5. Determination of chloride content of water sample by argentometric method [mohr's method]

5. DETERMINATION OF CHLORIDE CONTENT OF WATER SAMPLE BY ARGENTOMETRIC METHOD

[Mohr's Method]

Expt. No.

Date:

AIM

To determine the amount of chloride present in 100 ml of the given water sample, being supplied with standard solution of sodium chloride of strength ... N and an approximately N/20 solution of silver nitrate.

PRINCIPLE

Generally water contains chloride ions (CI) in the form of NaCl, KCI, CaCl₂ and MgCl₂. The concentration of chloride ions in water, more than 250 ppm, is not desirable for drinking purpose. The total chloride ions can be determined by argentometric method (Mohr's Method).

In this method Cl^- ion solution is directly titrated against AgNO₃ using potassium chromate (K₂C1O₄) as an indicator.

At the end point, when all the ci ions are removed. The yellow colour of chromate changes into reddish brown due to the following reaction.

MATERIALS REQUIRED

1. Std. NaCl solution 2. AgNO₃ solution 3. 2% K₂CrO₄ indicator 4. Burette, pipette, conical flask.

Step II: DETERMINATION OF CHLORIDE IONS

TITRATION - II

Water sample Vs. Std. AgNO₃

Calculation of Normality of the water sample (Chloride ions)

Volume of the water sample V₁ = 20 ml

Strength of the water sample (Chloride ions), N₁ = ….. ?

Volume of silver nitrate, V₂ = ................. ml

Strength of silver nitrate, N₂ = …… N

According to the law of volumetric analysis

PROCEDURE

Step I

TITRATION - I

Standardisation of Silver nitrate

The burette is washed well with distilled water and rinsed with the small amount of AgNO₃ solution. It is then filled with the same solution upto the zero mark without any air bubbles.

The pipette is washed with distilled water and rinsed with the small amount of standard NaCl solution. 20ml of this solution is pipetted out into a clean conical flask. 1ml of 2% K₂CrO₄ indicator solution is added and titrated against AgNO3 solution taken in the burette. The end point is the change of colour from yellow to reddish brown. The titration is repeated for concordant values.

Step II

TITRATION II

Determination of Chloride ions

20 ml of the given water sample is pipetted out into a clean conical flask and 1ml of 2% K₂CrO₄ indicator solution is added. It is then titrated against standardised AgNO₃ solution taken in the burette. The end point is the change of colour from yellow to reddish brown. The titration is repeated for concordent values.

RESULT

(i) Amount of chloride ion present in the whole of the given water sample = …….. gms/mgs/ppm

(ii) Amount of chloride ion present in the 100 ml of the given water sample = .... gms/mgs/ppm

TITRATION I: STANDARDISATION OF SODIUM THIOSULPHATE

Standard potassium dichromate Vs sodium thiosulphate

Volume of standard potassium dichromate(V₁ ) = 20 (ml)

Strength of standard potassium dichromate (N₁) = ...... N

Volume of sodium thiosulphate solution (V₂) = ...... ml

Strength of sodium thiosulphate solution (N₂) = ?

According to the law of volumetric analysis, V₁N₁ = V₂ N₂