A POTENTIOMETRIC AND SPECTROPHOTOMETRIC STUDY OF THE SYSTEM H+-AL3+-CHROME AZUROL-S

Equilibria in the Al3+-H+-chrome azurol S (GAS) system (pAl(3+)+qH(+)+rL(3-)reversible arrow Al(p)H(q)L(r)) were studied in 0.10 M (and 0.60 M) KCl at 25 degrees C. Proton-GAS equilibria were studied spectrophotometrically [log beta(1,-1,1)=-11.64 (-11.24); log beta(0,2,1)=6.93 (6.56)] and potentiometrically [log beta(0,1,1)=4.64 (4.43)]. Potentiometric titrations (p[H+]=2.5-6.0) and fixed pH spectrophotometric titrations (p[H+] 3.4 and 5.0) were performed under excess ligand and excess metal conditions. The ligand displayed both 'salicylate' and 'p-quinomethide-2-carboxylate' bonding; Results were interpreted in terms of a model containing five (six) species. These were a monomeric 1:1 complex AlH(-1)L [log beta(1,-1,1)=2.01 (1.80)] and, in excess ligand solutions, polymers with stoichiometries Al(x)H(-x)L(1+x) and Al(x)H(1-x)L(1+x), where x=4 or 5 [for example, with x=4, log beta(4,-4,5)=26.57 (24.72) and log beta(4,-3,5)=31.35 (28.56)], and in 0.6 M KCl Al(x)H(-1-x)L(1-x) (log beta(4,-4,5)=19.53). For solutions with excess metal additional species in the equilibrium model were Al(3)H(-2)L(2) (both ionic strengths), Al(2)H(-1)L(2) (0.1 M) and Al(4)H(-4)L(2) (0.6 M). Spectrophotometric titrations using malonate or oxalate as competitive ligands were performed to determine the polymer number x. Possible structures for the complexes are presented.