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Monday, April 1, 2019


(1)The amount of hardness causing substances (soluble salts of calcium or magnesium) in a certain volume of water measures the extent of hardness or degree of hardness.
(2) Hardness of water is always calculated in terms of calcium carbonate although this is never responsible for causing hardness of water because of its insoluble character.
(3) The reason for choosing CaCO3 as the standard for calculating hardness of water is the ease in calculation as its molecular weight is exactly 100.Thus the amount of various hardness causing substances in terms of CaCO3 can be calculated on the basis of the following relations.
Thus the various types of harnesses in a water sample may be calculated as below.

Temporary hardness =Hardness due to Ca(HCO3)2 + Hardness due to Mg(HCO3)2
Permanent hardness = Hardness due to CaCl2 + due to CaSO4 + due toMgCl2 + due toMgSO4

Degree of hardness is usually expressed as parts per million (ppm) and thus may be defined as the number of parts by weight of CaCO3 (equivalent to calcium and magnesium salts) present in a million (106) parts by weight of water.
From the above definition, we can say that;

ILLUSTRATIVE EXAMPLE (1): Determine the degree of hardness of a sample of water containing 30 ppm of MgSO4.
                      120 ppm = 100 ppm
                      Hence 30 ppm = 25 ppm   Ans

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