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 The alkaline earths calcium, magnesium, strontium and barium dissolved in water are designated as water hardness. Strontium and barium play a subordinate and negligible role. The earth alkalis are all grouped together as hardeners.
With ion-exchange softening, the calcium and magnesium ions are exchanged for sodium ions. For this purpose, the water is passed over a strongly acidic cation exchanger loaded with sodium ions.
Regeneration occurs with a common salt solution of an elevated concentration. The total salt content of water remains unchanged with this process.
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According to the international system of units, the earth alkali ion content must be given in mol/l or in mmol/l.
In Germany, the water hardness is still often given in °dH (degrees German hardness).
1 °dH is defined as 1mg CaO/100ml of water and corresponds to an earth alkali ion content of approximately 0.18 mmol/l.
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