Determination of the strength of Sodium Hypochlorite

 

Determination of the strength of sodium hypochlorite

Sodium Hypochlorite: (NaOCl) Eq Wt 35.5
Determination of active chlorine in Commercial Sodium Hypochlorite:
Active Chlorine is not liberated as such from alkaline hypochlorite bleach liquors.

The effective amount of Chlorine liberated by acidification of hypochlorite solution is defined by the equation as below.
NaOCl + 2KI + H2SO4 I2 + NaCl + K2SO4 + H2O
2Na2S2O3 + I2 2NaI + Na2S4O6

Procedure:
5 ml of concentrated hypochlorite is diluted to 100 ml with distilled water.
Pipette 10ml of the above solution in to a 500 ml conical flask containing about 200 ml distilled water and 20ml Potassium Iodide solution ( approximately 166 g/l)
Add 50ml dilute (10%) Sulphuric acid and titrate with 0.1N Sodium Thiosulphate until the solution is pale yellow color.
Add 5ml starch solution (soluble starch 1% w/w) and continue titration until the blue color formed is discharged. Let the Titre value be A ml of 0.1N Sodium Thiosulphate.

Calculation:
1 ml 0.1N Sod. Thiosulphate = 0.00355 gram active chlorine
Gram/liter Active Chlorine = 0.00355 x A x 1000 x 100/(10 x 5)

Standardisation of Sodium Thiosulphate:
Na2S2O3. 5H2O Mol Wt: 248.18 g/mol Eq. Wt: 248.18
Standard Solution : 6.205 gram of Thiosulphate is dissolved in 250 ml of D.D.water (Alkaline proof) 7.pH.
For alkaline proof, we have to add some phenolphthalein indicator and add 0.1N NaOH lye until pink colour appears.

Iodometric conversion table:
The table below provides easy conversion of titres to Sodium Hypochlorite or active chlorine content of bleach liquor when a 10ml sample of the liquor has been titrated with 0.1N Sodium Thiosulphate solution.

Volume of 01N Na2S3O4 Active  Chlorine content of 100% conc solution Active  Chlorine content of 26% conc solution Active Chlorine g/l
1  0.22 0.88 0.35
2 0.45 1.76 0.71
3 0.67 2.64 1.06
4 0.90 3.52 1.42
5 1.13 4.40 1.77
6 1.35 5.28 2.13
7 1.58 6.16 2.48
8 1.80 7.04 2.84
9 2.03 7.92 3.19
10 2.26 8.80 3.55
11 2.48 8.68 3.90
12 2.71 10.56 4.26
13 2.93 11.44 4.61
14 3.16 12.32 4.97
15 3.39 13.20 5.32
16 3.61 14.08 5.68
17 3.84 14.96 6.03
18 4.06 15.84 6.39
19 4.29 16.72 6.74
20 4.52 17.60 7.10