Alkylphosphoric Acids and Their Complexes with Metals

The object of this paper is to review some recent Harwell work on the extraction of metals and acids from aqueous solution by alkylphosphoric acids and to discuss the basic equilibria involved. The solubility and distribution of mono- and di-n-butyl phosphoric acid (H₂MBP and HDBP respectively) have been measured in various aqueous-organic solvent systems—in particular the TBP-kerosene-HNO₃ system. The data for HDBP can be interpreted in terms of a series of equations for the dissociation of the HDBP in the aqueous phase, the dimerization of the HDBP in the aqueous and organic phase, the association of HDBP with TBP and with nitric acid, and the distribution of the HDBP monomer and dimer, and the HDBP · HNO₃ complex, between the phases. Nitric acid and water are extracted by dialkylphosphoric acids and also by the commonly used diluents. Nitric acid is extracted by HDBP in toluene or kerosene largely as the HDBP · HNO₃ complex, which is in equilibrium with the hvdrated complexes HDBP · H₂O and HDBP · HNO₃ · H₂O. Metals are extracted from aqueous solution by dialkylphosphoric acids (HX) mainly in four forms containing as ligands: (i) X groups. (ii) X and HX groups. (iii) X and HX groups and also the anion, for example, NO₃⁻, present in the initial aqueous solution. (iv) HX groups and the anion in the aqueous phase. The occurrence of one or more of these tvpes of complex is illustrated for U(VI), Zr(IV), and Be(II).