Investigation 8: Redox Titration
A redox titration finds an unknown concentration by reacting it with an oxidizer or reducer of known concentration, using the balanced half-reactions' mole ratio instead of a simple 1:1 ratio.
Permanganate titrations are self-indicating. MnO4- is deep purple, but once it reacts it becomes nearly colorless Mn2+. As you drip KMnO4 into the flask, it reacts and decolorizes immediately. Once all the reducing agent is used up, the next drop of MnO4- has nothing left to react with, so it survives and tints the whole solution a faint persistent pink. That color change is the endpoint, no added dye needed.
The calculation runs through the balanced equation, not a bare 1:1 swap. Redox reactions trade electrons, and the coefficients encode how many electrons each species gives or takes. Write the balanced equation, find the mole ratio between titrant and analyte, then use buret volume x molarity of titrant to get moles of titrant delivered. Multiply by the mole ratio to get moles of analyte, then convert to mass or concentration as needed.
This sits alongside acid base titrations in the same lab unit, but the chemistry underneath is different: acid base relies on proton transfer and often needs phenolphthalein, while redox relies on electron transfer and sometimes needs no indicator at all. Both share the same math skeleton, volume and molarity to moles, then moles to moles by stoichiometry, then moles to whatever the question asks for. Getting comfortable with mismatched mole ratios here also prepares you for any multi-step stoichiometry problem with an unbalanced 1:1 instinct.
Key terms
- Titrant
- The solution of known concentration delivered from the buret until the reaction is complete.
- Analyte
- The substance of unknown amount or concentration being measured in the titration.
- Equivalence point
- The point where moles of titrant and analyte have reacted in the exact ratio given by the balanced equation.
- Self-indicating
- A titrant whose own color change signals the endpoint, so no separate indicator is needed.
- Mole ratio
- The ratio of coefficients from the balanced equation used to convert moles of one species to moles of another.
Watch it explained
- How it worksHow does a balanced redox equation turn a buret volume into a concentration?
- Worked exampleFind the percent of H2O2 in a drugstore bottle from a titration with 0.0200 M KMnO4.
- Worked exampleFind the iron content of an ore sample titrated with 24.6 mL of 0.0150 M dichromate.
- Common mistakeWhy isn't a redox titration calculated with a simple one to one mole ratio?
Check yourself
End of topic.