Question

Select indicators from Table 16-2 that would be suitable for finding the end point in Figure 16-3. What color changes would be observed?

Short answer

The most suitable indicators are Ferroin, Tris ($2,2\text{'}$bipyridine) iron, Diphenylbenzidine sulfonic acid, Diphenylamine sulfonic acid.

Step-by-step solution

Definition of Indicators

• Indicators are weak acids or bases (typically derived from plant pigments) that have unique colors in their ionized and non-ionized states and are proportional to the pH of the solution being tested.

• A material that can be added to the reaction mixture to show the titration's equivalence point. pH is frequently measured with colored indicators.

Find the Endpoint

• As we can see in figure $16-3$, the equivalence point is around $0.85\mathrm{V}$ and the step part of the curve is from $0.6\mathrm{V}$to $0.9\mathrm{V}$.

• The potential of the indicator should overlap with the steep part of the curve. The potential for redox indicators is with respect to standard hydrogen electrode.

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  • Since there is standard calomel $16-3$, to find the potential with respect to standard calomel electrode, we need to subtract the potential from the table with the potential of standard calomel electrode (0.241V ).

  • The most suitable indicators are:

  • Ferroin: potential around $0.91\mathrm{V}\Rightarrow$color change from pale blue to red

  • Tris (2,2'-bipyridine)iron: potential around $0.88\mathrm{V}\Rightarrow$color change from pale blue to red

  • Diphenylbenzidine sulfonic acid: potential around $0.63\mathrm{V}\Rightarrow$color change from violet to colorless

  Diphenylamine sulfonic acid: potential around $0.6\mathrm{V}\Rightarrow$color change from red-violet to colorless