The Influence of Entrainment on Distribution Ratios in Complexation Studies

ABSTRACT

Entrainment of a small amount of one phase in the other after phase separation is almost impossible to avoid in solvent extraction. The effect on distribution ratios, D-values, is that the measurement at high D-values results in lower values than the true D-value. The opposite happens with the aqueous phase. Entrainment reduces the maximum effective separation of two substances using countercurrent solvent extraction in mixer-settlers, pulsed columns, and centrifugal extractors. When solvent extraction is used to study the complex formation, the entrainment of a small amount of the organic phase in the aqueous phase reduces the limiting slope observed at low D-values and sometimes makes the accurate determination of one or more of the stability constants impossible.

On the other hand, the reduction of the expected limiting slope yields a tool for estimation of the entrainment in the specific case and hence makes it possible to get entrainment data that can be used in comparisons of the efficiencies of different phase separation techniques and equipment.

In this work, the effect of entrainment on “effective” D-values is derived in a quantitative way, yielding equations that can be either used to predict “effective” D-values from any given relative entrainment in any phase or used to estimate entrainment by comparing measured “effective” D-values and the corresponding data from test tube experiments with no entrainment.