The oxidation of arsenic(III) by chromium(VI) requires the postulation of uncommon oxidation states as reactive intermediates. The possible intermediates of chromium are the divalent, tetravalent, and pentavalent states. Recently arsenic(IV) has been postulated as a reactive intermediate in certain oxidations of As(III). A detailed study of the reaction rate law in various solutions has been carried out in order to determine the nature of the intermediates found in the Cr(VI)-As(III) reaction. The effects of hydrogen ion and buffer concentrations have been determined. The mechanism postulated depends upon the solution in which the reaction occurs. The kinetics of the chromium(VI)-arsenic(III) reaction have been measured in perchloric acid solutions, acetic acid solutions, acetic acid-acetate buffers, ammonium nitrate solutions, and dihydrogen phosphatemonohydrogen phosphate buffers. The rate laws are: Perchloric Acid - -d[Cr(VI)]/ dt = [k₀ + k_H[H⁺] + k_H² [H⁺]²] (K’ [As (III)][Cr(VI)])/(l+K’[As(III)]) Acetic Acid-Acetate Buffers - -d[Cr(VI)]/ dt = [k₀ + k_HOAc[HOAc]} (K’ [As (III)][Cr(VI)])/(l+K’[As(III)]) Acetic Acid - -d[Cr(VI)]/ dt = [k₀ + k’_HOAc[HOAc]] (K’ [As (III)][Cr(VI)])/(l+K’[As(III)]) H₂PO₄⁻- HPO₄⁻² Buffers - -d[Cr(VI)]/ dt = (kK”[As(III)][Cr(VI)][H₂PO₄⁻])/(1+k”[H₂PO₄⁻]) Ammonium Nitrate Solutions - -d[Cr(VI)]/ dt = k₂ [As(III)][Cr(VI)] The reaction studied was the oxidation of As(III) by Cr(VI} according to the stoichiometry 3 As(III) + 2 Cr(VI) = 3 As(V) + 2 Cr (III) Rate data were obtained spectrophotometrically and by iodometric titration depending upon the region being investigated. At high As(III)-low Cr(VI), the Cr(VI) concentration was followed as a function of time at 350 mu using a Beckman DU spectrophotometer. The validity of Beer's law with respect to Cr(VI) was checked. At low As(III)-high Cr(VI), the unreacted As(III) concentration was determined by iodometric titration. In phosphate buffers, the HCrPO₇⁻² complex is indicated as an oxidizing agent. In the other systems, the following mechanism is consistent with the experimental facts. As(III) + HCrO₄⁻ [stacked right and left arrows with K above] As(III) · HCrO₄ As (III), As(III) · HCrO₄⁻ [right arrow with kₒ above] products, rate determining As(III) · HCrO₄⁻ + HOAc [right arrow with K_HOAc above] products, rate determining As(III) · HCrO₄⁻ + H⁺ [right arrow with k_H] products, rate determining In perchloric acid, H₂CrO₄is indicated as participating in the oxidation by the following steps: HCrO₄⁻ + H⁺ [stacked right and left arrows with Kₐ above] H₂CrO₄, ₂CrO₄ + As(III) [stacked right and left arrows with K above] As(III) · H₂CrO₄ The decomposition of As(III}•H₂CrO₄ is also acid catalyzed. In the mechanism described above, the products of the rate determining steps are arsenic(V) and chromium(IV). The existence of chromium(IV) is supported by the induced oxidation of ferrocyanide by the As(III)-Cr(VI) reaction. The chromium(IV) reacts immediately with chromium(VI) to form chromium(V) which can oxidize As(III) to As(V) directly. This is in agreement with the experimentally observed stoichiometry and accounts for the use of K' in the rate laws where K’ is actually 2K. The value of K is obtained from plots of 1/k_m versus 1/As(III). The value of K which describes all data reported is 17.2 1.mole⁻¹. In all three systems, perchloric acid, acetic acid, and acetic acid-acetate, the same kₒ is obtained. The specific rate constant for the acetic acid catalysis is 1.99 x 10⁻³ 1.⁻² mole² sec.⁻¹. Comparison of k"_HOAc terms for acetic acid and acetic acid-acetate solutions shows that k”_HOAc for the acid solution is larger than that for the buffers. This difference is explained on the basis of a medium effect. / Doctor of Philosophy

The oxidation of arsenic (III) by chromium (VI) has been studied in pyridine - pyridinium buffers with and without added oxalate ion. The rate law at pH 4.6 for the reactions without oxalate is best described as - d[Cr(VI)]/dt = (0.535 [As(III)]² [Cr(VI)])/(1+187 [As(III)]) This is in accord with the general mechanism for chromium (VI) oxidation proposed by Beattie and Haight. The behavior in pyridine - pyridinium buffers is different from that observed in acetic acid acetate buffers of the same pH. In the presence of oxalate, the observed rate law is - d[Cr(VI)]/dt = (310 [As(III)] [C₂O₄²⁻] [Cr(VI)])/(1+31.0 [As(III)]) In each case the reaction stoichiometry was found to be 3As(III) + 2Cr(VI) = 3As(V) + 2Cr(III). The role of oxalate was found to be catalytic although it was incorporated in the final Cr(III) product. Complex kinetic behavior was observed as a function of pH, [AsIII], and buffer composition. / M.S.