The contemporary presence in aqueous solution of a ligand and a cation brings to an  equilibrium for the formation of new species. To investigate exhaustively the taking place of equilbria, it is necessary to explore a wide reagent concentration range in order to find the prevailing formed species and to evaluate the possibility of complex formation with the participation of hydrogen ions.

For this purpose, to obtain accurate results, it is necessary to use the constant ionic medium method proposed by Biedermann and Sillén, in order to minimize the activity  coefficients variations in spite of the change of the reagent concentrations. Based on this method, aminoacids as aminoacetate, 2-,3- aminopropanoate, serine, aspartate, glutamate, histidinem, ornithine and taurine, were studied as ligands of several cations like lead (II), cadmium (II), nickel (II), cobalt (II), zinc (II), iron (II), and (III) , calcium (II) magnesium (II), silver (I), copper (II). Among aminoacids those containing S atoms received little attention. In this group, cystine, cysteine and D- penicillamine disulfide are important compounds under different points of view. Their behaviour in aqueous solutions (solubility, acid constants….) also towards cations like alkaline ions and cadmium (II) was investigated.

The employed procedures to obtain suitable experimental data will be described so that the best way to perform equilibrium analysis will be explained. Furthermore, species assumed to explain to experimental data will be presented and discussed in order to stress analogies and differences.