The amount of calcium in milk and milk beverages is generally determined using atomic absorption spectroscopy (AA) or inductively coupled plasma atomic emission spectroscopy (ICP). Ion chromatography (IC) using acidic elute can also measure total calcium amount regardless of the form in which the calcium exists. Alternatively, the calcium ion selective electrode method measures the amount of free calcium ions. In addition, chelate titration allows measurement of free calcium ions and part of the calcium bound to other substances.
However, by ionizing protein-bound calcium using an acidizing pretreatment,, the calcium ion-selective electrode method (LAQUAtwin Ca2+) can be used to measure the total amount of calcium.
The following gives an explanation of this based on empirical data—measured by a variety of methods—on the calcium content of milk and milk beverages.For each type of sample, the following table shows the labeled value, together with the values measured by various methods. The diagram below shows the correlation between LAQUAtwin Ca2+ data and that from other methods for a pre-treated sample. Without the need for a large measurement system, LAQUAtwin Ca2+ produced values of total calcium amount that are approximately equal to those obtained by other methods.
Pretreatment of the lactic drink sample consisted of adjustment to a pH of around 2, followed by readjustment to a pH of 4 prior to measurement being carried out.
As a supporting electrolyte, add 0.0375 g of potassium chloride to 5 mL of the supernatant liquid from the pre-treated sample solution, and dissolve by stirring.
Measure the sample with a LAQUAtwin Ca2+ that has been calibrated against reference solution, and use the dilution ratio to calculate the result.
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