Hardness in water is that characteristic,
which prevent lathering of soap.
Originally,
It was defined as the soap consuming capacity of water sample. Hard water
comsumes lot of soap because of presence of certain salts of Ca, Mg and
other heavy metal ions like Al3+ , Fe3+ and Mn2+
in it.
A sample of hard water, when treated with soap (sodium or potassium salt of higher
fatty acid like oleic, palmitic or stearic) does not produce lather, but on the
other hand, forms insoluble white scum or precipitate, which do not possess any
detergent action. It is due to the formation of insoluble soaps of calcium and
magnesium. Typical reaction of soap (sodium stearate ) with calcium chloride and
magnesium sulphate are shown below :
2 C17H35COONa
+ CaCl2 à (C17H35COO)2 Ca↓ +2NaCl
2 C17H35COONa
+ MgSO4 à (C17H35COO)2 Mg↓ +Na2SO4
IN fact, any cation which produces insoluble
soap with soap solution will contribute to hardness.
Units for
expressing hardness:
(a)
Parts Per
millions (ppm). It is defined as
the number of parts by weight of calcium carbonate present per millions (106)
parts by weight of water,
i.e., 1 ppm
= 1 part of CaCO3equivalents hardness in (106) parts of
water.
Milligrams
Per litre(mg/L). It is defined
as the number of milligrams of CaCO3 present in one litre of water,
i.e. , 1 mg/L = 1mgof CaCO3
equivalents per litre of water. It can be easily proved that, for water
1mg/L
= 1 ppm
Assuming
unit density (=mass / volume ) for water,
Therefore 1 L =1kg = 106 mg
Therefore 1mg of CaCO3
equivalent hardness per liter of water
=
1mg of CaCO3 equivalent
hardness per 106 mg of water
=1
part of CaCO3 per 106 mg of water = 1ppm
Hence, 1mg/L =1ppm
Degree Clarke(◦Cl). It is defined as the parts of CaCO3
equivalent hardness per 70000 parts of water.
i.e. ◦Cl =1part of CaCO3 per70000
parts of water.
Degree French(◦Fr). It is defined as the parts of CaCO3
equivalent hardness per 105 parts of water.
i.e. ◦Fr =1part of CaCO3 equivalent
hardness per 105 parts of water