The oxidation states of vanadium

Somewhere over the rainbow, the different oxidation states of transition metal ions show different colours … and skies are blue.

This experiment should take approximately 30 minutes. 

Equipment

Apparatus

• Eye protection
• Conical flask, 1 dm³
• Filter funnel
• Boiling tube
• Dropping pipette
• Four petri dishes (optional)
• Access to an overhead projector (optional)
• Test-tubes and rack (optional)

Chemicals

The quantities given are for one demonstration.

•  Ammonium metavanadate (ammonium vanadate(V), NH₄VO₃), 11.7 g
• Zinc powder, 15 g
• Potassium permanganate (potassium manganate(VII), KMnO₄), 100 cm³, 0.25 mol dm^–3, in sulfuric acid, 1 mol dm^–3. (Dissolve 4 g of potassium permanganate in 100 cm³ of 1 mol dm^–3 sulfuric acid.)
• Powdered tin, 1 g (optional)
• Sodium thiosulfate solution, 1 mol dm^–3, 10 cm³ (optional (Dissolve about 25 g of sodium thiosulphate-5-water in 100 cm³ of water))
• Sulfuric acid, 1 mol dm^–3, 1 dm³ 

Health, safety and technical notes 

• Read our standard health and safety guidelines.
• Always wear eye protection.
• Ammonium metavanadate is extremely toxic if inhaled, toxic if swallowed, a skin/eye/respiratory irritant and a mutagen (see CLEAPSS Hazcard HC009b).
• Zinc powder is pyrophoric and water reactive and hazardous to the aquatic environment (see CLEAPSS Hazcard HC107).
• Potassium permanganate is an oxidiser, harmful if swallowed and hazardous to the aquatic environment (see CLEAPSS Hazcard HC081).
• Sodium thiosulfate solution is low hazard (see CLEAPSS Hazcard HC095a).
• Sulfuric acid is a skin/eye irritant (see CLEAPSS Hazcard HC098a).

Procedure

Before the demonstration

1. Make up a 0.1 mol dm^–3 solution of ammonium metavanadate by dissolving 11.7 g of solid in 900 cm³ of 1 mol dm^–3 sulfuric acid and making up to 1 dm³ with deionised water.
2. This yellow solution is usually represented as containing VO₂⁺(aq) ions (dioxovanadium(V) ions) in which vanadium has an oxidation number of +5.

The demonstration

1. Place 500 cm³ of the ammonium metavanadate solution in a 1 dm³ conical flask and add about 15 g of powdered zinc.
2. This will effervesce and give off hydrogen on reaction with the acid.
3. The solution will immediately start to go green and within a few seconds will turn pale blue, the colour of the VO₂⁺(aq) ion in which the vanadium has an oxidation number of +4.
4. The short-lived green colour is a mixture of the yellow of V^V and the blue of V^IV. The blue colour of VO²⁺ is similar to that of the Cu²⁺(aq) ion.
5. Over a further fifteen minutes or so, the colour of the solution changes first to the green of V³⁺(aq) ions and eventually to the mauve of V²⁺(aq) ions. The green of V³⁺(aq) is the most difficult to distinguish.
6. If desired, decant off a little of the solution at each colour stage, filter it to remove zinc and stop the reaction and place in a petri dish on the overhead projector to show the colour more clearly.
7. When the reaction has reached the mauve stage, filter off a little of the solution into a boiling tube and add acidified potassium permanganate solution dropwise. This will re-oxidise the vanadium through the +3 and +4 oxidation states back to V^V.
8. Take care with the final few drops to avoid masking the yellow colour of vanadium(V) with the purple of permanganate ions.

Notes

A white background is vital if the colour changes are to be clearly seen.

If desired, prepare solutions containing V^IV and V^III beforehand for comparison. This is recommended for teachers who are not familiar with these colours. This can be done as follows. V^IV: take a little of the original ammonium metavanadate solution in a test-tube and add approximately 1 mol dm^–3 sodium thiosulphate solution dropwise until no further colour change occurs and a light blue solution is obtained.

If too much thiosulphate is added, the solution will gradually go cloudy due to the formation of colloidal sulfur by reaction of the excess thiosulphate with acid, but this will not affect the blue colour of V^IV.

V^III: take a little of the original ammonium metavanadate solution in a test-tube and add a spatula-full of powdered tin. Leave this for about five minutes and then filter off the tin to leave a green solution containing V³⁺(aq) ions.

This demonstration can be used as an introduction to the idea that different oxidation states of transition metal ions often have different colours, and that electrode potentials can be used to help predict the course of redox reactions (via the ‘anticlockwise rule’ or otherwise). While waiting for the reaction to go to completion, some of the reactions can be discussed.

Theory

The relevant half reactions and redox potentials are as follows:

Zn2+(aq) + 2e– ⇋  Zn(s)	Eϴ = –0.76 V
V3+(aq) + e– ⇋ V2+(aq)	Eϴ = –0.26 V
Sn2+(aq) +2e–  ⇋ Sn(s)	Eϴ = –0.14 V
VO2+(aq) + 2H+(aq) + e– ⇋ H2O(I) + V3+(aq)	Eϴ = +0.34 V
S4O62– (aq) + 2e– ⇋ 2S2O32-(aq)	Eϴ = +0.47 V
VO2+(aq) + 2H+(aq) + e– ⇋ H2O(I) +VO2+(aq)	Eϴ = +1.00V

So zinc will reduce VO₂+(aq) to V²⁺(aq).

Tin will reduce VO₂+(aq) to V³⁺(aq) and no further.

Thiosulfate ions will reduce VO₂+(aq) to VO²⁺(aq) and no further.