Assay of Chlorinated Lime: Detailed Procedure, etc

ByDeepak Rajput

Assay of Chlorinated Lime: Chlorinated lime, chemically known as calcium hypochlorite (Ca(OCl)₂), is a white or grayish granular powder widely used as a disinfectant, bleaching agent, and water sanitizer. It is a major source of available chlorine, the active ingredient responsible for its oxidizing, germicidal, and bleaching properties.

Due to its strong oxidizing nature, chlorinated lime is extensively used in:

  • Water purification to eliminate pathogenic microorganisms
  • Bleaching textiles and paper
  • Pharmaceutical formulations as a disinfectant
  • Public health sanitation, especially in hospitals and swimming pools

Ensuring that chlorinated lime contains the right concentration of available chlorine is critical for its efficacy. This is achieved through a quantitative assay, which determines the amount of active chlorine present.

Definition of Assay of Chlorinated Lime

The assay of chlorinated lime is a quantitative analytical technique used to determine the strength or percentage of available chlorine in a sample of calcium hypochlorite. The assay is commonly performed using iodometric titration, a reliable and precise method that measures the chlorine content by reacting it with potassium iodide and titrating the liberated iodine with sodium thiosulfate.

In simpler terms, the assay measures how much oxidizing chlorine is present, which directly relates to its disinfecting power.

Chemical Principle

The iodometric assay of chlorinated lime relies on two main chemical reactions:

  1. Oxidation of Potassium Iodide by Calcium Hypochlorite

Ca (OCl)2 ​+ 4KI + 2H2​O → Ca (OH)2 ​+ 2KCl + 2I

Here, the calcium hypochlorite oxidizes iodide ions (I⁻) to free iodine (I₂), while being reduced to calcium hydroxide and potassium chloride.

  1. Titration of Liberated Iodine with Sodium Thiosulfate

I2​ + 2Na2​S2​O3 ​→ 2NaI + Na2​S4​O6​

  • The amount of sodium thiosulfate required to reduce iodine is directly proportional to the available chlorine content.
  • Starch solution is used as an indicator near the endpoint, forming a blue-colored complex with iodine, which disappears upon completion of the reaction.

This method is widely preferred because it is accurate, reproducible, and simple, making it suitable for both laboratory and industrial quality control.

Reagents Required

  • Standard Sodium Thiosulfate Solution (0.1 N) – Titrant for iodine
  • Potassium Iodide (KI) 10% solution – Reacts with chlorinated lime to liberate iodine
  • Acetic Acid (10%) – To acidify the reaction medium
  • Starch Indicator (1%) – Detects the endpoint of titration
  • Distilled Water – To dissolve the sample

Apparatus Required

  • 50 mL burette for titration
  • 250 mL conical flasks
  • 25 mL pipettes
  • Analytical balance (sensitivity 0.001 g)
  • Beakers and glass rods
  • Funnel
  • White tile (for better visibility of color changes)

Step-by-Step Procedure

  1. Weigh the Sample: Accurately weigh 1–2 g of chlorinated lime using an analytical balance.
  2. Dissolution: Dissolve the weighed sample in 100 mL of distilled water in a conical flask. Add 10 mL of 10% acetic acid to acidify the solution.
  3. Reaction with Potassium Iodide: Add 10 g of potassium iodide (KI) to the solution. Allow the reaction to proceed for 5–10 minutes in the dark, liberating free iodine.
  4. Titration: Titrate the liberated iodine with 0.1 N sodium thiosulfate solution from the burette. Swirl the flask continuously to ensure thorough mixing.
  5. Endpoint Detection:
  6. Near the endpoint, add a few drops of starch solution.
  7. The solution turns deep blue.
  8. Continue titration dropwise until the blue color disappears, indicating that all iodine has reacted.

Calculation of Available Chlorine

The percentage of available chlorine is calculated using the formula:

Available Chlorine (%) =

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Where:

  • (V) = Volume of sodium thiosulfate used (mL)
  • (N) = Normality of sodium thiosulfate
  • (35.5) = Equivalent weight of chlorine
  • (W) = Weight of chlorinated lime sample (mg)

Example Calculation:

  • Suppose 20 mL of 0.1 N sodium thiosulfate is used to titrate 1.5 g of chlorinated lime:

Available Chlorine (%) =

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Precautions

  • Chlorinated lime is a strong oxidizing agent; avoid contact with skin, eyes, and flammable materials.
  • Use freshly prepared solutions; hypochlorite decomposes over time.
  • Conduct the assay away from sunlight to prevent decomposition of iodine.
  • Add starch indicator only near the endpoint, as prolonged contact can darken the solution.
  • Ensure accurate weighing and titration for reliable results.

Applications Assay of Chlorinated Lime

  1. Pharmaceutical Quality Control: Ensures disinfectant strength and compliance with pharmacopoeial standards.
  2. Water Treatment: Measures the disinfecting power of chlorinated lime used in municipal water systems.
  3. Industrial Use: Determines bleaching strength in textile and paper industries.
  4. Public Health: Ensures safe and effective disinfection in hospitals, swimming pools, and sanitation programs.
  5. Research Applications: Used in chemical studies of oxidizing agents and reaction kinetics.

Advantages of Iodometric Assay

  • High accuracy and reproducibility
  • Suitable for low and high chlorine content samples
  • Cost-effective and relatively simple
  • Provides quantitative results for regulatory compliance
  • Applicable in laboratories and industrial settings

Conclusion

The assay of chlorinated lime is a vital quality control test that ensures the efficacy, safety, and potency of this widely used disinfectant. Using iodometric titration, chemists can accurately determine the available chlorine content, which directly correlates with its oxidizing and germicidal strength. Proper handling, storage, and careful execution of the assay are critical to obtaining precise and reproducible results, ensuring chlorinated lime remains effective for disinfection, water treatment, and industrial applications.

References

  1. British Pharmacopoeia, Calcium Hypochlorite Monograph, 2023.
  2. Indian Pharmacopoeia, Chlorinated Lime Assay, 2022.
  3. Beckett, A.H., Stenlake, J.B. Practical Pharmaceutical Chemistry, 5th Edition, CBS Publishers, 2002.
  4. Vogel, A.I. Vogel’s Textbook of Quantitative Chemical Analysis, 5th Edition, Longman, 1996.
  5. Gawande, S., et al. “Iodometric Determination of Chlorine in Calcium Hypochlorite.” Journal of Analytical Chemistry, 2019; 74(2): 121–129.

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