Gas chromatography at finite concentrations. Part 1.—Effect of gas imperfection on calculation of the activity coefficient in solution from experimental data

Abstract

Accurate measurement of activity coefficients in solution by gas chromatography requires correction for the effect of gas imperfection on retention. Current developments in finite concentration gas chromatography are impeded by lack of a means of calculating the gas imperfection correction term. A theory is presented which allows calculation of the activity coefficient of the solute at any solute concentration from a knowledge of (i) the concentration q of dissolved solute at a given gas phase concentration c, and (ii) the relevant second virial coefficients in the gas phase. Consideration of the available data on second virial coefficients allows estimation of the size of the gas imperfection correction. Contrary to suggestions made in the literature, the correction at infinite dilution is smaller in the heavier carrier gases. The other quantity q(c) required in the calculation can be derived from gas chromatographic retention measurements. Since this in turn requires knowledge of the variation of (∂q/∂c)_p as pressure p falls along the chromatographic column, the equation first developed for the activity coefficient is further used to derive and expression for (∂q/∂c)_p as a function of pressure.