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(H20);+/'+, ~e(~hen),)+/'+, F~(cN)~)-/"-, and ~o ( s e~) ' + / ' + upon pressure P (up to ca. 200 MPa) have been measured at 25.0°C relative to an AgCl/Ag(satd. KCI) electrode by cyclic voltammetry. For ~o ( s e~)~+ / ' + at ionic strength I = 0.28 and 1.00 rnol kg-', (dE/dP), is independent of P as well as I, giving a volume of reaction AV = + 13.7 ? 0.4 cm3 mol-' relative to AgCl/Ag; similarly, for F~( H~o ) ; + /~+ at I = 0.28 rnol kg-' (CF3S03H), AV = +5.0 * 0.3 cm3 mol-'. For F~(cN);-/~-in KC1 media, (dE/dP), is independent of P and I at I = 0.28 and 0.51 rnol kg-', giving AV = -36.1 ? 1.0 cm3 mol-', but shows some slight dependence on P at I = 1.00 rnol kg-' (low-pressure limit AVO = -38 * 1 cm3 mol-'; (dAV/dP), = +0.030 cm3 mol-' MP~-I).For ~e (~h e n )~~+ / ' + , (dE/dP), is markedly pressure and medium dependent, with AVO = +14.2 ? 0.5 and +6.2 ? 0.5 cm3 mol-' in KNO, media at I = 0.25 and 1 . OO rnol kg-', respectively. The peak-to-peak separation of the cyclic voltammograms increases with increasing P for Fe(cN)$-I4-, 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.

Electrochemistry at High Pressures: A Review

“…Pressure is transmitted to the pressure vessel through a separator with a free piston to keep the hydraulic fluid out of the electrochemical cell. The pressurizing fluid entering the electrochemical cell from the separator must be an electrical insulator and free from easily oxidizable or reducible impurities: hexanes [37], octanes, clean hydrocarbon white oil [1,2], heptane [38], or a mixture of isooctane and methylcyclohexane (10 : 90) [39] have given good performances and working pressures up to 10 kbar [39] are obtainable. These solvents are preferred to water or kerosene which were used in the past [40].…”

“…Doine et al [37] first developed a stainless steel pressure vessel which was pressurized with a handpump and intensifier and tested up to 300 MPa (ca. 3 kbar).…”

“…The Calgary cell [2] (Figure 6a) is an adaptation of Doine×s cell [37] in which compression of the reference electrode is allowed by mounting a Vycor frit in a free piston. Instead of saturated KCl solution, the reference compartment is filled with 4 M aqueous KCl to prevent dissolution or precipitation of KCl as the pressure is changed which may cause drift of the cell potential.…”

“…From these tables it can be seen that, despite the difficulty of obtaining reproducible electrochemical results from pressurized systems (see introduction), a general good agreement between the data reported by different authors is found. Furthermore, several studies for the ferri/ferrocyanide couple have been undertaken [37,52,53] in different electrolytes showing that for this couple the DV complex (complex-ion reduction volume) is almost independent of the nature or concentration of the electrolyte.…”

“…The estimated intrinsic and electrostrictive contributions for a range of couples were studied in aqueous solutions [37,53,54,81] and are summarized in Table 3. It can be seen that the electrostrictive volume change is around 20 cm 3 mol…”