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1.4 pH INDICATORS

Alizarin Red S (Mordant Red 3; Alizarin S) C₁₄H₇NaO₇S.H2O = 360.27

Use general reagent grade of commerce.

DESCRIPTION Yellowish brown or orange-yellow powder. Freely soluble in water with yellow colour; soluble in ethanol. Transition interval: from pH 3.7 to 5.2. Colour change: from yellow to red.

SENSITIVITY TO BARIUM To 5.0 ml of 0.05 M sulfuric acid add 5 ml of water, 50 ml of acetate buffer pH 3.7 and 0.5 ml of a 0.1 per cent w/v solution of the test substance. Add, dropwise, 0.05 M barium perchlorate: the colour changes from yellow to orange-red.

Alizarin Red TS A 0.1 per cent w/v solution of alizarin red S.

Alizarin Yellow GG C₁₃H₈N₃NaO₅ = 309.21

Yellow powder. Slightly soluble in water. Transition interval: from pH 10.0 to 12.0. Colour change: from colourless to yellow.

Bromocresol Green (Bromocresol Blue) C₂₁H₁₄Br₄O₅S = 698.01

White or pale buff-coloured powder. Slightly soluble in water; soluble in ethanol and in solutions of alkali hydroxides. Transition interval: from pH 4.0 to 5.4. Colour change: from yellow to blue.

Bromocresol Green TS Dissolve 50 mg of bromocresol green in 100 ml of ethanol, and filter if necessary.

Bromocresol Purple C₂₁H₁₆Br₂O₅S = 540.22

White to pink, crystalline powder. Insoluble in water; soluble in ethanol and in solutions of alkali hydroxides. Transition interval: from pH 5.2 to 6.8. Colour change: from yellow to purple.

Bromophenol Blue C₁₉H₁₀Br₄O₅S = 669.96

Pinkish crystals. Insoluble in water; soluble in ethanol and in solutions of alkali hydroxides. Transition interval: from pH 2.8 to 4.6. Colour change: from yellow to blue.

Bromothymol Blue C₂₇H₂₈Br₂O₅S = 624.38

Cream-coloured powder. Insoluble in water; soluble in ethanol and in solutions of alkali hydroxides. Transition interval: from pH 6.0 to 7.6. Colour change: from yellow to blue.

Bromothymol Blue TS Dissolve 100 mg of bromothymol blue in 100 ml of ethanol (50 per cent), and filter if necessary.

Congo Red C₃₂H₂₂N₆Na₂O₆S₂ = 696.66

Dark red or reddish brown powder. Soluble in 30 parts of water; slightly soluble in ethanol. Transition interval: from pH 3.0 to 5.0. Colour change: from blue to red.

Congo Red Paper Immerse strips of filter paper for a few minutes in a solution prepared by dissolving 0.1 g of congo red in a mixture of 20 ml of ethanol and 30 ml of water and adding sufficient water to produce 100 ml and allow to dry at room temperature. Transition interval: from pH 3.0 to 5.0. Colour change: from violet to orange.

Cresol Red (o-Cresolsulfonphthalein) C₁₂H₁₈O₅S = 382.43

Reddish brown powder. Slightly soluble in water; soluble in ethanol and in dilute solutions of alkali hydroxides. Transition interval: from pH 7.2 to 8.8. Colour change: from yellow to red.

Cresol Red TS Warm 20 mg of cresol red with 1.06 ml of 0.05 M sodium hydroxide and 2 ml of ethanol (90 per cent); after solution is effected, add sufficient ethanol (20 per cent) to produce 100 ml. The solution complies with the following test.

SENSITIVITY A mixture of 0.1 ml of the solution and 100 ml of carbon dioxide-free water to which 0.15 ml of 0.020 M sodium hydroxide has been added is purplish red; not more than 0.15 ml of 0.020 M hydrochloric acid is required to change the colour to yellow.

Crystal Violet C₂₅H₃₀ClN₃ = 407.98

Dark-green crystals. Slightly soluble in water; sparingly soluble in ethanol and in anhydrous glacial acetic acid. Its solutions are deep violet in colour.

SENSITIVITY Dissolve 100 mg in 100 ml of anhydrous glacial acetic acid, and mix. Pipette 1 ml of the solution into a 100-ml volumetric flask, and dilute with anhydrous glacial acetic acid to volume: the solution is violetblue in colour and does not show a reddish tint. Pipette 20 ml of the diluted solution into a beaker, and titrate with 0.10 M perchloric acid, adding the perchloric acid slowly from a microburette: not more than 0.10 ml of 0.10 M perchloric acid is required to produce an emeraldgreen colour.

Crystal Violet TS Dissolve 500 mg of crystal violet in 100 ml of anhydrous glacial acetic acid.

Dimethyl Yellow C₁₄H₁₅N₃ = 225.28

Yellow crystals. Sparingly soluble in water. Transition interval: from pH 2.8 to 4.6. Colour change: from red to yellow.

Dimethyl Yellow TS A 0.2 per cent w/v solution of dimethyl yellow in ethanol (90 per cent). The solution complies with the following test.

SENSITIVITY A solution containing 2 g of ammonium chloride in 25 ml of carbon dioxide-free water, to which is added 0.1 ml of the dimethyl yellow solution, is yellow. Not more than 0.10 ml of 0.10 M hydrochloric acid is required to change the colour to red.

Indigo Carmine (Indigotindisulfonate; Acid Blue 74) C₁₆H₈N₂Na₂O₈S₂ = 466.35

Deep blue powder or blue granules with a coppery lustre.

Indigo Carmine TS To a mixture of 10 ml of hydrochloric acid and 990 ml of a 20 per cent w/v solution of nitrogen-free sulfuric acid in water, add 0.2 g of indigo carmine to produce a solution that complies with the following test.

          Add 10 ml to a solution of 1.0 mg of potassium nitrate in 10 ml of water, rapidly add 20 ml of nitrogen-free sulfuric acid and heat to boiling. The blue colour is discharged within 1 minute.

Litmus Fragments of indigo blue pigment prepared from various species of Rocella, Lecanora or other lichen. The pigment has a faint, characteristic odour. Blue powder, cubes or pieces; odour, faint, characteristic. Partly soluble in water and in ethanol. Transition interval: from approximately pH 4.5 to 8.0. Colour change: from red to blue.

Litmus Paper Use red litmus paper or blue litmus paper, as appropriate.

Litmus Paper, Blue Boil 10 parts of coarsely powdered litmus under reflux for 1 hour with 100 parts of ethanol, decant the ethanol and discard. Add to the residue a mixture of 45 parts of ethanol and 55 parts of water. After 2 days decant the clear liquid. Impregnate strips of filter paper with the extract and allow to dry.

SENSITIVITY Immerse a strip measuring 10 mm × 60 mm in 100 ml of a mixture of 10 ml of 0.02 M hydrochloric acid and 90 ml of water. On shaking the paper turns red within 45 seconds.

Litmus Paper, Red To the extract obtained in the preparation of Blue litmus paper add 2 M hydrochloric acid dropwise until the blue colour becomes red. Impregnate strips of filter paper with the solution and allow to dry.

SENSITIVITY Immerse a strip measuring 10 mm × 60 mm in 100 ml of 0.002 M sodium hydroxide. On shaking the paper turns blue within 45 seconds.

Litmus TS Boil 25 g of coarsely powdered litmus with 100 ml of ethanol (90 per cent) under a reflux condenser for 1 hour, and pour away the clear liquid; repeat this operation using two 75-ml portions of ethanol (90 per cent). Digest the extracted litmus with 250 ml of water, and filter.

Malachite Green (Basic Green 4) C₂₃H₂₅ClN₂ = 364.92

Green crystals with a metallic lustre. Very soluble in water, soluble in ethanol, in methanol and in amyl alcohol. Transition interval: from pH 0.2 to 1.8. Colour change: from yellow to blue-green.

Malachite Green TS A 0.5 per cent w/v solution of malachite green in anhydrous acetic acid.

Metacresol Purple C₂₁H₁₈O₅S = 382.43

Dark brown glittering crystals or olive green powder. Very slightly soluble in water and in ethanol; soluble in methanol. Acid-transition interval: from pH 1.2 to 2.8. Colour change: from red to yellow. Alkaline-transition interval: from pH 7.4 to 9.0. Colour change: from yellow to purple.

Metacresol Purple TS A 0.1 per cent w/v solution of metacresol purple in 0.001 M sodium hydroxide.

Metanil Yellow C₁₈H₁₄N₃NaO₃S = 375.38

Brownish yellow powder. Soluble in water and in ethanol; sparingly soluble in ether; slightly soluble in acetone. Transition interval: from pH 1.2 to 2.4 Colour change: from red to yellow.

Metanil Yellow TS A 0.1 per cent w/v solution of metanil yellow in methanol. The solution complies with the following test.

SENSITIVITY A solution containing 0.1 ml in 50 ml of anhydrous acetic acid is pinkish red. Add 0.05 ml of 0.1 M perchloric acid VS; the colour of the solution changes to violet.

Methylene Blue TS Dissolve 125 mg of methylene blue in 100 ml of ethanol, and dilute with the same solvent to 250 ml.

Methyl Orange (Helianthin; Tropaeolin D) C₁₄H₁₄N₃NaO₃S = 327.33

Colour Changes of pH INdicators

Orange-yellow powder or crystalline scales. Slightly soluble in cold water; readily soluble in hot water; insoluble in ethanol. Transition interval: from pH 3.0 to

4.4. Colour change: from red to yellow.

Methyl Orange TS Dissolve 0.1 g of methyl orange in 80 ml of water and add sufficient ethanol to produce 100 ml. The solution complies with the following test.

SENSITIVITY A mixture of 0.1 ml of the solution and 100 ml of carbon dioxide-free water is yellow; not more than 0.10 ml of 1 M hydrochloric acid is required to change the colour of the solution to red.

Methyl Orange-Xylene Cyanol FF TS Dissolve 100 mg of methyl orange and 260 mg of xylene cyanol FF in 50 ml of ethanol and add sufficient water to produce 100 ml.

Methyl Red C₁₅H₁₆ClN₃O₂ = 305.76

Dark red powder or violet crystals. Sparingly soluble in water; soluble in ethanol. Transition interval: from pH 4.2 to 6.3. Colour change: from red to yellow.

Methyl Red TS Dissolve 100 mg of methyl red in 100 ml of ethanol, and filter if necessary.

Methyl Red-Methylene Blue TS (Methyl Purple TS) Add 10 ml of methyl red TS to 10 ml of methylene blue TS, and mix.

1-Naphtholbenzein (p-Naphtholbenzein) C₂₇H₂₀O₃ = 392.45

Brilliant brownish black crystals or brownish red powder. Practically insoluble in water; soluble in ethanol and in glacial acetic acid. Transition interval: from pH 8.8 to 10.0. Colour change: from orange to green.

          When used for titration in non-aqueous media, it changes from blue or green-blue (alkaline) through orange (neutral) to dark green (acidic).

1-Naphtholbenzein TS Dissolve 200 mg of 1- naphtholbenzein in 100 ml of anhydrous glacial acetic acid. The solution complies with the following test.

SENSITIVITY Add 0.25 ml of the solution to 50 ml of glacial acetic acid. Not more than 0.05 ml of 0.10 M perchloric acid is required to change the colour of the solution from brownish yellow to green.

Neutral Red C₁₅H₁₇ClN₄ = 288.78

Reddish to olive-green, coarse powder. Sparingly soluble in water and in ethanol. Transition interval: from pH 6.8 to 8.0. Colour change: from red to orange.

Neutral Red TS A 0.1 per cent w/v solution of neutral red in ethanol.

Phenolphthalein C₂₀H₁₄O₄ = 318.33 (TP p. 349)

White or faintly yellowish white, crystalline powder. Insoluble in water; soluble in ethanol. Transition interval: from pH 8.0 to 10.0. Colour change: from colourless to red.

Phenolphthalein TS Dissolve 1 g of phenolphthalein in 100 ml of ethanol (50 per cent).

Phenolphthalein TS, Dilute A 0.1 per cent w/v solution of phenolphthalein in ethanol (80 per cent). The solution complies with the following test.

SENSITIVITY A mixture of 0.1 ml and 100 ml of carbon dioxide-free water is colourless. Not more than 0.20 ml of 0.020 M sodium hydroxide is required to change the colour to pink.

Phenol Red C₁₉H₁₄O₅S = 354.38

Crystalline powder, varying in colour from bright to dark red. Very slightly soluble in water; freely soluble in solutions of alkali carbonates and hydroxides; slightly soluble in ethanol. Transition interval: from pH 6.8 to 8.2. Colour change: from yellow to red.

Phenol Red TS Warm 100 mg of phenol red with 1.42 ml of 0.2 M sodium hydroxide and 5 ml of ethanol (90 per cent); after solution is effected, add sufficient ethanol (20 per cent) to produce 250 ml. The solution complies with the following test.

SENSITIVITY A mixture of 0.1 ml and 100 ml of carbon dioxide-free water is yellow. Not more than 0.10 ml of 0.020 M sodium hydroxide is required to change the colour of the solution to reddish violet.

Resazurin (Sodium) C₁₂H₆NNaO₄ = 251.17

Brownish purple, crystalline powder. Soluble in 100 parts of water, forming a deep violet-coloured solution. Transition interval: from pH 3.8 to 6.4. Colour change: from orange to violet.

          Hydrogen sulfide and other compounds containing the thiol group decolourize solutions of resazurin sodium, forming dihydroresorufin. When the decolorized solution is shaken in the presence of air, a rose colour develops as a result of the formation of resorufin.

Thymolphthalein C₂₈H₃₀O₄ = 430.54

White to slightly yellow, crystalline powder. Insoluble in water; soluble in ethanol and in solutions of alkali hydroxides. Transition interval: from pH 9.3 to 10.5. Colour change: from colourless to blue.

Thymolphthalein TS Dissolve 100 mg of thymolphthalein in 100 ml of ethanol, and filter if necessary.

Thymol Blue (Thymolsulfonphthalein) C₂₇H₃₀O₅S = 466.59

Dark coloured, crystalline powder. Slightly soluble in water; soluble in ethanol and in dilute alkali solutions. Acid-transition interval: from pH 1.2 to 2.8. Colour change: from red to yellow. Alkaline-transition interval: from pH 8.0 to 9.2. Colour change: from yellow to blue.

Titan Yellow (Thiazole Yellow) C₂₈H₁₉N₅Na₂O₆S₄ = 695.7

Yellowish brown powder. Soluble in water and in ethanol to yield in each instance a yellow solution; soluble in dilute alkali to yield a brownish red solution.

Transition interval: from pH 12.0 to 13.0. Colour change: from yellow to red.

SENSITIVITY TO MAGNESIUM Add 0.2 ml of a 0.01 per cent w/v solution and 2 ml of 1 M sodium hydroxide to a mixture of 9.5 ml of water and 0.5 ml of a solution prepared by dissolving 1.014 g of clear crystals of smagnesium sulfate in water, diluting with water to 100 ml, and then diluting 10 ml of the resulting solution with water to 1000 ml: a distinct pink colour is produced within 10 minutes.

Store protected from light.

Titan Yellow TS A 0.05 per cent w/v solution of titan yellow.

Titan Yellow Paper Impregnate filter paper with titan yellow TS.

Tropaeolin O (Acid Orange 6) C₁₂H₉N₂NaO₅S = 316.26

Brown powder. Soluble in water and in ethanol. Transition interval: from pH 11.0 to 12.7. Colour change: from yellow to orange.

Tropaeolin OO C₁₈H₁₄N₃NaO₃S = 375.38

Yellow powder or orange-yellow scales. Transition interval: from pH 1.3 to 3.2. Colour change: from red to yellow.

Tropaeolin OO TS A 0.1 per cent w/v solution of tropaeolin OO.