Water quality is now a significant problem. Therefore, it is important to work out a technique for determination of natural waters general hardness. This method aims at obtaining of the more accurate values of hardness and at automatization the of the water quality monitoring process.

We suggest using a digital indicator that is the value of red color channel of photographic image of an analyzed solution for determining of the general water hardness.

At complexon III titrations of the model solutions (n = 5, P = 0.95, pH 9, indicator is eriochrome Т black) with MgSO₄ concentration equals 0.05M and 0.1М, the following results have been obtained: C(Mg²⁺) = (0.050 ± 0.002)М, s_r = 0.025 и C(Mg²⁺) = (0.100 ± 0.003)М, s_r = 0.002, accordingly.

The proposed method let us to determine the cold water-pipe general hardness. At titration of 40 ml solutions of analyte (n = 5, P = 0.95) it gives (6.3 ± 0.1) mmol/l, s_r = 0.015.