A process of diminishing the concentration of a transition metal complex from a first solution by adding a solubility-enhancing compound that enhances the solubility of said complex in a second solution and extracting the first solution with the second solution. The solubility-enhancing compound is a compound of formula A ##STR00001## wherein, R.sup.a is SH, SO.sub.3H, OH or COOH; R.sup.b is SH, OH or COOH; R.sup.c each independently is H, SH, OH or COOH; R.sup.d each independently is H or COOH; n is 1, 2, 3, 4 or 5; or a salt or an activated form thereof.