Calcium Ion Concentration in Milk

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Abstract
The murexide method for measuring calcium ion concentration ([Ca++]), was modified to increase its sensitivity and to permit its use with concentrated milk products. Skimmilk contained from 2.5 to 3.4 m[image]/l Ca++. Addition of Ca to milk precipitated both phosphate and citrate but increased [Ca++]; addition of phosphate precipitated Ca and decreased [Ca++], and addition of citrate dissolved colloidal phosphate but decreased [Ca++]. The apparent solubility products of di- and tri-calcium phosphate in milk appear to be approximately 1.5 X 10-5 and 1 X 10-24, respectively. Heating skimmilk caused precipitation of calcium phosphate and lowered [Ca++]. Concentration of milk also caused precipitation of calcium phosphate, but [Ca++] increased.