POLAROGRAPHIC STUDY OF THE INTERACTION OF CHOLATE AGGREGATES WITH CU2+, PB2+ AND CD2+ IONS

When the concentration of sodium cholate in a solution containing a constant concentration of a heavy metal ion (Cu2+, Cd2+, Pb2+) is gradually increased and polarographic current voltage curves, corresponding to the reduction of the metal ion, are recorded, the observed changes in these curves can be classified into four categories. For unbuffered solutions containing 0.15 M NaNO3 and 1 x 10(-4) M metal ion, these changes occur in the following ranges of cholate concentration. (1) At cholate concentrations below 1 x 10(-3) M, no measurable interaction between the cholate anions and the metal ion occurs- This is indicated hy the constant value of the half wave potential and the constant height of the limiting current of the reduction wave of the metal ion. (2) For cholate concentrations between about 1 x 10(-3) M and 1 x 10(-2) M, complex formation. probably with small cholate aggregates, occurs, as shown bv the shift of the half-wave potential of the metal ion. Some of these complexes have limited solubility, as indicated by the decrease in the limiting current of the metal ion and the turbidity of the solutions. (3) In the third range for cholate concentrations between about 1 x 10(-2) and 2 x 10(-2) M the current reaches its minimum value, corresponding to the solubility of the complex between the metal ion and some smaller cholate aggregates. (4) At cholate concentrations higher than about 2 x 10(-2) M larger aggregates ("micelles") are formed which solubilize the slightly soluble complexes with smaller aggregates. This is indicated by an increase in limiting current and clarification of the Solution. These changes do not result from hindered transport of metal ions to the electrode surface. The complex formation involves rhe carboxylate group.