Evaluation of colour changes of sulphamphthalein indicators: synthesis, purification and characterisation of phenol- and o-cresolsulphamphthalein

Abstract

A new series of indicators, which are analogous to the existing phthalein and sulphonephthalein indicators, is described. The conditions for the synthesis of phenolsulphamphthalein (PSP) and o-cresolsulphamphthalein (CSP), with high yields, are given. Purification of the indicators in a multi-purpose solvent extractor, to chromatographic purity, has been accomplished. Evaluation of the colour changes of these indicators in terms of specific colour discrimination (SCD) values, pH of maximum colour change (pH_mcc) and half band width of colour changes in terms of pH units (ΔpH_½SCD) have been calculated. The correlation of the SCD values at pH_mcc and the concentration of the indicators are shown. The limiting SCD values are given. The pK values were determined spectrophotometrically and the pH_mcc–pH_½trf values (pH of 50% transformation of o-cresolsulphamphthalein) were found to be –0.01 and –0.02 pH unit in the acidic and alkaline region of colour transformation, respectively.