The influence of pH on the stability constants of lanthanum and europium complexes with humic acids

Complex formation of humic acids (HA)(n) with La3+ and Eu3+ was studied. Commercial (HA)(n) was purified and characterized. The stability constants were determined at several pH values and 0.2 M NaClO4 ionic strength by the Shubert's method of radiochemical ionic exchange. The slopes of the lines were dependent on the [(HA)(n)]. The values of log for j = 1 were the following: 6.29 +/- A 0.04 (pH 4.9 +/- A 0.4) and 7.61 +/- A 0.03 (pH 5.9 +/- A 0.1) for lanthanum and 7.31 +/- A 0.01 (pH 5.9 +/- A 0.2) for europium. Log was determined as well for higher values of the j parameter and these values were: 12.2 +/- A 0.1 (j = 2, pH 7.7 +/- A 0.2), 15.6 +/- A 0.2 (j = 3, pH 4.9 +/- A 0.4) and 16.05 +/- A 0.07 (j = 3, pH 5.9 +/- A 0.1), for lanthanum and 13.18 +/- A 0.03 (j = 2, pH 5.9 +/- A 0.1) for europium. A discussion is presented about the complex formation regarding pH and [(HA)(n)].