Page 296 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

1-Naphtholphthalein. Dissolve 1 g of the indicator in 500 mL of ethanol and
dilute with 500 mL of water.

Phenolphthalein. Dissolve 5 g of the reagent in 500 mL of ethanol and add
500 mL of water with constant stirring. Filter, if a precipitate forms.
Alternatively, dissolve 1 g of the dry indicator in 60 mL of 2-ethoxyethanol
(Cellosolve), b.p. 135 OC, and dilute to 100 mL with distilled water: the loss by
evaporation is less with this preparation.
Thymolphthalein. Dissolve 0.4 g of the reagent in 600 mL of ethanol and add
400 mL of water with stirring.
Sulphonphthaleins. These indicators are usually supplied in the acid form. They
are rendered water-soluble by adding sufficient sodium hydroxide to neutralise
the sulphonic acid group. One gram of the indicator is triturated in a clean
glass mortar with the appropriate quantity of 0.1 M sodium hydroxide solution,
and then diluted with water to 1 L. The following volumes of 0.1 M sodium
hydroxide are required for 1 g of the indicators: bromophenol blue, 15.0 mL;
bromocresol green, 14.4 mL; bromocresol purple, 18.6 mL; chlorophenol red,
23.6 mL; bromothymol blue, 16.0 mL; phenol red, 28.4 mL; thymol blue,
21.5 mL; cresol red, 26.2 mL; metacresol purple, 26.2 mL.

Quinaldine red. Dissolve 1 g in 100 mL of 80 per cent ethanol.
Methyl yellow, neutral red, and Congo red. Dissolve 1 g of the indicator in 1 L
of 80 per cent ethanol. Congo red may also be dissolved in water.
4-Nitrophenol. Dissolve 2 g of the solid in 1 L of water.
Alizarin yellow R. Dissolve 0.5 g of the indicator in 1 L of 80 per cent ethanol.
Tropaeolin O and tropaeolin 00. Dissolve 1 g of the solid in 1 L of water.
Many of the indicator solutions are available from commercial suppliers
already prepared for use.

For some purposes it is desirable to have a sharp colour change over a narrow
and selected range of pH; this is not easily seen with an ordinary acid-base
indicator, since the colour change extends over two units of pH. The required
result may, however, be achieved by the use of a suitable mixture of indicators;
these are generally selected so that their pK;, values are close together and the
overlapping colours are complementary at an intermediate pH value. A few
examples will be given in some detail.
(a) A mixture of equal parts of neutral red (0.1 per cent solution in ethanol)
and methylene blue (0.1 per cent solution in ethanol) gives a sharp colour
change from violet-blue to green in passing from acid to alkaline solution
at pH 7. This indicator may be employed to titrate acetic acid (ethanoic
acid) with ammonia solution or vice versa. Both acid and base are
approximately of the same strength, hence the equivalence point will be at
a pH N 7 (Section 10.15); owing to the extensive hydrolysis and the flat
nature of the titration curve, the titration cannot be performed except with
an indicator of very narrow range.