Measurement of calcium and magnesium ions

Cation-exchange resins. Using ion-exchange resins, and are 

Interaction with murexide. The murexide method depends on the formation of a complex between Ca and ammonium purpurate (murexide, M)  

The free dye (M) has an absorption maximum at 520 nm while Ca M absorbs maximally at 480 nm. The concentration of Ca can be calculated from a standard curve in which A480 is plotted as a function of [Ca ] or preferably from a standard curve of (A520 — A4.80) function of [Ca ] 

The murexide method measures only Mg, at the concentration in milk, does not affect the indicator appreciably. Calculation of Mg concentration is possible when the total calcium and magnesium (obtained by EDTA titration) is known. This is based on the assumption that the same proportion of each cation is present in the ionic form, which is justifiable since the dissociation constants of their citrate and phosphate salts are virtually identical. 

Ca-ion electrode. Ca activity (rather than concentration) can be determined rapidly and accurately using a Ca ion-specific electrode. Care must be exercised to ensure that the potentiometer is properly standardized using solutions that simulate the composition of milk serum. The Ca activity is lower than the Ca concentration - values of about 2mM have been reported. 

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