Gel-phase absorptiometry of phosphate with molybdate and Malachite Green and its application to flow analysis

The ion associate formed by the reaction of molybdophosphate with Malachite Green was adsorbed on Sephadex LH-20, a hydroxypropyl derivative of a Sephadex dextran gel. Direct absorptiometric measurements of the ion associate in the gel phase were carried out after either batch or on-line concentration of the species. For the batch method, the coloured gel beads, on which the ion associate was adsorbed, were packed into a specially designed 5-mm cell; the attenuance at 627 and 750 nm was measured and the difference was used for the determination of trace amounts of phosphate. For the flow method, after mixing the sample with the molybdate-Malachite Green solution, the coloured species was concentrated on-line on to the gel packed in a flow-through cell; the increase in the attenuance at 627 nm was used for the determination of phosphate. As both methods are highly sensitive, phosphate can easily be determined in water samples at µg dm–3 levels or lower.