Extraction of carboxylic acids by amine-based extractants: Apparent extractant basicity according to the pH of half-neutralization

This article analyzes the extraction of hydrochloric, trichloroacetic, propionic, lactic, and acetic acids by nine amine-based extractants, including Primene JMT, tris(2-ethylhexyl)amine (TEHA), and tri-n-octylamine (TOA), in various diluents. The extractants were half-loaded with the acids, and the pH values (pH(hn)) of the aqueous phases were determined. The IR spectra of the organic phases were also recorded. The results were analyzed according to our theory, which was developed in a previous article, and a good correlation was found. Ion-pair formation is the main mechanism for systems in which the basicity of the amine is higher than that of the extracted acid's anion, and H-bonding of the undissociated acid becomes important only for the extraction of weak acids by weak bases. Thus, one can predict the acid form in the organic phase from the difference between the apparent basicity of the extractant on extracting HCl, pH(hn(HCl,D)), and the pK(a) of the extracted acid.