Residual chlorine is the amount of chlorine that remains in the water after a certain period or contact time.
The chlorine residual is usually tested in finished water which is ready to be released into the distribution system, although operators must also ensure that there is adequate residual at the extreme ends of the distribution system.
Sampling & Storage
Chlorine is not stable in aqueous solution. Exposure to sunlight or other light or agitation will accelerate the reduction of chlorine. Therefore it is recommended that chlorine determinations be started immediately after sampling and exposure to light and agitation are to be avoided.
Interferences
Oxidized forms of manganese and other oxidizing agents interfere. Reducing agents, such as organic sulphides also interfere. Although the neutral titration minimizes the interfering effect of ferric & nitric ion, the acid titration is preferred because some forms of combined chlorine do not react at pH 7. Use only acetic acid for titration; sulphuric acid will increase interferences; never use hydrochloric acid.
Procedure
Select a sample volume which will require no more than 20 ml of 0.01 N sodium thiosulphate. Thus for residual chlorine concentration of 1 mg/L or less, take 1000 mL of sample; for range of 1 to 10 mg/, a 500 mL of sample and above 10 mg/L proportionately less sample.
Take appropriate amount of sample and add acetic acid to bring down the pH to 3 to 4 in the flask. Add about 1 g of potassium iodide crystals & mix with a glass rod. Add chlorine free distilled water if larger volume is preferred for titration.
Titrate with 0.01 N of sodium thiosulphate until yellow colour of the liberated iodine is almost discharged.
Add 1 ml of starch indicator and titrate until the blue colour is discharged.
In many cases residual chlorine is very low and starch needs to be added before starting up the titration.
Calculation
Environmental Significance
Chlorine residuals determination is used to control chlorination of domestic and industrial wastewaters.
Chlorination of water & wastewater serves primarily to destroy or deactivate disease-producing microorganisms.
Chlorine determination is important to avoid bad odor and change in the taste of water.
It improves the overall water quality.
Conclusion
To fulfill the primary purpose of chlorination and to minimize any adverse effects, it is essential that proper test methods be used with a foreknowledge of limitations of analytical determination.