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Redox colour tests – KMnO₄ and KI, vanadium oxidation states and Fe²⁺ titration

ChemistryReactions & RedoxAges 15–16

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Tests screen: add acidified KMnO₄ (purple → colourless) or acidified K₂Cr₂O₇ (orange → green) drop by drop to identify reducing agents, and KI (colourless → yellow-brown, blue-black with starch) to identify oxidising agents, with a control tube, an unknowns mode and a results table. Vanadium screen: zinc in acid reduces VO₂⁺ (yellow, +5) through VO²⁺ (blue, +4) and V³⁺ (green, +3) to V²⁺ (violet, +2), and standard electrode potentials show how far tin or iron(II) can go. Titration screen: titrate Fe²⁺ with KMnO₄ or K₂Cr₂O₇, record concordant titres and calculate the concentration.

Lesson: Oxidising and reducing agents, oxidation numbers and half-equations; redox colour tests; vanadium oxidation states; redox titrations

What it shows

An oxidising agent gains electrons and is reduced; a reducing agent loses electrons and is oxidised. Colour changes make these easy to see. Acidified manganate(VII) is purple, but its reduced form Mn²⁺ is almost colourless, so a reducing agent decolourises it; acidified dichromate(VI) turns from orange to green Cr³⁺. Colourless iodide ions are oxidised to brown iodine, which starch turns blue-black, so iodide tests for oxidising agents. Vanadium shows four oxidation states with four colours, and standard electrode potentials predict which reducing agent can reach each one.

How to use

On Tests, choose a Reagent and a Substance, press +1 drop or +5 drops and compare the tube with the control tube. Read the conclusion, the half-equations and the oxidation numbers, then press Record. Tick Unknowns to hide the names and press Reveal at the end. On Vanadium, change the Reducing agent and watch the colours and the graph. On Titration, add the Burette solution until the end point, press Record titre and repeat until two titres agree.

Parameters you can change

  • Screen Tests for oxidising and reducing agents, Vanadium, Fe²⁺ titration
  • Reagent acidified KMnO₄ (tests for reducing agents), acidified K₂Cr₂O₇ (tests for reducing agents), KI (tests for oxidising agents), KI + starch (tests for oxidising agents)
  • Substance tested FeSO₄ – iron(II) sulfate, KI – potassium iodide, Na₂SO₃ – sodium sulfite, H₂O₂ – hydrogen peroxide (acidified), FeCl₃ – iron(III) chloride, Cl₂ – chlorine water, Br₂ – bromine water, acidified K₂Cr₂O₇, CuSO₄ – copper(II) sulfate, NaCl – sodium chloride, distilled water (control)
  • Unknowns (names hidden)
  • Reducing agent for vanadium(V) zinc (Zn), tin (Sn), iron(II) (Fe²⁺)
  • Speed of the vanadium simulation 1–10
  • Solution in the burette KMnO₄ 0.0200 mol/dm³, K₂Cr₂O₇ 0.0167 mol/dm³
  • Fe²⁺ sample unknown concentration, known (0.0800 mol/dm³)

Questions to explore

  1. Why does hydrogen peroxide decolourise acidified potassium manganate(VII) but turn potassium iodide brown?
  2. Why does the purple colour stop disappearing after a certain number of drops?
  3. Why does tin reduce vanadium(V) only to vanadium(III) while zinc reaches vanadium(II)?

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