A photometric probe is a sensor that generates a steep millivolt potential when a solution with an indicator changes color after 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 calcium binds with it instead of the indicator. The solution will change from red to blue when there is no free calcium 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.