Extraction of phosphoric acid with long-chain tertiary amines-I. Extraction from aqueous phosphoric acid solutions

The extraction of phosphoric acid from its aqueous solutions with the long-chain tertiary amine Alamine 336 in toluene was studied at 25°C. Alamine 336 is intermediate in its extraction properties between trioctylamine and tridecylamine, but the spread in the distribution coefficients between these two does not exceed 10%. Admixing 20% by volume of dodecane with the toluene diluent produces a notable synergistic effect, but high concentrations of the aliphatic diluent lead to third phase formation. Some amine is extracted from the organic phase into dilute aqueous phosphoric acid solutions, affecting the titration of this acid with base. The distribution data can be quantitatively explained by the formation in the organic phase of two amine phosphate species with stoichiometries R₃N:H₃PO₄ of 1 : 1 and 1 : 2. At high acid concentrations (> 4.2 M), excess acid over the 1 : 2 species is extracted.