Metal-metallochromic indicator complexes as acid-base indicators

Abstract

When the formation of metal-metallochromic indicator complexes is accompanied by the release of more than one proton per indicator molecule complexed, the colour change at the end-point is sharper than that of a conventional acid-base indicator, when only one proton is released. The larger the number of protons released per indicator molecule, the sharper the colour change. Because complexes of different stabilities are formed between metals and metallochromic indicators, one such indicator can provide a whole range of pH indicators by variation of the metal ion used. The pK value for the indicator system is a conditional constant, and can be lowered by increasing the amount of free metal ion present or by increasing the concentration of the indicator complex. The nature of the metal-indicator complex can be deduced from a study of the absorbance-pH curve for different ratios of metal ion and indicator concentrations. Either component of the indicator system can be determined by potentiometric (pH) titration with the other. Limitations are imposed by the necessity to avoid the presence of species that form more stable complexes with the metal ion, by hydrolysis, and by the acid-base characteristics of the free indicator. In the most favourable instances it is possible to achieve a complete colour change over 0·5 pH unit or less. The pK value is relatively indifferent to changes in ionic strength.