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 \(\mathrm{Fe}(\mathrm{CN})_{6}^{4-}\) was investigated by measuring the formal potentials of the \(\mathrm{Fe}(\mathrm{CN})_{6}^{3-}\)/\(\mathrm{Fe}(\mathrm{CN})_{6}^{4-}\) 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 \(\mathrm{Fe}(\mathrm{CN})_{6}^{4-}\) 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 \(\mathrm{HFe}(\mathrm{CN})_{6}^{3-}\) and \(\mathrm{H}_{2}\mathrm{Fe}(\mathrm{CN})_{6}^{2-}\), already described in the literature, but also the species \(\mathrm{H}_{3}\mathrm{Fe}(\mathrm{CN})_{6}^{-}\) (associated with the acid–base equilibrium \(\mathrm{H}_{3}\mathrm{Fe}(\mathrm{CN})_{6}^{-}\rightleftharpoons \mathrm{H}_{2}\mathrm{Fe}(\mathrm{CN})_{6}^{2-} + \mathrm{H}^{+}\)). The acidic dissociation constants of \(\mathrm{HFe}(\mathrm{CN})_{6}^{3-}\), \(\mathrm{H}_{2}\mathrm{Fe}(\mathrm{CN})_{6}^{2-}\) and \(\mathrm{H}_{3}\mathrm{Fe}(\mathrm{CN})_{6}^{-}\) were found to be \(\mathrm{p}K^{\mathrm{II}}_{1}= 3.9\pm0.1\), \(\mathrm{p}K^{\mathrm{II}}_{2} = 2.0\pm0.1\), and \(\mathrm{p}K^{\mathrm{II}}_{3} = 0.0\pm0.1\), respectively. These constants were determined by taking into account that the activities of the species are independent of the ionic strength.