Nature of the yellow copper complex produced in certain analytical methods for the determination of malathion

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Abstract
Evidence is presented to show that the yellow copper complex, cupric O, O‐dimethylphosphorodithioate (C), which is formed in certain colorimetric methods for the determination of malathion, can exist in reversible equilibrium with its two dissociation products, cuprous O, O‐dimethylphosphorodithioate (A) and bis‐(dimethoxyphosphorothiono) disulphide (B). At 20°, in solution in chloroform, equimolar proportions of a and B were found to be 24.86% associated to form C, and the equilibrium constant for the association reaction, a + B → 2C, was found to be 0.4378.In view of these findings, a correlation is possible between colour intensities of yellow complex solutions obtained by two differing methods. This indicates that the yellow complex obtained by previous authors is initially undissociated. Hence, dissociation is considered to account, at least partly, for the instability of the yellow complex that has been reported by previous authors. the incorporation of B in the extract ant, at a level of 450 moles of B per mole of yellow complex, is proposed as a means of limiting dissociation to a maximum of 1 %.

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