Pressure dependences of the electrode potentials of some iron(III/II) and cobalt(III/II) couples in aqueous solution

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Abstract
The dependences of the electrode potentials E of aqueous Fe(H2O)63+/2+, Fe(phen)33+/2+, Fe(CN)63−/4−, and Co(sep)3+/2+ upon pressure P (up to ca. 200 MPa) have been measured at 25.0 °C relative to an AgCl/Ag(satd. KCl) electrode by cyclic voltammetry. For Co(sep)3+/2+ at ionic strength I = 0.28 and 1.00 mol kg−1, [Formula: see text] is independent of P as well as, giving a volume of reaction ΔV = +13.7 ± 0.4 cm3 mol relative to AgCl/Ag; similarly, for Fe(H2O)63+/2+ at I = 0.28 mol kg−1 (CF3SO3H), ΔV = +5.0 ± 0.3 cm3 mol−1. For Fe(CN)63−/2− in KCl media, [Formula: see text] is independent of P and I at I = 0.28 and 0.51 mol kg−1, giving ΔV = −36.1 ± 1.0 cm3 mol−1, but shows some slight dependence on P at I = 1.00 mol kg−1(low-pressure limit ΔV0 = −38 ± 1 cm3 mol−1; [Formula: see text]. For Fe(phen)33+/2+, [Formula: see text] is markedly pressure and medium dependent, with ΔV0 = +14.2 ± 0.5 and +6.2 ± 0.5 cm3 mol−1 in KNO3 media at I = 0.25 and 1.00 mol kg−1, respectively. The peak-to-peak separation of the cyclic voltammograms increases with increasing P for Fe(CN)63−/4−, but decreases slightly for the cationic couples; these trends can be accounted for in terms of the volumes of activation for the corresponding self-exchange reactions. Keywords: high pressures, electrode potentials, volume of reaction, iron complexes, cobalt complexes.

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