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Chapter: Environmental Engineering - Quality of water

Hardness of Water: Definition, Causes, Determination, Types

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.


HARDNESS

 

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.

 

CAUSES

 

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.

 

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