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Ion Selective Electrode Application Methods: Determination of Calcium in Milk
Determination of Calcium in Milk using Ion Selective Electrodes
This application note covers the determination of Calcium in milk. Calcium ions are responsible for an important role in the stability of casein in milk. Measurement of Calcium concentration is made by sample addition.
Equipment Required
- lon analyser with a Calcium ISE function, or pH meter with millivolt scale.
- Calcium ion selective electrode
Reagents
- Calcium stock solution 0.1M Ca2+(4000 ppm): Dissolve 11.1 g of CaCl2 into distilled water in a volumetric flask and dilute to 1000 ml.
- Prepare calcium standard solutions of 10 ppm, 100 ppm and 1000 ppm by serial dilution of the stock solution (approx. 100 ml of each).
- lonic strength adjustment buffer (ISAB)4.0M KCI: Dissolve 298.2 g of potassium chloride into distilled water in a volumetric flask and dilute to 1000 ml.
- Outer filling solution 1.0M KNO3: Dissolve 101.1 g of potassium nitrate into distilled water in a volumetric flask and dilute to 1000 ml.
Method
Take 100 ml of standard and immerse the electrodes. Record the electrode potential mV1, and then pipette 10 ml of sample solution into sample, stir thoroughly and allow for stabilisation. Record the electrode potential mV2. The change in electrode potential ΔE is given by mV2-mV1. Use the sample addition formula below to calculate the concentration of unknown.
Sample Addition
In this case a small known volume of the sample is added to a known volume of a more dilute standard solution and the electrode potential difference ΔE measured. The equation in this case is:
This technique finds application when the samples to be measured are of rather high concentration and the standard addition procedure produces very little change in the electrode potential.
Calculation
The slope of the electrode needs to be accurately known. This can be achieved by taking two standards 100 and 1000 ppm Ca2+ and record the electrode potential difference between the two. The theoretical value for Ca2+ISE is +29.2 mV. The result from the sample addition formula is a direct measurement.
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