Spectrophotometric determination of manganese II with benzohydroxamic acid

Abstract

Manganese II reacts with benzohydroxamic acid in ammoniacal solution to form a stable reddish-brown complex (λ max 500 mμ). The reagent does not absorb at this wavelength and the concentration of ammonium hydroxide is not critical above pH 10. The sensitivity is 0.015 μg of manganese per cm 2 for log I 0/I = 0.001 ; the spot-plate sensitivity is 0.2 μg of manganese per 0.05 ml. Beer's law is obeyed up to a manganese concentration of 10 ppm in 1-cm absorption cells. The colour reaction takes place in non-aqueous solvents; optimum conditions for the reaction in dimethylformamide have been established. Interference by iron, cobalt and copper is avoided by the separation of manganese from these ions by ion-exchange. Development of the method included a study of the effect of temperature, pH, reagent: manganese ratio, stability of the complex and the rate of colour formation. The spectrophotometric method has been successfully applied to the determination of manganese in steel, bronze and magnesium alloy.