SOP: Determination of Residual Chlorine in Water by the Iodometric Method
GIFSQ — Global Institute for Food Safety & Quality
Principle
Chlorine liberates free iodine from potassium iodide solutions at pH 8 or less. The liberated iodine is titrated with a standard solution of sodium thiosulphate, using starch as the indicator. The reaction is preferably carried out at pH 3 to 4. The iodometric method is generally used in preparing temporary standards of chlorine; it is more precise than the orthotolidine method for determining residual chlorine when the concentration is greater than 1 mg/litre. It determines the total residual chlorine only, not the free or the combined residual chlorine.
Reagents
a. Acetic acid (glacial).
b. Potassium iodide crystals.
c. 0.1 N standard sodium thiosulphate solution (stock solution).
d. 0.01 N or 0.025 N sodium thiosulphate (Na₂S₂O₃) solution, working standards: the stability of 0.01 N or 0.025 N Na₂S₂O₃ is improved if it is prepared by diluting an aged 0.1 N solution; dilute with freshly boiled distilled water; add a few ml of chloroform to prolong the storage life.
e. Starch indicator.
Procedure
Measure into a porcelain dish or flask a suitable volume of sample depending on the chlorine concentration, as indicated below:
| Residual chlorine | Volume to be taken |
|---|---|
| 1 mg/litre or less | 1000 ml |
| 1 to 10 mg/litre | 500 ml |
| 10 mg/litre or more | Proportionately less |
The titre value should not be more than 20 ml of 0.01 N thiosulphate.
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Add 5 ml or more of glacial acetic acid to reduce the pH between 3 and 4
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Add about 1 g of potassium iodide, and titrate with 0.01 N or preferably with 0.025 N thiosulphate from a burette until the yellow colour of liberated iodine is almost discharged
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Add 1 ml of starch solution and titrate until the blue colour is discharged; do not carry out the titration in direct sunlight
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Blank titration: carry out a blank to correct for the titre contributed by impurities in the reagents, such as free iodine or iodate in the potassium iodide (which liberates extra iodine), or the traces of reducing agents that might reduce some of the liberated iodine; measure a volume of distilled water corresponding to the sample used for titration, add 5 ml of acetic acid, 1 g of potassium iodide and 1 ml of starch solution. Blank titration A: if a blue colour occurs, titrate with 0.01 or 0.025 N sodium thiosulphate until the blue colour is discharged and note the volume (titration A). Blank titration B: if no blue colour occurs, titrate with 0.025 N iodide solution until a blue colour appears; back-titrate with 0.01 N or 0.025 N sodium thiosulphate until the blue colour is discharged and record the difference (titration B).
Notes: chlorine in aqueous solution is not stable and will rapidly decrease, particularly in weak solutions; chlorine determinations must therefore be made immediately after sampling, avoiding excessive light and agitation. The minimum detectable concentration using a 500-ml sample and 0.01 N sodium thiosulphate is approximately 0.04 mg of chlorine per litre.
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Before calculating the chlorine from the titre value of the sample, either subtract from it blank titration A or add the net equivalent value of blank titration B, as the case may be.
Calculation
mg/litre chlorine = (Titre × Normality of Na₂S₂O₃ × 35.46 × 1000) / (ml of sample taken)
Where,
- Titre = corrected titre value (after the blank correction above)
- Normality of Na₂S₂O₃ = normality of the standard thiosulphate
- ml of sample taken = volume of the water sample taken
If this method is used for preparing a standard chlorine solution for temporary standards, calculate the chlorine content using:
mg/ml chlorine = (Titre × Normality of Na₂S₂O₃ × 35.46) / (ml of sample taken)
References
- Hallinan, F.J. & W.R.A. Thompson, J. Am. Chem. Soc., 61, 265 (1939).
- Standard Methods for the Examination of Water and Wastewater, American Public Health Association, Washington D.C., p. 110 (1971).