Estimation of the reduction potential of transition metal atoms in aqueous solutions

The reduction potentials of the M2+/M0 couples of transition metals Mn, Fe, Co, Ni, Cu, and Zn were calculated by subtracting from their standard electrode potentials in liquid water the standard molar Gibbs energy of formation of atoms in the gas phase ΔfG0(Mgas0) and the Gibbs energy of their hydration in solution ΔfG0(Maq0). The potentials are –2.503 V for Mn0, –2.454 V for Fe0, –2.333 V for Co0, –2.335 V for Ni0, –1.291 V for Cu0, and –1.335 V for Zn0. The disproportionation of ions in the intermediate oxidation state M+ with the formation of the M0 atom is thermodynamically favorable in the solution volume only for zinc.