4. Redox titrations. a. (1 pt) Explain how a redox indicator works. Why is it im

Question

4. Redox titrations. a. (1 pt) Explain how a redox indicator works. Why is it important to consider the standard reduction potential of the indicator-what happens if it is more easily oxidized or reduced than the analyte? b. (1 pt) Explain why starch is not really a redox indicator. How does starch help you detect the end point of an iodimetric titration, and why is it OK to add the starch at the beginning unlike the iodometric titrations you did in lab? Why must starch solutions be freshly prepared?

Explanation / Answer

4.a) A redox indicator is an indicator compound undergoes colour change at specific potential difference.Thus it is essential for a redox indicator to have different oxidized and reduced forms with different colours.The redox process must be reversible.

When a titration is planned it is important to consider the reduction potential of the indicator as it will change the colour to mark the end point of the titration. If the reduction potential is lower than the analyete it will undergo colour change instantly when it comes in contact with the analyte and if the reverse happens then no colour change will take place at the end point. So it becomes difficult to mark the end point properly.

b) Starch is not a redox indicator but it is the indicator for the presence of iodine. With molecular iodine starch gives deep blue colour which marks the end point of an iodimetric titration. Thus the use of starch as indicator becomes limited only for iodine. So starch cannot be considered as a redox indicator since it does not have universal application.

In oiodometry I- ions are allowed to oxidize to form molecular I2  which turns starch solution into deep blue colour, so starch solution can be added at the beginning of the titration. But in iodimetry free iodine is allowed to get reduced to form iodides. If starch is added at the beginning the whole solution will turn blue or black and with the slight reducing power of starch some iodine will be reduced to iodide resulting an erronious result. So starch solution is not added in the beginning.

The starch solution has some reducing properties and it is redily biodegraded, so it needs to be prepared freshly for best results.

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