Assay of Chlorinated lime or Bleaching Powder.

To determine the amount of “available chlorine” in chlorinated lime (bleaching powder), the Indian Pharmacopoeia (IP) recommends using an iodometric titration method. Chlorinated lime must have at least 30.0% w/w of accessible chlorine, according IP requirements.

Principle

The assay is based on the principle that the “available chlorine” present in chlorinated lime. Chlorinated  lime release slowly when it is added with  water but on trituration and shaking with water it release chlorine easily, So the aqueous suspension of the sample is made  to treat with acetic acid in presence of Potassium iodide and it release chlorine from the sample and displace the equivalent amount of Iodine from the KI. Than the liberated  iodine is titrate against 0.1N Sodium thiosulphate using mucilage starch as indicator for determination of end point.

Chemical reactions

Liberation of Chlorine:

Ca(OCl)Cl+2CH₃​COOH→Ca(CH3​COO)₂​+HOCl+HCl

HOCl+HCl→H₂​O+Cl₂​

Liberation of Iodine: Cl_2​+2KI→2KCl+I_2​

Titration of Iodine: I₂​+2Na_2​S₂​O₃​→2NaI+Na₂​S₄​O_6​

Each ml Of  0.1N sodium Thiosulphate equivalent to 0.003545 gm of available chlorine.

Procedure

1. A specified amount(about 4gm) of the chlorinated lime sample is weighed and triturated with a small amount of water to form a paste, then diluted to a known volume in a volumetric flask.
2. The 100 ml of prepared suspension is transferred to an iodine flask.
3. 3gm of KI added on it and 5ml of acetic acid   is added  than shake for 2-3 minutes .The flask is then stoppered and allowed to stand for 15 minutes.
4. The content liberated iodine is titrated with a standardized 0.1 M sodium thiosulfate solution.
5. A few drops of starch indicator solution are added when the solution turns a pale yellow color (just before the endpoint). This forms a blue-black complex with the remaining iodine.
6. The titration is continued until the blue-black color disappears, leaving a colorless solution. This is the endpoint.

Calculation

The percentage of available chlorine is calculated based on the volume of sodium thiosulfate solution consumed, using the following relationship: Each mL of 0.1 N Na2​S2​O3​ is equivalent to 0.003545 g of available chlorine.

The percentage of available chlorine can be calculated using the formula:

(V×N×0.03545)/W×100​

V = Volume of 0.1 N Na2​S2​O3​ consumed (in mL)

N = Normality of Na2​S2​O3​ solution

W = Weight of the sample (in g)

Dr.Dash Srimanta

Hi…! Currently, I am working as an  Professor at Department of Pharmaceutical Chemistry(H.O.D),The Pharmaceutical College, Barpali, Odisha. I have more than 19 years of teaching & research experience in the field of Chemistry & Pharmaceutical sciences.