Abstract: The stability of compfex anion Fe(CTS)^--composed of cation Fe³⁺and anion (CTS)^4-in mixed solvents of ethanol-water with weight percentage of ethanol 0, 10, 20,30,40 and 50 was measured spectrophotometrically at 293 ± 1 Kby using NaClO₄ as the supporting electrolyte with ionic strength ranging from 0.1～3. 0 mol/L. The thermodynamic stability constants of Fe(CTS)^-in various mixed solvents have been obtained by a polynomial approximation equation based on Pitzer's theory. As a comparison, the traditional De bye-Hckel equation was used to treat the same data. The results show that this polynomial approximate method is obviously superior to the Debye-Hckel equation. It was discovered that there is a linear relation between the thermodynamic stability constants of Fe(CTS)^-and the mole fraction of organic component in mixed solvents or the reciprocal of dielectric constants of the solvents. The medium effect and the standard free energy of transfer for the coordination reaction:Fe³⁺+(CTS)^4-=Fe(CTS)^-is also discussed.

Key words: Stability of complexes, Pitzer's equation, Medium effect