Determination of Residual Chlorine
1. OBJECTIVE AND IMPORTANCE OF EXPERIMENT
Chlorine residuals determination is used to control chlorination of domestic and industrial
wastewaters.
Active chlorine (free and combined) should be determined at each stage in the treatment
process of drinking water and in the water mains in order to guarantee bacteriologically
impeccable water.
Chlorine determination is important to avoid bad odour and change in the taste of water.
It is determined in the swimming pools to avoid ill effects due to excess chlorination.
Determination of chlorine residual in water distribution is useful to find the source of
contamination or leakage points, so as to supply wholesome water to the consumer.
Thus, the main purpose for the chlorination of water supplies and polluted waters serves
primarily to destroy or deactivate disease-producing micro-organisms.
Chlorine applied to water in its molecular or hypochlorite form initially undergoes hydrolysis
to form free chlorine consisting of aqueous molecular chlorine, hypochlorous acid, and
hypochlorite ion. The relative proportion of these free chlorine forms is pH and
temperature-dependent. At the pH of most waters, hypochlorous acid and hypochlorite ion
will predominate.
Free chlorine reacts readily with ammonia and certain nitrogenous compounds to form
combined chlorine. With ammonia, chlorine reacts to form the chloramines:
monochloramine, dichloramine, and nitrogen trichloride. The presence and concentrations
of these combined forms depend chiefly on pH, temperature, initial chlorine-to-nitrogen
ratio, absolute chlorine demand, and reaction time. Both free and combined chlorine may be
present simultaneously. Combined chlorine in water supplies may be formed in the
treatment of raw waters containing ammonia or by the addition of ammonia or ammonium
salts. Chlorinated wastewater effluents, as well as certain chlorinated industrial effluents
normally contain only combined chlorine. Historically, the principal analytical problem has
been to distinguish between free and combined forms of chlorine.
Şekil 1 Chlorine breakpoint curve
1.1 Chlorine Chemistry
Chlorine combines with water to form hypochlorous and hydrochloric acids.
Cl2 + H2O
HOCl
HOCl + H+ + Cl- (hydrolysis)
H+ + OCl-
1.2 EXPERIMENTAL PROCEDURE (IODOMETRIC METHOD)
Materials and Equipment











Erlenmeyer Flask
Pipette
Graduated Cylinder
Stirring rod (magnet)
Stirrer
Burette
Volumetric flask
Acetic acid, conc (glacial).
Potassium iodide, KI, crystals.
Standard sodium thiosulfate, 0.1 N (titrant)
Starch indicator solution
Standardize 0.1 N Na2S2O3 by one of the following:
1) lodate method: Dissolve 3.249 g anhydrous potassium biiodate, KH(IO3)2, primary
standard quality, or 3.567 g KIO3 dried at 103 ± 2°C for 1 h, in distilled water and
dilute to 1000 mL to yield a 0.1 N solution. Store in a glass-stoppered bottle.
To 80 mL distilled water, add, with constant stirring, 1 mL conc H2SO4, 10.00 mL 0.1
N KH(IO3)2, and 1 g KI. Titrate immediately with 0.1 N Na2S2O3 titrant until the
yellow color of the liberated iodine almost is discharged. Add 1 mL starch indicator
solution and continue titrating until the blue color disappears.
2) Dichromate method: Dissolve 4.904 g anhydrous potassium dichromate, K2Cr2O7, of
primary standard quality, in distilled water and dilute to 1000 mL to yield a 0.1 N
solution. Store in a glass-stoppered bottle.
Proceed as in the iodate method, with the following exceptions: Substitute 10.00 mL
0.1 N K2Cr2O7 for iodate and let reaction mixture stand 6 min in the dark before
titrating with 0.1 N Na2S2O3 titrant.
Normality Na2S2O3 = 1/ mL Na2S3O3 consumed
Steps of the Experiment




For a chlorine range of 1 to 10 mg/L, use a 500-mL sample; above 10 mg/L, use
proportionately less sample.
Place 5 mL acetic acid, or enough to reduce the pH to between 3.0 and 4.0, in a flask
or white porcelain casserole. Add about 1 g KI estimated on a spatula. Pour sample in
and mix with a stirring rod.
Add 0.025 N or 0.01 N Na2S2O3 from a buret until the yellow color of the liberated
iodine almost is discharged. Add 1 mL starch solution and titrate until blue color is
discharged.
Blank titration: Correct result of sample titration by determining blank contributed by
oxidizing or reducing reagent impurities. The blank also compensates for the
concentration of iodine bound to starch at the end point.
Perform blank titration as in 1) or 2) below, whichever applies.
1) If a blue color develops, titrate with 0.01 N or 0.025 N Na2S2O3 to disappearance of
blue color and record result. B is negative.
2) If no blue color occurs, titrate with 0.0282 N iodine solution until a blue color
appears. Back-titrate with 0.01 N or 0.025 N Na2S2O3 and record the difference. B is
positive.
Before calculating the chlorine concentration, subtract the blank titration of 1) from the
sample titration; or, if necessary, add the net equivalent value of the blank titration of 2).
According to the iodometric method, the reactions:
Cl2 + 2I-
I2 + 2Cl-
I2 + starch
I2 + 2Na2S2O3
I2 + 2S2O3-
blue color
Na2S4O6 + 2NaI or
S4O6-2 + 2I-
Şekil 2 Steps of the Experiment
Calculations
mg Cl as Cl2/L = [(A ± B) x N x 35450] / mL sample
A = mL titration for sample,
B = mL titration for blank (positive or negative), and
N = normality of Na2S2O3.
Resources


http://nitttrc.ac.in/Four%20quadrant/eel/Quadrant%20-%201/exp11_pdf.pdf
Standard Methods for the examination of water and wastewater