Experiment Overview

Dichromatometry is a redox titration technique that uses potassium dichromate as a strong oxidizing agent. It is commonly used to determine the concentration of reducing agents such as iron(II) ions in acidic medium.

What This Experiment Demonstrates

• Oxidation–reduction reactions involving chromium compounds
• Use of an external indicator in redox titrations
• Stoichiometry of electron transfer reactions

Apparatus Required

• Burette
• Pipette
• Conical flask
• Volumetric flask
• White tile

Chemicals Required

• Potassium dichromate solution
• Iron(II) salt solution (e.g. iron(II) sulfate)
• Dilute sulfuric acid
• Diphenylamine or diphenylamine sulfonate indicator
• Distilled water

Theory

In acidic medium, potassium dichromate acts as a strong oxidizing agent. The dichromate ion is reduced from chromium(VI) to chromium(III), while the reducing agent undergoes oxidation.

The reduction half-reaction for dichromate in acidic solution is:

\[\ce{Cr2O7^{2-} + 14H^+ + 6e^- -> 2Cr^{3+} + 7H2O}\]

When iron(II) ions are used as the reducing agent, the oxidation half-reaction is:

\[\ce{Fe^{2+} -> Fe^{3+} + e^-}\]

Combining the half-reactions gives the overall equation:

\[\ce{Cr2O7^{2-} + 6Fe^{2+} + 14H^+ -> 2Cr^{3+} + 6Fe^{3+} + 7H2O}\]

Unlike permanganometry, potassium dichromate is not self-indicating, because the color change from yellow to green is not sharp enough to be able to precisely find and endpoint. An external indicator such as diphenylamine is used to detect the endpoint, which is marked by a distinct color change.

Method

1. Rinse and fill the burette with potassium dichromate solution.
2. Pipette a known volume of the iron(II) solution into a conical flask.
3. Add dilute sulfuric acid to acidify the solution.
4. Add a few drops of diphenylamine indicator.
5. Titrate with potassium dichromate until the endpoint color change appears.
6. Record the final burette reading.

Observations

The solution changes color sharply at the endpoint due to oxidation of the indicator. This indicates that all iron(II) ions have been oxidized to iron(III).

Calculations

• Use the balanced redox equation to determine mole ratios.
• Calculate the moles of potassium dichromate used.
• Determine the concentration of the iron(II) solution.

Result

The concentration of the reducing agent is determined using the volume of potassium dichromate required to reach the endpoint.

Video Demonstration

Video courtesy of Dr. Vishnu Thumma (YouTube). Video used for educational purposes under YouTube’s embedding policy.