Deprotonation and transfer energetics of diglycine and triglycine in aqueous mixtures of urea and glycerol from emf measurements at different temperatures

Abstract

The two-stage deprotonation constants of diglycine and triglycine have been determined from emf measurements of cells without liquid junction comprising H₂/Pt and Ag-AgCl electrodes at five equidistant temperatures in the temperature range of 15°to 35°C in aqueous mixtures of urea and glycerol and also in water in the case of triglycine. Medium effect on deprotonation of the acids, i.e. <em>δ</em> (<img src='http://www.niscair.res.in/jinfo/delta.gif' border=0>X<img src='http://www.niscair.res.in/jinfo/equation154.gif' border=0>) (X = G or S) has been dissected into transfer free energies (<img src='http://www.niscair.res.in/jinfo/delta.gif' border=0><em>G</em><img src='http://www.niscair.res.in/jinfo/zerot.gif' border=0> and entropies (<img src='http://www.niscair.res.in/jinfo/delta.gif' border=0><em>H</em><img src='http://www.niscair.res.in/jinfo/zerot.gif' border=0>) of the species involved using the pre-evaluated values of the zwitterions (RH^±) and of H⁺ based on the widely used tetraphenylarsonium tetraphenylborate (TATB)reference electrolyte assumption. Attempts have been made to discuss the free energy and entropic contributions of the different species involved in the protolytic equilibria in terms of the their solvation behaviour as guided by the acid-base, dispersion and electronic characteristics of the acid-base species and of the cosolvent molecules and of their aqueous mixtures, besides the superimposed cavity and Born-type electrostatic effects.