Temporary vs Permanent Water Hardness
Temporary hardness (calcium and magnesium bicarbonates) precipitates on heating; permanent hardness (their sulphates and chlorides) is not removed by boiling.
The total hardness of water is the sum of the dissolved calcium (Ca²⁺) and magnesium (Mg²⁺) ions, and is expressed as mg/l of CaCO₃ or in degrees (1 ºHF = 10 mg/l of CaCO₃). Part of it is associated with bicarbonates: this is temporary or carbonate hardness. When the water is heated, the bicarbonates break down, release CO₂ and precipitate as calcium carbonate. That is why limescale appears mainly in water heaters, heating elements, dishwashers and hot water circuits. The other part, linked to sulphates, chlorides or nitrates, is permanent or non-carbonate hardness: it does not precipitate with heat and remains present in hot water.
The distinction helps in choosing the treatment. Heating only reduces temporary hardness, and at the cost of leaving scale in the equipment. To reduce both, ion-exchange water softeners, which replace calcium and magnesium with sodium, or reverse osmosis are used. In washing processes, the hardness that has not been removed is controlled with sequestrants and chelating agents included in the detergent, which keep calcium and magnesium in solution so that they do not react with the surfactants or form deposits. Scale that has already formed is removed with acid products.
Water analyses usually give total hardness (for example, by EDTA titration according to ISO 6059) and alkalinity. When the alkalinity (as CaCO₃) is lower than the total hardness, it is roughly equivalent to the temporary hardness and the rest is permanent. Detergent technical data sheets state the maximum working hardness as total hardness, without distinguishing between the two types.
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