Carbonate hardness

Carbonate hardness, or carbonate alkalinity is a measure of the alkalinity of water caused by the presence of carbonate (CO2−
3) and bicarbonate (HCO−
3) 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) CaCO
3, e.g. one degree KH corresponds to the carbonate and bicarbonate ions found in a solution of approximately 17.848 milligrams of calcium carbonate (CaCO
3) per litre of water (17.848 ppm). Both measurements (mg/L or KH) are usually expressed as mg/L CaCO
3 – meaning the concentration of carbonate expressed as if calcium carbonate were the sole source of carbonate ions.

Carbonate and bicarbonate anions contribute to alkalinity due to their basic nature, hence their ability to neutralize acid. Mathematically, the carbonate anion concentration is counted twice due to its ability to neutralize two protons, while bicarbonate is counted once as it can neutralize one proton. Therefore, bicarbonates that are present in the water are converted to an equivalent concentration of carbonates when determining KH. For example:

An aqueous solution containing 120 mg NaHCO₃ (baking soda) per litre of water will contain 1.4285 mmol/L of bicarbonate, since the molar mass of baking soda is 84.007 g/mol. This is equivalent in carbonate hardness to a solution containing 0.71423 mmol/L of (calcium) carbonate, or 71.485 mg/L of calcium carbonate (molar mass 100.09 g/mol). Since one degree KH = 17.848 mg/L CaCO₃, this solution has a KH of 4.0052 degrees.

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