Method for determining the moment of saturation of an ion exchange filter with calcium ions, wherein the capsule is filled with strongly basic anionite AB-17-8, pretreated with a reagent solution, water is passed through it after a cationic filter, and the color change of the AB-17-8 anionite is monitored. Wherein, an ammonium oxalate solution is used as a reagent for the treatment of AB-17-8 anionite.