Determination of Hardness in Water by Ion Exchange
Water which requires an excessive amount of soap to form a lather or forms much incrustation on vessels in which it stands or is heated is commonly called hard water. Soap is precipitated chiefly by the calcium and magnesium ions commonly present in water; but it may also be precipitated by the ions of other polyvalent metals, such as iron, aluminum, manganese, strontium, and zinc. Therefore, the theoretical hardness of a water is defined as the sum of the concentrations of all the metallic cations other than the cations of the alkali metals, expressed as equivalent calcium carbonate concentration. Since calcium and magnesium are usually the only ions present in significant concentrations in natural water, hardness is defined as the total concentration of just the calcium and magnesium ions. However, if the other hardness-producing metallic ions are present in significant amounts, they should be included in the total hardness of the water.
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