Complex formation of iron (III) with chrome azurol s

The complex formation of iron(IIl) with 3”-sulpho-2”,6”-dichloro-3,3'-dimethyl-4'-hydroxy-fuchson-5,5'-dicarboxylic acid (chrome azurol S) was studied by spectrophotometric, conductometric and potentiometric methods. The pure tetrabasic acid of the ligand was prepared from the impure trisodium salt (commercially availalile), and the dissociation constants of the ligand were redetermined. At 20° ± 1° and in the presence of 0.10 M potassium chloride the dissociation constants were: pk₁ < 0.0, pk₂ = 2.25 ± 0.05, pk₃ = 4.71 ± 0.03 and pk₄ = 11.81 ± 0.03.In the pH range 2–4, four complexes were detected (the absolute stability constants at 20° ± 5° and at an ionic strength of 0.10 M are given in parentheses) : a ring-formed dimer complex Fe(H₂O)₂]₂Ch₂^2- (log k_2,2 = 36.2); a monomer of composition Fe(H₂O)₄]HCh or Fe(H₂O)₄]HCh^- (the absolute stability constant was calculated as log k_1,1 = 15.6 for the latter composition); a complex Fe(H₂O)₄]₂Ch²⁺ (log k_3.1=20.2) and, finally, a complex of composition Fe(H₂O)₂]H_xCh₂^x-5 (the value of x being unknown). In addition, hydroxo complexes of the dimer were formed at higher pH values.