The presence of humic acid seems to decrease the rate of CaSO3 precipitation during clarification of cane juice in the sugar manufacturing. The first and second ionisation constants of H2SO3 were found to decrease by 8 and 13% at RT in presence of humic acid in solution, respectively. However, the first and second ionisation constants of the acid were affected in the range 19-16% respectively across the temperature range 50-90degreesC. In the presence of humic acid the activation energy was found to increase. The nature of positive slope of Arrhenius plot indicates the diffusion controlled reaction in the precipitation reaction which is consistent with low value of activation energy of ionisatiion of sulphurous acid.