In terms of water hardness, the concentration of the anion hydrogen carbonate (HCO3-) is of particular importance. This concentration is referred to as carbonate hardness, transient hardness, residual alkalinity, m-value or acid capacity up to a pH value of 4.3 (KS 4.3).

The term m-value is based on the use of the indicator methyl orange. The m-value thus corresponds to the turnover point of methyl orange (pH 4.3). The use of methyl orange enables a highly accurate determination.