Excess potassium iodide is used in bleach titration primarily to aid in the clear detection of the endpoint during the titration process.
In the quantitative analysis of bleach, the active ingredient, sodium hypochlorite (NaClO), is reacted with an excess amount of potassium iodide (KI) in an acidic solution. This reaction produces iodine (I₂). The amount of iodine produced is directly proportional to the amount of hypochlorite initially present in the bleach sample.
The generated iodine is then titrated using a standard solution of sodium thiosulphate (Na₂S₂O₃). The thiosulphate reacts with the iodine, reducing it back to iodide ions (I⁻).
$$ \text{I}_2 \text{ + 2 S}_2\text{O}_3^{\text{2-}} \rightarrow \text{2 I}^{\text{-}} \text{ + S}_4\text{O}_6^{\text{2-}} $$
As the thiosulphate is added, the concentration of iodine in the solution decreases. Iodine produces a distinct yellow-brown color in solution, which fades as it reacts with thiosulphate. However, as the titration nears completion and the iodine concentration becomes very low, the color becomes too pale to accurately judge the endpoint.
This is where the excess potassium iodide plays a crucial role.
Role of Excess KI in Endpoint Detection
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Ensuring Complete Reaction: Initially, excess KI ensures that all the hypochlorite reacts completely to form iodine.
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Enhanced Indicator Performance: Starch solution is typically used as an indicator in this titration. Starch forms a deep blue complex with iodine (specifically, triiodide ions, I₃⁻, which are formed when iodine reacts with iodide ions). The reaction of iodine with iodide ions is:
$$ \text{I}_2 \text{ + I}^{\text{-}} \rightleftharpoons \text{I}_3^{\text{-}} $$
The presence of excess iodide shifts this equilibrium to the right, forming more triiodide ions. This complex is much more effective at interacting with starch to produce the intense blue color compared to iodine alone.
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Sharper Endpoint: As the thiosulphate reacts with the iodine/triiodide complex, the blue color persists until nearly all the iodine is consumed. When the last trace of iodine reacts, the blue complex instantly disappears, leaving a clear or very pale yellow solution (depending on other substances present). This transition is sharp and easy to observe.
According to the reference:
"Redox titration occurs between iodide and aqueous sodium thiosulphate. With each addition of thiosulphate, the concentration of iodine decreases, it becomes difficult to detect the endpoint. So, excess potassium iodide will only help in detecting end point and does not affect the titration."
This highlights the key benefit: excess KI ensures the endpoint is clearly visible. It doesn't interfere with the stoichiometry of the reaction between iodine and thiosulphate; its purpose is solely to enhance the visual signal for the endpoint, making the titration results more accurate.
Summary of Benefits
- Ensures initial reaction completeness (hypochlorite to iodine).
- Favors the formation of the triiodide-starch complex for a vibrant blue color.
- Provides a sharp, easily observable color change at the endpoint.
- Does not alter the fundamental stoichiometry of the titration reaction between iodine and thiosulphate.
Using excess KI ensures that the disappearance of the blue color, marking the titration endpoint, is unambiguous, leading to more reliable results in determining the hypochlorite concentration in the bleach sample.
