Studies of the Conductances and Energetics of Transport of Sodium Acetate, Benzoate and Salicylate in an uncharged Microporous Membrane

Department of Chemistry, University of Burdwan, Burdwan-713 104

Manuscript received 2 March 1994. revised 27 June 1994, accepted 11 August 1994

Studies on the conductance behaviour of an 'uncharged' microporous membrane bathed in sodium salt solutions of organic anions, viz. acetate (Ac^-), salicylate (Sl^-) and benzoate (Bz^-) at different concentrations (1-200 mol m^-3 ) and in the temperature range 298-323 K are reported. The value! show a smooth increase in conductance at all concentrations, tending towards limiting values, and maintain the order, Sl^- > Bz.^- > Ac^- reverse to that in aqueous solution over the temperature range studied here. This reversal of order has been correlated with the kind of ion-pairing termed as 'localised hydrolysis  supposed to be more predominant within the micropores. The pK_a values and hence the proton binding affinity of the conjugate bases showing the order, Ac^- >Bz^- > Sl^- may justify the conductance in the reverse order. The temperature variation of conductance has been utilised to calculate the activation energies assuming the applicability of Arrhenius equation. The energy values follow the order E_Ac- > E_Sl^- > E_Bz^-. The highest energy for the acetate ion is in conformity with its lowest conductance, but the higher activation energy value for Sl^- than that of Bz^- may well be explained in the light of extra energy required for endothermic opening up of intramolecular hydrogen bonding of Sl^- before undergoing localised hydrolysis during the conductance.