TABLE OF CONTENTS Water chemistry is a very complex and specialized subject. This short section touches on some aspects of water softening and water stabilization, and is no more than a very simplified introduction to water softening. For more detail, reference should be made to an appropriate specialized textbook (for example, Ref. 1). In this section only calcium and magnesium are considered which is reasonable for most waters.
Hard water is water that requires the use of a large amount of soap to produce lather. Soap is normally the sodium salt of a fatty acid. When soap is added to water containing polyvalent metal ions the salt formed by the polyvalent metal ions and the fatty acid precipitates, forming a scum. Only when all the polyvalent metal ions have precipitated will a lather form. Thus hardness is caused by dissolved polyvalent metal ions. In practice it is only bivalent metal ions that occur to a significant extent in normal water, notably calcium and magnesium. Waters that are high in bivalent ions can also cause scaling problems in hot-water systems.
Hardness is defined by the total concentration of bivalent metal ions present but expressed as mg CaCO 3/l. This is the equivalent calcium carbonate concentration that would have the same effect as the bivalent metal ions actually present. Calcium carbonate has an equivalent weight of 50, and thus the equivalent hardness due to a concentration M of a bivalent metal ion is:
Hardness as CaCO 3 = M
