The previously proposed mechanism of bicarbonate ions decomposition by the H-mechanism (with the formation of H+ and CO32-) or OH-mechanism (with the formation of OH- and CO2) is confirmed experimentally without and with heating water up to 190oC. The change decomposition mechanism depends on hardness, alkalinity and pH. The H-mechanism is observed at higher pH values (pH decreases) and the OH-mechanism is observed at lower values (pH increases) in particular solutions. The developed technique is based on the measurement of changing the pH (ΔрНt) of the solution (at a fixed temperature of 15 to 25oC) after heating it to a given temperature (ranging from 40 to 190oC) and maintaining this temperature for at least 30 minutes. A decrease in the pH of the water after heating (ΔрНt>0) indicates the formation of carbonate ions and the need for additional water treatment to reduce the hardness, alkalinity or pH. The greater the ΔрНt, the greater the concentration of carbonate ions formed in water. If ΔрНt is zero then this temperature can be used as a water quality index. This temperature is 86.4oC for tap water in Kyiv (pH 7.4). Measurement of ΔрН in water meeting standards for water in German heating supply systems (pH 9.06, Hardness 45 μmol/dm3 - 5 times less than in Ukraine) shows that such water is not scale-safe (ΔрН150=0.15). The proposed technique will be able to replace the Langelier index (up to 90oC) and the carbonate index (up to 190oC) under the condition of systematic processing of experimental data obtained for the water of different compositions on its basis. This method can be used in addition to the expensive experimental method of dynamic tube blocking.