HYDROLYSIS OF THIOCYANIC ACID .I. DEPENDENCE OF RATE ON ACIDITY FUNCTION

The rate of formation of ammonia, hydrogen sulfide, and carbon dioxide from potassium thiocyanate in 0.05-11.0 M hydrochloric, phosphoric, and sulfuric acids, measured at 13.5-90.1°, shows a linear correlation of log k with H0. The slope is 1.0 at high acidities where the predominant species is HNCS and about 1.4 below H0 = -2. The pKA of thiocyanic acid is -2.3 to -2.0 at 25°, depending on the estimated H_ of hydrochloric acid solutions. There is a solvent isotope effect on the absorption spectrum of thiocyanate ion in D2O. The hydrolysis rates are correlated by log k = [h_/(KA + h_)] (k1 + k2h0), an equation consistent with concurrent mechanisms involving thiocyanic acid and one of its conjugate acids.