Micellar and salt effects on the rate of the condensation between ninhydrin and [Cr(his)(H2O)3]2+

The rates of reaction between ninhydrin and [Cr(his)(H2O)3]2+ (his, histidine) have been determined in water and in micelles formed by the surfactants cetyltrimethylammonium bromide (CTAB; cationic), sodium dodecyl sulphate (SDS; anionic) and octylphenoxy polyethoxy ethanol (Triton X-100, TX-100; non-ionic) in the presence and absence of various salts at 80°C and pH 5.0. SDS micelles show no effect on the reaction rate but increase in the total concentration of CTAB from 0.0 to 20.0×10−3 mol dm−3 results in an increase in the pseudo-first-order rate constant (kψ) by a factor of ca. 3. The micellar catalysis is explained in terms of the pseudophase model of the micelles. CTAB micelles decrease the activation enthalpy and make the activation entropy less negative. At constant concentration of CTAB, the rate constant decreases with increase in concentration of added anions (bromide, chloride, sulphate and nitrate). The kinetic data have been used to calculate the micellar binding constants KS for [Cr(his)(H2O)3]2+ and KN for ninhydrin and the respective values are 28 and 73 mol−1 dm3 at 80°C.