Kinetic and Thermodynamic Effects of Auxiliary Complexing Agents on the Color Reactions of Niobium and Tantalum with 4-(2-Pyridylazo)resorcinol

摘要

Kinetics and thermodynamics of the ligand substitution reactions of niobium and tantalum complexes involving oxalic, tartaric or citric acid as an auxiliary complexing agent with 4-(2-pyridylazo)resorcinol have been studied spectrophotometrically. The equilibrium constant and the rate equation for the respective color reactions were determined at 25°C and I=0.1 (NaClO4). A kinetic consideration of the values of the second-order rate constants for the niobium complexes shows that the order of labilizing power of the auxiliary complexing agents is: tartaric acid_??_citric acidoxalic acid. For a given auxiliary complexing agent, the niobium complex reacts faster with this metallochromic reagent than does the corresponding tantalum complex, irrespective of this order. The thermodynamic effect of the auxiliary complexing agents on this color reaction is discussed.