Carbonate hardness, or
carbonate alkalinity is a measure of the
alkalinity of water caused by the presence of
carbonate and
bicarbonate anions. Carbonate hardness is usually expressed either as
parts per million (
ppm or
mg/L), or in
degree KH (
dKH) (from the German
"Karbonathärte"). One degree KH is equal to 17.848 mg/l (ppm) , e.g. one degree KH corresponds to the carbonate and bicarbonate ions found in a solution of approximately 17.848 milligrams of
calcium carbonate per litre of water (17.848 ppm). Both measurements (mg/L or KH) are usually expressed as mg/L – meaning the concentration of carbonate expressed as if calcium carbonate were the sole source of carbonate ions.