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