Electrochemical Measurements on Milk with Cation and Anion Sensitive Membrane Electrodes

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Abstract
Total cationic activities (TCA) in milk were measured with membranes prepared from the resin Amberlite IR-120-K, selective clay membranes prepared from potassium montmorillonite were used to measure monovalent activities, and total anionic activities were measured with Amberlite IRA-400-C1 membranes. Data are given on reproducibility of selected membranes when used in measuring total cationic potential of a series of milk samples. TCA of milk expressed as an "activity of monovalent equivalents" was calculated to be 0.047. TCA of milk expressed similarly was 0.028, and the monovalent activity of K+ and Na+ ions as measured with selective clay membranes was 0.035. A "generalized activity coefficient" for milk of 0.72 was calculated from potential measurement data on a ternary solution containing KC1, NaCl, and MgCl2. Concentration of ionic Ca in milk was calculated from total cationic and selective potential measurements and is a minimum of 14 mg%. When appropriate corrections are made for Mg content, values of at least 20 mg% were obtained. There were no significant differences in either total cationic or anionic potentials for heated, as compared to raw, skim milk. Results indicate that either changes are small or ionic equilibrium of milk compensates for some insolubilization of calcium phosphate. Membrane potential measurements were made on raw skim milk after the addition of salts common to milk. Chloride salts of K, Na, and Ca were added in levels ranging from 0.02 to 0.20 eq/l. There was no apparent chelation or chemical reaction of these salts with milk components. The only type of cation-protein binding would be that due to formation of a double electrical layer with a resultant drop in activity.