Sulphonphthalein indicator

Several indicators have been suggested for the determination of pH, but the most suitable are the sulphonphthalein indicators such as m-cresol purple, thymol blue and cresol red. They all exist in three forms, with the chemical equilibria following reactions (7-17) and (7-18)... 

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. 

Various sulphonphthalein dyes which are used as acid-base indicators, e. g., th3nnol blue, bromophenol blue and phenol red, may be separated using solvents 11—13 (Table 131). Waldi [82] has determined the hB/-values of indicator dyes with a view to identification and purity testing he used standard conditions on alumina G-silica gel G [1 1) layers, with ethyl acetate-methanol-6N ammonium hydroxide solution (60 + 30 + 10). The time of run was 30 min. Results are in Table 132. 

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