Box 12-3 Water Hardness

Hardness is the total concentration of alkaline earth (Group 2) ions, which are mainly Ca2+ and Mg2+, in water. Hardness is commonly expressed as the equivalent number of milligrams of CaC03 per liter. Thus, if fCa2+J + [Mg2+] = 1 mM, we would say that the hardness is 100 mg CaC03 per liter because 100 mg CaC03 = 1 mmol CaC03. Water whose hardness is less than 60 mg CaCO, per liter is considered to be soft. If the hardness is above 270 mg/L. the water is considered to be hard.  

Hard water reacts with soap to form insoluble curds  

Enough soap to consume Ca2+ and Mg2+ must be used before soap is useful for cleaning. Hard water leaves solid deposits called scale on pipes when it evaporates. It is not believed that hard water is unhealthy. Hardness is beneficial in irrigation water because alkaline earth ions tend to flocculate (cause to aggregate) 

To measure hardness, the sample is treated with ascorbic acid (or hydroxylamine) to reduce Fe3+ to Fe2+ and with cyanide to mask Fe2+, Cu+, and several other minor metal ions. Titration with EDTA at pH 10 in NH3 buffer then gives the total concentrations of Ca2+ and Mg2+. Ca2+ can be determined separately if the titration is carried out at pH 13 without ammonia. At this pH, Mg(OH)2 precipitates and is inaccessible to EDTA. Interference by many metal ions can be reduced by the right choice of indicators.21 

Insoluble carbonates are converted into soluble bicarbonates by excess carbon dioxide  

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