Fish Microbiology and Quality Assurance

19. Determination of available chlorine in bleaching powder, sodium hypochlorite and process water

Chlorine is one of the commonly used chemical disinfectants in fish processing industry and maintaining specified chlorine level in process water is necessary to attain disinfectant effect.
When a sample containing chlorine is treated with potassium iodide under acidic condition, iodine is liberated and the amount of iodine liberated is equivalent to the amount of chlorine present in the sample. The liberated iodine is estimated by titrating against standard sodium thiosulphate solution using starch as indicator.

Materials required

1. Standard (0.02N) Potassium dichromate (K₂Cr₂O₇)
Dissolve 0.09806 g of K₂Cr₂O₇ in 100 ml distilled water.
2. 0.02 N Sodium thiosulphate solution: Dissolve 2.5 g sodium thiosulphate in 500 ml distilled water.
3. 20% Potassium iodide solution: Dissolve 20 g KI in 100 ml distilled water and store in amber coloured bottle.
4. 0.5% Starch solution: Dissolve 0.5 g starch in 100 ml distilled water by boiling.
5. 2N H₂SO₄ acid: Dilute 2.85 ml of conc. H₂SO₄ acid to 50 ml with distilled water.
6. 4 N HCl acid: Dilute 35.6 ml of conc. HCl to 100 ml with distilled water.

Standardization of sodium thiosulphate solution
Take 25 ml of 0.02 N K₂Cr₂O₇ solution, 5 to 7 ml of KI solution and 10 to 12 ml of 2 N H₂SO₄ acid in a conical flask and titrate them against 0.02 N sodium thiosulphate solution by adding few drops of starch solution when it becomes faint yellow. Again continue the titration till the solution turns colorless. By noting the titre value correct normality of sodium thiosulphate solution is determined using the following formula: V₁N₁ = V₂N₂

Procedure

Determination of available chlorine in bleaching powder

About 0.5 to 1g of bleaching powder is dissolved in 1 litre distilled water by grinding. To 25 ml of this solution taken in a conical flask 5 to 7 ml of KI solution and 20 ml of N HCl acid is added and titrated then against standard sodium thiosulphate solution using starch indicator. Available chlorine is calculated by using the following formula
Available chlorine (g %) =[(X x N ) /V] X [35.46 /1000] X [1000 /W ] X 100

Where,
X - Titre value
N - Normality of sodium thiosulphate used
V - Volume of sample taken for titration
W - Weight of sample dissolved in 1000 ml distilled water
35.46 - Equivalent weight of chlorine

Determination of available chlorine in Sodium hypochlorite
Dilute 5 ml of sodium hypochlorite stock solution to 100 ml with distilled water. Then take 25 ml of the diluted solution for analysis and carry out the analysis as mentioned for bleaching powder. Calculate the available chlorine using the formula given below:
Available chlorine (ml %)=[(X x N ) /V₂] X [35.46 /1000] X [1000 /V₁ ] X 100

Where,
X - Titre value
N - Normality of sodium thiosulphate used
V1 - Volume of stock solution diluted to 100 ml with distilled water
V2 - Volume of diluted solution taken for titration/ analysis

Determination of available chlorine in process water
Preparation of chlorinated water
Required level (in ppm) of chlorine in process water can be prepared by adding the required the quantity of bleaching powder or sodium hypochlorite to 1000 litres of water by using the following formula:

Q = (100 x X)/Y
Where,
X - Required or desired ppm level in the water
Y - Available chlorine (%) level in the stock (bleaching powder or
sodium hypochlorite)
Q - Quantity (g or ml) of bleaching powder or sodium hypochlorite to be added to 1000 litres of water.