SOP for Determination of Free residual Chlorine

Scope

This SOP is applicable for the determination of free residual Chlorine.

Table of Contents

Iodometric Method

Interference

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 and nitric ions, 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.

Reagents

Glacial Acetic Acid
Potassium Iodide Crystals
Standard Sodium Thiosulphate-0.01N
Standard Potassium Dichromate – 0.1 N
Starch Indicator Solution 5 g/l

Triturate 0.5 gm of Soluble starch with 5 ml of water and add this with const. stirring to sufficient boiling water to produce about 100 ml. Boil for few minute and cool.

Procedure

Select the sample volume which will require no more than 20 ml of 0.01 N Sodium thiosulphate.

Sample Volume Condition:

If FRC is 1 mg/l or less, sample should be taken 1000 ml.
If FRC is 1 to 10 mg/l, sample should be taken 500 ml.
If FRC is above 10 mg/l Sample should be taken proportionally less.

Take appropriate volume of the sample and add acetic acid to bring down the pH to 3 to 4 in the flask.
Add about 1.0 g of potassium iodide crystals and mix with a glass rod. Add chlorine-free distilled water if larger volume is preferred for titration.
Titrate with 0·01 N sodium Thiosulphate until yellow colour of the liberated iodine is almost discharged.
Add 1·0 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

FRC mg/l = V1 x N x 35450/ Volume of sample

V1 = Volume of Standard Sodium Thiosulphate

N = Normality of sodium Thiosulphate

Safety Guidelines

Proper safety & health precaution are taken while doing the analysis by using safety goggles, apron, safety shoes & using fume chamber for fume generating chemicals

Reference

IS 3025 (part-26): 2021