Complexometric determination of calcium and magnesium in calcite and limestone after selective precipitation of magnesium quinolin-8-olate from a homogeneous solution

Abstract

A simple complexometric method has been developed for the determination of magnesium directly and of calcium by difference at pH ∼ 10, using EDTA in a single aliquot of solution. The method is based on the joint titration of calcium and magnesium with EDTA at pH ∼ 10 and the release of EDTA from the magnesium-EDTA complex by selective precipitation of magnesium quinolin-8-olate from a homogeneous solution with potassium quinolin-8-olate and a known excess of calcium ions at 50–60 °C. The EDTA released is a direct measure of magnesium, and is titrated with calcium solution at 10 °C. The interfering ions, such as zinc, cadmium, manganese and iron are masked by potassium cyanide during the joint titration of calcium and magnesium. Precipitation of the dark coloured iron(III) quinolin-8-olate from the iron(III)-cyanide complex has effectively been prevented by reducing the latter to the iron(II)-cyanide complex with ascorbic acid. Aluminium and titanium, unless present in large amounts (more than 5 mg), have no effect on the method. The method has been successfully applied to the determination of calcium and magnesium in calcite and limestone.