its roots remain embedded in the pool surface and pipework, which allows the spots to grow back.
A commercial algaecide is then recommended.
Other professional pool stain removers recommend scrubbing with a nylon brush and then scrubbing with powder from broken cghlorine tablets.
Stain inhibitor: also called a sequestering or chelating agent.
A chemical that will combine with dissolved metals in the swimming pool water to prevent the metals from coming out of solution and so avoiding discoloration of the water or stains.
Iron is a natural element that can cause the swimming pool water to become clear brown or green in colour.
It can also result in staining of the swimming pool surfaces.
Iron can be controlled by the addition of a suitable sequestering or chelating agent.
Treating a metal stain, e.g. copper, with high strength chlorine and algaecides will oxidize the metal staining and make it even darker.

18.1.10 Lithium hypochlorite
Lithium hypochlorite, LiClO, is a dry granular chlorine compound with 35% available chlorine and has a pH of 10.7.
It dissolves quickly and completely.
It can be used to superchlorinate vinyl liner swimming pools, painted swimming pools, fibre glass swimming pools and "hot tubs".

18.1.11 ORP, Oxidation-Reduction Potential
ORP (Oxidation-Reduction Potential, Redox Potential) is a measure of the ability of the oxidizer, e.g. chlorine, to oxidize contaminants versus the ability of the contaminants, e.g. algae, to reduce the oxidizer.
It is an indication of the level of free available ability of the oxidizer in the swimming pool water.
ORP is generally used with automated dosage systems and can give a fair idea of the sanitation of the swimming pool water.
It is not a measure of the total or available chlorine.

18.1.12 Oxidizing agents
See diagram 13.4.2: Reduction potential of chlorine in water and pH.
The ability of a material to oxidize is measured by the standard half-cell reduction potential, in volts.
This is an equilibrium value given for very specific conditions and so gives only a general indication for a practical situation.
The larger the half-cell reduction potential, the stronger the oxidizing agent.
The standard electrode potentials, E0 are reference to a standard electrode, usually the hydrogen electrode, which is arbitrary defined as 0 V.
The oxidizing agent HOCl is stronger than OCl-.
HOCL + H3O+ + 2e- < = > Cl- + 2H2O, E0 = 1.49 V
ClO-+ 2H2O + 2e- < = > Cl- + 2OH-, E0 = 0.94 V
It is dangerous to store hypochlorite powder swimming pool chlorine near materials that can burn, because it is a strong oxidizing agent.
If the material comes in contact with something that can be oxidized with chlorine, e.g. brake fluid, then a fire can occur.
Chlorine gas is released when solid hypochlorite powder comes in contact with moisture at any pH.

18.1.13 Oxidizers
An oxidizer is any compound that removes or destroys organic waste and organic compounds in the swimming pool water.

18.1.14 Phenol red
Phenol red: C19H14O5S (acid-base indicator), A chemical reagent dye used to test pH.
It can measure pH from 6.8 to 8.4.
The tablet form usually incorporates a chlorine neutralizer for more accurate results.

18.1.15 QAC (quaternary ammonium compounds)
QAC, Quaternary ammonium compounds (Quats, QAC) are a type of Algaecide composed of ammonia compounds.
They are also effective algaestats for certain types of ammonium compounds.)

18.1.16 Redox potential
Redox potential (Reduction-Oxidation Potential) is a measurement of the ability of the chlorine oxidizer to oxidize contaminants versus the ability of the contaminant, e.g. algae, to reduce the oxidizer.
It is an indication of the level of free available oxidizer in the swimming pool water.
ORP is generally used with automated dosage systems and can give a fair idea of the sanitation of the swimming pool water.
It is not a measure of the total or available chlorine.
Sometimes called ORP.

18.1.17 Ozonator
An ozonator is an electrical device that produces ozone that is introduced into the swimming pool water as a sanitizer.

18.1.18 Sequestering agents, chelating agents
A chemical or compound that combines with dissolved metals or minerals in the swimming pool water to prevent them from coming out of solution, thus colouring the swimming pool water or causing stains.
A sequestrant is a chemical that holds metals in solution and helps prevent scaling.

18.1.19 Sodium bicarbonate
A base used to raise total alkalinity in swimming pool water with only a slight effect on the pH.
Sodium bicarbonate can only raise the pH of the swimming pool water to 8.5, regardless of the amount used.
Do not add large quantities at one time.
A correspondent from New Zealand reports that he got great results, sparking water, excellent control and minimal chemical use by raising the total alkalinity to 200 ppm + using sodium bicarbonate and in the process got a stable pH of 8.3, which he almost never had to test for.
Also, the chemical was safe to use.
The correspondent compensated for the higher pH by running the swimming pools with a FAC of around 4 ppm and not letting it drop below 1 ppm.
He only used sodium hypochlorite diluted to 8% to give a good shelf life.
This procedure was satisfactory for about 20 swimming pools.
In this procedure the ORP is above 650 mV at pH 8.4 and 4 ppm free chlorine, but less than 750 mV.
650 mV to 750 mV is the recommended range for effective sanitation.
Assuming he did not use stabilizer or did not need to, he probably needed to top up the bicarbonate relatively frequently to keep it at around 200.
A lower alkalinity and a weekly dose of acid, to give lower pH and higher ORP could allow free chlorine to be reduced to about 1 ppm.
To maintain a stable pH level, add about 100 mg / kg of sodium bicarbonate to water, i.e. about 6 kg in a 60, 000 litre pool.
Or
Add 1.5 pounds of baking soda for each 10, 000 gallons of water to raise total alkalinity by 10 ppm, for total alkalinity within 80 to 150 ppm.
Maintain a proper level of total alkalinity to minimize changes in pH when acidic or basic chemicals or contaminants enter the water, reducing chloramine formation, corrosivity of water, eye irritation and unpleasant odours, while improving bactericidal effectiveness.

18.1.20 Sodium hypochlorite
Sodium hypochlorite, NaClO, highly toxic by all routes, forms chlorine gas, skin irritant, corrosive.
Sodium chlorate (I), domestic bleach, oxidizing agent
Sodium hypochlorite, is highly toxic by all routes, forms chlorine gas, skin irritant, corrosive.
Sodium hypochlorite solution, 16%, bleach, is highly toxic, forms chlorine gas, skin irritant, corrosive.
Sodium hypochlorite solution, 5-16%, bleach, is highly toxic, forms chlorine gas, skin irritant, corrosive.
Sodium hypochlorite solution, < 5%, is not hazardous.
Low cost: from supermarkets as laundry bleach having about 6% solution of sodium hypochlorite + sodium hydroxide + "whiteners", scents etc.
For example, Clorox® Regular-Bleach contains sodium hypochlorite, sodium chloride, sodium carbonate, sodium hydroxide, sodium polyacrylate.
The cheapest laundry bleach may be the purest!
Sodium hypochlorite, NaOCl (aq), sodium chlorate (I), domestic bleach, bleaching fluid, e.g. "Chlorox" bleach, oxidizing agent, 12.5% solution, liquid bleach, colourless, strong odour, heat decomposes (4% Cl2), solution (store below 25oC) (household bleach,domestic bleach.
5% NaOCl, bleaching fluid, disinfecting sinks and drains), chlorine water, chloride of soda, chlorine bleach, highly toxic by all routes, skin irritant, corrosive.
Sodium hypochlorite (aq) bleach evolves chlorine gas that is highly irritant to the lungs.

18.1.21 Sodium carbonate
Sodium carbonate heptahydrate, Na2CO3.7H2O, (not a mineral)
Low cost: from supermarkets as washing soda, from pottery supply stores as soda ash, by heating a thin layer of sodium bicarbonate in the oven for an hour
Sodium carbonate, Na2CO3, calcined soda, carbonic acid disodium salt, soda ash Na2CO3.10H2O, sodium carbonate decahydrate crystals, formerly "nitre" (US niter)
Sodium carbonate, soda crystals, sal soda, crystal carbonate, black ash, trona, E500, colourless or white crystals, odourless powder, hygroscopic
Sodium carbonate is readily soluble in water to form strong alkaline solution.
Sodium carbonate is NOT decomposed by heat (also anhydrous sodium carbonate, soda ash anhydrous, Na2CO3, RD. 2.53, MP. 851oC), It is used in glass production, softens "hard" water, removes grease, in washing powders and bath salts.
Over half the weight of the crystals is water because of the water of crystallization.
Soda ash is sold as "sodium carbonate".
If sodium carbonate solution is left on the skin it may produce blisters.
Sodium carbonate is aso called soda ash.
A base that is used to raise the pH of acidic (below pH 7.0) swimming pool water.

18.1.22 Sodium dichlor
Sodium dichlor is a granular, stabilized organic chlorine compound providing 56% or 62% available chlorine that has a pH of 6.9, used for regular chlorination.
It should be used with caution for superchlorination, because as it can cause the stabilizer level to rise too high, resulting in chlorine lock.

18.1.23 Sodium monopersulfate
Sodium monopersulfate is the active ingredient and chemical name of a non-chlorine shock treatment or non-chlorine oxidizer of swimming pool water.

18.1.24 Sodium persulfate
Active ingredient and chemical name of a non-chlorine shock treatment or non-chlorine oxidizer of swimming pool water.

18.1.25 Sodium sulfite
A chemical that can be used to neutralize chlorine or dechlorinate swimming pool water.

18.1.26 Sodium thiosulfate: A chemical that can be used to neutralize chlorine or dechlorinate swimming pool water.

18.1.27 Trichlor
Trichlor is a slow dissolving, tablet or granular stabilized organic chlorine compound that provides 90% available chlorine and has a pH of 2.9.
It must be dispensed using a floating feeder or an in-line chlorinator.
Trichlor contains cyanuric acid that prevents the chlorine from being destroyed by the ultraviolet rays of the sun.
When using trichlor, the cyanuric acid level needs to be checked regularly to avoid chlorine lock.

18.1.28 Zeolite
Zeolite is an alternative to quartz or silica for use in sand filters for swimming pools.
It is typically clinoptilolite, the specific zeolite suitable for swimming pool water conditions.
Zeolite can provide filtration down to 3 microns, is able to absorb heavy metals such as iron or manganese, and absorbs ammonia and nitrogen compounds.
Zeolite can also absorb chloramines and is regenerated by washing in salt.
Being less dense than quartz, 60% to 70% of the weight of quartz is used.
Zeolite is also claimed to reduce back washing by 50%.

18.2.1 Available chlorine
Available chlorine content is the term used to compare the amount of oxidizing power that products containing chlorine have compared to gas chlorine Cl2.
It permits easy comparison of relative values of chlorine compounds.
Available chlorine is the amount of free chlorine that is available to sanitize or disinfect the water.
It is also called Residual Chlorine and Free Available Chlorine.
When chlorine gas dissolves in water, it forms hydrochloric acid and hypochlorous acid.
Since only the hypochlorous acid sanitizes, only half the chlorine added to the water is usable.
Chlorine gas is defined as 100% available chlorine, so compounds for which all the chlorine in solution is active will have percentages twice the value based on composition.
Ca(OCl)2, which is 99.2% available chlorine for the pure material may be quoted as 100% available chlorine.
It produces two moles of active chlorine compared to only one mole from Cl2.
However, it has more than twice the molecular mass, a ratio of 143: 71.
So, on a mass basis, both chlorine gas and calcium hypochlorite are equally effective.
Materials releasing other oxidizing agents when dissolved in water have available chlorine defined in the same manner.
Approximate percentage available chlorine of substances added to swimming pool water are as follows:
Cl2 chlorine gas 100% (by definition)
Bleaching powder (chloride of lime) 35%
Ca(OCl)2 calcium hypochlorite 99%, industrial products 70%
NaOCl, sodium hypochlorite 95% (solution 100%)
Bleach (industrial product) 12%
Bleach (household product) 3%
ClO2, chlorine dioxide 263%
NH2Cl, monochloramine 138%
NHCl2, dichloramine 165%
CONClCONClCONCl, C3Cl3N3O3, trichloroisocyanuric acid 91%
CONClCONClCONH, dichloroisocyanuric acid 72%
CONClCONClCON-Na+, sodium dichloroisocyanurate 64%.

18.2.2 Chlorination of swimming pools
See diagram 13.4.2: Distribution of HOCl and OCR-in water at pH levels.
Most swimming pools are sanitized with chlorine-based compounds.
When chlorine compounds are dissolved in water, hypochlorous acid forms, which does the actual sanitizing.
In most cases, the non-chlorine part of the chlorine compound serves no other purpose than to hold the chlorine until the product dissolves.
The three categories of sanitizer contain chlorine in different forms.
All three sanitizers are compatible and effective with other chemicals in swimming pool water.
Each will do its function without causing objectionable tastes, odours or colours in the water, if properly applied.
Swimming pool chlorine may be calcium hypochlorite or sodium hypochlorite.

18.2.3 Chlorine and water
When chlorine is added to water, a mixture of hypochlorous acid, HOCl, the active sanitizing species, and hydrochloric acid, HCl, forms within seconds at room temperature.
Cl2 (aq) + 2H2O <=> HOCl + Cl- + H3O+
chlorine + water <=> hypochlorous acid + chloride ion + hydronium ion.
In dilute solution and pH > 4, the equilibrium displaces to the right, and little Cl2 exists in solution.
The chlorine added to swimming pool water does not produce a concentrated solution of a strength to yield such a low pH.
However, the oxidizing property of the added chlorine is in the HOCl formed and produces the main disinfecting action of added chlorine solutions.
Hypochlorous acid dissociates almost instantaneously into hydrogen and hypochlorite ions.
The reaction is reversible.
The dissociation depends on the pH and temperature.
Hypochlorous acid exists in a pH dependent equilibrium with hypochlorite ion (OCl-) in swimming pool water.
H2O + HOCl <=> H3O+ + OCl- water + hypochlorous acid <=.> hydronium ion + hypochlorite ion.

18.2.4 Chlorine demand
Chlorine demand is the amount of chlorine necessary to oxidize all organic matter (bacteria, algae, chloramines, ammonia and nitrogen compounds) in the swimming pool water.
Residual chlorine is the amount of free available chlorine remaining in the swimming pool water after the chlorine demand has been satisfied.

18.2.5 Chlorine enhancer
A chlorine enhancer is a chemical compound used in conjunction with chlorine, which makes the chlorine do better as an Algaecide.

18.2.6 Chlorine gas in swimming pools
Chlorine is a member of the halogen family of sanitizers.
Its use in swimming pools is in the form of a gas, as a liquid, in granular or tablet forms.
When added to swimming pool water it acts as an oxidizer, sanitizer, disinfectant and biocidal agent.
Chlorine gas is referred to as having 100% available chlorine, is relatively low cost and is used in public swimming pools and most bulk drinking and waste water treatment systems.
However, it is a gas that must be delivered in bulky metal cylinders and has to be applied to the water through sophisticated metering systems operated by trained personnel.
It is highly corrosive, toxic, and very acidic, because of the H3O+ and Cl-.
Operators of regulated public swimming pools may be required to install separate feeding equipment to add soda ash to neutralize the acidity from the chlorine gas.
If chlorine gas were the only chemical available to disinfect water, there would be few home swimming pools.

18.2.7 Chlorine generator:
A chlorine generator is an electrical device that generates chlorine from a salt solution.
The salt solution may be in a separate tank or may be in the swimming pool itself.

18.2.8 Chlorine levels in swimming pools
Free chlorine must remain in the swimming pool after all the organic material has been oxidized, so measuring free chlorine is necessary.
This is usually done with test kits, e.g. using OTO, ortho-tolidine (4,4'-diamino-3,3'-dimethylbiphenyl) or DPD (Diethyl-paraphenylene diamine).
A chemical reagent used to test the total chlorine level in water at normal temperatures.
It can measure free available chlorine if the water is first cooled to 1oC.
Its results depend on pH, time and concentration of chlorine.
Because of its toxicity, the use of OTO is restricted or prohibited in many western countries.
It is chemically similar to benzidine without the methyl groups, a bladder carcinogen.
The colour test with chlorine works well with laboratory solutions of chlorine in water.
The reagent also reacts slowly with chloramines, the compounds that chlorine forms when reacting with nitrogenous body waste products in swimming pools.
So the free chlorine must be measured above what has reacted with waste otherwise under dosing of chlorine may cause ear, eye and throat infections increases, because insufficient free chlorine exists in solution.
Breakpoint chlorination occurs when free chlorine and combined chlorine are simultaneously minimized.
Addition of further chlorine gives free chlorine.
Beyond which point nitrogenous compounds have been oxidized, principally to nitrogen gas.
The methyl orange test measures free residual chlorine (HOCl + OCl-), but the OTO test kit measures the combined chlorine, as well, but you must do the test quickly.
Add chlorine solution to ammonia to form chloramines.
Make a stock solution of 0.2 g of Ca(OCl)2 per litre (1.78 × 10-3 M) as stock solution.
Dilute by a factor of five for use (3.5 × 10-6 M or 25 mg /kg Cl2 equivalent).
Add the stock solution in 20 mL aliquots to 1 mL of 0.015 M NH3 solution (255 mg / kg).
A concentrated ammonia solution keeps the volumes almost constant.
Test with indicators.
Methyl orange will not bleach until about 2 mole equivalents of chlorine have been added (about 80 mL), and free HOCL is present.
OTO, on the other hand, will show off scale from the beginning, indicating reaction with chloramines as well.
Note the time taken by the oxidation reactions.
Measure a chlorine solution as a control to monitor any loss of chlorine.
Chlorine meters do not measure HOCl concentration.
They measure ORP and via an algorithm approximate HOCL concentration.
The reading may be unreliable.

18.2.9 Chlorine lock:
Chlorine lock occurs if the level of cyanuric acid (stabilizer) in the swimming pool water is much over 80 ppm, the chlorine becomes trapped and is unable to oxidize effectively.
Despite being able to measure normal chlorine levels, the redox potential is very low, indicating a lack of oxidizer.
The only way to fix this is to drain some of the swimming pool water and refill the swimming pool.
Care should be taken when using stabilized chlorine products (dichlor or trichlor) to avoid the level of cyanuric acid increasing too much.

18.2.10 Chlorine lost by ultraviolet radiation
See diagram 13.4.2: Ultraviolet absorption spectrum of swimming pool chlorine.
Reports suggest that in strong sunlight, up to half of the HOCl is destroyed within 17 minutes.
Chlorinated sanitizer will produce HOCl, but up to 97% of that residual can be lost in 2 hours so when 4-5 ppm of free available chlorine is added as calcium hypochlorite at 6 am, it will be completely gone
by 12 noon, whether or not anyone uses the swimming pool.
This same wasteful chlorine consumption occurs with chlorine gas and sodium hypochlorite.
The free available chlorine residual could be maintained by testing the water at noon, adding another 4-5 ppm residual for safe afternoon swimming, then repeat the process at 6 pm to use the swimming pool in the evening or install a chemical feeding system to constantly add sanitizer.
Photolysis reaction
2OCl- + νuv --> 2Cl- + O2 (g)
2HOCl- + νuv --> 2HCl + O2 (g).

18.2.11 Chlorine neutralizer
A chlorine neutralizer is a chemical used to deactivate or destroy chlorine.
It is used in better test kits to prevent the bleaching effect of the chlorine and so to increase the accuracy of the tests.

18.2.12 Chlorinated isocyanurates
Chlorinated isocyanurates, stabilized chlorine, are a group of chlorine swimming pool sanitizers that contain stabilizer (cyanuric acid or isocyanuric acid) as the granular form dichlor 56% available chlorine and the tablet or stick form trichlor 90% available chlorine, the latter usually used in a chlorine feeder.
Chemical feeder: a device that dispenses chemicals into the swimming pool water at a predetermined rate.
Some provide chlorine or bromine, while others add cyanuric acid (CNOH)3, also called conditioner and stabilizer.
A granular chemical added to the swimming pool water, which provides a shield to chlorine for protection from the UV radiation from the sun.
It is also found in dichlor / trichlor products.

18.2.13 Chlorinating concentrates
Both the granular and tablets forms are based on cyanuric acid (CNOH)3, composed of alternating C and N atoms.
In the granular form two atoms of chlorine are added, giving an available chlorine of 56%.
It is a sodium salt, so it has excellent solubility at 26.1% w / v and a nearly neutral pH of 6.7.
It may be added directly to the swimming pool by hand broadcasting or it can be pre-dissolved and added as a hypochlorite solution.
The tablet form contains three atoms of chlorine, giving it 90% available Cl2.
It has a relatively low pH of 2-3 and high available chlorine content, so much less needs to be added.
It has a minimal impact on pH and its low solubility of 1.2% w / v makes it ideal for use in tablet form in continuous feeding systems.
The trichloro product should not be added directly to the swimming pool in either its tablet or granular form, because its high available chlorine, slow solubility and acidic pH give it the potential to bleach, or pit any swimming pool surfaces.
Both products are free of insoluble residues, produce a minimal impact on pH and do not contribute to water hardness or scaling.
After the chlorine has been consumed in performing the sanitizing functions, the cyanuric acid remains dissolved in the water to provide maximum stabilization for the free chlorine residual.
Secondary chemical treatment with substances that control pH and buffer the swimming pool, is also needed for swimmer comfort.
Depending on swimming pool conditions, secondary treatment might include decolorizing, additional Algaecide, and chemicals to adjust mineral levels (hardness) or retard evaporation.

18.2.14 Chlorinating tablets, chlorinating concentrate
Pool chlorinating concentrates provide the effectiveness of HOCl, the ease and convenience of concentrated solids, and the benefits of stabilization, to provide outstanding water quality with minimum effort and expense.
Granular compounds react with water to produce the same active sanitizing species, hypochlorous acid.
Therefore, it is an effective bactericide and Algaecide that will oxidize organic contaminants.
Tablets react similarly, but produce three units of hypochlorous acid.
Both the granular compounds and tablets have a by-product, cyanuric acid, which is supposed to stabilize free chlorine residual without interfering with its sanitizing effectiveness.
However, the binding action of cyanuric acid reduces the concentration of free available chlorine and demonstrably reduces sanitizer efficiency.
Pool owners who use cyanuric acid must have higher free residual chlorine to compensate for this fact so that sanitizer efficiency is maintained.
Some Australian States have banned the use of cyanuric acids.
So to describe cyanuric acid products as a "conditioner" is misleading.

18.2.15 Chlorinator
A chlorinator is a mechanical or electrical device for adding chlorine to a swimming pool at a controlled rate.
It is usually a floater filled with tablets of chlorine or an in-line feeder.

18.2.16 Combined chlorine
Combined chlorine is the measure of chlorine that has attached itself to other molecules or organisms, usually ammonia or nitrogen compounds.
Most of these compounds are present as unwanted chloramines.
So combined chlorine is the chlorine that has combined with ammonium compounds or organic matter containing nitrogen to form chloramines.
Combined chlorine residual is the chlorine combined with simple nitrogen compounds such as ammonia and urea as chloramine that is non-effective as a sanitizer compared to free available chlorine.
Bactericidal properties of combined chlorine (i.e. chloramines) is only about one hundredth that of a similar level of free chlorine in water.
With a DPD test kit, DPD1 shows free available chlorine and DPD3 shows total chlorine.
The difference, if any, is the level of combined chlorine.

Recent studies have focussed attention on combined chlorine in swimming pools, a common name for mono-, di- and trichloramine.
Chloramines and trihalomethanes, with chloroform, as the best known compound in this group, are generally characterized as disinfection by-products, because they emerge as secondary pollutants out of the reaction between chlorine disinfectants and organic pollutants in water.
Chloramines are formed in water by reaction between nitrogen hydrocarbons, more especially ammonia compounds like urea (NH2)2CO and chlorine following reactions (1) (5)
1. NH4+ + H2O, H3O+ + NH3 (decomposition of urea from urine, sweat)
2. NH3 + Cl2 --> NH2Cl monochloramine
3. NH2Cl + Cl2 --> NHCl2 dichloramine
4. NHCl2 + Cl2 --> NCl3 trichloramine
5. NH2R + Cl2 --> NHRCl alkylmonochloramine
Trichloramine NCl3, formed in swimming pool water at pH < 6, is very volatile and responsible for a pungent, acid smell, which is generally confused with a strong chlorine odour and, which is the main cause for eye irritation in covered swimming pools.
Chloramines can be prevented forming by using CO2 as a pH corrector to limit the low pH needed for trichloramine formation, or by using alternative disinfection products, bromine, ozone or chlorine dioxide (in pure form or as a complex compound, e.g. Hydroxan, all with a low chloramine forming potential.
Once formed, chloramines can be reduced with:
1. active charcoal in granular form in filters, in powder or as hydroanthracite in double layer filters
2. breakpoint chlorination, with chlorine at least ten times above the chloramine concentration in swimming pool water.
As chloramines can hardly be found in open air swimming pools, because of the almost immediate destruction by incident sunlight, the idea grew years ago to use artificial sunlight as a chloramine reduction tool.
The chloramine separator of P. Kaas used ultraviolet and infrared light and was used with changing success from the years 1975-80 on.
The use of UV light was promoted as best available technique in water treatment of swimming pools by the Flemish Technology Research Institute VITO.
Recently, new UV systems became available on the market using low or medium pressure UV lamps.
Low pressure lamps contain mercury vapour at about 10-3 bar and emit monochromatic light of 254 nm while the medium pressure.
One type operates at mercury vapour pressures that emits polychromatic light, including the 254 nm wavelength.
This is near the optimum wavelength of 245 nm at which monochloramine, the precursor of dichloramine and trichloramine, breaks down by photochemical oxidation.
Thus low pressure lamps focus on the breakdown of monochloramine, while medium pressure lamps attack dichloramine and trichloramine.

18.2.17 Cost of chlorination
Assess the cost at, say, 1.5 KW for 6 hours for 4 months and 1.5 KW for 4 hours for 8 months at $0.12 per kWh.
Amortize the cost of the equipment over, say, 7 years.

18.2.18 Dissolve chlorine in swimming pool water by electrolysis
Add common salt to water at concentration of approximately 4000 ppm to generate hypochlorous acid continuously with an electrolysis cell.
The water will gradually become basic so you must add an acid to keep the pH between 7.2 and 7.8 for optimum swimmer comfort and to maintain sanitizer efficiency.
Cathode (-) reaction: 2e- + 2H2O --> 2OH- + H2 (g)
water --> hydroxyl ion + hydrogen gas
Anode (+) reaction: 2Cl- --> Cl2 (g) + 2e-
chloride ion --> chlorine gas
Overall Reaction: 2Cl- + 2H2O --> Cl2 (g) + H2 (g) + 2OH-
chloride ion + water --> chlorine gas + hydrogen gas + hydroxyl ion.

8.2.19 Green hair and faded hair from swimming pools
Low calcium ion level, with low pH and high levels of copper and chlorine from cheaper copper-based Algaecide, overuse of this Algaecide or corrosive water may cause green hair, blue fingernails, green, brown or blue water, stained swimming pool surfaces, and leaching of copper from copper pipes, copper heating elements and bronze pump impellers.
It is not the chlorine that bleaches hair.
Oxidized metals in the water bind to the protein in the hair shaft and deposit their colour.
The copper in Algaecide turns blond hair green.
Bleach added to a swimming pool may oxidize the copper, but the bleach itself is not the cause of the colour.
The green precipitate is a mixture of copper sulfate and copper hydroxide and is most noticeable in blond hair and especially bleached hair where the hair shaft may lack the natural oils that reduce water absorption.
To remove the green tint in hair, 1. Use folk remedies, e.g. wash the hair in aspirin, tomato juice, lemon juice, baking soda, Alka Seltzer, high pH shampoo.
2. Rinse the hair immediately after leaving the swimming pool.
3. Use a shampoo that chelates the copper.
Before swimming, prevent copper from binding to the hair by sealing the hair cuticle with a hair conditioner.
Swimming pool chemicals, especially chlorine and bromine, react with the sebum and oils that protect the hair, leaving the hair's cuticle exposed.
This allows other chemicals to react with the hair, such as copper compounds, which can give the hair a greenish tint.
Hair also becomes more susceptible to damage from the ultraviolet rays of sunlight that break the bonds in keratin the protein in the hair to cause roughness and split ends.
Pigment molecules also succumb to swimming pool chemicals and the sun, so even if the hair does not turn green, it can become lighter or faded.
The best way to prevent swimmers hair is to keep the swimming pool water from soaking into the hair with a swim cap.
Limit exposure to swimming pool water by taking only occasional dips, not letting the hair get wet, and by showering after leaving the swimming pool.
The best treatment is to use a shampoo, but even a quick rinse in fresh water will help remove the swimming pool chemicals.
Follow up with a conditioner to seal the cuticle of the hair's cuticle and replenish its protective coating.
Healthy hair is less susceptible to swimmers hair than hair that already has damage.
Also, coloured, permed or heat-treated hair is at greater risk for dryness and colour loss from swimming than untreated hair.
If you swim a lot, reduce hair processing and keep up the cut so that chlorine will not get in through split ends.
A special shampoo made just for swimmers may contain ingredients that will chelate copper and other metals so that they do not discolour the hair.
The shampoo may leave a waxy coating on the hair to prevent it from soaking up swimming pool water.
Another shampoo is a clarifying shampoo to prevent build-ups that can weigh the hair down and dull its shine.
Another option is to use a regular shampoo and follow-up with a leave-in conditioner that contains a UV filter that give protection from both the sun and the swimming pool water.

18.2.20 Methyl orange test for chlorine in swimming pools
Free chlorine bleaches methyl orange solution quantitatively
Make a stock solution of methyl orange:
Dissolve 0.05 g methyl orange in 100 mL of water.
Make a standard solution: Add 0.2 g NaCl to 10 mL of stock solution then dilute to 100 mL.
Swimming pools will have different amounts of dissolved salt depending on their treatment history.
The addition of 0.2 g of NaCl is an attempt to ensure roughly the same ionic strength of the test sample as this may affect the accuracy of the measurement.
The reagent appears to be stable in the dark for years.
Make a test solution: Add 3 mL of 6 M hydrochloric acid to 24 mL of standard methyl orange solution.
Step 1: Put 0.25 mL of test solution in a test-tube.
Add 10 mL of swimming pool water.
1.1 It decolorizes instantly so at least 1 mg / kg chlorine present.
1.2 It does not decolorize instantly so insufficient chlorine in the swimming pool.
Add more chlorine to the swimming pool and repeat the test.
Step 2: Put 0.5 mL of test solution in a test-tube.
Add 10 mL of swimming pool water.
2.1 It decolorizes instantly so at least 1.5 mg / kg chlorine present.
2.2 It does not decolorize instantly.
The swimming pool contains between 1.0 and present.
1.5 mg / kg of chlorine.
This is the best concentration.
Step 3: Put 0.75 mL of test solution in a test-tube.
Add 10 mL of swimming pool water.
3.1 It decolorizes instantly so at least 1.75 mg / kg chlorine present.
3.2 It does not decolorize instantly.
The swimming pool contains between 1.5 and 1.75 mg / kg of chlorine.
This concentration is too high.

18.2.21 Stabilized chlorine
Stabilized chlorine is a family of organic chlorine compounds that contain stabilizer (cyanuric acid or iso-cyanuric acid) to protect the chlorine from the degrading UV rays in sunlight.
Most common types are dichlor and trichlor.
The granular form is dichlor and the tablet or stick form is trichlor.
Stabilized chlorine should be used with care, to avoid problems such as chlorine lock that may be caused by overstabilization.

18.2.22 Superchlorination, shock treatment, breakpoint chlorination
See diagram 13.4.2: Chlorine dosage and residual chlorine for breakpoint chlorination.
Superchlorination is adding 7 to 10 times the normal dose of chlorine to the swimming pool water to destroy ammonia, nitrogen, chloramines and other contaminants.
Shock treatment is adding large amounts of an oxidizer such as chlorine, hydrogen peroxide or potassium peroxymonosulfate to the swimming pool water to destroy ammonia and nitrogen compounds, chloramines and other contaminants.
Superchlorination is used to get rid of chloramines.
When HOCl is added to pool water at first chloramines form by reaction with nitrogen compounds from humans.
With increased concentration of HOCl the chloramines are destroyed as in the following equation with the products being off-gassed.
However, with excess HOCl, the chlorine concentration in the water rises as chlorine no longer combines with organic compounds.
2NH3 + 3HOCl --> N2 (g) + 3HCl + 3H2O
ammonia gas + hypochlorous acid --> nitrogen gas + hydrochloric acid + water
When chlorine in its various forms is added to water, it is used up in oxidizing any material for which it is a sufficiently strong oxidizing agent, e.g. iron II, sulfide, nitrite.
Next, chloramines, called combined chlorine, form from reactions of chlorine with organic nitrogen compounds.
The breakpoint when further addition of chlorine is not used to oxidize nitrogenous compounds, but remains as free available chlorine.
When you superchlorinate or shock treat the swimming pool, the goal is to reach a high enough level of free chlorine to break apart.
All molecular bonds, specifically the combined chlorine molecules, ammonia or nitrogen compounds and to completely oxidize all organic matter.
If you add enough chlorine to achieve this breakpoint chlorination, chlorine added after that point will be free available chlorine.
Superchlorination refers to further additions of chlorine that will remain in the water as residual chlorine to react with any material later added to the swimming pool water.
Maintain a free chlorine residual at all times to achieve sparkling clear, sanitary swimming pool water by periodic super chlorination of the swimming pool water.
When chloramines are removed, better efficiency of chlorine is achieved.
More of the chlorine residual can then exist as the free or active form, rather than as the less effective combined form.
You can use potassium monopersulfate for strong oxidation reactions to oxidize any chloride ion back to chlorine.
Superchlorination is the remedial action that should be taken when the DPD test detects the presence of 0.2 ppm or greater of combined chlorine in the swimming pool.
Superchlorination simply involves the addition of 5 to 10 × normal daily dose of chlorine.
Super chlorination should be routinely performed on very hot days under heavy loads after heavy rains or if DPD test shows combined chlorine in excess of 0.2 ppm combined chlorine.
Superchlorination is a remedial action and is recommended to remove gross amounts of chloramines, correct eye and nasal discomfort, or destroy a visible algae growth.
It also eliminates waiting, vacuuming, scrubbing and back washing to remove algae appeared while chloramines accumulated.
However, although it works well, eventually if you keep loading chemicals and contaminants in a swimming pool you problems.

18.2.23 Test kit for chlorine levels in swimming pools
A test kit is a manual or electrical device used to measure specific chemical residuals, levels or demands in swimming pool water.
Kits usually contain reagents, vials, titrants and colour comparators for the tests.
The most common tests are as follows: pH, free available chlorine, combined chlorine, total alkalinity, calcium hardness, cyanuric acid and metals.
Test strips are small plastic strips with pads attached that have been impregnated with reagents to test swimming pool water.
The strips are dipped into the water and the resulting colours are compared to a colour scale to find the values.
Balanced water is the result when all the chemical parameters are within the tolerance limits.
The most important parameters of water balance are pH, total alkalinity, calcium hardness and temperature, as measured using the Langelier Saturation Index.

18.2.24 Tests for free chlorine in water
Free chlorine (free available chlorine, free residual chlorine, residual chlorine) is the amount of chlorine left in the swimming pool water after the chlorine demand has been satisfied.
It is the amount of free chlorine in the swimming pool water that is available to oxidize, sanitize or disinfect the water.
Free chlorine is active chlorine and is not combined with any other molecule e.g. ammonia.
It is free to kill bacteria and algae in a swimming pool and is the chlorine available to do its job of sanitizing the water.
A portion of the free available chlorine is present as hypochlorous acid, which reacts to destroy organic material in the swimming pool water.
The standard method for determining free chlorine is to measure the amount of oxidant by its ability to liberate iodine from acidified iodide solution.
Titrate a water sample with a standard iodide solution and detect the iodine released by the blue colour formed with a fresh starch indicator.
Find the amount of iodine released by back titration with sodium thiosulfate, hypo.
The reagent is only oxidized to tetrathionate by the iodine.
With chlorine it is oxidized to sulfate.
HOCl + 2I- + H3O+ --> Cl- + I2 + 2H2O
hypochlorous acid + iodide ion + hydronium ion --> chloride ion + iodine + water
I2 + 2S2O32- --> S4O62 -+ 2I-
iodine + thiosulfate ion --> tetrathionate ion + iodide ion
Devices called rapid test meters can be used to test for chlorine and pH.
With a DPD test kit, DPD1 shows free available chlorine and DPD3 shows total chlorine.
The difference, if any, is the level of combined chlorine.

18.2.25 Total chlorine in swimming pools
Total chlorine is the sum of combined chlorine and free chlorine, i.e. the total amount of chlorine in the swimming pool water.
When chlorine is added to water in a newly filled swimming pool, some of it is consumed in the process of destroying algae, bacteria and other oxidizable material in the water.
The amount of chlorine consumed in this process is referred to as the chlorine demand of the water.
Once the chlorine demand is satisfied, any additional chlorine added is referred to as chlorine residual.
With a DPD test kit, DPD1 shows free available chlorine and DPD3 shows total chlorine.
The difference, if any, is the level of combined chlorine.

18.3.1 Acid demand of swimming pools
Acid demand is the amount of acid required to bring high pH or total alkalinity down to their proper levels is determined by an acid demand test.
The acid demand test is a reagent test usually used in conjunction with a pH test to find the amount of acid needed to lower pH or total alkalinity.
Acid demand is the amount of acid that needs to be added to swimming pool water to lower the pH and total alkalinity to acceptable levels.

18.3.2 Alkalinity, total alkalinity and buffer capacity
See diagram 13.4.2: Titration of sodium carbonate with hydrochloric acid.
Alkalinity is the total amount of dissolved alkaline compounds in swimming pool water.
Alkalinity is used a measure of the buffer capacity of swimming pool water, i.e. the resistance to change in the pH of water when strong acids or bases are added.
If the alkalinity is too low, pH control is difficult, because the pH is sensitive to small amounts of acid and base.
Calculate how much 0.1 M hydrochloric acid would be needed to change the pH by one unit if there is no buffering capacity.
The pH 8.4 of sodium bicarbonate in water lies between the pK values of the first and second dissociation constants of carbonic acid, 3.60 and 10.25.
Carbon dioxide constitutes about 0.033% of the atmosphere and dissolves in rainwater to form carbonic acid and produce theoretical pH value pH 5.65.
Carbonic acid has two acidic hydrogens and hence has two dissociation constants, PKa1 and PKa2.
H2O (l) --> H+ (aq) + OH- (aq)
2H+ (aq) + CO32- (aq) --> H2CO3 (aq) carbonic acid
CO2 + H2O --> H3O+ + HCO3-
HCO3- + H2O --> H3O+ + CO32-
CO2 + H2O --> H2CO3 carbonic acid
H2CO3 --> HCO3- + H+
Ka1 2.5 × 10-4 mol / L, Pka1 3.60
HCO3- --> CO32- + H+
Ka2 5.61 × 10-11 mol / L, PKa2 10.25
Total equation: CO2 + H2O --> HCO3- + H+
Total equation KA = 4.30 × 10-7, PKa = 6.36, but call this value the acidity constant.
Total alkalinity is a measure of the buffering capacity of swimming pool water, i.e. resistance to change in pH of the water when acid is added.
It is usually expressed as carbonate alkalinity, the total amount of alkaline materials present in the water, principally hydroxides, carbonates and bicarbonates.
Control of pH can be simplified by maintaining total alkalinity in the range of 80 to 150 ppm.
Total alkalinity is composed of carbonates, bicarbonates, and hydroxides, and functions as a buffer to help keep pH in the proper range.
Total alkalinity is easily measured with a test kit, and can be adjusted with alkalinity control or acid according to label directions.
When you measure the total alkalinity, only measure the carbonate alkalinity level.
The term "alkalinity" comes from the common use of sodium bicarbonate as a buffer and its alkaline pH of 8.4.
However, you can also use acid buffers and so "buffer capacity" may be a better term than "alkalinity".
If the swimming pool has an unstable pH, the total alkalinity should be tested and adjusted.
The ideal total alkalinity range for swimming pools containing either calcium or sodium hypochlorite is between 60 and 120 mg / L with the optimum between 80 and 100 mg / L.
Total alkalinity too low
A swimming pool with low alkalinity, below 60 mg / L, will require the addition of a little acid to cause a rapid reduction in pH.
A constant pH in low alkalinity water is difficult to maintain after rain.
Low buffering capacity causes corrosion and pH bounce, i.e. the pH resumes previous levels soon after adjustment, because the buffering capacity is too low.
Raise total alkalinity by addition of an alkalinity control base, e.g. sodium bicarbonate, that in turn increases the buffering capacity of the swimming pool.
To raise alkalinity with sodium bicarbonate, the dosage calculation is based on tables prepared for the swimming pool industry.
For example, a 25 000 L swimming pool would need 445 g of 57% sodium bicarbonate to increase alkalinity by 10 mg / L.
You can add 80 to 120 mg / kg of the white powder sodium bicarbonate as a buffering agent to raise the total alkalinity of swimming pool water without having much affect on pH.
It will only increase pH up to 8.4, regardless of the quantity used.
Avoid adding large quantities at one time.
Total alkalinity too high, A swimming pool with high alkalinity, over 120 mg / L, requires the addition of large quantities of acid to produce a drop in pH.
It will tend to retain pH in most conditions and cause staining, scale deposits and difficulty in adjusting pH.
High buffer capacity requires large amounts of acid or base to change the pH of the solution.
If the pH rises rapidly even after the addition of large amounts of acid, the buffering capacity is too high.
Reduce total alkalinity to 80 ppm by addition of granular addition of PH minus or hydrochloric acid (muriatic acid) or sodium bisulfate (sodium bisulfate, sodium hydrogen sulfate) granular dry acid, a dry white crystal that is safer to handle than hydrochloric acid.
To lower alkalinity with hydrochloric acid or sodium bisulfate, the dosage calculation uses tables to find the volume of hydrochloric acid required to lower alkalinity.
The weight of sodium bisulfate, dry acid, required is found by multiplying the volume of hydrochloric acid by 1.2.

18.3.3 Adjusting the pH of swimming pool water
The ideal range for pH in swimming pools is 7.0 to 7.6.
The pH of human tears is 7.2 to 7.4.
Measure the pH with an indicator that changes colour at a suitable pH.
The indicator is a weak acid that shifts from one colour to the other just as hypochlorous acid shifts from HOCl to OCl-.
Use an indicator with the same pKa as hypochlorous acid.
The pH of natural water is about pH 5.6, because dissolved carbon dioxide from the air forms carbonic acid that lowers the pH.
The continuous addition of hypochlorite powder will raise the pH, so you must add "swimming pool acid" after some time, e.g. the solid "swimming pool acid" sodium hydrogen sulfate, NaHSO4.
You can add hydrochloric acid to salt water swimming pools with electrolyzers.
A neutralizer is a chemical used to deactivate or destroy chlorine or bromine used in better test kits to increase the accuracy of the swimming pool water tests.
Base demand is a titration test used to find the amount of a base (alkali) required to increase the pH to the correct level.
Base demand determined by a titration to find the amount of a base (alkali) required to increase the pH to the correct level.
Sodium bicarbonate is used as a base to raise the pH, but sodium carbonate is more effective and more commonly used.
The adding of sodium bicarbonate to water gives pH 8.4, but adding sodium carbonate to water gives pH 11.6.
The addition of acid changes the pH according to the titration curve diagram.
The pH of swimming pool water may be changed from pH 7.2 to 7.8 by rain, dust, swimmer wastes and addition of sanitizer.
However, a stabilized granular form of sanitizer, with a pH of between 6 and 7, will have little effect on the pH of swimming pools.
Stabilized swimming pool chlorine tablets, although low in pH, have a minimal effect on pH, because their high available chlorine and stabilization means far less is needed to provide effective sanitation.
Chlorine gas, calcium hypochlorite, and liquid bleach all require greater pH adjustment, because their pH is far from the desired operating range of pH range 7.2 to 7.8.
This pH range must be maintained, because of its impact on sanitizer efficiency, swimmer comfort, corrosion and scaling.
Since total alkalinity affects the amount of pH adjusting chemical added, it should first be adjusted to the 80-125 ppm range.
After adjusting swimming pool chemistry, pH balance should be checked with pH adjusting chemicals to protect chlorine from UV rays.
18.3.4 Hypochlorous acid, HOCl

18.3.4 Hypochlorous acid, HOCl
Hypochlorous acid, HClO, is the active form of chlorine in swimming pool water.
Hypochlorous acid is a weak acid and dissociates poorly below pH 6, so it exists mainly as HOCl at low pH.
Between pH 6.0 and 8.5, a very sharp change occurs from undissociated HOCl to almost complete dissociation.
At 20oC and pH greater than 7.5, hypochlorite ions, OCl-, predominate.
HOCl is about 100 times more effective as a sanitizer than OCl-, because the negative charge on the OCl- ion hinders it entering living cellular structures and oxidizing the contents.

18.3.5 Swimming pool water and pH
Lower the pH of the swimming pool below pH 7 to increase the oxidizing strength of the HOCl, but more acidic solutions will corrode many components .
For tiled swimming pools, the recommended pH range is pH 7.4-8.0 and you must add about 200 mg / L calcium chloride to offset removal of calcium salts from the grouting between the tiles.
A test for chlorine in water may measure the free residual chlorine or the free residual chlorine plus the chloramines, which are termed combined chlorine.
The lower the pH, the more readily chloramines form.
Above pH 7 few chloramines form, so keep the pH above 7.

18.4.1 Swimming pool chemistry
A swimming pool system lacks the three purifiers that protect water quality in natural bodies of water:
1. Aeration, the addition of oxygen to the water from the continuous flow of water through lakes, streams and rivers.
2. Dilution of sediment from continuous water flow.
3. Prevention of contaminant build-up by water flow, movement and dilution, and biodegradation by aquatic organisms.
As a swimming pool lacks these purifiers, it is subject to rapid stagnation.
Also, it is usually contaminated with bacteria, algae, dust and dirt, and organic materials from swimmers wastes, sweat, urine and faeces.
A swimming pool must be disinfected as part of a total system to remove bacteria, algae and organic contaminants leaving water with acceptable clarity and colour.
Volume of a swimming pool length × width × depth.
If length 25 m, width 10 m, depth 1 m to 2 m (average 1.5 m), then volume = 25 × 10 × 1.5 = 375 cubic metres (375 000 litres).

18.4.2 Contaminants in swimming pools
Contaminants is the general name for any micro particle or organism that reduces swimming pool water clarity, quality or presents health hazards.
Filtering, oxidizing and sanitizing are necessary to destroy the contaminants.

18.4.3 Corrosion in swimming pools
The effects of an acidic swimming pool environment when the pH and / or alkalinity are very low.
Corrosion in the form of etching, pitting or erosion of swimming pool equipment and surfaces is the result.
Corrosion may also be caused by misuse of acid or by soft water.

18.4.4 Defoamers
Foam is a froth of bubbles on the surface of the swimming pool water.
It usually comes from overuse of Algaecides, but may also be caused by soaps, oils or other contaminants carried by swimmers.
Enzymes may be used for foam control.
A defoamer (anti-foam) is a chemical added to swimming pool water to destroy the foam.
These products do not remove the source of the foaming.
Shocking and superchlorination may help prevent foaming.
Controlled use of certain of the cheaper Algaecides can prevent their resulting in foaming.

18.4.5 Disinfectants
Disinfectants are chemicals, elements or processes that destroy vegetative forms of micro-organisms and other contaminants.
Examples are chlorine, bromine, ionizers, ozonators and copper and silver Algaecides.
Only chlorine and bromine are used as oxidizers, disinfectants and sanitizers in swimming pools.
Hydrogen peroxide is an unstable, colourless liquid, which is used as an antiseptic in the home.
It can be used as an oxidizing agent in swimming pools.
It is NOT compatible with the swimming pool wizard.

18.4.6 Make-up water, fill water
Make-up water (fill water, top up water, refill water), is the water used in filling or topping up the swimming pool.
It is the water used to replace swimming pool water lost to evaporation, splash out, leaks and back-washing in swimming pools.

18.4.7 Filters
The three main types of swimming pool filters used in swimming pools are sand filters, cartridge filters and diatomaceous earth, DE filters.
Filter aids include the clarifiers, flocculents and coagulants that are the inorganic salts of aluminium (alum) or organic polyelectrolytes, and diatomaceous earth, called filter powder.
Sand filters use hard sharp silica or quartz or the zeolite clinoptilolite.
Filter micron size is in a unit of length equal to 1 millionth of a metre.
Microns are used to describe the pore size of swimming pool filter media.
Sand filters have openings of 25 to 30 microns, cartridge filters have openings of 8 to 16 microns, and DE (diatomaceous earth) filters have openings of 1 to 5 microns.
Zeolite used in sand filters have openings of 3 to 6 microns.
Flocculents, e.g. alum, and coagulants cause minute particles to clump together to be trapped by the filter or fall to the bottom as "floc".
Flocculent are also used to clarify muddy swimming pool water with the charged aluminium ion from alum.
Aluminium ion is also used in sticks to coagulate blood from shaving cuts and in antiperspirants to coagulate sweat from pores under the arm.
Alum sinks everything to the bottom to be vacuumed to waste and a small amount of alum can also be used as a sand filter additive.

18.4.8 Ionizers for swimming pools
A swimming pool water sanitizer that uses electricity to generate metal ions, usually copper and silver.
It works by passing a current through a set of electrodes.
The copper is an Algaecide and algaestat, while the silver is a bactericide.
Ionizers can significantly reduce chlorine consumption.
If the ion levels get too high, problems with staining or discoloration of the swimming pool water occur.

18.4.9 Langelier saturation index
Langelier saturation index (LSI, Langelier index, saturation index), is used to find swimming pool water balance according to the levels of pH (total alkalinity) and calcium.
It measures the relative concentration of Ca2+ and CO32- to indicate if CaCO3 will precipitate out of solution and affect the clarity of the pool water or form an unsightly scale that may damage pumps.

18.4.10 Liquid acid for swimming pools
Liquid acid (hydrochloric acid, muriatic acid), is used for lowering pH, total alkalinity and for acid washing in swimming pools.

18.4.11 Magnesium hardness
Magnesium hardness is a measure of the amount of magnesium dissolved in swimming pool water.
It is part of total hardness.

18.4.13 Marbelite
Marbelite ("plaster"), is a mixture of white cement and white marble dust used as an interior finish of a swimming pool.
It can be given a colour or it may be left white.

18.4.14 Nitrogen pollution
Nitrogen pollution causes algae to bloom and disables chlorine in swimming pools.
It is introduced into the swimming pool water by rain and by swimmers.
Maintaining proper chlorine levels will prevent nitrogen from becoming a problem.
Superchlorination can destroy nitrogen and nitrogenous compounds.

18.4.15 Non-chlorine shock, potassium peroxymonosulfate
Non-chlorine shock is a granular form of potassium peroxymonosulfate (potassium monopersulfate, MPS, "Oxone"), commonly used triple salt oxidizing agent, 2KHSO5KHSO4K2SO4, used to oxidize materials such as micro-organisms, contaminants (ammonia, nitrogen, swimmer waste) or chloramines in swimming pools.

18.4.16 Organic waste
Organic waste (swimmer waste, bather waste), refers to the soap, deodorant, suntan lotion, body oils, sweat, spit, urine.
introduced into the swimming pool water by swimmers, as well as the leaves, dust and insects that end up in the swimming pool.
The organic waste may form undesirable chloramines, which require large amounts of chlorine or non-chlorine shock to be destroyed.
There is a famous myth that a special substance may be added to swimming pool water to reveal the presence of urine and embarrass the urinating swimmer.
No such substance exists.
Such a substance would probably react to other body excretions, e.g. sweat, and use of it would tempt naughty children to discolour the the water near another swimmer than swim away.

18.4.17 Sanitizers
The role of sanitizers is to kill all micro-organisms, including bacteria and algae, and to remove unwanted contaminants.
Ammonia NH3 quickly combines with chlorine to form bad smelling chloramines.
The chloramines are undesirable smelly compounds formed when insufficient levels of free available chlorine react with ammonia and other nitrogen containing compounds (swimmer waste, sweat, urine).
Chloramines are a threat to human health and are very poor sanitizers.
Chloramines can be destroyed by superchlorination.
Since all chlorinated sanitizers react with water to produce HOCl, chlorine consumption depends on the amount of contamination that is present.
Enough sanitizer must be added to meet the chlorine demand of the water before a measurable residual can be maintained.
This amount depends on the amount of contamination present in make up water, plus whatever is added by swimmer loading, rain, dust and other external sources, e.g. nitrogenous wastes from swimmers' bodies.
Whether they are as simple as ammonia in urine, or as complex as the components in perspiration or saliva, they present special problems when they accumulate in swimming pool water.
These contaminants react with HOCl to form compounds called chloramines, or combined chlorine.
The combined chlorine reaction begins with one unit of ammonia, combining with one unit of HOCl to form monochloramine NH2Cl.
This reacts with another unit of HOCl to form dichloramine and finally with a third unit of HOCl to produce trichloramine NCl3.
It takes a fourth unit of HOCl to finally convert the original molecule of ammonia into harmless nitrogen gas N2, water and chloride ion C l- and a fifth unit of HOCl before a free available chlorine residual can be measured.
These chloramines cause trouble in swimming pool water, because they are stable and persistent.
The monodichloramine and trichloramine from this first unit of ammonia will survive and accumulate with the chloramines formed from subsequent units of ammonia.
This is actually chlorine consumption, because HOCl combined with ammonia forms chloramines.
Chloramines have very poor sanitizing power, so algae and bacteria can grow.
In fact, they have such poor swimming pool sanitizing power that they would be rated at only 0 to 10 on a relative activity scale with HOCl rated at 10 000.
Chloramines could provide germ fee water if they were present at a concentration of at least 25 to 50 ppm.
However, this concentration would create additional problems in a swimming pool, because chloramines are very pungent and irritating, causing eye irritation and chlorine odours at very low concentrations.
Unfortunately, the chlorine odours generated by chloramines lead many people to think that too much chlorine has been added.
So, they stop adding chemicals and problems grow worse.
These symptoms are a signal to test and adjust pH, and add enough chlorine to oxidize all the chloramines, establishing a free available chlorine residual.
It is often very difficult to convince a swimming pool owner that insufficient chlorination is the cause of chlorine odours, eye burn and algae, because the owner probably tested the swimming pool when a problem was noticed, and got a very positive chlorine test according to the test kit.
This is the most confusing problem caused by combined chlorine.
Some test methods measure it as part of a total chlorine residual.

18.4.18 Scale
The precipitate that forms on surfaces in contact with swimming pool water when the calcium hardness, pH or total alkalinity levels are too high.
Scale may appear as grey, white or dark streaks on the plaster, fibreglass or vinyl.
It may also appear as a hard crust at the waterline.

18.4.19 Scum
The foreign matter that floats to the surface of the swimming pool water and forms a layer or a film.
It can also refer to a residue deposited on the tiles or walls of the swimming pool.

18.420 Sediments
The solid material that precipitates out of the swimming pool water and settles to the floor of the swimming pool.

18.1.54 Precipitation
Precipitation is material forced out of solution will settle, stain, scale or remain suspended in the swimming pool water.

18.4.21 Stabilized and unstabilized swimming pools
In swimming pool waters, the free chlorine, HOCl, may be consumed by the following:
1. Destroying bacteria and algae introduced by swimmers and by wind and rain borne contamination,
2. Reacting with reduced metals such as Fe2+ to produce the oxidized Fe3+ and chlorine ions,
3. Action of the ultraviolet energy of sunlight that converts free available chlorine to the inactive chloride ion,
4. Oxidizing nitrogenous compounds such as ammonia NH3 and urea introduced into the water as components of perspiration, urine and other bodily excretions.
Maintenance of free available chlorine residual at 1 ppm to 3 ppm.
Pre-conditioning the swimming pool with 30 ppm of conditioner, and using a sanitizer to maintain that minimum, automatically provides protection for a swimming pool against the effects of sunlight.
Most overseas countries require swimming pool operators to measure free chlorine and ORP.

18.4.22 Starting to use a filled swimming pool
1. Adjust the water level.
2. Check pump, filter, dosing equipment and gauges, and replace or repair if faulty.
Check the pump strainer basket on the suction side of the pump, which traps large debris in the swimming pool water flow without causing restriction.
It is sometimes called a Pump Leaf Trap.
Swimming pool pumps can cause severe injury to small children if they are small enough to sit over the pump.
Some children in Australia have had their lower colon and anus pulled out by a swimming pool pump.
3. Check for algae infestation on walls and floor.
4. Superchlorinate until free chlorine residual of 10 milligrams per litre is obtained.
This may be achieved by the addition of calcium hypochlorite or by running the sodium hypochlorite dosing pumps while the recirculation system is running.
5. Start up the pump and filter system.
6. Run for at least 48 hours before allowing swimmers into the swimming pool.
7. Test swimming pool and make adjustment to pH water balance if required.

18.5.1 Parts per million
It is a method of assigning value to concentrations of chemicals in the swimming pool water.
Many of the common swimming pool water tests, as well as acceptable ranges, are stated as ppm.
One ppm is equivalent to 1 milligram of something per litre of water (1mg / litre).
ppt: parts per thousand, approx. g / kg of salt in solution, salinity sensors, total dissolved salt content in water

18.5.2 DPD test for swimming pools
DPD, N,N-Diethyl-P-phenylenediamine, C10H16N2, (C2H5)2NC6H4NH2
DPD is an indicator reagent used to measure free available chlorine (DPD1) and total chlorine (DPD3) bromine, ozone and other oxidizers in water.
DPD is far superior to OTO.
The efficacy of chlorine is affected by many factors, including the sun, temperature, water balance and the chlorine demand of the water.
DPD, is the phenylenediamine used to determine chlorine in water.
DPD free chlorine reagent powder pillows, DPD No. 1 tablets, DPD No. 3 tablets
DPD total chlorine reagent powder pillows.
1. In the DPD calorimetric method to measure total free chlorine, the intensity of the colour is directly proportional to the amount of chlorine present in the sample.
When DPD is added, the chlorine oxidizes it to magenta-coloured "Wurster Dye", and colourless imine compound.
However, the test does not differentiate between the two forms of free chlorine: HOCl, and OCl- and at high pH gives an inaccurate reading.
DPD is a 2+ cation (NH3+ and N+H), e.g. AQUANAL, often as a sodium salt.
2. Measure chlorine levels in the range 0 -2 ppm by simple colour comparison
colourless DPD amine cation + Cl2 --> magenta colour Wurster dye free radical cation + colourless imine cation.
3. Measure "combined chlorine" as monochloramine, NH2Cl and dichloramine, NHCl2,
3.1 To measure monochloramine, add 0.1 mg KI to a 10 mL test sample acidified to pH 6.2 to 6.5
NH2Cl + 3I- + H2O + H+ --> NH4OH + Cl- + I3-
Then the I3- (and free chlorine in the sample) forms the magenta Wurster dye
3.2 To measure the dichloramine, add >0.1 g of KI to the 10 L sample
NHCl2 + 3I- + H2O + 2H+ --> NH4OH + 2Cl- + I3-
This last reaction gives total chlorine in the sample so the dichloamine is measured by subtraction.
4. When DPD, is added to the sample, chlorine oxidizes the DPD to form two possible oxidation
product, magenta coloured compound called Wurster Dye, and a colourless imine compound.
When small amounts of chlorine react, Wurster Dye is the favoured product.
When high levels of chlorine react the colourless imine compound is favoured, leading to the apparent fading of the solution.
This method does not differentiate between the two forms of free chlorine: HOCl, and OCl-.
The amounts of HOCl and OCl- present in solution are pH dependent.
If the process pH is allowed to rise too high, all free chlorine in solution will be present as OCl-, the less efficient sanitizer, yet the total free chlorine levels, as reported accurately by the DPD test, will remain the same.
5. The DPD test kit is in very common use.
DPD will distinguish free from combined chlorine.
A DPD residual will be fee available chlorine, not some combination of HOCL, monochloramine, dichloramine and trichloramine.
It will effectively protect the swimming pool from contamination.
The method is simple and rapid.
A DPD NO. 1 tablet is dissolved in a measured amount of swimming pool water to produce a coloured solution.
This colour is compared with the colour standards in the comparator to find the amount of FAC that is present.
The amount of chloramine that is present can easily be determining by adding a DPD NO. 3 tablet to the same test sample and determining whether any additional colour develops.
The difference between this total chlorine measurement and the FAC test result equals the chloramine content of the water.
Example: DPD No. 3 minus DPD No. 1 Chloramines 1.5 ppm to 1.3 ppm 0.2 ppm chloramines.
6. Total Free Chloramine residual
This test method indicates precisely when remedial action is necessary to prevent the accumulation of chloramines in swimming pool water.
Bleaching out occurs when at above 10 ppm free available chlorine in a swimming pool, a DPD test kit often indicates zero chlorine. because the reagent is being destroyed.
If you observe an initial pink colour that then rapidly fades, you probably have far too much residual chlorine in the water.

18.5.4 OTO test for swimming pools
(OTO, ortho-tolidine, ortho-toluidine, o-Toluidine, 2-Aminotoluene, 2-Methylaniline, CH3C6H4NH2)
The OTO test kit is seen as unreliable and is no longer used extensively.
If you find a yellow colour in the comparator and a clear liquid reagent, then it is clear that this swimming pool is being tested by the orthotolidine, or OTO method.
The OTO method has some advantages that have made it popular and widely used.
It also has some major deficiencies.
OTO reacts easily with hypochlorite, but reacts more slowly with chloramines, so does not reliably measure "free chlorine", the chlorine in the form of OCl- and HOCl.
The fact that it cannot easily distinguish free from combined chlorine makes it a very doubtful aid to swimming pool operation.
Even worse, it creates a false sense of security, leading to erroneous diagnosis of swimming pool problems, which delays remedial action.
The swimming pool owner could test once, twice, or three times daily and still have no idea whether enough free available chlorine was present to protect the quality of the water.
A 1 ppm residual measured by OTO will provide far less protection to swimming pool than a 1 ppm residual measured by the DPD method.
Ortho-toluidine may be carcinogenic.

18.5.6 Total dissolved solids (TDS) water hardness, scale
TDS, Total Dissolved Solids, is a measure of everything that has ever dissolved in the swimming pool water and all the matter that is in solution.
They stay in the water and may cause staining, scaling and reduced chlorine efficiency, if allowed to accumulate beyond recommended limits.
All swimming pool water contains total dissolved solids.
Stabilizing and using chloroisocyanurate produces the least amount of TDS.
It is estimated that TDS should be maintained at less than 1500 ppm.
Cyanuric acid causes no ill effects in swimming pool water, but that it may, by accumulation, indicates the onset of problems, because of TDS in the swimming pool.
The only way to lower TDS is to drain part of the swimming pool water and replace it.
The recommended rate of water removal per week is 1 to 3%.
In a 10 000 gallon swimming pool, this represents 100 gallons per week.
Hardness, total hardness is a measure of the calcium and magnesium content of the water.
All water contains some natural hardness.
The amount will vary regionally, and from source to source.
Calcium hardness refers to the calcium content of the water.
If the calcium hardness level is too low, the water may be corrosive and attack the materials of construction.
So a certain amount of hardness is necessary in water to control its tendency to dissolve.
Hardness treatment will increase low water hardness and prevent etching, pitting and corrosion of surfaces and metallic components.
The soluble white salt calcium chloride is used to raise the calcium hardness or total hardness level of swimming pool water.
If the calcium hardness level is too high, the water may have a tendency to form scale visible as crusty grey deposits and cloudy water or deposits in piping that will y not function properly if their diameters are decreased by scale formation.
Scale on swimming pool surfaces is unsightly and unattractive.
Scale is formed from calcium carbonate crystals when the calcium hardness, pH or total alkalinity levels are too high.

18.5.7 Suspended solids
Insoluble solid particles that either float on the surface or are in suspension in the swimming pool water causing cloudiness.
They may be removed by filtration, but if the particles are too small, a flocculent or coagulant is necessary to enable the filter to trap them.

18.5.8 Turbidity
Turbidity is the cloudy condition of the swimming pool water, because of the presence of extremely fine particles in suspension that are able to pass through the filter.
Adding a flocculent or coagulant will clump the particles together so they can be trapped in the filter.
Devices called turbidity meters can be used to measure turbidity.

18.5.9 Water hardness
Soft water
Soft water has a low calcium content that can cause etching of swimming pool surfaces, so water hardness should be increased with calcium chloride.
Hard water
Swimming pool water that is high in calcium, magnesium or other salts, which makes it difficult for soap to lather.
Hard water also has a tendency to form scale.
Calcium hardness: The amount of calcium dissolved in the swimming pool water.
It is usually measured as calcium carbonate.
Total hardness:
The amount of calcium, magnesium and other salts dissolved in the swimming pool water.
Hardness and water temperature.
When all the parameters are in balance, the swimming pool water will neither be corrosive nor scaling.