Kinetics and mechanism of the reaction of aquachromium(III) with L-ascorbic acid in aqueous media

The kinetics of the reaction of L-ascorbic acid (H(2)A) with aquachromium(III) has been studied over the range 0.04 less than or equal to [H(2)A](T) less than or equal to 0.16 mol dm(-3), 3.5 less than or equal to pH less than or equal to 6.0, 25.0 less than or equal to t less than or equal to 50.0 degrees C, 0.03 less than or equal to I less than or equal to 1.0 mol dm(-3) (KNO3) and 0% less than or equal to [MeOH] less than or equal to 20% (v/v). The reaction takes place via an outer-sphere association between Cr3+/CrOH2+ and H(2)A, followed by transformation of the outer-into an inner-sphere complex by slow interchange. The anation rate constants for Cr3+ and CrOH2+ species (k(1) and k(2)) at 25 degrees C and I = 1.0 mol dm(-3) (KNO3) are 1.68 x 10(-4) and 2.14 x 10(-4)s(-1), respectively. Delta H-double dagger and Delta S-double dagger for the k(1) and k(2) paths are 27.0 +/- 2 KJ mol(-1) -227 +/- 5 JK(-1) mol(-1) and 57.2 +/- 4 kJ mol(-1), -123 +/- 14 JK(-1) mol(-1), respectively. Anation of Cr(H2O)(6)(3+) and Cr(H2O)(5)OH2+ follow an I-a path.