Determination of chloride content of water by Mohr’s method

Introduction


Apart from hardness causing ions, water has a lot of other ions mixed in it, for example, chlorides, sulphates, phosphates, nitrates, flurides, ions,etc., which add to the impurity of water. In order to check the purity of water, a range of experiments are carried our, among which hereby we are measuring the chloride content.


Generally water contains chloride ions (Cl–) in the form of NaCl, KCl, CaCl2 and MgCl2. The concentration of chloride ion in water >250 ppm, is not desirable for drinking purpose. The total chloride ion can be determined by argentometric method (Mohr’s Method). In this method Cl– ion solution is directly titrated against AgNO3 using potassium chromate (K2CrO4) as an indicator. The reaction involved in the titration is given below:

$$\text{AgNO}_3 + \text{NaCl} \rightarrow \text{AgCl} \text{(white ppt)} \downarrow + \text{NaNO}_3 \quad \text{Eqn. (1)}$$
$$2\text{AgNO}_3 + \text{K}_2\text{CrO}_4 \rightarrow \text{Ag}_2\text{CrO}_4 \text{(Reddish brown ppt)} \downarrow + 2\text{KNO}_3 \quad \text{Eqn. (2)}$$


According to the above mentioned reactions, when all the Cl– ions are removed as AgCl, at the same point yellow color of chromate ions changes to reddish brown color, and at the end point no further change is observed in color of solution.

structure
Fig.1 Before Titration

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Fig.2 After Titration-Brick red colour

Applications:

1. Water Quality Monitoring

2. Environmental Monitoring

3. Industrial Processes

4. Desalination Processes

5.Corrosion Control