Ligand-exchange separations of amino acids. I. Distribution equilibria of some amino acids between ammoniacal copper(II) nitrate solutions and phosphonic, carboxylic and iminodiacetic ion exchangers in the copper(II) form.

The ligand-exchange process for the distribution of glycine, alanine and leucine between a copper(II) phosphonate resin and ammoniacal solutions of copper nitrate has been studied. The formation in both resin and solution phases of mixed CuA(NH3)+i complexes (i = 1-3) was demonstrated. A theoretical expression for the distribution coefficient, taking into account the various parameters upon which it depends, has been established and verified experimentally. Some experiments carried out with carboxylic and iminodiacetic resin in the Cu(II) form confirmed the validity of the theoretical model. The distribution of an amino acid by ligand exchange occurs as a complex phenomenon that brings many equilibria into play. However, by simplifying the theory with some approximations, it is possible to describe the distribution by means of two equilibria, one of ion exchange and the other of the formation in solution of mixed complexes. This simplified theory clearly displays how the ammonia and copper(II) concentrations affect the amino acid fixation. Some experimental graphs illustrate the variation of the distribution coefficients for different amino acids with each of these parameters. Finally, some observations on the selectivity that may be expected from this process are made. In particular, it is shown how the optimal conditions for the separation of two amino acids can be determined.