Definition of Hardness

Soft water is relatively free of calcium and magnesium ions. It produces a rich foamy lather with soap. This is essential for the soap to be an effective emulsifying agent for oils and dirt. With hard water, the soluble sodium salt of soap reacts with the alkaline earth metal ions and precipitates as the useless and undesirable calcium or magnesium soap (Scheme 8.2). The cleaning ability is lost. Hardness is defined as the presence of soluble calcium and magnesium salts in the water. If these are present in the form of bicarbonates, the hardness is temporary. Heating hard water containing bicarbonates eliminates dissolved carbon dioxide and the reverse of Scheme 8.1 causes precipitation of calcium carbonate. Magnesium carbonate is slightly soluble in water but heating will cause its hydrolysis into the much less soluble magnesium hydroxide (Scheme 8.3). Simply boiling and filtering the water therefore eliminates temporary hardness. In regions where water has high temporary hardness, and is used directly without treatment, it is not uncommon to see hot water rinsing and washing baths with a generous crust of chalk (CaCO3) on the inner surfaces. This type of precipitation inside a boiler is also undesirable because the scale reduces the efficiency of heat transfer. Mg(HCO3)2(aq)= MgCO3(s) + CO2(g) + H2O MgCO3(aq) + H2O =Mg(OH)2(s) + CO2(g) Permanent hardness arises when water contains soluble salts of calcium and magnesium such as chlorides and sulphates. It is unaffected by boiling the water. The total hardness of water is determined by the amount of dissolved calcium and magnesium, but expressed in the form of CaCO3. This is possible because calcium and magnesium carbonates have identical stoichiometry. Therefore, water that contains 1.0 mM calcium and 0.5 mM magnesium ions, has a total concentration of 1.5 m M . Since the molecular weight of CaCO3 is 100 g mol–1, 1.5 m M corresponds to 150 mg l–1 of CaCO3, or 150 ppm (parts per million), assuming that 1.0 l of the water has a mass of 1.0 kg. The permanent hardness is that remaining after the water has been boiled and filtered to remove the precipitated calcium and magnesium carbonates and magnesium hydroxide. The temporary hardness is the difference between the total and permanent hardness.Occasionally, hardness due to magnesium alone is of interest, again expressed as the equivalent amount of CaCO3. For the water mentioned above, 0.5m M of magnesium ions is equivalent to 50 mg l–1 or 50 ppm CaCO3. Understanding water hardness values is often a tedious exercise in manipulation of units. In the past, the hardness of water was given in units called degrees. The definition of one degree of hardness, however, varied from one country to the next (Table 8.2). It is much simpler to express hardness in mg l–1 or ppm of CaCO3.