Dynamics of multiphasic glass powder catalyzed autoxidation of sulphur dioxide in bulk aqueous phase

Abstract

Kinetics of autoxidation of sulphite has been studied in unbuffered as well as ac tate-buffered aqueous suspensions of glass powder. The buffer concentration has an inhibitory effect n the reaction and the nature of the rate law depends on the buffer concentration. At[CH₃COONa] = 0.14 mol dm^-3 and varying [CH₃COONa], the kinetics in the two <i>p</i>H regimes, 5 &lt; <i>p</i>H <i> <img src='http://www.niscair.res.in/jinfo/lessequals.gif' border=0> </i> 5.9 and 4.2 &lt; <i>p</i>H &lt; 5.0, are different. In the first <i>p</i>H regime, with increasing [glass], the rate profile passes through a maximum. For the region having linear dependence on [glass],the initial rate, R_obs,is defined by the rate law (A)at atmospheric O₂ pressure: <br>

R_obs= R_uncat + (K₄+ K₅[H+]^-1)[glass][S(IV)]² ... (A) <br>

where R_uncat, is the rate of uncatalyzed reaction. The rate has an almost inverse dep ndence on particle size. On the other hand, in the regime 4.2 &lt; <i>p</i>H&lt; 5.0, an inducation period is observed which decreases on decreasing buffer concentration and on increasing <i>pH </i>and [S(IV)]. It also decreases on passing oxygen. When the buffer concentration is lower, viz., [CH₃COONa] = 0.07 mol dm^-3 and [CH₃COONa] is varied, the kinetics are in conformity with the rate law (B): R_obs=( K₆+ K₇[H⁺]^-1)[glass][S(IV)] ... (B)<br>

In unbuffered solutions, an initial rapid drop in [S(IV)]in the high <i>p</i>H region (6.2-6.8 ) and an induction period in the low <i>p</i>H region (3.49-4.44) have been noted. In the middle <i>p</i>H region (4.8-5.5), the kinetics obey the rate law (C): R_obs= K₈[S(IV)] [glass] ... (C)<br>

The suggested autoxidation mechanism is of Langmuir-Hinshelwood type which envisions the adsorption of both dioxygen and S(IV) on glass surface.