Chlorides estimation by Mohrs Method

Two methods are prescribed for the determination of chloride. One of them is Mohr’s method,  it is suitable for use when 0.15 to 10mg of chlorine (Cl) is present in the portion of sample titrated.

Chloride is determined in neutral or slightly alkaline solution by titration with STD silver nitrate solution in the presence of potassium chromate indicator. Silver chloride is precipitated and at the end point, red silver chromate is formed.

The sample, after neutralization, is titrated against STD silver nitrate solution using potassium chromate indicator end colour is from yellow to bricks red.

1. Aluminium hydroxide suspension:
Dissolve 125g of potassium or ammonium alum in 1lt of distilled water. Precipitate the aluminium by adding ammonium hydroxide slowly along with stirring. Wash the precipitate by successive decantation with several portions of distilled water until free from Sulphates.
2. Hydrogen peroxide 30%
3. Calcium carbonate.
4. Dilute nitric acid 0.1N
5. Potassium dichromate:
Dissolve 5g of potassium chromate in distilled water and makeup to 100ml. Add silver nitrate solution to produce a slight red precipitate and filter.
6. Std silver nitrate solution:
Dissolve 4.791g of silver nitrate, dried at 105oC in distilled water and makeup to 1 lit. The solution to stored at the dark place.

100ml of the sample is pretreated with decolourization, adjusting pH, filtrations for removal of suspension. Decolourize by adding 3ml of aluminium hydroxide suspension. Stir thoroughly and after a few minutes filter and wash with 10 to 15ml of distilled water. If Sulphites are present, add 1ml of hydrogen peroxide with stirring.

Place the treated sample in a porcelain basin. If the pH of the sample is less than 6.8, add a small amount of calcium carbonate to the sample in the basin so as to neutralize the acidity. If the pH is above 10, determine the amount of dilute nitric acid required to neutralized 100ml of the sample, and this amount of the acid to the portion used for the chloride determination, and then add a trace of calcium carbonate. Add 1ml of Potassium dichromate and titrated with std silver nitrate solution with constant stirring until there is perceptible reddish colouration. Substrate 0.2 ml from the titration figures to allow for the excess of reagents required to from silver chromate.

Chloride (as Cl), mg/l = 10 V
Where V= volume in ml of std silver nitrate sol consumed in terms of CaCO3
Cl- (mg/l) = (S-B) ×N×Eq Wt of Cl-×1000 / vol of sample
S = sample value
B = blank sample value