Quantitatively, ozone is measured by reacting it with potassium iodide (KI) solution and then titrating the liberated iodine. Here's a breakdown of the process:
The Potassium Iodide Method
This method relies on ozone oxidizing iodide ions (I-) to iodine (I2).
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Ozone Reaction: Ozone (O3) is bubbled through or allowed to react with a solution of potassium iodide (KI) buffered with a borate buffer at a pH of 9.2. This buffered solution is crucial to maintain a stable pH and ensure the reaction proceeds efficiently. The ozone oxidizes the iodide ions to iodine:
O3 + 2I- + H2O → O2 + I2 + 2OH-
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Iodine Liberation: The reaction liberates iodine (I2) in direct proportion to the amount of ozone present.
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Titration with Sodium Thiosulfate: The liberated iodine is then titrated against a standard solution of sodium thiosulfate (Na2S2O3) using starch as an indicator.
I2 + 2Na2S2O3 → 2NaI + Na2S4O6
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Endpoint Detection: Starch is added near the end of the titration as an indicator. Iodine forms a blue-black complex with starch, and the endpoint is reached when the blue-black color disappears, indicating that all the iodine has reacted with the sodium thiosulfate.
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Calculation: The amount of ozone can then be calculated based on the volume of sodium thiosulfate used in the titration. The stoichiometry of the reactions provides the relationship between the amount of thiosulfate consumed and the amount of ozone initially present.
Key Considerations
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Buffering: Maintaining the correct pH (around 9.2 using a borate buffer) is essential for the efficient reaction between ozone and potassium iodide.
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Interferences: Other oxidizing agents present in the air sample can also oxidize iodide ions, leading to overestimation of ozone levels. Therefore, careful sample preparation and analysis are necessary.
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Accuracy: The accuracy of the method depends on the accuracy of the sodium thiosulfate standard solution and the precision of the titration.
Summary
The potassium iodide method provides a reliable way to quantitatively determine ozone levels by measuring the amount of iodine liberated when ozone reacts with potassium iodide, followed by titration with a standard solution of sodium thiosulfate.
