Ion-selective determination of total metal ion concentration in an excess of complexing ligand using the standard additions method

Abstract

An ion-selective potentiometric determination of total calcium and silver ion in the presence of an excess of complex-forming ligands, nitrilotriacetic acid and sodium thiosulphate, respectively, is described. A standard additions method with a Philips plastic calcium and solid-state silver ion-selective electrodes was used. This method has been applied to 1 mg l^–1 of calcium and 10^–3–10^–5 mol l^–1 silver solutions containing an ionic strength adjustor and an excess of complex-forming ligand. Calcium measurements in solutions containing an excess of ligand and other complexing ions are reported, e.g., HPO₄^2– and NO₃^–. This method has also been applied to the determination of total silver in black and white photographic paper containing 0.8 g m^–2 of silver. Dissolution of the paper's silver in an excess of thiosulphate is followed by a standard additions procedure. Suitable conditions for calcium and silver determinations are reported and advantages gained in precision of measurement, electrode sensitivity and speed of analysis are discussed.