Effect of Zn(II) on acid hydrolysis of methylparathion

Methylparathion (O,O,­dimethyl O­4 nitrophenylphosphorotioate) kinetic transfor­ mation in presence of Zn(II) was studied at pH range from 2 to 7. Between pH 2­5 methylparathion degraded, but at pH 6 and 7 Zn(II) precipitated and the pesticide hydrolysis did not occur. The reaction was first order and the rate constant values were 3.38 x 10 ­2 h ­1 (pH 2), 7.69 x 10 ­3 h ­1 (pH 3), 1.31 x 10 ­2 h ­1 (pH 4) and 1.02 x 10 ­ 2 h ­1 (pH 5). Paranitrophenol appeared as the only electroactive product at all pH values. It is proposed that the main degradation pathway for methylparathion in acid medium was a hydrolysis reaction of the pesticide catalyzed by Zn(II), via an electro­ philic mechanism.