2. Determination of free residual chlorine(FRC) in a given water sample for BSC Environment Science

5 Min Read


Apparatus required:

  1. Conical Flask
  2. Burette
  3. Burette stand
  4. Measuring Pipette
  5. Measuring Cylinder

Chemical Required:

  1. Conc. acetic acid
  2. Potassium iodide crystals
  3. Sodium thiosulphate
  4. Starch indicator
  5. Chlorinated Water Sample


In swimming pools, water treatment facilities, and water waste, chlorine is frequently employed as a disinfectant. It is a crucial component of many laundry bleaches and is also used as a bleaching agent in paper mills and textile industries. Even though not all germs are eliminated by chlorine, the vast majority of them are, making it the most potent available emergency disinfectant. Chlorine destroys microorganisms in water and is consumed by organic matter and ammonia if enough is introduced.

After all potential organisms have been eliminated, some chlorine will still be present in the water. Free chlorine is the remaining chlorine.

Until it itself disappears or is used to eliminate fresh contamination, free chlorine will continue to exist in water. To disinfect water, chlorine must be in contact with it for at least 30 minutes. The optimal time to apply chlorine is after any treatment step and before storage and usage. Even in trace concentrations, free chlorine is hazardous to fish and other aquatic life. The ideal level of chlorine residual in drinking water should be between 0.2 and 0.5 mg/l.


The free residual chlorine can be determined by Jodometric method. In this method chlorine liberates free iodine from potassium iodide solution at pH 8 or less. The liberated iodine is titrated with a standard solution of sodium thiosulphate with starch as the indicator 

Working formula:

Free residual chlorine (mg/l) = ((ml * N) of Na₂S₂O3 x 35.5×1000) / volume of sample taken


ml = Vol of Na₂S₂O3 consumed during titration

N =Normality of No₂S₂03(0.025)


A conical flask was filled with 100 ml of chlorinated water, 5 ml of acetic acid, and 1 gram of potassium iodide crystal. The solution was thoroughly shaken for 5 to 10 minutes, resulting in a light yellow color. A few drops of starch indicator were afterwards added, turning the solution blue. After that, sodium thiosulphate (0.025N) was added in titrations until the solution’s end point, which is indicated by a change in color to colorless, was reached. Up until the contemporaneous reading was achieved, the process was repeated.

S.N.Volume of Sample Taken(ml)Vol. of Na₂S₂O3Concurrent Reading
Initialfinal difference


free residual chlorine (mg/l) = ((ml * N) of Na₂SO3 x 35.5x 1000) / volume of sample taken

= (0·1×0.025 x 35.5 x 1000)/100

= 0.8875 mg/L

free residual chlorine (FRC) = 0.8875mg/l


The Free residual chlorine (FRC) in the given chlorinated water sample was found to be 0.8875 mg/l.

In the above calculation, the free chlorine / free residual chlorine was found to be 0.8875 mg/l. Thus the given chlorinated water sample is not suitable for drinking purpose because the optimum chlorine residual in drinking water must be in the range of 0.2-0.5 mg/L. 100 the calculated data surpassed this range, the water is toxic and have harmful effect on human health.

Similarly, chlorine is strong oxidants and is lethal to most fish at level between 0.2-0.3 mg/l . So the given chlorinated water sample is lethal to fish as it has a value of 0.8875 mg/l. It is best to maintain level of free residual chlorine below 0.001- 0.003 mg/l in aquatic systems. Higher levels of chlorine affect aquatic organisms.


Thus, the free residual Chlorine (FRC) in given water sample was determined by Jodometric method.

Share This Article
Leave a comment

Leave a Reply

Your email address will not be published. Required fields are marked *