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Cation and Pressure Effects on the Electrochemistry of 12-Tungstocobaltate and 12-Tungstophosphate Ions in Acidic Aqueous Solution

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journal contribution
posted on 09.02.2004, 00:00 by Mitsuru Matsumoto, Nancy I. Neuman, Thomas W. Swaddle
The effects of supporting electrolytes and of pressure on the electrode reactions of the aqueous CoW12O405-/6- couple at 25 °C are reported, together with limited data on PW12O403-/4- and PW12O404-/5-. The half-wave potentials E1/2 for the CoW12 couple become moderately more positive with increasing electrolyte concentration and cationic charge, and also in the sequences Li+ ≈ Na+ < NH4+ ≤ H+ < K+ < Rb+ < Cs+ and Na+ < Mg2+ < Ca2+ < Eu3+. The mean diffusion coefficients for CoW12 with the 1:1 electrolytes are independent of electrolyte concentration and rise only slightly from Li+ to Cs+, averaging (2.4 ± 0.3) × 10-6 cm2 s-1. Neither the volumes of activation for diffusion ΔVdiff (average −0.9 ± 1.1 cm3 mol-1) nor the electrochemical cell reaction volumes ΔVAg/AgCl (average −22 ± 2 cm3 mol-1) for the CoW12 couple show significant dependence on electrolyte identity or concentration. For the PW123-/4- and PW124-/5- couples, ΔVAg/AgCl = −14 and −26 cm3 mol-1, respectively, suggesting a dependence on Δ(z2) (z = ionic charge number) as predicted by the Born−Drude−Nernst theory of electrostriction of solvent, but comparison with ΔVAg/AgCl for CoW12 and other anion−anion couples shows that the Born−Drude−Nernst approach fails in this context. For aqueous electrode reactions of CoW12, as for other anionic couples such as cyanometalates, the standard rate constants kel show specific cation catalysis (Na+ < K+ < Rb+ < Cs+), and ΔVel is invariably positive, in the presence of supporting electrolytes. For the heavier group 1 cations, ΔVel is particularly large (10−15 cm3 mol-1), consistent with a partial dehydration of the cation to facilitate catalysis of the electron-transfer process. The positive values of ΔVel for the CoW12 couple cannot be attributed to rate control by solvent dynamics, which would lead to ΔVel ≤ ΔVdiff, i.e., to negative or zero ΔVel values. These results stand in sharp contrast to those for aqueous cationic couples, for which kel shows relatively little influence of the nature of the counterion and ΔVel is always negative.