Standard potentials of Ag-AgX (X = Br and I) electrodes at different temperatures and the related transfer energetics of HX in quasi-isodielectric aqueous ethylene carbonate solvents

Abstract

Ag-AgI electrodes have been determined in quasiisodielectric aqueous mixtures of 10, 30, 50 and 70 (wt %) dipolar aprotic ethylene carbonate (EC) at 25°,30°,35°,40° and 45°C. These E⁰ values have been utilized to compute transfer free energies (∆<i>G</i><img src='http://www.niscair.res.in/jinfo/zerot.gif' border=0>) and entropies (∆<i>S</i><img src='http://www.niscair.res.in/jinfo/zerot.gif' border=0>) of HBr and HI. These values when coupled with previously determined transfer energetics of H⁺ yield the transfer energetics of Br^- arid I^-. The chemical parts of transfer free energies ∆G<img src='http://www.niscair.res.in/jinfo/zerot.gif' border=0>_,ch (i), obtained after correcting the cavity effect and Born-type and ion-dipole type electrostatic effects, are found to be in accord with the dictates of increased destabilizing chemical effect of dipolar aprotic nature of EC and the superimposed soft-soft interactions on these large sized halide ions. The observed T∆S<img src='http://www.niscair.res.in/jinfo/zerot.gif' border=0> (i) versus composition profiles as well as those of T∆G<img src='http://www.niscair.res.in/jinfo/zerot.gif' border=0>_,ch (i) versus composition, as obtained after correcting for the cavity effect and Born-type electrostatic effects, have been examined in the light of Kundu <i>et al.'s </i> semiquantitative theory based on a four-step transfer process, It is found that addition of cosolvent EC induces breakdown of three dimensional water structure due to possible formation of H-bonded EC-(H₂O)_x complexes and EC-EC aggregates and the packing imbalance in solvent mixtures containing higher proportion of EC.