DEGREE OF HARDNESS:
The degree of hardness of water is conveniently expressed in terms of equivalent amount of CaCO3. Although hardness of water is never present in the form of calcium carbonate because it is insoluble in water and hardness is actually caused by bicarbonates, chlorides and sulphates of calcium and magnesium. Choice of CaCO3 as the standard for reporting hardness of water is due to its molecular weight which is exactly 100 and equivalent weight is 50 (easy for calculations). Moreover it is the most insoluble salt that can be precipitated in water treatment. Thus equivalent of CaCO3 for hardness producing substance can be calculated by formula.
For calculating the multiplication factor of hardness producing substances to convert into equivalent of CaCO3, according to following steps given below:
For example: To find the multiplication factor of Mg (HCO3)2
Step1. Molar mass of Mg (HCO3)2 = 146
Step2. Find molar mass of CaCO3 (100)
Step3. Multiplication factor =2/2*chemical equivalents
Step4. Find molar mass of CaCO3
Multiplication factor = 100/2*73 = 100/146
NOW, we calculate hardness of Mg (HCO3) in terms of CaCO3 equivalents.
Suppose, x quantity of Mg (HCO3)2 = x.100/146 amount of CaCO3.
Thus, the factor 100/146 is multiplication factor for Mg (HCO3)2. Multiplication factors for different compounds are given in table.
Table: Formulae and multiplication factor of various dissolved salt/Ion
Dissolved Molar Chemical Multiplication factor for converting into equivalents of
Salt/ion Mass Equivalent CaCO3
Ca (HCO3)2 162 81 100/162
Mg (HCO3)2 146 73 100/146
CaSO4 136 68 100/136
CaCl2 111 55.5 100/111
MgSO4 120 60 100/120
MgCl2 95 47.5 100/95
CaCO3 100 50 100/100
MgCO3 84 42 100/84
Mg (NO3)2 148 74 100/148
Ca++ 40 20 100/40
Mg++ 24 12 100/24
HCO3- 61 61 100/2*61
CO2-2 60 30 100/2*17
OH- 17 17 100/2*17
H+ 1 1 100/2
CO2 44 22 100/44
HCl 36.5 36.5 100/36.5*2
H2SO4 98 49 100/98
FeSO4.7H2O 278 139 100/278
Al (SO4)3 342 57 100/114
NaAlO2 82 82 100/164
Thus the various types of hardness in a water sample may be calculated as below:
Temporary hardness = [(hardness due to Ca (HCO3)2 + hardness of due to Mg (HCO3)2]
Permanent hardness = [(hardness due to CaCl2 + CaSO4 + MgSO4 + MgCl2 + Mg (NO3)2 + Ca (NO3)2]
Total hardness = [temporary hardness + permanent hardness]