Accurate determination of the Ca 2+ activity in milk-based systems by Ca-ISE: Effects of ionic composition on the single Ca 2+ activity coefficient and liquid junction potentials.

Calcium ion selective electrode (Ca-ISE) was found to underestimate the actual Ca ²⁺ ion activity in simulated milk ultrafiltrate (SMUF) and milk. It is shown that the ionic compositional difference between conventional calibration solutions and milk type samples had a significant effect on the single Ca ²⁺ activity coefficient, which generates the erroneous estimate of Ca ²⁺ activities in SMUF and milk. This study tests new standards with ionic profiles similar to SMUF, aiming at the reduction of the errors generated by the compositional difference between conventional standards and milk samples. As a result, the new standards showed a significant improvement in the accuracy of Ca ²⁺ activity and Ca ²⁺ activity coefficient over the conventional standards. The systematic error is reduced from 20% to 5% for SMUF and from 44% to 15% for milk. In addition, the new standards generate liquid junction potentials that are practically insignificant.
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