pH dependence of tartaric acid extraction

The dependence of extraction equilibrium on pH of the aqueous phase was studied in the system: aqueous solution of D-tartaric acid - solution of trialkylamine (TAA) in binary diluent. TAA was a mixture of tertiary amines with C 7 -C 9 carbon chains, the binary diluent was a mixture of w=0.7 of n-octan-1-ol in n-heptane. Measurements were performed at three levels of equilibrium pH 2.0, 2.5 and 3.0, the pH value required being adjusted with aqueous sodium hydroxide solution. The range of pH values and of acid concentrations investigated was limited by the low solubility of the sodium hydrogentartrate forming. In the range of pH values examined an increase of acid extraction with increasing pH was found, if only free acid in the aqueous phase was taken into account. The results of the extraction at constant initial total acid molality and varying pH exhibited diverse trends according to the way of expressing the degree of extraction. All these effects could be explained by assuming that only the free, undissociated acid can be extracted.