Chromium(III)-catalysed cerium(IV) oxidation of arsenic(III) in aqueous sulphuric acid

Abstract

The chromium(III)-catalysed cerium(IV) oxidation of arsenic(III) studied in aqueous sulphuric acid follows the rate law (i) where the right-hand side represents the catalysed path. In equation (i), the –d[Ce^IV]//_dt–k_u[As^III][Ce^IV]=2k_ak_cβ₄K_IP[Ce^IV][Cr^III][As^III][SO₄^2–]³[HSO₄^–]²[H⁺]²//_{(1 +β1[SO4^2–]+β2[SO4^2–]²+β3[SO42)]²[HSO4^–]+β4[SO4^2–]²[(HSO4^–]²[H⁺])(1 +KIP[SO4^2–])}×{k_bK₅[Ce^III][SO₄^2–]³//_{(1 +K5[SO4^2–]³)}+k_c[As^III][H⁺]}^–1(i)β's are the stability constants of the different cerium(IV) sulphate complexes and K_IP and K₅ are the equilibrium constants of the species Cr³⁺SO₄^2– and Ce(SO₄)₃^3– respectively. A chromium(IV) species is involved in the catalysed path. Increasing sulphuric acid concentrations effect rate increases of both the uncatalysed and catalysed paths and the active oxidant is understood to be the species H₃Ce(SO₄)₄^–.