Kinetics of the oxidation of hydroxyurea with vanadium(V) ions in acidic aqueous solution

Abstract

Hydroxyurea (HU) effectively reduces vanadate (VO2+) into vanadyl (VO2+) species in acidic aqueous solution acting as a two-electron donor. The reaction starts by the formation of a transient complex followed by an electron transfer process that includes the formation and subsequent fading out of a free radical, U• (U•≡H2N–C(=O)N(H)O•). The rate determining step of the redox reaction is the formation of a free radical either by the inner-sphere one-electron transfer within the formed VO2+-complexes, or by an outer sphere one-electron transfer from VO2+-complex to the second VO2+ ion. Assuming a rapid pre-equilibrium for the proton-transfer and complexation reactions, an inner-sphere electron transfer pathway has been proposed (VO2-U2+\({\mathop{\longrightarrow}\limits^{k_{1}}}\) VO2+-U•2+, k1 = 1.4 s−1) when HU is in excess. When VO2+ in excess, an additional reaction possibly proceeds through two outer-sphere electron-transfer pathways: VO2-U2+ + VO2+ + 2H2O \({\mathop{\longrightarrow}\limits^{k_{4}}}\) VO2+ + VO2+ + U•, and VO2-U2+ + VO(OH)2+ + 2H2O \({\mathop{\longrightarrow}\limits^{k_{5}}}\) VO(OH)2+ + VO2+ + U•, characterized by k4 = 22 s−1 mol−1 dm3, and k5 = 3.8 × 103 s−1 mol−1 dm3.