Definition: Hardness is the property of water which prevents the formation of lather or foam and needs large quantities of soap'. It forms scales in not water pipes, heaters, boilers where the temperature of water is increased.
It is caused by 'DIVALENT METALLIC CATIONS' the principal hardness causing cations are calcium and magnesium there are two types of hardness temporary and permanent hardness
TEMPORARY HARDNESS Caused due to presence of carbonates and bicarbonates of calcium and magnesium this can be removed by boiling or by adding lime solution in water. Temporary hardness is also called carbonate hardness.
PERMENET HARDNESS of water is caused due to the presence of sulphates, chlorites and nitrates of calcium and magnesium. They cannot be removed by simple boiling and require special treatment of water softening it is also called as Non-=carbonate hardness.
DETERMINATION OF HARDNESS
Hardness is generally defined as the caco3 equivalent of ca and Mg ions present in water and expressed in mg/llitas caco3
Hardness can be determined by EDTA titrometric method ( EthyleneDiamine Tetra acidic acid), Ferrochrome black - T is used as indicator
Erichrome Black (Blue colour) + water (Ca++,Mg++) = Less stable ions(wine red colour)
EDTA+ Less stable ion = Erichrome black T (Blue colour) + more stable lon
Colour change = Wine red to purple to blue
Water are commonly classified interms of degree of hardness.
Milligram / Litre as CaCo3 Degree of hardness
0 - 75 Soft
75 -150 Moderately soft
150 - 300 Hard
300 and a bove Very hard
However the permissible Units of hardness for potable water ranges between 75 - 115 mg/lit as CaCo3.