Kinetics and mechanism of the reduction of hydroxylamine-O-sulfonic acid by the hexaaquochromium(II) ion

The title reaction takes place with a 2:1 (Cr{sup 2+}):(HSA) stoichiometry (HSA = hydroxylamine-O-sulfonic acid) and yields CrNH{sub 3}{sup 3+}, Cr{sup 3+}, and CrSO{sub 4}{sup +} as the main products. The kinetics conform to the rate law {minus}d(HSA)/dt = {minus}d(Cr{sup 2+})/2(dt) = k{sub obs}(Cr{sup 2+})(HSA), where k{sub obs} = kK{sub a}/(K{sub a} + (H{sup +)}). At 25{degree}C and 1.0 M ionic strength (CHlO{sub 4} + LiClO{sub 4}) the parameter k has the value of 20.5 {plus minus} 0.3 M{sup {minus}1} s{sup {minus}1} when K{sub a} is set at the value (6.8 {plus minus} 0.8) {times} 10{sup {minus}2} M, as determined by pH titration. In the proposed mechanism, Cr{sup 2+} attacks at the nitrogen end of the anion, NH{sub 2}OSO{sub 3}{sup {minus}}, to form CrNH{sub 3}{sup 3+} and SO{sub 4}{sup {center dot}{minus}}. The sulfate radical anion then oxidizes rapidly the second mole of Cr{sup 2+} to yield Cr{sup 3+} and some CrSO{sub 4}{sup +}. In solutions containing Br{sup {minus}}, SO{sub 4}{sup {center dot}{minus}} oxidizes it to Br{sub 2}{sup {center dot}{minus}}. The latter reacts with Cr{sup 2+} to yield CrBr{sup 2+}. 19 refs., 2 figs.