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, temporary hardness, 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, which has its transition point at a pH value of 4.3.