Stoichiometry and equilibrium study of ion-ligand conducted in aqueous solution by spectrophotometry. Stability constants of the complexes are determined at 25 °C, ionic strength was adjusted to 0.2 (% w/v) with NaCl. The pH was adjusted to 6. 2-Thiobarbituric acid-ion complexesʼ stability was calculated in aqueous solution using DATAN (Data Analysis program). The results of this method denote formation specie ML2. The log K for ions Cu+2,Co+2, Ni+2 and Zn+2 were 4.89, 4.69, 4.47 and 4.82, respectively.