A pH-stat study of the reaction of some transition metal cations with disodium ethylenedinitrilotetraacetate (EDTA) and its analytical application

The pH-stat titration technique is an autonomous and very powerful tool for performing and monitoring chelatometric titrations of metal cations with great accuracy, poorly known, however, and seldom exploited. Based on measurement of the amount of strong base required to keep the pH of the test solution at a selected value during stepwise known additions of ethylenedinitrilotetraacetate (EDTA), it requires a glass electrode as the only sensor and is easily implemented on potentiometric titrators. It was introduced a quarter of century ago on an empirical basis for a very peculiar purpose (determination of calcium in diary products), but only very recently it was generalised and its fundamentals were thoroughly examined. In this work, pH-static titrations of some transition metal cations of analytical relevance (Co²⁺, Cu²⁺, Mn²⁺, Zn²⁺) were thoroughly investigated in the acid pH range between 2.3 and 5 or 7 (the highest pH depending on the metal hydroxide or carbonate solubility). The results at higher acidity showed unsuspected properties of such chelation reactions. At moderately acid pH (generally ≥4), indeed, pH-static titrations yield results of high precision and accuracy. On decreasing pH, however, the reaction stoichiomety deviates more and more from the 1:1 ratio between chelating agent and cation, seemingly because of formation of binuclear complexes, an occurrence very seldom mentioned in the current literature. The optimal titration conditions for each metal are defined, and directions for establishing a laboratory protocol for quantitative determinations are given.