The mean spherical approximation methodology applied to the acid-base equilibria of glycine in artificial seawater

We present results obtained by use of a statistical mechanics approximation based on integral equations, the mean spherical approximation, to predict equilibrium constants for glycine in artificial seawater and to illustrate the effect of the ionic medium. A relationship between the dissociation con stants of glycine and the molar concentration of the seawater solution is d erived in the range of salinities varying from 0.5 to 4%. The MSA is a simp le analytical theory for electrolytes that is very useful in representing t he thermodynamic properties of ionic solutions over a wide range of concent rations in terms of one simple parameter, Gamma, and the charges, diameters , and concentrations of the ions. A concentration dependent dielectric cons tant rather than the pure solvent permittivity together with the concentrat ion-dependent diameters of the cations are incorporated in the calculations . This is a way of including the effect of soft repulsions and attractions and the effect of solvation. In calculations, an optimized diameters for th e protonated glycine ion and the proton, both of which are involved both in the equilibria of glycine, have been obtained.