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.