Equilibrium modelling of interferences in the visible spectrophotometric determination of aluminium(III): Comparison of the chromophores chrome azurol S, eriochrome cyanine R and pyrocatechol violet, and stability constants for eriochrome cyanine R aluminium complexes

Equilibria in the Al3+-H+-eriochrome cyanine R (ECR) system were studied by potentiometry and spectrophotometry in 0.10 M KCl at 25 degrees C and p[H+] 2.5-6.5. Potentiometric titration data were interpreted in terms of a monomeric 1:1 complex AlH(-1)L (log beta(1,-1,1) = 1.75), an excess metal polymer Al(3)H(-2)L(2) (log beta(3,-2,2) = 13.44), and excess ligand polymers with stoichiometries Al(4)H(-3)L(5) and Al(4)H(-4)L(5) (log beta(4,-3,5) = 29.07; log beta(4,-4,5) = 25.30). For titrations involving more rapid addition of alkali there was evidence for an additional polymeric conjugate base, Al(4)H(-5)L(5) (log beta(4,-5,5) = 20.67). The ECR-aluminium stability constants, along with literature values for those of chrome azurol S (GAS) and pyrocatechol violet (PCV), were used to calculate interference effects in the spectrophotometric determination of Al. Calculations established that interference by fluoride, citrate, oxalate and salicylate in aluminium assays was a minimum for PCV at pH 6.5 and maximum attainable chromophore concentration. CAS and ECR were more subject to interference and had pH values of minimum interference which varied with the interferent according to its acid-base properties. The use of mass action effects to suppress interferences was limited by the comparatively high ligand-only absorbances for CAS and ECR.