Potentiometric titration systems can use a calcium ISE or a photometric probe for the titration of magnesium. The use of a titration system allows for a precision measurement that can be automated for multiple measurements.
A photometric probe is a sensor that generates a steep millivolt potential when a solution with an indicator changes color after adding a titrant. Magnesium can be determined with EDTA and Eriochrome Black T indicator, which changes from blue to pink when magnesium binds to it. In this complexometric reaction an excess of EDTA is added then a back titration with magnesium chloride is performed with the end point being the excess magnesium reaction with the indicator to change color. The photometric probe has a wavelength of 625 nm in which there is high transmittance when the solution is pink but has a low transmittance when the solution turns blue.
The calcium ISE is a liquid membrane sensor in which an ionophore is embedded in a matrix that reacts with calcium in a lock and key fashion. An EDTA solution is used as a titrant to determine calcium concentration and calcium and magnesium in a sample. The difference between the two represents the magnesium that is present. The calcium ISE is available as either a single half-cell or combination design that is complete with a reference cell.