11.2 Permanent Hardness

1.1 Significance of Chemical Equations

3.1 Mass-Mass Relationship
3.2 Mass-Volume Relationship
3.3 Volume-Volume Relationship

4.1 Titration
4.2 Molarity $(M)$
4.3 Molality $(m)$
4.4 Normality $(N)$
4.5 Equivalent Weight

5.1 Acid-Base Reaction

6.1 Precipitation/Double Decomposition Reactions

7.1 Simple Titration
7.2 Double Titration
7.3 Method
7.4 Titration of the solution containing both $N{a_2}C{O_3}$ and $NaHC{O_3}$
7.5 Titration of the solution containing both $NaOH$ and $N{a_2}C{O_3}$
7.6 Back Titration

8.1 Iodimetric Titrations
8.2 Iodometric Titrations

11.1 Temporary Hardness
11.2 Permanent Hardness

It is due to the presence of dissolved $CaC{l_2},{\text{ }}CaS{O_4},{\text{ }}MgC{l_2}$ and $MgS{O_4}$ in water. A known volume of hard water is taken and an excess of known equivalents of $N{a_2}C{O_3}$ are added in it. $N{a_2}C{O_3}$ reacts with $C{a^{ + + }}$ and $M{g^{ + + }}$ forming precipitates of $CaC{O_3}$ and $MgC{O_3}.$

These precipitates are filtered off. The filtrate is titrated with a strong acid $\left( {HCl{\text{ }}or{\text{ }}{H_2}S{O_4}} \right).$ Knowing the equivalents of $N{a_2}C{O_3}$ added and left unreacted, the equivalents of $N{a_2}C{O_3}$ consumed by hard water is known.

The equivalents of $N{a_2}C{O_3}$ consumed is equal to the total equivalents of $C{a^{ + + }}$ and $M{g^{ + + }}$ ions present in hard water.

Hardness of water is represented in ppm (mg/litre) of $CaC{O_3}$ i.e. milli grams of $CaC{O_3}$ present per litre of hard water. But hard water does not contain $CaC{O_3}$ Hard water contains $CaC{l_2},{\text{ }}MgC{l_2},{\text{ }}Ca{\left( {HC{O_3}} \right)_2}$ etc.

One mole ${\text{ }}CaC{l_2}$ $ \equiv $ one mole $CaC{O_3}$

or

$111g{\text{ }}CaC{l_2}$ $ \equiv $ $100g{\text{ }}CaC{O_3}$

Similarly,

\[120g{\text{ }}MgS{O_4} \equiv 100g{\text{ }}CaC{O_3}\]

Thus mass of $CaC{O_3}$ corresponding to the mass of $CaC{l_2},{\text{ }}MgS{O_4}$ etc., present in hard water is calculated. Milligrams of $CaC{O_3}$ per litre of hard water is called hardness of water in ppm.