Kinetics of anation of hexaaquachromium(III) by thiocyanates – specific salt effects

Abstract

The kinetics of anation of hexaaquachromium(III) by thiocyanates follows the rate law: −d[complex]/dt=k[NCS] (20–40 °C, [NCS−]=0.1–0.6 M, I=2.0 M, pH=1.0). The specific salt effect has been studied for five media: NaCl, NaBr, NaClO4, KCl and CsCl. The series of chloride (Na+, K+ and Cs+) salts show a negligible effect on the anation rate. On the contrary, the series of sodium salts (Cl−, Br− and ClO4−) reveal a marked difference in the reaction rate. The anation rate decreases sharply with the ionic strength increase (I=0.2–2.0 M, NaCl). The results were interpreted within the frame of fast equilibria of ion-pair formation followed by an interchange mechanism step. The difference of reaction rate is a result of competition between anions (thiocyanates and supporting electrolyte anions) to the complex cation at an ion-pair formation process.