Thermodynamic and Kinetic Effects Involved in the pH Titration of As(III) with Iodine in a Buffered Malonate System

Abstract

The pH titration of As(III) in the buffered malonate system with V mL of I2+KI solution is considered. At V≤V eq (V eq denotes the equivalence volume), the course of the titration curve depends on thermodynamic effects and the malonate species act only as an acid–base buffer. For V>V eq the course of this curve depends on kinetic effects and the malonic acid acts both as a buffering and a reducing agent. The V eq value in the titration was obtained as the point of intersection of two segments approximated by hyperbolic functions. The difference in the slopes of the segments results from the difference in the number of protons evolved in thermodynamic reaction of iodine with HAsO2 (for V≤V eq) and in the kinetics of the reaction with malonate species (V>V eq). The slope of the segment at V>V eq depends on the titration rate within this volume interval. This paper illustrates an application of the GATES methodology, and provides the possibility of making reasonable inferences from comparison of preliminary physicochemical knowledge with experimental data.