Solvent extraction 349 Xa concentration of A in raffinate phase The distribution coefficient, m, is a measure of the affinity of the solute (A) for one phase(E, S)over the other phase(F, R). The concentration of A may be expressed in various units, but for ease of subsequent calculations it is preferable to express the concentration on a solute-free basis for both phases. For example, in the extraction of acetone from water with toluene weight acetone-free wate ight aceto Although the units of m appear to be dimensionless, they actually (weight acetone-free water /weight acetone-free toluene) If more than one solute is present, the preference, or selectivity, of the solvent for one(A)over the other( B)is the separation factor(a) aar The separation factor(aaB)must be greater than unity in order to separate A from B by solvent extraction, just as the relative volatility must be greater than unity to separate A from b by distillation The analogy with distillation can be carried a step further. The extract phase is like the vapor distillate, a second phase wherein the equilibrium distribution of A with respect to B is higher than it is in the feed liquid(liquid bottoms Extraction requires that the solvent and feed liquor be at least partially immiscible( two liquid phases), just as distillation requires both a vapor andSolvent Exlraction 349 m=yA= concentration of A inextract phase X, concentration of A in raflinate phase The distributioncoefficient, m, is a measure ofthe affinity ofthe solute (A) for one phase (E, 5') over the other phase (E R). The concentration ofA may be expressed in various units, but for ease of subsequent calculations, it is preferable to express the concentration on a solute-free basis for both phases. For example, in the extraction of acetone from water with toluene: weight acetone weight acetonefree water X= weight acetone weight acetonefree toluene Y= Although the units of m appear to be dimensionless, they actually are If more than one solute is present, the preference, or selectivity, of the (weight acetone-free water)/(weight acetone-free toluene). solvent for one (A) over the other (B) is the separation factor (a). The separation factor (arn) must be greater than unity in order to separate A from B by solvent extraction, just as the relative volatility must be greater than unity to separate A from B by distillation. The analogy with distillation can be carried a step further. The extract phase is like the vapor distillate, a second phase wherein the equilibrium distribution ofA with respect to B is higher than it is in the feed liquid (liquid bottoms). Extraction requires that the solvent and feed liquor be at least partially immiscible (two liquid phases), just as distillation requires both a vapor and a liquid phase