Kinetics of the interaction of Cd(II)-histidine complex with ninhydrin in absence and presence of cationic and anionic micelles

Kinetics of the interaction of Cd(II)-histidine complex with ninhydrin has been carried out at pH 5.02 (acetic acid-sodium acetate buffer) under varying conditions of reactant concentrations, temperature, and surfactant concentrations. The order of the reaction with respect to Cd(II)-histidine complex was unity while it was fractional with respect to ninhydrin. On the basis of these studies a mechanism has been proposed. In the absence of the surfactants, the reaction followed rate equation: while, in presence of surfactants, the following rate equation was obeyed: Anionic micelles of sodium dodecyl sulphate catalyze the reaction with the rate reaching a maximum at ca. 0.10 mol dm⁻³ surfactant. The surfactant decreases activation enthalpy and makes it more negative. Cationic micelles of cetyltrimethylammonium bromide strongly inhibit the reaction and increase the activation enthalpy but make the activation entropy more positive than the SDS micelles. Added salts (KNO₃ and NaCl) inhibit the catalysis, and the effect is more with the latter. The rate constants, binding constants with surfactants, and the index of cooperativity have been evaluated. © 1997 John Wiley & Sons, Inc.