This study focused on determination of free Ca2þ, Mg2þ, Naþ and Kþ concentrations in a series of CaCl2
solutions, simulated milk ultrafiltrate and reconstituted skim milk using a recently developed Donnan
Membrane Technique (DMT). A calcium ion selective electrode was used to compare the DMT results. The
study showed that the free Ca2þ concentrations measured by the DMT agreed well with calcium electrode
data. Concentrations of free Ca2þ, Mg2þ, Naþ and Kþ measured by the DMT agreed with concentrations
predicted by existing ion speciation models. It is concluded that the DMT is suitable to measure
various free metal ion concentrations simultaneously in complex milk-type systems.