Reaction between Chlorine Dioxide and Sulfur(1V)
Inorganic Chemistry, Vol. 17, No. 11, 1978 3115 Contribution from Miami University, Oxford, Ohio 45056
Stoichiometry and Kinetics of the Reaction between Chlorine Dioxide and Sulfur(1V) in Basic Solutions KAZUNORI SUZUKI and GILBERT GORDON* Received June 19, 1978
The reaction between chlorine dioxide and sulfur(1V) has been studied in phosphate and borate buffer solutions in the pH 8-13 range where, in the presence of a slight excess of sulfur(IV), the major products are chlorite ion and sulfate ion with only minor amounts of chlorate ion and chloride ion. In the presence of phosphate buffer, the relative amount of chlorate ion produced is less than 3% whereas in the presence of borate buffer as much as 21% chlorate ion is produced. These results should be compared directly with the disproportionation of chlorine dioxide in basic solution which is considerably slower and results in a 50% yield of chlorite ion and 50% chlorate ion. The rate of reaction has also been measured in phosphate buffer solutions at 10 OC. The rate of reduction of chlorine dioxide is first order in total sulfur(1V) concentration. The second-order rate constants at pH 8.7, 10.0, and 11.5 are calculated to be 7.5 X lo5, 8.6 X lo5, and 1.2 X lo6 M-' SI, respectively. Introduction Recent studies on the reactions of sulfur(1V) with various metal ions such as Fe(CN),3-,1-4 F e ( ~ h e n ) , ~ +Cr(VI),6 ,~ and IrC162-7,8 have suggested that the sulfur(V) radical is formed from sulfur(1V) via a one-electron process and the resulting sulfur(V) is either oxidized further by these oxidants or dimerized to form dithionate ion. These metal ion oxidants, except Cr(VI), act as one-electron oxidants and cannot be reduced to the lower oxidation states. On the other hand, in the reaction between sulfur(1V) and an oxidant which has more than one stable low oxidation state, it is expected that the oxidation of sulfur(1V) to sulfur(V1) may proceed via a single two-electron transfer process or a combination of one- and two-electron proces~es.~ The reaction between sulfur(1V) and chlorine dioxide seems to be consistent with this expectation, since chlorine dioxide is reduced to chlorite ion via a one-electron reduction and/or to hypochlorite ion or hypochlorous acid presumably via two-electron or more complicated reduction pathways.'O The reaction between sulfur(1V) and chloride dioxide has been studied in acidic solutions" but the mechanistic details have not been elucidated owing to the rapid successive reductions by sulfur(1V) of the chlorous acid and hypochlorous acid intermediates. In contrast with the acidic solutions where these reactions are rapid, we have discovered that chlorite ion reacts only very slowly with either sulfur(1V) or hypochlorite ion in basic solutions.'&'* Experimental Section Materials. Reagent grade chemicals were used throughout. Borate buffer solutions were prepared by neutralizing boric acid solutions with sodium hydroxide solution. Chlorine dioxide was generated from the oxidation of chlorite ion by peroxodisulfate ion in aqueous solution and was collected10in distilled water at 0 'C by passing a constant stream of air through the reacting solution. The stock solution was stored at 0 OC. The chlorine dioxide and sulfur(1V) concentrations were determined iodometrically. The amount of chlorite ion found in the stock solution was negligibly small. The chlorine dioxide concentrations in the kinetic measurements were determined spectroph~tometrically'~at 360 nm (e 1.24 X lo3 M-' cm-' 1. Stoichiometry. The stoichiometric measurements for the chlorine dioxide-sulfur(1V) reaction were carried out in 0.17 M borate or phosphate solutions involving a slight excess of sulfur(1V) which had been previously deoxygenated. The reaction was initiated by injecting the chlorine dioxide solution into these solutions with a calibrated glass syringe.I4 The reactions were stirred with a magnetic stirrer and were relatively rapid such that they were finished within 5 s. The remaining sulfur(1V) was determined iodometrically at pH 10-1 1 under 1 atm of nitrogen. The main chlorine-containing product of the reaction was chlorite ion, and the side products were chloride 0020-1669/78/ 1317-3115$01.OO/O
Table I. Reduction of C10, by Excess Sulfite' .
7.03 8.14 9.28 10.13 11.37 11.62 12.00 12.73
0.29 0.49 0.58 0.63 0.78 0.83 0.91 0.98
-..
1.31 0.96 0.89 0.82 0.69 0.64 0.58 0.49
0.21 0.18 0.15 0.13 0.08 0.06 0.03
