In recent years many proton transfer systems including various diacids and bases such as piperazine and their complexes were studied in solid state and solution [1-2] but the proton transfer systems including thio acids less was attended [3]. In this work the protonation constants of 2,2'- thiodiacetic acid and piperazine as the building blocks of the proton transfer system, the stability constants of this system and its complexes with Cu2+, Cd2+, Zn2+, Ni2+ and Ce3+ ions were determined by potentiometric studies in aqueous solution at 25.0 ± 0.1°C and 0.1M NaNO3 were investigated by potentiometric pH titrations. The overall stability constant, logarithm of β-values, of all species present were evaluated by computer refinement of pH-volume data using the BEST computer program. The concentration distribution diagrams of binary and mixed systems were obtained in terms of percent of cited ions as a pH function. The obtained results was compared to the isolated crystalline in solid state.
