HARDNESS OF WATER
1. If the water produces lather with soap, it is soft water.
If the water does not produce lather with soap, it is called as Hard water. The property is known as hardness. But it will produce a scummy white precipitate.The hardness is due to Ca2+ , Mg2+ and SO42- , Cl- , CO32-, HCO3- ions and their salts.
2.Test for hardness:
a) Eriochrome Black – T indicator gives red colour in hard water.
b) With soap, hard water gives a scummy precipitate.
2C17H35COONa + CaCl2 à (C17H35COO)2Ca + 2NaCl
(Sodium stearate) (Scummy precipitate)
3. Types of Hardness:
a) Temporary Carbonate hardness : The hardness due to carbonates and bicarbonates can be removed by simple boiling. So, they are known as temporary or carbonate or alkaline hardness.
Ca(HCO3)2 (On heating) à CaCO3 + H2O + CO2
b) Permanent hardness ( Non-carbonate): The hardness due to chlorides and sulphates cannot be removed by simple boiling. They need special methods like lime soda process and zeolite process. This hardness is called as “Permanent hardness or Non-carbonate or non alkaline hardness.
CaCl2 + Na2CO3 à CaCO3 + 2NaCl ( Lime soda process)
CaSO4 + Na2Ze (Zeolite) à CaZe + Na2SO4 (Zeolite process)
4. Units of hardness: i) ppm ii) mg/L iii) degree Clarkes iv) Degree French
1 ppm – 1 part by weight of CaCO3 equivalent hardness / 106 parts of water
1 mg/L – 1mg of CaCO3 / 1 Lr of water
Clarkes Degree - 0Cl-1 part of CaCO3 equivalent hardness / 70,000 parts of water
1 Degree French – 0F - 1 part of CaCO3 equivalent hardness / 105 parts of water
Therefore , 1ppm = 1mg/L = 0.07 0Cl = 0.10F
Hardness is expressed in CaCO3 equivalence. Because,
i) It is the most insoluble and easily precipitatable salt.
ii) Its molecular weight is 100 and equivalent weight is 50. As these are whole numbers, it is very easy to use them in calculations.