Accurate determination of the Ca2+ activity in milk-based systems by Ca-ISE: Effects of ionic composition on the single Ca2+ activity coefficient and liquid junction potentials

► The compositional difference between calibration and sample solutions has large influence on estimation of Ca2+ activity. ► New standards, resembling milk-like systems, remarkably improve the accuracy of Ca2+ activity estimation by Ca-selective electrode. ► Ionic composition has no significant effect on the liquid junction potential of a Ca-selective electrode. ► The dynamic ion speciation (DIS) model helps in developing new calibration solutions for the Ca-selective electrode. Calcium ion selective electrode (Ca-ISE) was found to underestimate the actual Ca2+ 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 Ca2+ activity coefficient, which generates the erroneous estimate of Ca2+ 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 Ca2+ activity and Ca2+ 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.