Determining the chlorine content in bleaching powder, specifically the available chlorine, is typically achieved through a method involving chemical titration.
Bleaching powder's effectiveness comes from its "available chlorine," which represents its oxidizing power. This isn't the total chlorine content but the amount that is active and can be used for bleaching or disinfection. The standard method for determining available chlorine is an iodometric titration.
The Iodometric Titration Method
This method relies on a series of chemical reactions where the available chlorine oxidizes iodide ions (I⁻) to iodine (I₂), and then the liberated iodine is quantified using a standard solution of sodium thiosulfate (Na₂S₂O₃).
Here's a breakdown of the process:
- Sample Preparation: A known amount of bleaching powder is dissolved or suspended in water.
- Reaction with Potassium Iodide (KI): A measured amount of sample solution is added to an acidified solution of potassium iodide, liberating an equivalent amount of iodine. The acidity is crucial to facilitate the reaction. The available chlorine (often represented as hypochlorite ions, OCl⁻, in solution) reacts with iodide ions in the acidic medium:
OCl⁻ + 2I⁻ + 2H⁺ → Cl⁻ + I₂ + H₂O
This step quantitatively converts the available chlorine into elemental iodine. - Titration with Sodium Thiosulfate: This free iodine is determined by titration with standardized sodium thiosulfate solution. Sodium thiosulfate reacts with iodine, reducing it back to iodide ions:
I₂ + 2S₂O₃²⁻ → 2I⁻ + S₄O₆²⁻ - Indicator: A starch solution is used as an indicator. Starch forms a deep blue complex with iodine.
- Endpoint Detection: Starch indicator is added when most of the iodine has reacted (the solution turns pale yellow). Titration with sodium thiosulfate continues until the blue color disappears completely, indicating that all the free iodine has been reduced. This point is the starch indicator endpoint.
By knowing the volume and concentration of the standardized sodium thiosulfate solution used, the amount of iodine liberated can be calculated, and subsequently, the amount of available chlorine in the original bleaching powder sample can be determined.
Key Reagents Used
Understanding the roles of the chemicals involved is important:
- Bleaching Powder Sample: Contains the available chlorine to be measured.
- Potassium Iodide (KI): Provides iodide ions that are oxidized by the available chlorine to form iodine.
- Acid (e.g., dilute sulfuric acid or acetic acid): Provides H⁺ ions necessary for the reaction between hypochlorite and iodide.
- Standardized Sodium Thiosulfate (Na₂S₂O₃): The titrant used to reduce the liberated iodine. Its concentration must be accurately known.
- Starch Solution: The indicator that signals the presence of iodine by forming a blue complex.
Practical Considerations
- The accuracy of the determination depends on the proper standardization of the sodium thiosulfate solution.
- The sample solution should be handled carefully as bleaching powder is an irritant.
- Titration should be done slowly, especially near the endpoint, for accurate results.
This method provides a reliable way to assess the strength and quality of bleaching powder based on its available chlorine content.
