Carbonate hardness

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Carbonate hardness is the measure of Calcium and Magnesium and other hard ions associated with carbonate (CO₃^2-) and bicarbonate (HCO₃^-) ions contained in a solution, usually water. It is usually expressed either as parts per million (ppm or mg/L), or in degrees (KH - from the German "Karbonathärte"). One German degree of carbonate hardness is equivalent to about 17.8575 mg/L. Both measurements (mg/L or KH) are usually expressed "as CaCO₃" – meaning the amount of hardness expressed as if calcium carbonate was the sole source of hardness. Every bicarbonate ion only counts for half as much carbonate hardness as a carbonate ion does. If a solution contained 1 liter of water and 50 mg NaHCO₃ (baking soda), it would have a carbonate hardness of about 18 mg/L as CaCO₃. If you had a liter of water containing 50 mg of Na₂CO₃, it would have a carbonate hardness of about 29 mg/L as CaCO₃.

Carbonate hardness supplements non-carbonate (a.k.a "permanent") hardness where hard ions are associated with anions such as Chloride that do not precipitate out of solution when heated (contributed by Rami E. Cremesti M.Sc., rami@cremesti.com).

Carbonate hardness is removed from water through the process of softening. Softening can be achieved by adding lime in the form of Ca(OH)₂, which reacts first with CO₂ to form calcium carbonate precipitate, reacts next with multi-valent cations to remove carbonate hardness, then reacts with anions to replace the non-carbonate hardness due to multi-valent cations with non-carbonate hardness due to calcium. The process requires recarbonation through the addition of carbon-dioxide to lower the pH which is raised during the inital softening process.