A study of the rates of sulfur production in acid thiosulfate solutions using S-35

Abstract

Initial rates of formation of colloidal sulfur in solutions containing HCl and Na 2S 2O 3 were measured using S-35 labeled thiosulfate. Measurements were made at an ionic strength of 0.035 and, because of the method used to form the labeled thiosulfate, the reaction systems contained 0.002 to 0.02% dioxane. Sulfur production occurs as a third-order process with the equation, d( S)/ dt = k( H +) ( S 2 O 3 =)1 2. The third-order rate constant is κ = (1.64 ± .13) × 10 11 exp[(—15,930 ± 160)/RT] −2 mole −2 sec −2 . The results indicate a bimolecular reaction between HS 2O 3 − and S 2O 3 = as the dominant rate limiting step. Initial reaction rates based upon the appearance of turbidity of colloidal sulfur did not agree with the rates based upon sulfur formation. The temperature coefficient for the “precondensation” rate was also much smaller and corresponded to an apparent activation energy of 11.0 kcal/mole.