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Calcein as an Indicator for the Determination of Total Calcium and Magnesium and Calcium Alone in the Same Aliquot of Milk

Department of Food Science, University of Illinois, Urbana

ABSTRACT

A method has been developed for calcium and magnesium determination in milk on the basis of the different behavior of these two cations towards the indicator calcein. The determination of both cations can be performed by back titration with calcium chloride solution in the same aliquot of a diluted milk containing an excess of Ethylenediamine tetraacetate (EDTA). Total calcium and magnesium can be obtained in milk at pH range 12.0 to 12.5, whereas calcium alone can be obtained from pH 13.0 to 13.5. Comparison of the values obtained for total calcium and magnesium by the calcein method with those obtained by the Eriochromeblaek T method indicates no significant difference. The recoveries of calcium added to milk are complete. Concentrations of phosphates and proteins four times those present in the average milk and of heavy metals as present in average milk do not influence the results. Variations in the added amount of indicator between 0.05 and 0.15 ml and in the dilution of the sample with water in the ratio from 1:10 to 1:30 do not influence significantly the results. The method is rapid and reproducible, with a standard deviation of ±0.14 meq/liter of milk for total calcium and magnesium, or ±0.19% of the measured value, and ±0.12 meq/liter of milk for calcium alone, or ±0.18% of the calcium content.