Acid–Base Properties of the $\mathrm{Fe}(\mathrm{CN})_{6}^{3-}$ / $\mathrm{Fe}(\mathrm{CN})_{6}^{4-}$ Redox Couple in the Presence of Various Background Mineral Acids and Salts

Abstract

The acid-base behavior of Fe(CN) 4− 6 was investigated by measuring the formal potentials of the Fe(CN) 3− 6 /Fe(CN) 4− 6 couple over a wide range of acidic and neutral solution compositions. The experimental data were fitted to a model taking into account the protonated forms of Fe(CN) 4− 6 and using values of the activities of species in solution, calculated with a simple solution model and a series of binary data available in the literature. The fitting needed to take account of the protonated species HFe(CN) 3− 6 and H 2 Fe(CN) 2− 6 , already described in the literature, but also the species H 3 Fe(CN) − 6 (associated with the acid-base equilibrium H 3 Fe(CN) − 6 H 2 Fe(CN) 2− 6 + H + ). The acidic dissociation constants of HFe(CN) 3− 6 , H 2 Fe(CN) 2− 6 and H 3 Fe(CN) − 6 were found to be pK II 1 = 3.9 ± 0.1, pK II 2 = 2.0 ± 0.1, and pK II 3 = 0.0 ± 0.1, respectively. These constants were determined by taking into account that the activities of the species are independent of the ionic strength.