Question

You are a teaching assistant in a general chemistry lab. The next experiment is to be an oxidation-reduction titration involving iodine. You get a starch indicator from the stockroom. Why do you need it?

Short answer

Starch forms a blue-black complex with iodine, helping to visibly indicate the end-point of the titration.

Step-by-step solution

Understanding the Experiment

Identify that the experiment involves an oxidation-reduction titration with iodine.

Role of Starch in the Experiment

Determine that starch is used as an indicator in iodine-based titrations.

Interaction of Starch and Iodine

Understand that starch forms a blue-black complex with iodine, which is visually detectable.

Purpose of the Indicator

Conclude that the starch indicator is needed to visibly indicate the end-point of the titration by showing a color change.

Key concepts

starch indicator

The starch indicator plays a crucial role in iodine-based titrations. When starch interacts with iodine, it forms a distinct blue-black complex. This complex is very easy to see, even at low concentrations of iodine.
The starch indicator is especially helpful because iodine alone can sometimes be difficult to detect when its concentration is low.
Using the starch indicator ensures that even minimal amounts of iodine will still produce a visible change.
This makes it much easier to determine the end-point of the titration.

iodine titration

An iodine titration is a type of redox titration where iodine is used as the titrant. During the reaction, the iodine either oxidizes or reduces another substance in the solution.
It is essential for quantitative analysis since it helps in determining the concentration of substances in solution.
The process generally involves adding a known concentration of iodine to the solution until the reaction is complete.
The starch indicator is then used to help measure when all the analyte has reacted with the iodine.

end-point detection

Detecting the end-point in an oxidation-reduction titration with iodine is crucial for accurate results. The end-point is the moment when the reactants have completely reacted with each other.
For iodine titrations, this is visually detected with the help of the starch indicator.
When the starch is added to the solution, it forms a blue-black complex with any free iodine present.
As you approach the end-point, the solution will first turn a dark blue-black and then, upon adding the last drop of titrant, will suddenly become colorless if the sample runs out of reactable iodine.
This sudden change helps clearly and accurately identify the end-point of the titration.