Determination of Chlorides in Water
Scope
This method is applicable to determination of chlorides in water and waste water
Principles
In a neutral or slightly alkaline solution, potassium chromate can indicate the end point of the silver nitrate titration of chloride. Silver chloride is precipitated before red silver chromate is formed.
Interference
Bromide. Iodide and cyanide register equivalent chloride concentrations. Sulphite, Thiosulphate and sulphide ions interfere but can be removed by treatment with hydrogen peroxide. Orthophosphates in excess of 25 mg/I interfere. Iron in excess of 10 mg/I interfere. by masking the end point.
METHOD : Argentometric Method
Reagents
Potassium Chromate Indicator Solution
Dissolve 50 g of potassium chromate in a little distilled water. Add silver nitrate solution until a definite red precipitate is formed. Let it stand for 12 h, filter and dilute to 1 litre with distilled water.
Standard silver nitrate titrant (0.0141 N)
0.0141 N. Dissolve 2·395 g of silver nitrate in distilled water and dilute to 1 litre. Standardize against 0·014 1 N sodium chloride solution. 1·00 ml = 500 µg of chloride. Store in a brown bottle
Standard sodium chloride solution (0.0141 N)
Dissolve 824·0 mg of sodium chloride (dried at 140°C) in distilled water and dilute to 1 litre. 1.0 ml =500 µg of chloride.
Special reagent for removal of interference
Aluminium Hydroxide Suspension
Dissolve 1-2 5 g of aluminium potassium sulphate or aluminium ammonium sulphate in 1 Lit DM water. Warm to 60°C and add 55 ml of concentrated ammonium hydroxide slowly with stirring. Let it stand for 1 h, transfer to a large bottle and wash precipitate by successive additions, with thorough mixing and decanting with distilled water, until free from chloride. When freshly prepared, the suspension occupies a volume of about 1 litre.
Phenolphthalein indicator solution
Sodium hydroxide (1N)
Sulphuric acid (1N)
Hydrogen peroxide (30 %)
Procedure
Standardization of silver nitrate(0.0141) against the Sodium Chloride(0.0141) by taking 10ml of sodium chloride in conical flask and add pinch of potassium chromate and titrate with silver nitrate.
Use 100 ml sample or a suitable portion diluted to 100 ml.
IF sample is turbid and coloured, filter the sample with filter paper.
(NOTE: If the sample is highly coloured. Add 3 ml of aluminium hydroxide suspension, mix, let settle and filter. If sulphide, sulphite or Thiosulphate is present, add 1 ml of hydrogen peroxide and stir for 1 minute directly titrate the samples in the pH range 7 to 10. Adjust sample pH to 7-10 with sulphuric acid or sodium hydroxide if it is not in the range. )
Add 1 ·0 ml of potassium chromate indicator solution.
Titrate with standard silver nitrate solution Yellow to a Brownish asend point.
Calculation
Chlorides (mg/l) = (V1-V2) x N x 35.45 x1000/ V3
Where
V1 = volume in ml of silver nitrate used by the sample,
V2 = volume in ml of silver nitrate used in the blank titration,
V3 == volume in ml of sample taken for titration, and
N = normality of silver nitrate solution.
Safety guidelines
Proper safety & health precaution are taken while doing the analysis by using safety goggles, apron, safety shoes & using fume chamber for fume generating chemicals.
Reference
IS: 3025 (P-32) 1988 RA 2019