On the Ag(II, I) and Co(III, II) standard redox potentials

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Investigating the solution chemistry of transition metal ions in high oxidation states, at low temperature (−5 °C) and in strongly acidic medium (5 M HClO 4), our first success was, the evaluation of the standard redox potential of the Ag(II, I) pair. This is the e.m.f. of the hypothetic cell: ▪ at −5 °C, with [Ag 2+] = [Ag +], C HClO 4 → 5 M, and E = O for the half cell: Pt, H 2 (1 atm)/H + ( a = 1). Owing to the experimental impossibility of building such a cell, and thanks to the good working of the glass electrode in such conditions [1], we measured the e.m.f.s of the cells without junction: ▪ and ▪ whose combination and rearrangement leads to: ▪ where F is the activity coefficients ratio and all concentrations are expressed in molality. All the terms on the left side of eqn. ( 1) are directly measurable, while we call standard redox potential of the Ag(II, I) pair, at −5 °C and in 5 M HClO 4 (6.5 M), the sum of the right side terms. The method of preparation of Ag 2+ solutions can be summarized as follows: ▪ Once a Ag 2+ solution was obtained, e.m.f. measurements of cell (G) were carried out at −5 °C, after successive additions of a cold Ce(ClO 4) 3 in 5 M HClO 4 solution. The collected data enabled us to build a Gran diagram from which to draw the Ag 2+ concentration at each point of the titrations and, by eqn. ( 1), a set of E° values for each titration. The mean value, derived from 5 titrations at different Ag 2+ initial concentration, Ag total concentration, and varying the [Ag 2+]/[Ag +] ratio over a wide range, is 1977.5 ± 1.4 mV for E°, the standard redox potential of the Ag(II, I) pair at −5 °C and in 5 M (6.5 M) HClO 4. Likewise, adding to the Ag 2+ solution, at −5 °C, successive amounts of a cold Co(CLO 4) 2 in 5 M HClO 4 solution, we generated Co + by the reaction: Ag 2+ + Co 2+ → Ag + + Co 3+. After the equivalent point, the cell ( G) becomes: ▪ for which, from E R′ = E G′ + E H, holds equation ( 2): ▪ where E° is the standard redox potential of the Co-(III, II) pair at −5 °C in 5 M (6.5 M) HClO 4. Preliminary titrations at −5 °C using cell ( G′ gave, by eqn. ( 2), E° = 1.84 V.