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

Abstract

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 constants of glycine and the molar concentration of the seawater solution is derived in the range of salinities varying from 0.5 to 4%. The MSA is a simple analytical theory for electrolytes that is very useful in representing the thermodynamic properties of ionic solutions over a wide range of concentrations in terms of one simple parameter, Γ, and the charges, diameters, and concentrations of the ions. A concentration dependent dielectric constant rather than the pure solvent permittivity together with the concentration-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 the protonated glycine ion and the proton, both of which are involved both in the equilibria of glycine, have been obtained.