VisualChem The interactive chemistry textbook
中文

The four oxidation states of vanadium: a rainbow in one flask

Add zinc powder to acidified ammonium vanadate and watch the solution run from yellow through green, blue and green to violet; then try tin, an iron(II) salt and potassium iodide, and use standard potentials to explain where each reducing agent stops.

Difficulty ●●○ ≈ 25 min

Aim Match vanadium’s +5, +4, +3 and +2 states to their colours, and use E° data to predict — then test — how far different reducing agents take the reduction.
Reducing VO₂⁺ step by step: colours computed from absorption bands

The four colours of vanadium

Reducing agent

0.0 min (illustrative)

Share of each oxidation state

  • VO₂⁺ (+5, yellow) 100 %
  • VO²⁺ (+4, blue) 0 %
  • V³⁺ (+3, green) 0 %
  • V²⁺ (+2, violet) 0 %

The solution now holds mainly VO₂⁺ (+5, yellow); computed colour rgb(255 253 32).

Thermodynamic end point: V²⁺ (+2, violet)

Potential ladder (V)

0.991V(V)/V(IV)0.337V(IV)/V(III)−0.255V(III)/V(II)−1.175V(II)/V(0)Zn²⁺/Zn −0.762 V
  • VOX2X++2 HX++eX−→VOX2++HX2O\ce{VO2+ + 2H+ + e- -> VO^2+ + H2O} Goes: E°cell = +1.753 V
  • VOX2++2 HX++eX−→VX3++HX2O\ce{VO^2+ + 2H+ + e- -> V^3+ + H2O} Goes: E°cell = +1.099 V
  • VX3++eX−→VX2+\ce{V^3+ + e- -> V^2+} Goes: E°cell = +0.507 V
Which steps go is decided by CRC standard potentials alone: a step happens when the vanadium couple lies above the reductant’s couple. The colour adds the teaching bands of the four vanadium species (the near-UV charge-transfer tail of VO₂⁺, the 760 nm band of VO²⁺, the d–d bands of V³⁺ and V²⁺) in the current proportions and passes them through the CIE observer, with 0.050 M vanadium and a 1.5 cm path. The time course is an assumed chain of first-order steps whose rate constants are purely illustrative — real rates depend on the zinc’s surface area, the acidity and the stirring. In a real flask V²⁺ is reoxidised by air within minutes, so air must be kept out.

Procedure

  1. Look at the starting solution first: ammonium vanadate dissolved in dilute sulfuric acid gives yellow VOX2X+\ce{VO2+} (+5). Note its colour.
  2. Choose zinc powder and press “Add and swirl”. Every few minutes record the colour and the share of each oxidation state, until the colour stops changing.
  3. Compare the colour with the bar: is the first green a new oxidation state? When is the solution pure blue?
  4. Repeat with tin, iron(II) sulfate and potassium iodide. First predict each end point from the potential ladder below, then check the simulation.
  5. Work out Ecell∘=E∘(vanadium couple)−E∘(reductant couple)E^\circ_\mathrm{cell} = E^\circ(\text{vanadium couple}) - E^\circ(\text{reductant couple}) for every step and tabulate the results.

What to notice

  • Zinc reduces VO₂⁺ (yellow) to VO²⁺ (blue), V³⁺ (green) and V²⁺ (violet) in turn; the first “green” is a mixture of the yellow and blue species.
  • Tin (−0.14 V) stops at V³⁺; iron(II) (+0.77 V) and iodide (+0.54 V) only take V(V) to V(IV).
  • Zinc’s potential (−0.76 V) lies above V²⁺/V (−1.18 V), so no amount of waiting gives vanadium metal.
  • Every step’s EMF is computed from CRC standard potentials: a step goes if and only if the vanadium couple lies above the reductant’s couple.

In a real lab

  • Vanadium compounds are toxic — avoid breathing dust and skin contact; concentrated sulfuric acid is corrosive, so always add acid to water. Zinc and acid give off hydrogen: keep flames away.
  • V²⁺ is reoxidised by air to V³⁺ within minutes: use a flask loosely plugged with cotton wool or fitted with a Bunsen valve, or work under nitrogen, to keep the violet colour.
  • The rate depends on the zinc’s particle size, the acidity and the shaking; the simulated time axis is only illustrative, and a real run may be much faster or slower.
  • The simulated colours come from teaching-fit absorption bands; real colours also depend on which anions (sulfate, chloride) coordinate, so the green of V³⁺ solutions varies.
  • The waste contains vanadium: collect it in the heavy-metal waste container, never down the drain.

Related lessons

All experiments