Learn how to prepare useful acid-base indicators.

School Science Lessons
(UNChemAcidBase)
2024-12-24

Acid-base indicators
Please send comments to: j.elfick@uq.edu.au
Contents
5.6.1 pH and acid-base indicators
5.6.2 Indicator Table
5.6.3 Indicator pH Range
5.6.4 Prepare acid-base indicators

5.6.2 Indicator Table
1.00 Acid fuchsin
2.00 Alizarin yellow R
3.00 Benzopurpurin 4B
4.00 Brilliant yellow
5.00 Bromocresol green
6.00 Bromocresol purple
7.00 Bromophenol blue
7.01 Bromophenol red
8.00 Bromothymol blue
9.00 Chlorophenol red
10 Clayton yellow
11 Congo red
12 Cresol red (IV)
13 Erythrosin
14 Indigo carmine
15 Litmus
16 Malachite green
17 Metacresol purple
18 Metanil yellow
19 Methyl green
20 Methyl orange
21 Methyl red
22 Methyl violet 10B
23 Methyl yellow
24 Neutral red
25 Nitramine
26 Orange IV, Tropeolin OO
27 Paramethyl red
28 Paranitrophenol
29 Phenol red
30 Phenolphthalein
31 Quinaldine red
32 Resazurin
33 Thymolphthalein
34 Thymol blue (1st range)
35 Thymol blue (2nd range)
36 Trinitrobenzene
37 Universal indicator

5.6.4 Prepare acid-base indicators
3.01 Prepare beetroot (beet) juice acid-base indicator
4.01 Prepare berry juice acid-base indicators
5.01 Prepare bromocresol green indicator
6.01 Prepare bromocresol purple indicator
7.01 Prepare bromophenol, blue acid-base indicator
8.01 Prepare bromothymol blue acid-base indicator
11.01 Prepare congo red indicator solution
12.01 Prepare cresol red acid solution
12.03 Prepare cresol red basic solution
12.04 Prepare crystal violet solution
13.01 Prepare goldenrod indicator, electric writing
15.01 Prepare litmus solution
16.01 Prepare malachite green oxalate solution
17.01 Prepare metacresol purple basic solution
18.01 Prepare metanil yellow solution
19.01 Prepare methyl green solution
20.01 Prepare methyl orange acid-base indicator
21.01 Prepare methyl red acid-base indicator
22.01 Prepare neutral red indicator solution
23.01 Prepare phenolphthalein acid-base indicator
24.01 Prepare plant extracts acid-base indicators
25.01 Prepare red cabbage acid-base indicator
26.01 Prepare rose petal acid-base indicator
27.01 Prepare universal indicator solution

1.00 Acid fuchsin, acid-base indicator
Acid fuchsin, Andrade's acid fuchsin, acid violet 19, fuchsin S, fuchsin acid, rubine S, C20H17N3Na2O9S3
Add 5 g of acid fuchsin to deionized water.
Add 150 mL of M sodium hydroxide solution.
Mix the solution and leave to stand for one day.
The colour should change from red to brown.
If the solution is not decolorized enough, add 10 mL of M sodium hydroxide solution.
Mix the solution and leave to stand for one day.
Repeat the process until the solution has a straw yellow colour.
pH colour change: 5.0-8.0
Acid: pink, Base: yellow

2.00 Alizarin yellow R, acid-base indicator
Alizarin yellow GG
pK: 11.0
pH colour change: 10.0-12.0
Acid: yellow, Base: red
Alizarin yellow, mordant orange l (sodium salt), azo dye, rust colour, Harmful if ingested.
Alizarin yellow as acid: C13H9N3O5, O2NC6H4N=NC6H3-2-(OH)CO2H, [5-(4-Nitrophenylazo)salicylic acid].
Alizarin yellow as sodium salt, C13H8N3NaO5, [sodium 5-(p-nitrophenylazo)benzenesulfonate].

3.00 Benzopurpurin, acid-base indicator
Benzopurpurin 4B, acid-base indicator, 1-naphthalene sulfonic acid, benzopurpurin 4b, C34H26N6O6S2, sodium salt,
direct red, cotton red 4b, red acid dye, changes from violet to red in the pH range 1.2-4.0, formerly used as a stain and
as an indicator, for liquid crystal displays, inks, dyes, micro-organism stains.
pH colour change: 1.2-4.0
Acid: violet, Base: red

4.00 Brilliant yellow, acid-base indicator
Brilliant yellow, direct yellow 4, C26H18N4Na2O8S2
pH colour change: 6.6-7.8
Acid: yellow, Base: orange

5.00 Bromocresol green, acid-base indicator
Bromocresol green (3',3',5',5'-tetrabromo-m-cresol-sulfonephthalein), C21H14Br4O5S
pK: 4.90
pH colour change: 3.8.0-5.4
Acid: yellow, Base: blue
It is used as a tracking dye.
5.01 Prepare bromocresol green indicator
Dissolve 0.1 g bromocresol green in 14.3 mL of M NaOH + 235.7 mL of water.

6.00 Bromocresol purple, acid-base indicator
Bromocresol purple, acid-base indicator, C21H16Br2O5S, dibromo-0-cresolsulfone phthalein
pH colour change: 5.2-6.8
Acid: yellow, Base: purple
6.01 Prepare bromocresol purple indicator
Dissolve 0.1 g bromocresol purple in 18.5 mL 0.01 M NaOH + 231.5 mL of water.

7.00 Bromophenol blue, acid-base indicator
Bromophenol blue, C19H10Br4O5S, (tetrabromophenolsulfonephthalein, 2-bromo-2-methylbutane, t-butyl bromide
pK: 4.10, dye, [dichromatism.
Dichromatism, bromophenol blue, resazurin, pumpkin seed oil.
pH colour change: 3.0-4.6
Acid: yellow, Base: blue

7.01 Bromophenol red, acid-base indicator
Bromophenol red, C19H12Br2O5S, 3'3"-Dibromophenolsulphonphthalein
pH colour change: 5.2-6.8
Acid: yellow, Base: red

8.00 Bromothymol blue, acid-base indicator
Bromothymol blue, bromthymol blue (dibromothymolsulfophthalein), C27H28Br2O5S, weak acid.
pK: 7.30
pH colour change: 6.0-7.6
Acid: yellow, Base: blue
8.01 Prepare bromothymol blue acid-base indicator
pH < 6.0 yellow to pH > 7.6 blue, in 20% alcohol solution.
Dissolve 0.5 g of bromothymol blue in 500 mL of water.
Add a drop of ammonia solution to turn the solution deep blue in colour.

9.00 Chlorophenol red, acid-base indicator
Chlorophenol red (dichlorophenolsulfonephthalein), C19H12Cl2O5S
pK: 6.25
pH colour change: 4.8-6.4
Acid: yellow, Base: red
It can detect chlorine dioxide in drinking water, green to brown-green or brown powder, crystalline powder, crystals, granules or lumps
It is soluble in ethanol, and has a "pharmacy smell".

10.00 Clayton yellow, acid-base indicator
Clayton yellow, direct yellow 9, titan yellow, thiazol yellow G, mimosa, C28H19N5O6S4Na2
pH colour change: 12.2-13.2
Acid: yellow, Base: amber

11.00 Congo red, acid-base indicator
Congo red, C32H22N6Na2O6S2
Congo Red 4B, Cosmos Red, Cotton Red B, Cotton Red C, Direct Red 28, Direct Red R
pH colour change: 3-5
Acid: blue, Base: red
11.01 Prepare congo red indicator solution
Dissolve 01% congo red indicator in water.
Microscopy stain, blue in acid and red in alkali, made from coal tar for dialysis diffusion experiments and as a yeast stain.

12.00 Cresol red (IV), acid-base indicator
Cresol red, indicator grade (o-cresolsulfonephthalein), C21H18O5S
pH colour change: 7.2-8.8
Acid: red, Base: yellow

12.01 Prepare cresol red acid solution
Dissolve 0.1 g cresol red in 26.2 mL 0.01 M NaOH + 223.8 mL water

12.02 Cresol Red, Na salt, indicator grade, C21H17NaO5S
pH colour change: 7.0-8.8
Acid: yellow, Base: red
Triarylmethane dye, used to monitor the pH in aquarium water, electrophoresis, irritates eyes, do not ingest it.
m-cresol, 3-hydroxytoluene, Highly toxic by all routes, lung irritant vapour, highly corrosive to skin
m-cresol, Solution < 12%, Not hazardous
12.03 Prepare cresol red basic solution
Dissolve 0.1 g cresol red, Na salt, in 26.2 mL 0.01 M NaOH + 223.8 mL water.

13.00 Erythrosin, acid-base indicator
Erythrosin B, Solvent Red 140, C20H8I4O5
Erythrosine, sodium salt, C20H6I4Na2O5, Red No.3
Cherry pink food colouring, E127, used to detect dental plaque
pH colour change: 2.2-3.6

14.00 Indigo carmine, acid-base indicator
Indigo carmine (acid-base indicator), microscopy stain, Toxic if ingested, decolorized by oxidizing agents, e.g. ozone.
Indigo carmine, sodium indigotin, disulfonate soluble indigo blue
Indigo carmine, Oxidation of glucose with sodium hydroxide and methylene blue.
Indigo carmine solution becomes brown-red on gentle shaking and pale green on more shaking.
On standing the sequence of colours reverses.
pH colour change: 11.5-14
Acid: blue, Base: yellow
Synthetic food colours approved for use in Australia, Blue shadow, CI73015 Indigo carmine

15.00 Litmus, acid-base indicator
1. Litmus paper contains several dyes, including the very sensitive purple-red dye azolitmin
Use red books below pH 5 and blue books above pH 8.
Litmus is also available as granulated litmus (tournesol), powdered litmus.
Litmus, litmus solution, granulated litmus, tournesol
Litmus paper, 20 sheets per booklet, pack / 10 booklets.
Litmus paper has the acid / base indicator absorbed into it.
Litmus powder blue colouring agent may irritate the eyes.
Used to colour beverages.
Litmus acid / indicator is extracted mainly from the lichen Rocella tinctoria

15.01 Prepare litmus solution
1. Use red books pH 5 and blue books pH 8 or boil powder litmus for five minutes.
Add drops of nitric acid until a purple colour appears.
Then filter and store in a bottle, but keep the solution exposed to the air.
Use a fresh solution before testing pH.
2. Boil 10 g crushed litmus powder in 500 mL water for five minutes.
Leave to stand, then filter the solution and store in a bottle.
Add drops of nitric acid until a purple colour appears.
Then filter and store in a bottle, but keep the solution exposed to the air.
Use a fresh solution before testing pH.
Litmus solution can from blue to red.
Put a finger width of litmus powder in the test-tube, and add water.
Shake to make the powder dissolve.
Add a finger width of tartaric acid powder until the colour changes to red.
Sodium carbonate can change this red liquid to blue.
Add a finger width of sodium carbonate to the red liquid until the colour changes back to blue.
Shake the test-tube to help the mixing.
3. Grind 250 g of granular litmus and put it in a flask with 500 mL of 40% ethanol.
Heat and boil the solution for one minute.
Decant the liquid to storage leaving a residue in the flask.
Add 500 mL of 40% ethanol to the residue.
Heat and boil the solution for one minute, then add it to the stored decanted liquid.
Centrifuge the solution and adjust the volume of the supernatant to 1000 mL with 40% ethanol.
Add M hydrochloric acid drop by drop until the solution becomes purple.
Test the solution by boiling 10 mL of deionized water, leave to cool, add one drop of the litmus solution.
Mix the drop with the water and the water should become mauve in colour.
For laboratory use, make a 2.5% solution on the litmus indicator in deionized water.

16.00 Malachite green, acid-base indicator
Malachite green, [C6H5C(C6H4N(CH3)2)2]Cl
pK: 1.0
pH colour change: 0.2-1.8
Acid: yellow, Base: blue-green
Used with carbol fuchsin in Sigma Acid Fast Stain as a counterstain and a differentiation.
Malachite Green oxalate salt (N, N, N, N-Tetramethyl-4, 4-diaminotriphenylcarbenium oxalate), Basic Green 4
(C23H25N2.C2HO4.0.5C2H2O4), counterstain for paraffin-embedded botanical material, spore stain for bacteria.
Food dye banned in food industry, same colour as malachite mineral, copper carbonate hydroxide, Cu2CO3(OH)2.
16.01 Prepare malachite green oxalate solution.
Dissolve malachite green oxalate 1.5% + acetic acid, 10% + glycerol, 17%, in deionized water.

17.00 Metacresol purple, acid-base indicator
Metacresol purple (in acid solution), C21H18O5S, m-cresol purple, m-cresolsulfonphthalein, cresol purple
pK: ~2.0
pH colour change: 1.2-2.8
Acid: red, Base: yellow
m-cresol purple sodium salt, C21H17NaO5S, m-cresolsulfonphthalein sodium salt, stain and dye
Metacresol purple (in basic solution) (m-cresol purple, cresol purple, m-cresolsulfonephthalein)
pK: 8.32
pH colour change: 7.6-9.2
Acid: yellow, Base: purple-red
17.01 Prepare metacresol purple basic solution
Dissolve 0.1 g metacresol purple in 26.2 mL 0.01 M NaOH + 223.8 mL water.

18.00 Metanil yellow, acid-base indicator
Metanil yellow, [3-(4-Anilinophenylazo)benzenesulfonic acid sodium salt], acid yellow 36, C18H14N3NaO3S
(m-(p-anilinophenylazo)benzene sulfonate), Na - salt
pH colour change: 1.5-2.7
Acid: red, Base: yellow
18.01 Prepare metanil yellow solution
Dissolve : 0.01% metanil yellow solution in water.

19.00 Methyl green, acid-base indicator
Methyl green, pyronin 100, ethyl green
monobromomonochloro ZnCl2 salt: C27H35BrClN3.ZnCl2
acid-base indicator
pH 0.2 yellow - pH 1.8 blue
Methyl green, Use as 0.1% solution in water.
It is used to stain chromatin in microscopy.
19.01 Prepare methyl green solution
Dissolve 0.1% methyl green solution in water.

20.00 Methyl orange, acid-base indicator
Methyl orange, C14H14N3NaO3S
pK: 3.46
pH colour change: 3.2-4.4
Acid: red, Base: yellow
Prepare as methyl orange solution: 0.01% solution in water.
20.01 Prepare methyl orange acid-base indicator
It is best for solutions with concentration > M/5.
Dissolve 1 g of methyl orange powder in water.
Use 2 drops for each 25 mL of solution in a titration.
1. Use as a 0.1% solution in 20% ethanol
2. Put a finger width of the methyl orange powder in the test-tube and add water to half fill the test-tube.
Shake to make the powder dissolve.
Add tartaric acid powder until the colour changes to red.
Sodium carbonate change this red liquid to orange.
Add a finger width of sodium carbonate to the red liquid until the colour changes back to orange again.
Shake the test-tube to help the mixing.
3. Methyl orange is best for solutions with concentration > M/5.
Mix 1 g of methyl orange powder with water.
Use 2 drops for each 25 mL of solution in a titration.

21.00 Methyl red, acid-base indicator
Methyl red, [2-(4-Dimethylaminophenylazo)benzoic acid, 4-Dimethylaminoazobenzene-2-carboxylic acid], acid red 2
(CH3)2NC6H4N=NC6H4CO2H
pK: 5.00
pH colour change: 4.4-6.2
Acid: red, Base: yellow
21.01 Prepare methyl red acid-base indicator
1. It is a sensitive indicator for titration of weak organic bases and ammonia.
Dissolve 1 g of methyl red powder in 500 mL of 60% alcohol.
Use 2 drops for 25 mL of liquid in a titration.
2. Dissolve 0.04 g of methyl red in 40 mL of ethanol and make up to 100 mL with water.
3. For a 0.05% solution of methyl red, dissolve 0.025 g in 20 ml of 95% ethanol, and add water to bring the volume close to 50 mL.
If any of the solid methyl red has not dissolved, add drops of 0.1 m sodium hydroxide until the volume reaches 50 mL.
4. For a basic solution of methyl red, add to 1mL of solution 3. (above), 10.0 mL of 0.1 M sodium acetate solution and 9.0 mL of 1.0 M potassium chloride solution.
Then dilute to 100 mL with distilled water and mix the solution.
5. For an acidic solution of methyl red add to 1mL of solution 3. (above), 10.0 mL of 0.1 M acetic acid solution and 10.0 mL of 1.0 M potassium chloride solution.
Then dilute to 100 mL with distilled water and mix the solution.
6. To avoid turbidity, dissolve 50 mg of methyl red in a mixture of 1.86 mg of 0.1 M sodium hydroxide and 50 mL of 95% ethanol, then dilute to 100 mL.

22.00 Methyl violet, acid-base indicator
Methyl violet, 2B, basic violet 1, C24H28N3Cl, Paris violet, green crystalline powder, metal lustre
pH 0.15 yellow to pH 3.2 blue-violet
Use as 01 - 0.05 % in water
Use as 0.1 % solution microscopy stain
Methyl violet 10B, basic violet 3, C25H30N3Cl, gentian violet B, crystal violet
pH colour change: 0.0-2.0
Acid: yellow Base: blue-violet

Crystal violet (C.I. 42555, C.I. basic violet 3), C25N3H30Cl, Toxic if ingested
Crystal violet 10% W / V alcoholic
12.04 Prepare crystal violet solution
Dissolve 0.02% methyl violets olution in water.
Gentian violet 10B, C25H30ClN3 (CI: 42535) (1% solution), bactericide, anti-fungal

23.00 Methyl yellow, acid-base indicator
Methyl yellow (dimethyl yellow), C14H15N3
pK: 3.25
pH colour change: 2.9-4.0
Acid: red, Base: yellow
C.I. 11020
In aqueous solution at low pH, methyl yellow appears red. pH 2.9 and 4.0, becomes yellow above pH 4.0.

24.00 Neutral red, acid-base indicator
Neutral red, C15H17ClN4
(3-Amino-7-dimethylamino-2-methylphenazine hydrochloride), toluylene red, amino dimethyl amino toluminozine
C.I. Red Base 5, toluidine red, Toxic if ingested, microscopy stain
red pH < 6.8 to yellow pH > 8.0

22.01 Prepare neutral red indicator solution
1. Dissolve 0.1 g neutral red indicator in 100 mL deionized water, Dilute 1 in 10.
2. Mix 0.1 g stain + 0.2 mL 1% acetic acid + 100 mL water. Use as 1% solution in 59% ethanol.
pH colour change: 6.8-8.0
Acid: red, Base: amber

25.00 Nitramine, acid-base indicator
Nitramine (acid-base indicator, 3.5), C7H5N5O8, tetryl, colourless-yellow crystals, explosive formerly for munitions
pH colour change: 11-13
Acid: colourless, Base: brown

26.00 Orange IV, acid-base indicator
Orange IV (acid-base indicator, 1.3), C18H14N3O3SNa, monosodium salt, orange G, acid orange, tropaeolin 00, benzenesulfonic acid
orange-yellow powder, soluble in water and alcohol

27.00 Paramethyl red, acid-base indicator
Paramethyl red, p-methyl red, [4-dimethylaminoazobenzene-4'-carboxylic acid], [sodium p-(p-dimethylaminophenylazo)-benzoate]
C15H15N3O2
pH colour change: 1.0-3.0
Acid: red, Base: yellow

28.00 Paranitrophenol, acid-base indicator
Paranitrophenol, p-nitrophenol, C6H5NO3
pK: 7.15
pH colour change: 5.6-7.6
Acid: colourless, Base: yellow

29.00 Phenol red, acid-base indicator
Phenol red, C19H14O5S, phenolsulfonephthalein, Toxic if ingested
pK: 8.00
pH colour change: 6.8 - 8.2
Acid: yellow, Base: red
Used to test pH in swimming pools.
The tablet form usually incorporates a chlorine neutralizer for more accurate results.
Phenol red, some preparations give a bright pink colour at pH 8.1.

30.00 Phenolphthalein, acid-base indicator
Phenolphthalein, C20H14O4, [3,3-Bis(4-hydroxyphenyl)-1(3H)-isobenzofuranone]
pK: 9.5
pH colour change: 8.0-10
Acid: colourless, Base: pink
See diagram Phenolphthalein.
Phenolphthalein, C20H14O4, weak acid, former laxative, was prepared from phthalic anhydride, C6H4(CO)2O.
It is a yellow-white to pale orange fine crystalline powder, odourless, tasteless, acidic in water,
colourless to pH 8.5, pink to red above pH 9, but colourless if large amount of alkali.
It is used as a cathartic medicine stimulating the intestinal mucosa and constricting smooth muscles, but a suspected carcinogen.
23.01 Prepare phenolphthalein acid-base indicator
Phenolphthalein, C20H14O4, indicator in acid-base titrations (acid- colourless, base-pink),
a weak acid, former laxative, prepared from phthalic anhydride, C6H4(CO)2O (FLAM) + 13 o C 1170),
pH 10 red, with excess alkali colourless again.
1. Dissolve 5 g phenolphthalein in 500 mL of ethanol, add 500 mL water, then stir.
2. Dissolve 1 g of phenolphthalein powder in 500 mL of 50% alcohol.
Add drops of this phenolphthalein solution to 100 mL of 0.5 mol / litre sodium hydroxide solution until a deep pink colour appears.
Divide this solution into 3 test-tubes.
Leave the first test-tube as a control.
Add drops of HCl to the second test-tube until the pink colour disappears.
Add 3 pellets of solid sodium hydroxide to the third test-tube.
Shake to dissolve.
The pink colour reappears.
3. Add colourless phenolphthalein indicator to lime water.
The liquid turns pink.
Blow into the liquid through a drinking straw.
The pink colour disappears and the liquid becomes cloudy.
4. Dissolve 1 g of phenolphthalein powder in 500 mL of 50% alcohol.
Add drops of this phenolphthalein solution to 100 mL of 0.5 mol / litre sodium hydroxide solution until a deep pink colour appears.
Divide this solution into 3 test-tubes.
Leave the first test-tube as a control.
Add drops of HCl to the second test-tube until the pink colour disappears.
Add 3 pellets of solid sodium hydroxide to the third test-tube 3 and shake to dissolve.
The pink colour reappears.
5. Add colourless phenolphthalein indicator to lime water.
The liquid turns pink.
Blow into the liquid through a drinking straw.
The pink colour disappears and the liquid becomes cloudy.
Phenolphthalein, C20H14O4, indicator in acid-base titrations (acid-colourless, base-pink), a weak acid, former laxative
Phenolphthalein is synthesised from by condensation of phthalic anhydride, C6H4(CO)2O (FLAM) with phenol.
pH 0 to pH 8.3 colourless, pH 8.3 to pH 10.0 pink, pH >10.0 with excess alkali colourless again.

31.00 Quinaldine red, acid-base indicator
Quinaldine red, [2-(4-Dimethylaminostyryl)-1-ethylquinolinium iodide], C21H23IN2
pH colour change: 1.0-2.2
Acid: clear, Base: red

32.00 Resazurin, acid-base indicator
Resazurin, C12H7NO4, sodium salt, C12H6NNaO4
pH colour change: 3.8-6.4
Acid: orange, Base: violet
Resazurin (red to colourless) is dark blue when first added to the solution to be tested then becomes pale blue then becomes pink purple on shaking.
Resazurin has dichromatism, the hue of the colour depends on the concentration of the absorbing substance and the thickness of the medium the light passes through.
Resazurin stain, test for milk
Resazurin tablets (tests: oxygen in solution, freshness of food, e.g. milk, viability of cells), large dichromatism.
Purchase: Resazurin sodium salt (7-Hydroxy-3H-phenoxazin-3-one-10-oxide sodium salt ), C12H6NNaO4, powder, BioReagent, suitable for cell culture.
Resazurin (7-Hydroxy-3H-phenoxazin-3-one 10-oxide), blue and weakly fluorescent, is reduced to pink and highly red fluorescent.
Prepare a 0.005% solution of the blue redox indicator dye resazurin by dissolving 1 tablet in 50 mL of deionized water.
Resazurin is pink when oxidized and colourless when reduced.
The Resazurin test for the quality of raw bulked milk is a rapid bacteria estimation, done either as a 10 minute pass or fail,
or a 3 hour triple reading test,
or as the standard and generally-accepted 1 hour test.
Resazurin gives milk a characteristic blue colour and the test is based on the ability of Lactobacillus, in the milk to reduce the blue dye.
The quality of the milk is judged by noting the degree of colour change, from blue through mauve and purple and pink and finally colourless,
after a stated period of incubation, or the time required to reduce the dye to a predetermined colour.

33.00 Thymolphthalein, acid-base indicator
Thymolphthalein, C28H30O4, ACS reagent, (5, 5-Diisopropyl-2, 2-dimethylphenolphthalein)
pK: 9.7
pH colour change: 9.4-10.6 Acid: colourless, Base: blue
Tests for carbon dioxide with thymolphthalein indicator
Thymolphthalein, C28H30O4, acid-base indicator, pH 9.4 colourless, pH 10.6 blue.
Thymolphthalein, pH RangepH: 9.4-10.6, Quantity per 10 ml: 1 drop 0.1% solution in 90% alcohol, Colour change: colourless blue
Put 125 mL of ethanol in a beaker and add 5 drops of thymolphthalein indicator.
Add drops of dilute sodium hydroxide solution until the solution turns blue.
Blow through a tube into the solution until it becomes colourless.
CO2 (g) + H2O (l) --> H2CO3 (aq) <--> H + (aq) + HCO3 - (aq)
CO2 (g) + 2NaOH (aq) + CO2 (g) --> Na2CO3 (aq) + H2O (l)
The sodium hydroxide is added to make the solution slightly alkaline at the beginning of the experiment and to absorb any initial carbon dioxide or any other acid.
Na2CO3 is less basic than NaOH.

34.00 Thymol blue, 1st range, acid-base indicator
Thymol blue, 1st range, acid solution (thymolsulfonephthalein), C10H16, Harmful if ingested
pK: 1.65
pH colour change: 1.2-2.8
Acid: red, Base: yellow

35.00 Thymol blue, 2nd range, acid-base indicator
Thymol blue, 2nd range, basic solution (thymolsulfonephthalein), C10H16, Harmful if ingested
pK: 9.20
pH colour change: 8.0-9.6
Acid: yellow, Base: blue

36.00 Trinitrobenzene, acid-base indicator
Trinitrobenzene, C6H3N3O6, (1,3,5-trinitrobenzene)
Used in chromatography

37.00 Universal indicator, acid-base indicator
Indicator papers, pH 0 to 14, strips, pack / 100
Indicator papers, pH 0 to 14, 30 second, 5 m reel
27.01 Prepare universal indicator solution.
1. Dissolve the following in 500 mL ethanol: 0.0250 g thymol blue, 0.0625g methyl red, 0.5 g phenolphthalein, 0.25 g bromothymol blue.
Dilute this solution to 1 litre with deionized water.
Add drops of 0.05 M sodium hydroxide until mixture is green.
2. Universal indicator test paper (FLAM 1142), is mixture of acid-base indicators that causes a colour change for each change in pH value over a wide range.
Note the colour chart on the bottle or package: Red pH 1-3 (strong acid solution)
Orange pH 4-5 (weak acid) (Pink pH 4), Yellow pH 6 (weaker acid), Green or pale green pH 7 (neutral)
Blue or green-blue pH 8 (very weak base), Indigo pH 9-10 (weak base) (Blue pH 9) (Blue-violet pH 10), Violet pH 11 to 14 (very basic solution)
3. Use 2 drops of Universal Indicator to 10 mL of test solution.
Test the pH value of the following substances: baking soda solution, demineralized water, dill pickle juice, distilled water, household ammonia, liquid soap,
pineapple juice, sodium bicarbonate solution, tap water, lemon juice, lime water, sodium hydroxide solution, vinegar, washing soda.
4. Dissolve in 500 mL ethanol: 0.0250 g thymol blue, 0.0625 g methyl red, 0.5000 g phenolphthalein, 0.2500 g bromothymol blue.
Dilute this solution to 1 litre with deionized water.
Add drops of 0.05 M sodium hydroxide until the mixture is green.
5. Test colours of universal indicator
Slowly neutralize lime water containing universal indicator, by adding acid drop by drop.
Describe what you see.
The starting colour is blue.
As citric acid is added, the colour changes, because the acid keeps weakening the alkali, neutralizes it (pale green colour), and gradually builds up its own strength.
Red: strong acid | orange: weak acid | yellow : weaker acid | pale green: neutral | green: weaker acid | blue: weak acid | violet: strong alkali.
6. Make a very dilute solution of citric acid by adding 7 g of citric acid to one litre of water.
Put two test-tubes of lime water in a beaker.
Put a piece of universal indicator paper in each beaker.
Stir until an inky blue solution forms then remove the universal indicator paper.
Add citric acid to the beaker, drop by drop.
Note that although much acid has to be added to form the different colours, it is the last drop that causes one colour to change into another.
If you miss a colour by adding the acid too quickly, add lime water to the beaker to restore the blue colour and start again.
To make the colours more easily seen, put the beaker on a white tile.
Citric acid is not a strong acid.
7. Universal Indicator is a mixture of indicators that causes a colour change for each change in pH value over a wide range.
Note the colour chart on the bottle or package:
Red pH 1-3 very acidic solution, orange pH 4 to 5 weak acid, yellow pH 6 very weak acid, green pH 7 neutral, blue pH 8 very weak base, indigo pH 9 to 10 weak base, violet pH 11 to 14 very basic solution.
Use 2 drops of Universal Indicator to 10 mL of test solution.
Test the pH value of lemon juice, vinegar, sodium bicarbonate solution, washing soda, lime water, NaOH solution, tap water, demineralized water.

3.01 Prepare beetroot (beet) juice acid-base indicator
1. Crush beetroot and use the juice as an acid-base indicator.
Add ammonia solution for red to dark purple colour change.
Adding vinegar for red colour to return.
Some teachers report yellow with sodium hydroxide, purple with ammonia solution and pink with acids.
2. The red colour of beetroot comes from betacyanin pigments.
Beetroot acid-base indicators have optical stability at pH 4 to 5, but are structurally unstable at extremes of pH.
So the red colour in urine after eating beetroot depends on urine pH and the pigments not being broken down by digestion processes.
Eating excess beetroot as in borscht soup usually causes red or pink urine.

4.01 Prepare berry juice acid-base indicators
Test the juices from stewed blackberries, black currants, and raspberries.
Also, mix a spoonful of fruit jam with warm water then filter it to get a colourless liquid.
Test the solution with acids and bases.

13.01 Prepare goldenrod indicator, electric writing
Direct Yellow 4 "Goldenrod"
Goldenrod paper is coated with a dye from turmeric that acts as an acid-base indicator.
It turns bright red in contact with bases and back to its normal golden colour in contact with acids (vinegar, lemon juice).
Use it to test household items, e.g. (soaps, milk, juice, drinks, food, to determine if they are acidic or alkaline (basic).
Experiment: Electric writing
Dip goldenrod indicator paper in salt water and place it on aluminium foil.
Connect the positive terminal of a battery to the foil using crocodile clips.
Connect a loose wire to the negative battery terminal.
Drag the end of the loose wire across the wet goldenrod indicator paper to create red writing.
If you reverse the polarity of the battery to erase the red writing.
Repeat the experiment with goldenrod indicator paper dipped in dilute ammonia solution.

24.01 Prepare plant extracts acid-base indicators
1. Use plant extracts to "indicate" whether a substance is acidic or basic.
Select brightly coloured flowers or leaves:
Rose, Bougainvillaea, hibiscus, geranium, red carnation (light red with acid and bright green with alkali), sweet pea, snapdragon, pansy, tulip, willow herb.
The colours are usually caused by anthocyanin water-soluble pigments that change colour with change in pH.
Boil a fresh unboiled beetroot, red cabbage, tomato skins (colourless in acid and deep yellow in alkali), blackberry or blackberry jam, damson, elderberry.
Squeeze or grind the plant material with a mortar and pestle with a mixture of 2 mL of acetone and 2 mL of methylated spirit.
Filter the solution, collect the filtrate, and label the indicator, e.g. "rose extract".
Rose extract colours may be scarlet-pink at pH 1, pale pink at pH 3, green at pH 4, yellow-brown at pH 7 and orange at pH 12.
Use universal indicator solution to test the plant indicators.
Indicators made from plants are mostly red with acids, but yellow, green or purple with alkalis.
2. Test common substances and note the colour change of the plant extract indicator:
Ammonia solution, antacid tablet solution, baking soda solution, bleaching powder solution, coconut milk, coffee grounds,
fertilizer solution, fruit juice, lemon juice, lemonade, lime water, red cabbage juice, saliva, soap solution, sugar solution, vinegar, tap water, tea bag in hot water, whitewash.
Estimate the range of pH tested by the plant extract indicators.
3. Fresh grape juice turns red in acid lemonade and blue in alkaline dishwater.
4. Put spots of plant extract indicators on absorbent paper and leave to dry.
Put one drop of lemon juice on each spot and note the colour change.
Note the colours given by sodium bicarbonate solution, washing soda, lime water and a dilute solution of sodium hydroxide.
These are alkaline, basic, substances.
Note whether they all give the same colour.
Plant extracts can act as indicators to test whether a substance is acidic or basic.
5. Add a few drops of sodium bicarbonate solution to 1 mL of flower extract indicator in a test-tube.
Then add lemon juice and note any colour change.
6. Repeat the experiment with lime water and indicator followed by dilute hydrochloric acid.
Note any colour change.
Note whether the original colour returns after by adding more lime water.
Note how many times the indicator colour can change before the test-tube is full.
7. Soak cut pieces of red cabbage leaf in boiling water for 30 minutes then remove them.
Pour red cabbage water into the following solutions:
1. water, stays violet, 2. white vinegar, turns red, 3. baking soda solution, turns green.
Also, use the liquid from a container of pickled cabbage.
8. Fresh grape juice turns red in acid lemonade and blue in alkaline dishwater.
9. Put spots of plant extract indicators on absorbent paper and leave to dry.
Put one drop of lemon juice on each spot and note the colour change.
These are alkaline, basic, substances.
Note whether they all give the same colour.
Plant extracts can act as indicators to test whether a substance is acidic or basic.
10. Put some original filtrate on to another piece of filter paper.
When the filter paper is dry, note the colours given by sodium bicarbonate solution, washing soda, lime water and a dilute solution of sodium hydroxide.
These are alkaline, basic, substances.
Note whether they all give the same colour.
Plant extracts can act as indicators to test whether a substance is acidic or basic.
11. Add a few drops of sodium bicarbonate solution to 1 mL of flower extract indicator in a test-tube.
Then add lemon juice and note any colour change.
12. Repeat the experiment with lime water and indicator followed by dilute hydrochloric acid.
Note any colour change.
Note whether the original colour returns after by adding more lime water.
Note how many times the indicator colour can change before the test-tube is full.
13. Do NOT taste these acids in the laboratory.
Taste lemon juice, vinegar and cream of tartar at home.
Each acid has a sour taste that is a characteristic of acids.
Lemon juice contains the white crystalline citric acid.
Vinegar contains ethanoic acid (acetic acid, CH3COOH).
Cream of tartar contains the acid salt potassium hydrogen tartrate, the purified form of argol that occurs as brown crystals in fermenting wine.
14. Wave a bluebell flower closely over an ant nest.
The angry ants rush out to squirt formic acid on the bluebell flower and the blue pigment in the petals turns red.

25.01 Prepare red cabbage acid-base indicator
1. Soak cut pieces of red cabbage leaf in boiling water for 30 minutes then remove them.
Pour cabbage water into:
1.1 water, it stays violet,
1.2 white vinegar, it turns red,
1.3 baking soda solution or ammonia solution, it turns green.
2. Boil shredded red cabbage for 15 minutes then squeeze out the juice. Fry an egg.
When the "white" of the egg is about to change from colourless to white as the protein albumin denatures, add some red cabbage juice.
The "white" of the egg turns green.
3. Use the liquid from a container of pickled cabbage as an acid-base indicator
4. Use the green water juices from boiled cabbage as an acid-base indicator.

26.01 Prepare rose petal acid-base indicator
1. Boil red rose petals in some water until the petals have almost lost their colour and a pink solution forms.
Test this pink solution with acids and bases.
2. Wave a bluebell flower closely over an ant nest.
The angry ants rush out to squirt formic acid on the bluebell flower and the blue pigment in the petals turns red.

5.6.1 pH and acid-base indicators
The pH tests use an indicator that changes colour with changes in the concentration of hydrogen ions, or the acidity of the solution.
The pH scale (Peter Sorensen 1868 - 1939) is a scale of acidity and alkalinity that runs from pH 0, most acid, to pH 14, most alkaline.
A neutral solution has pH = 7, an acid solution has pH < 7, and a basic or alkaline solution has pH >7.
The term "pH" stands for "power of hydrogen" and measures the concentration of hydrogen ions in water.
The pH scale is logarithmic so a pH 4 solution is ten times more acidic than a pH 5 solution.
pH = -log10(H + ), where (H + ) = concentration of hydrogen ions.
(OH - ) = concentration of hydroxyl ions.
For water (H + )(OH - ) = 1 X 10 -14 at 25 o C.
Pure water is neutral where (H + ) = (OH - ) = 1 X 10 -7 M, i.e. at pH 7.
For acid solutions (H + ) is greater than (OH - ), so pH is less than 7 (0 to 7).
For alkaline solutions (H + ) is less than (OH - ), so pH is greater than 7 (7 to 14).
To convert pH to hydrogen ion molar concentration, [H3O + ] = Antilog(-pH), so if pH = 2.55, [H3O + ]
= Antilog(-2.55) = 2.8 X 10 -3 M.
Ionization of water
2H2O <--> H3O + + OH - (25 o C) Ka =1.00 X 10 -14 , pKa = 14.00
2H2O <--> H3O + + OH - (0 o C) Ka = 0.11 X 10 -14 , pKa = 14.94
Tests for common solutions with acid-base indicators
Acid-base indicators change colour in acidic or basic solutions.
They may be weak acids that dissociate and change colour in alkaline solutions.
Test acid-base indicators with dilute HCl, lemon juice, vinegar, ammonia solution, dilute sodium hydroxide solution, lime water, tap water, demineralized water.
Tests for acid-base indicators
Acid-base indicators change colour in acidic or basic solutions.
They may be weak acids that dissociate and change colour in alkaline solutions.
Indicators and colours
Litmus: pH 6 red, pH 7 purple, pH 8 blue
Methyl orange: pH 3.2 red, pH, pH 4.4 yellow
Methyl red: pH 4.4 red, pH 6.2 yellow
Phenolphthalein: pH 8.2 colourless, pH 10 pink
Universal Indicator: pH 1, 2, 3 red, pH 4 pink, pH 5 orange, pH 6 yellow, pH 7 green, pH 8 green-blue, pH 9 blue, pH 10 blue-violet, pH 11 to 14 violet.
Test the following indicators: dilute HCl, lemon juice, vinegar, NH3 solution, dilute NaOH solution, lime water, tap water, demineralized water: