CATALYTIC-OXIDATION OF SULFUROUS ACID BY MOLECULAR-OXYGEN - INVESTIGATIONS ON THE ELECTROCHEMICAL ESTIMATION OF THE CATALYTIC ACTIVITY

The catalytic oxidation of sulfurous acid S(IV) to sulfuric acid S(VI) can be monitored by measuring the increasing conductivity of the solution while the reaction proceeds. The turnover-frequency and thereby the activity of the catalyst sample examined is calculated from the conductivity derivative by multiplying it with a factor (f); this factor is based on the difference between the molar conductivities of the reaction product S(VI) and the educt S(IV). The factor was found to depend on both the temperature and slightly on the starting concentration [SIV]0. An empirical interpolation formula for calculation of the factor was derived from the calibration experiments. The molar conductivities (lambda) of S(IV) and S(VI) were determined at 6 different temperatures from 228 to 313 K in the concentration range from 0.1 to 2.0 mM. To illustrate the method some applications are presented. These include measurements with Fe-graphite and NH3 treated carbon black samples as catalysts. For the latter the catalytic activity is discussed in relation to XPS surface characterisation and elemental chemical analysis. The exemplary experimental runs prove the method's plausibility but indicate that such reactions are very complex in their kinetics: when estimating catalyic activities reaction parameters like especially the temperature and the S(IV)-concentration should be adjusted carefully, to warrant reproducible results.