THE RATE OF DECOMPOSITION OF THE FERRIC-THIOSULFATE COMPLEX IN ACIDICAQUEOUS-SOLUTIONS

The rate of decomposition of the ferric thiosulfate complex varies as the square of the concentration of the complex. The rate of decomposit ion of the complex was determined by following the change in absorbanc e due to the complex as a function of time. Regression of log rate vs. log m(FeS2O3+) at 20-degrees-C produced the rate law dm(FeS2O3+)/dt = -10(2.0(+/-0.3)m(FeS2O3+)2.0(+/-0.1). The decomposition of this compl ex in acid solutions is strongly dependent on temperature, E(a) = 120 (+/-15) kJ mol-1. The rate of reaction increases with increasing ionic strength, which is consistent with the interaction of two positively charged ions to form the activated complex. This study resolves many o f the inconsistencies found in earlier studies and shows that reaction with H+ is a more important sink for S2O32- than reaction with Fe3+ w hen pH > approximately 1.7.