Potentiometric titration systems can use a calcium ISE or a photometric probe for the titration of hardness. The use of a titration system allows for a precision measurement that can be automated for multiple measurements.
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. The calcium ISE is available as either a single half-cell or combination design complete with a reference cell.
A photometric probe is a sensor that generates a steep millivolt potential when a solution with an indicator changes color when adding a titrant. Calcium hardness is determined with EDTA as a titrant and calconcarboxylic acid (Patton-Reeder) indicator. In this complexometric reaction calcium binds to the indicator to change the color of the solution from blue to red. As EDTA is added to the solution the hardness binds with it instead of the indicator. The solution will change from red to blue when there is no free hardness available to react with the indicator. The photometric probe has a wavelength of 625 nm in which there is high transmittance when the solution is red but has a low transmittance when the solution turns blue.