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 drinpurpose. The total chloride ion can be determined by argentometric method (Mohr’s Method). In this meCl^– ion solution is directly titrated against AgNO3 using potassium chromate (K₂CrO) as an indicator. The reaction involved in the titration is given below:

AgNO₃ + NaCl → AgCl (white ppt) ↓ + NaNO₃     Eqn. (1)

2AgNO₃ + K₂CrO₄ → Ag₂CrO₄ (Reddish brown ppt) ↓ + 2KNO₃    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.

Fig. 1 Before Titration

Fig. 2 After Titration — Brick Red Colour

Applications

1. Water Quality Monitoring
2. Environmental Monitoring
3. Industrial Processes
4. Desalination Processes
5. Corrosion Control