Collect. Czech. Chem. Commun. 1982, 47, 2110-2120

Complex-formation reactions of 7-(o-carboxyphenylazo)-8-hydroxyquinoline-5-sulphonic acid with copper(II) ions: An application in EDTA photometric microtitrations

Jaromíra Chýlková, Václav Říha and Stanislav Kotrlý

Department of Analytical Chemistry, Institute of Chemical Technology, 532 10 Pardubice


Colour reactions of 7-(o-carboxyphenylazo)-8-hydroxyquinoline-5-sulphonic acid (OKFAZOX) with various metal ions are described. The formation of copper(II) complexes has been studied spectrophotometrically within a wide pH range by molar ratio method using computer-assisted regression matrix analysis of the absorbance sets. The following complexes have been identified, in agreement with the results of non-linear regression of continuous variation plots, as predominating species: CuLH2 within pH 1.3-2.9; CuLH within pH 3-7. Then, between pH 7 and 10, the complexes CuL and CuL2 take part in a stepwise equilibrium. Relevant conditional stability constants have been determined. A new photometric EDTA microtitration of copper(II) using OKFAZOX as indicator is proposed with the following optimum arrangement: pH about 5 (acetate buffer) and ionic strength 0.01; indicator concentration approx. 5 μmol l-1 or slightly higher; absorbance followed at 510 nm. End-point extrapolation gives precise results even within micromolar range:e.g. 2 μmol l-1 Cu2+ can be determined with a relative error less than 2%.