Oxidation of hydrazine by Cr(VI) oxide. kinetic and mechanistic studies in the presence of complexing agents

The kinetics of the Cr(VI) oxidation of hydrazine sulphate have been investigated both in the presence and absence of aqueous sulphuric acid. At low concentrations of hydrazine sulphate (< 0.02 M), the reaction exhibits a total second order dependence being first with respect to the gross [Cr(VI)] and also first with respect to [hydrazine sulphate]. But a plot of 1/k₁ versus 1/[substrate] is linear with an intercept when [N₂H₄·H₂SO₄] is > 0.02 M pointing to the formation of an equilibrium complex between the reactants prior to the main reaction. The effect of added H₂SO₄, sulphate, bisulphate and chloride ions has been analysed. A stoichiometry of [Cr(VI]: [N₂H₄] = 4:3 and the absence of ammonia in the reaction product (nitrogen being the only product) indicate that Cr(VI) behaves as a two-electron oxidant. Effect of added EDTA on this system is quite interesting and has been investigated thoroughly. A study of the UV spectrum of Cr(VI) solution in the presence of EDTA indicates the formation of mono and binuclear complexes of Cr(VI) with EDTA. Suitable mechanisms have been proposed to account for these observations.