How is Sulfite in Water Determined by Titration?

Sulfite concentration in water is determined by titration using a potassium iodide-iodate titrant in an acidified sample with a starch indicator. The titration continues until a faint permanent blue endpoint is reached, indicating the complete exhaustion of the sample's reducing power.

Here's a breakdown of the process:

1. Principle:

The method relies on the redox reaction between sulfite (SO₃^2-), iodide (I^-), and iodate (IO₃^-) in an acidic environment. Iodate oxidizes iodide to iodine (I₂), which then reacts with the sulfite.

2. Reagents:

• Standard Potassium Iodide-Iodate Titrant (KIO₃/KI): A solution of known concentration is prepared. The iodate ions (IO₃^-) will be reduced to iodine in the presence of excess iodide ions (I^-) and acid.
• Acid (e.g., Sulfuric Acid): Provides the acidic medium necessary for the reaction to proceed.
• Starch Indicator: Added near the endpoint. Starch reacts with iodine (I₂) to form a deep blue-black complex, providing a visual indication of the endpoint.

3. Procedure:

1. Sample Preparation: A known volume of the water sample containing sulfite is acidified by adding sulfuric acid.
2. Indicator Addition: A small amount of starch indicator solution is added to the acidified sample.
3. Titration: The potassium iodide-iodate titrant is slowly added to the sample while continuously mixing.
4. Endpoint Determination: As the titrant is added, the generated iodine (I₂) is immediately consumed by the sulfite in the sample. When all the sulfite has reacted, the excess iodine will react with the starch indicator, resulting in a faint, permanent blue color. This color signals the endpoint of the titration.

4. Chemical Reactions:

The overall reaction can be represented as follows:

1. Iodate and Iodide Reaction: IO₃^- + 5I^- + 6H⁺ -> 3I₂ + 3H₂O
2. Sulfite and Iodine Reaction: SO₃^2- + I₂ + H₂O -> SO₄^2- + 2I^- + 2H⁺

5. Calculation:

The concentration of sulfite in the water sample can be calculated from the volume of the potassium iodide-iodate titrant used, the concentration of the titrant, and the stoichiometry of the reaction. The calculations involve accounting for the amount of iodine generated by the titrant that reacted with the sulfite in the sample.

6. Importance of Endpoint:

The sharpness and accuracy of the endpoint are crucial for precise sulfite determination. The use of starch indicator helps in visualizing even small amounts of excess iodine, enabling accurate endpoint detection. The endpoint needs to be faint and permanent, because a large amount of iodine will turn the solution nearly black, making endpoint determination nearly impossible.

7. Considerations:

• The titration should be performed soon after sampling, as sulfite can be oxidized by air.
• Interfering substances that also react with iodine should be absent or accounted for.
• The strength of the potassium iodide-iodate titrant should be appropriate for the expected sulfite concentration in the sample.