THE RATE OF DECOMPOSITION OF THE FERRIC-THIOSULFATE COMPLEX IN ACIDIC AQUEOUS-SOLUTIONS

The rate of decomposition of the ferric thiosulfate complex varies as the square of the concentration of the complex. The rate of decomposition of the complex was determined by following the change in absorbance 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 complex 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 of the inconsistencies found in earlier studies and shows that reaction with H+ is a more important sink for S2O32- than reaction with Fe3+ when pH > approximately 1.7.