General Test Procedure for Estimation of Free Chlorine

PURPOSE

To lay down the procedure for determining the presence of free chlorine in water samples.

Chlorine will liberate free iodine from potassium iodide (KI) solutions at pH 8 or less. The liberated iodine is titrated with standard solutions of sodium thiosulphate (NA2S2O3) with starch as the Indicator. Titrate at pH 3 to 4 because the reaction is not stoichiometric at neutral pH due to partial oxidation of thiosulphate to sulfate)

APPARATUS

Burette

Pipettes

Conical flask and stopper bottles

REAGENTS

Potassium iodide crystals

Standard sodium thiosulphate 0.1 N

Starch indicator solution

Glacial Acetic Acid

PROCEDURE

Preparation of Reagent

Standardization of 0.1N Sodium Thiosulphate

• Dissolve 3.249 gms of anhydrous potassium bio date KH(IO3)2 dried at 103°C for 1 hr.
• Dissolve it in 100 ml and dilute it to 1000 ml to make a final concentration of 0.1000 N solutions.
• Store it in a glass-stoppered bottle.

Standardization

• Take 80 ml of distilled water.
• To it add with constant stirring 1.0 of conc. H2SO4, 10. 0 ml of 0.1 000 N KH (I O3)2 and Add 1.0 gram of KI.
• Titrate immediately with 0.1 N Na2S2O3 until the yellow color of iodine is liberated and iodine is discharged.
• Add 1.0 ml of the freshly prepared starch indicator and continue titrating until the blue color disappears.

Method for Estimation of Free Chlorine

• Take a 500 ml sample of a clean and dried conical flask.
• Add 5.0 ml of glacial acetic acid to acidify the sample it brings the pH 3 to 4, in a flask.
• Add 1.0 gm of KI into the sample and mix with a stirring rod to dissolve.
• Take a clean and dried burette and fill it with 0.1 N Na2S2O3 up to the zero mark.
• No air bubbles should be present in the burette.
• Add 1.0 ml of freshly prepared solution of starch indicator into the sample.
• Titrate till the blue color disappears. Note the reading of the burette.

Blank Titration

• Take 500ml of distilled water in clean dried conical flask.
• Add 5.0 ml of glacial acetic acid to acidify the sample its brings the pH3.0 to 4.0, in a flask.
• Add 1.0 gm of KI into the sample and mix with a stirring rod to dissolve.
• Take a clean and dried burette and fill it with 0.1 N Na2S2O3 up to the zero mark.
• No air bubble should be present in the burette.
• Add 1.0 ml of starch indicator in to the sample.
• Perform the blank titration.
• If blue color develops , titrate with 0.01N Na2S2O3 to disappear of blue color and record the results B is Negative
• If no blue color occurs, titrate with 0.0282N iodine solution until a blue color appears. Back titrate with 0.01N NA2CO3 and record the difference. B is positive

CALCULATION

For determining the total available residual Chlorine in the water sample:

mg Cl as Cl2/L= (A - B) x N x 35.45 / ml sample

Where:

35.45 is the chlorine factor.

A= mL titration for sample

B= mL titration for blank

N= Normality of Na2S2O3

ACCEPTANCE CRITERIA

The sample should not contain Not More Than 5.0 ppm

REFERENCE

American Public Health Association

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