Determination of some thiophosphorus insecticides based on coulometric titration with the anodically generated chlorine: a further insight in the reaction mechanism in aqueous medium.

A new method of analytical determination of some thiolo (PS) and thiono (PS) group containing phosphorus insecticides using a coulometric titration with the anodically generated chlorine and biamperometric end-point detection was described. The method worked well for compounds containing one (methamidophos) or two PS bonds (iso-malathion) or one PS bond (fenitrothion) but failed for compounds containing both PS and PS bonds (malathion, dimethoate) or additional sterically hindered CSC bond (fenthion). Based on the found number z of electrons transferred per mol of oxidized compounds, mechanistic pathways for chlorination of PS and PS containing thiophosphorus insecticides with molecular chlorine in aqueous, acidic medium were proposed.