Flow injection spectrophotometric determination of calcium, phosphate and chloride ions in milk

Flow injection procedures for the determination of calcium, phosphate and chloride ions in milk samples are described. The reactions are based on the formation of coloured complexes and their spectrophotometric monitoring. A sample pre-treatment with acetate buffer was carried out owing to the complexity of the sample matrix. For chloride, a rapid and reliable automated procedure for direct measurement of its content in milk (using a dialyser to eliminate interferences) is also described. After optimizing the sample pre-treatment and flow injection variables, the procedures were applied to commercial milk; the results obtained agreed satisfactorily with those of the reference methods. With 50 mm3 samples, a working range of 0-15 ppm for calcium, 50-150 ppm for phosphate and 5-100 ppm for chloride is covered with a precision of better than 1.1%. The sample throughput was higher than 50 samples h-1. These preliminary experiments are the basis for the automation of the determination of calcium, phosphate and chloride ions using a computer-controlled, self-designed and laboratory-built autoanalyser.