Freezing Technology

Estimation of available chlorine

The available chlorine content of sodium hypochlorite solutions form the market varies with different samples. The method for the estimation of available chlorine in sodium hypochlorite solution is as follows. For strong solution, pipette 10ml into a 250ml graduated flask containing about 100ml of distilled water, keeping the tip of the pipette beneath the surface of the water. Dilute to the mark and mix thoroughly. Use a measured portion for the determination. For weak solutions (below 3.5% available chlorine) use the sample as received. Dissolve 2-3 gms of potassium iodide crystals in 50ml of distilled water in a 250ml Erlenmeyer flask. Introduce the sample under the surface of the solution. (The volume of the sample should be such that it will titrate about 40.0ml of N/10 sodium thiosulphate solution). Acidify slightly with acetic acid.

Titrate with N/10 sodium thiosulphate until the yellow colour of iodine is nearly destroyed. Add 5ml of starch solution and titrate until the blue colour entirely disappears.

$$ml \ N/10 \ sodium \ thiosulphate \ \times \ 0.3546 \% (W/V) = {Available chlorine in sodium hypochlorite solution \over ml of sample}$$