Calcium Ion Concentration in Milk

Abstract

Summary The murexide method of Smeets (15) , modified to give increased sensitivity and coupled with a pressure ultrafiltration technique that provides sufficient sample in approximately 1 hr., is a simple, convenient method for the determination of calcium ion in milk on a semiroutine basis. With suitable adjustment of the ionic strength and sodium content of the reference solutions, the method is applicable also to concentrated milk products. Skimmilk was found to contain from 2.5 to 3.4m M /liter Ca ++ . Addition of calcium to milk precipitated both phosphate and citrate but increased (Ca ++ ); addition of phosphate precipitated calcium and decreased (Ca ++ ), and addition of citrate dissolved colloidal phosphate but decreased (Ca ++ ). The apparent solubility products of di- and tricalcium phosphate in milk appear to be approximately 1.5×10 −5 and 1×10 −24 , respectively. Heating skimmilk caused precipitation of calcium phosphate and lowered (Ca ++ ). Concentration of milk also caused precipitation of calcium phosphate, but (Ca ++ ) increased.