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.