In this study, the overall stability constants of copper(II) complexes with some a-amino acids (glycine, DL-alanine, DL-valine, L-leucine, L-asparagine, L-glutamine) were determined by potentiometric titration in water, 25% dioxan-75% water, 35% dioxan-65% water, 50% dioxan-50% water, and 60% dioxan-40% water. The titrations were performed at 25 degreesC, under nitrogen atmosphere, and the ionic strength of the medium was maintained at 0.10 M by using sodium perchlorate. The formation curves of their complexes ((n) over barp[L]) were obtained by means of the titration data. Then the stability constants were determined in relation to these curves. The mol ratio of copper(II) to a-amino acid was also determined and it was found that the complexes were CuL2 type. Another important result obtained was that the tendency of amino acids to form complexes with copper(II) was greater in dioxan-water mixtures compared to water. (C) 2001 Academic Press.