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

Abstract

The dependences of the electrode potentials E of aqueous Fe(H 2 O) 6 3+/2+ , Fe(phen) 3 3+/2+ , Fe(CN) 6 3−/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 cm 3 mol relative to AgCl/Ag; similarly, for Fe(H 2 O) 6 3+/2+ at I = 0.28 mol kg −1 (CF 3 SO 3 H), ΔV = +5.0 ± 0.3 cm 3 mol −1 . For Fe(CN) 6 3−/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 cm 3 mol −1 , but shows some slight dependence on P at I = 1.00 mol kg −1 (low-pressure limit ΔV 0 = −38 ± 1 cm 3 mol −1 ; [Formula: see text]. For Fe(phen) 3 3+/2+ , [Formula: see text] is markedly pressure and medium dependent, with ΔV 0 = +14.2 ± 0.5 and +6.2 ± 0.5 cm 3 mol −1 in KNO 3 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) 6 3−/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.