Box 13-2 What Is Hard Water

Hardness refers to the total concentration of alkaline earth ions in water. The concentrations of Ca and Mg are usually much greater than those of other Group 2 ions, so hardness can be equated to [Ca ] + [Mg ]. Hardness is commonly expressed as the equivalent number of milligrams of CaCOs per liter. Thus, if [Ca ] + [Mg ] = 1 mM, we would say that the hardness is 100 mg CaC03 per liter, because 100 mg CaCOa = 1 mmol CaC03. Water whose hardness is less than 60 mg CaC03 per liter is considered to be soft. Hard water reacts with soap to form insoluble curds  

Enough soap to consume the Ca and Mg must be used before the soap is useful for cleaning. Hard water is not thought to be unhealthy. Hardness is beneficial in irrigation because alkaline earth ions tend to flocculate (cause to aggregate) colloidal particles in soil and thereby increase the permeability of the soil to water. Colloids are soluble particles that are 1-500 nm in 

To measure hardness, water is treated with ascorbic acid to reduce Fe to Fe and with cyanide to mask Fe, Cu, and several other minor metal ions. Titration with EDTA at pH 10 in ammonia buffer gives [Ca ] + [Mg ]. [Ca ] 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. 

Insoluble carbonates are converted into soluble bicarbonates by excess carbon dioxide  

(a) A 50.0-mL sample containing Ni was treated with 25.0 mL of 0.050 0 M EDTA to complex all the Ni and leave excess EDTA in solution. How many millimoles of EDTA are contained in 25.0 mL of 0.050 0 M EDTA  

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