Equilibrium studies on chromium(III) complexes of salicylic acid and salicylic acid derivatives in aqueous solution

The complexes of chromium(III) ion formed by salicylic acid, SA(H2L), and its derivatives (H2L): 5-nitrosalicylic acid (5-NSA), 5-sulphosalicylic acid (5-SSA) were investigated by means of potentiometry and spectroscopy, at 25degreesC and in ionic strength of 0.1 m KNO3 and 0.1 m KCI, respectively. Over the acidic pH range, the coordination of Cr(III) ion to SA and its derivatives in 1:1 mole ratio occurs, CrL+ type complex is formed. In the excess of ligand, the coordination of the second ligand molecule is somewhat hindered; as a result CrL(HL) type complex occurs. Their existences were verified and their formation constants were determined. At near neutral pH, CrL(OH) and CrL(HL)(OH)(-) type hydroxo complexes formed by hydrolytic equilibria and their formation constants were also defined. The stabilities of Cr(III) complexes of SA and its derivatives decrease in the following order: SA>5-SSA>5-NSA. The formation constants of Cr(III) complexes of SA and its derivatives are in comparable ranges with the corresponding complexes of the 2,x-dihydroxybenzoic acid (2,x-DHBA) of Cr(III) ion. The stabilities of SA complexes for V(IV), Cr(III) and Fe(III) ions that have similar ionic radii, increase in the order VO