Use of pH electrode for precipitation titration analysis: Theory and practice

Abstract

A theoretical model for the potentiometric analysis of precipitation titrations using a pH electrode has been developed and tested. The new analytical method is possible by introducing a mediator which must be a weak acid and must be able to form an insoluble salt with a cation (a titrant). Theoretical expressions of pH, as well as the concentrations of all other species, were derived and solved numerically using bisection method. Among the various factors that influence pH during the titration, concentrations of the mediator and initial values of pH were proved to be very critical. The experimental potentiometric titration curves agree well with those predicted from the theoretical model. Crossing point method was adopted to determine an equivalence point from the titration curves. The method is tested using a known system of chloride determination. Among the several mediators tested (bisulfite, chromate, phosphate, cyanide, arsenate and EDTA), phosphate yielded the best results with an error of 0.1%; bisulfite, chromate and arsenate yielded comparably good results with an error of 0.3∼0.5%, but cyanide and EDTA yielded unsatisfactory results. The optimum mediator concentration found to be in a range of 1–2 mM.