Determination of Chloride ions in a given water sample PDF

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Determination of Chloride ions in a given water sample

Titrimetric method in which silver nitrate is used as the titrant are called ( argentimetric ) method .Argentimetric method are the most important of those based on precipitation processes. One of general methods of argentimetry is the Mohr’s method which is a direct method using potassium chromate as indicator and the formation of a colored precipitate.

Principle:The principle of Mohr’s method depends on the fractional precipitation of silver chloride and silver chromate .since the former is considerably less soluble, silver chromate does not precipitate until the solution is free from chloride ions. Red silver chromate than precipitation and imparts a faint red – orange color to the white precipitate of silver chloride . silver chromate does not precipitate until its solubility product has been exceeded. So the Mohr procedure is used for the determination of chloride or bromide. In the titration of a neutral solution of ,say, chloride ions with silver nitrate solution a small quantity of potassium chromate solution is added as indicator. At the end point the chromate ions combine with silver ions to form the sparingly soluble ,red silver chromate .So as mentioned before we have here a case of fractional precipitation , the two sparingly soluble salts being silver chloride (solubility product = 1.2 x 10-10 ) and silver chromate ( solubility product = 1.7 x 10 -12 ) the silver chloride is the base – soluble salt and furthermore , the initial chloride concentration is high ,hence silver chloride will be precipitated first.

Glassware:    

Burette Pipettes Erlenmeyer flasks Measuring cylinder

Reagents

   

Chloride free distilled water. Standard silver nitrate solution (0.0141N) Potassium chromate indicator. Acid or alkali for adjusting pH.

Equations:The ionic equations covering the titration are :Ag + + Cl Ag + + CrO4-

AgCl (white ppt) Ag2CrO4 (red ppt.)

SOLUTION PREPARATION  Dissolve 1g of K2CrO4 in 20ml distilled water.  5g of solid AgNO3 and dissolve it in 250ml of distilled water in a conical flask.  In case of unavailability of seawater, use 5ml NaCl solution and pour it into the 20ml unknown water sample.

Procedure:Stander solution of silver nitrate must be prepared by titrated it with standard solution of 0,1 N of NaCI. 1. Take 50mL of sample (V) and dilute to 100mL. 2. If the sample is coloured add 3mL of aluminium hydroxide, shake well; allow to settle, filter, wash and collect filtrate. 3. Sample is brought to pH 7-8 by adding acid or alkali as required. 4. Add 1mL of indicator (Potassium chromate). 5. Titrate the solution against standard silver nitrate solution until a reddish brown precipitate is obtained. Note down the volume (V1). 6. Repeat the procedure for blank and note down the volume (V2).

Note:- avoid the oxidation of Ag+ to oxide (black).

OBSERVATIONS AND CALCULATIONS: Titration with the Blank solution Sr. No.

Volume of distilled water in ml

Volume of AgNO3 solution in ml

1 2

25 25

0.1 0.2

3

25

0.2

Concordant Volume of AgNO3 solution, V1 = 0.17 ml Titration with the Sample Water: Sr. No. Volume distilled water in ml 1 2 3

25 25 25

of Burette Readings in ml Initial

Final

Volume of AgNO3 solution in ml

0 7.5 14.8

7.5 14.8 22.4

7.5 7.3 7.6

Concordant Volume of AgNO3 solution, V2 = 7.46 ml

CALCULATIONS Volume of titrant used (VA) = V2-V1 = 7.46-0.17 ml = 7.26 ml Volume of given water sample (VW) = 25 ml Normality of standard silver nitrate solution, NSN = 0.02N Volume of silver nitrate solution, VSN = 7.26 ml Volume of collected water sample, Vw = 25 ml Normality of collected water sample, Nw =? Now, Nw can be calculated from the following normality formula

Nw×Vw=NSN×VSN Nw = NSN × VSN / Vw =

7.26 × 0.02 25

= 0.005808

Amount of Chloride ions = Nw × equivalent weight of chlorine =N w×35.45g/lit = N w × 35.45 × 1000 mg/lit or ppm = 0.005808× 35.45 × 1000 mg/lit or ppm

RESULT: The amount of chloride ion in the given water sample is 206.184 ppm.

Precautions:  Lab coats, safety glasses and enclosed footwear must be worn at all times in the laboratory.  Silver nitrate solution causes staining of skin and fabric (chemical burns).  Any spills should be rinsed with water immediately.  Concentrated nitric acid is very corrosive, take great care using the 6 molL-1 solution....