27. Methods to remove permanent hardness
Treatment with washing soda( sodium carbonate): Washing soda reacts with hard water forming insoluble metal carbonate which can be filtered and removed
MCl + Na CO � MCO
2 2 3
� 2NaCl
3
MSO + Na CO � MCO � Na SO
4 4 2 3 3
2
( M = Mg, Ca)
Calgon’ s method: Calcium and magnesium ions are rendered ineffective by addition of sodium hexa metaphosphate which is commercially known as Calgon
�
Na P O � 2Na + + Na P O 2- 6 6 18 4 6 18
( M � Mg, Ca)
M2� � Na P O 2� � [ Na MP O ] 2� � 2Na � 4 6 18 2 6 18
The complex anion keeps the Mg 2 + and Ca 2 + ions in solution
Ion-exchange method: In this method ions responsible for hardness of
water are exchanged by certain less damaging ions present in some chemical compound like sodium aluminum silicate( NaAlSiO4) which can also be written as NaZ( also called as zeolites). When this is added in hard water, exchange reactions take place. Zeolite is said to be exhausted when all the sodium in it is used up. It is regenerated for further use by treating with an aqueous sodium chloride solution
2NaZ( s) + M 2 + 2( s) + 2Na +( aq)( M = Mg, Ca)
MZ2 2( aq)
Synthetic resins method: These are insoluble polymeric solids having giant hydrocarbon network containing reactive acidic or basic groups. They perform function similar to zeolites but they are superior to zeolites because they can remove all types of ion in water.
28. Hydrogen peroxide can act as oxidizing agent as well as reducing agent in both acidic and alkaline medium