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