Abstract The kinetics of oxidation of L-Aspartic acid  (ASP) by dihydroxydiperiodatoargentate(III) (DPA) was studied spectrophotometrically between 25ºC and 45ºC in alkaline medium. The reaction rate showed first order dependence in oxidant and fractional order in reductant. A plausible mechanism involving a pre-equilibrium of adduct formation between the complex and reductant was proposed. The rate equations derived from mechanism can explain all experimental observations. The activation parameters along with rate constants of the rate-determining step were calculated.
Keywords dihydroxydiperiodatoargentate (III) L-Aspartic acid (ASP) redox reaction, kinetics and mechanism

Recently, the study of the highest oxidation state of transition metals has intrigued many researchers. Transition metals in a higher oxidation state generally can be stabilized by chelation with suitable polydentate ligands. Metal chelates such as diperiodatoargentate(III)^[1], ditelluratoargentate(III)^[2], diperiodatonickelate(IV)^[3] are good oxidants in a medium with an appropriate pH value. The use of complexes as good oxidizing agents in analytical chemistry has been reported^[4].The oxidation of a number of organic compounds and metals in lower oxidation state by Ag(III) has also been performed^[1], but no further information on the kinetics is available. Because Ag(III) is in the highest oxidation state and the reaction is complicated in this kind of reaction system, it is of significance to have a further study on this kind of reaction system. Investigation on them will certainly provide us with more dynamical parameters, and will provide theoretical foundation for the design of reaction route in the organic synthesis and quantitative analysis in analytical chemistry. In this paper, the mechanism of oxidation of L-Aspartic acid by dihydroxydiperiodatoargentate (III) is reported.

1.EXPERIMENTAL SECTION
1.1 Materials
All the reagents used were of A.R. grade. All solutions were prepared with doubly distilled water. Solution of [Ag(OH)₂(H₂IO₆)₂]^3- (DPA) was prepared and standardized by the method reported earlier^[5]. Its UV spectrum was found to be consistent with that reported. The concentration of DPA was derived by its absorption at l =351nm. Solution of DPA was always freshly prepared before use with solution and doubly-distilled water. The ionic strength m was maintained by adding KNO₃ solution and the pH value of the reaction mixture was regulated with KOH solution.
1.2 Apparatus and Kinetics Measurements
All kinetics measurements were carried out under pseudo-first order conditions. Solution (2 mL) containing definite [Ag (III)], [OH^-], [IO₄^-] and ionic strength m and reductant solution (2mL) of appropriate concentration were transferred separately to the upper and lower branch tubes of a type two-cell reactor. After it was thermally equilibrated at desired temperature in thermobath (made in Shanghai-Techcomp Scientific Instrument Co., Ltd. ), the two solutions were mixed well and immediately transferred to a 1 cm thick rectangular cell quartz in a constant temperature cell-holder (±0.1ºC). The reaction process was monitored automatically by recording the disappearance of Ag(III) with time (t) at 351 nm with a UV-8500 spectrophotometer (made in Shanghai-Techcomp Scientific Instrument Co., Ltd.). All other species did not absorb significantly at this wavelength. Details of the determinations are described elsewhere ^[6].
1.3 Product Analysis
Solution having known concentrations of [Ag (III)] and [OH^-] was mixed with an excess of L-Aspartic acid. The completion of the reaction was marked by the complete discharge of Ag (III) color. After completion of the reaction, the oxidation product was identified^[7] as acid ketone.

2. RESULTS AND DISCUSSION
2.1Evaluation of Pseudo-First Order Rate Constants
Under the conditions of [Reductant]₀>>[Ag(III)]_0.,the plots of ln(A_t-A_∞) versus time are lines, indicating the reaction is first order with respect to [Ag(III)], where A_t and A_∞ are the absorbance at time t and at infinite time respectively. The pseudo-first-order rate constants k_obs were calculated by the method of least squares (r≥0.999). To calculate k_obs generally 8-10 A_t values within three times the half-life were used. k_obs values were at least averaged values of three independent experiments and reproducibility is within ±5%.
2.2 Rate Dependence on L-Aspartic acid
At fixed [Ag(III)], [OH^-], [IO₄^-], ionic strength m and temperature. k _obs values increased with the increase of [ASP] and the order in [ASP]was found to be 0< n_ap <1. The plots of 1/k_obs versus 1/ [ASP] are straight lines with a positive intercept (Table 1) .
2.3 Rate Dependence on [OH^-]
At constant [Ag(III)], [ASP], [IO₄^-], ionic strength m and temperature, k_obs values decreased rapidly with the increase in [OH^-], and then increased slowly with the increase in [OH^-], The concentration of OH^- was about 0.15 mol·L^-1 at the turning point in which the rate was the slowest.(Fig 1)
2.4 Rate Dependence on [IO₄^-] and Ionic Strength m
At constant [Ag(III)], [ASP], [OH^-], ionic strength m and temperature, k_obs values decreased slowly with the increase in [IO₄^-]. (Table2). The rate was increased by added KNO₃ solution (Table3), which is consistent with the common regulation of the kinetics^[8].