Potentiometric and Thermodynamic Studies of Some Metal-Cysteine Complexes

A new and simple potentiometric method is developed to determine the stability constants of cysteine complexes with Mn(II), Cu(II), Zn(II), Cd(II), and Ce(III) metal ions using a modified Bjerrum method. Potentiometric titrations were performed in water and water/dioxane mixtures at five different temperatures. The least and highest stable complexes were those of Cu(II) and Zn(II), respectively. Furthermore, by conducting the experiments at temperatures of 15, 20, 25, 35, and 45 degrees C, the thermodynamic parameters H degrees, S degrees, and G degrees for the pertinent complexes were calculated. The results show that the H degrees values of the complexes are positive except for Zn(II). Negative values of G degrees are evidence for the spontaneity of the reactions.