Extraction of lower carboxylic acids from aqueous solution by tri-n-octylamine.

To investigate the mechanism of extracting organic acids from water by amine, the extraction equilibria of the ternary systems of tri-n-octylamine (TOA), water, and each of four lower carboxylic acids were measured at 30°C. The experiments show that the distribution ratio of an organic acid strongly depends on the kind of acid when the concentration in the aqueous phase is very low. An extraction model where both association and physical distribution are taken into consideration has been proposed. By this model the experimental data can be explained well, while the association mechanism of the model is estimated from the excess volumes of TOA/organic acid binary mixtures. The model suggests that the 1:1 complex between amine and an organic acid strongly contributes to the distribution and selectivity of the acid in the region of very low concentration.