Illustrating the oxidation states of vanadium
Requirements per student (or pair)
Conical flask, 100cm3
Filter funnel
Measuring cylinder, 25cm 3
5 test tubes
Bunsen burner, tripod and gauze
Heat proof mat
Filter papers
Safety glasses
Disposable gloves
Spatula
Test tube holder
Test tube rack
Wash-bottle of distilled water
Notes
Ammonium polytrioxovanadate (ammonium
metavanadate), NH4VO3
~1g
Sodium sulphite, Na2SO3
~1g
Zinc powder, Zn
~2g. Small granules may be used, but powder
is better. Avoid dust – it is dangerously
flammable
Sulphuric acid, concentrated, H2SO4
~10cm3
Potassium iodide solution, 0.05 mol dm -3 KI
~2cm3. Dissolve 0.8g of solid in water and
make up to 100cm3
Potassium manganate (VII) (permanganate)
solution, 0.02 mol dm-3 KMnO4
~10cm3. Dissolve 3.2g of solid in water and
make up to 1000cm3
Sodium thiosulphate solution, 0.1 mol dm -3
Na2S2O3
~2cm3. Dissolve 25g of the pentahydrate in
water and make up to 1000cm 3
Sulphuric acid, dilute, 1 mol dm -3 H2SO4
~25cm3. Carefully add 55cm3 of concentrated
acid, slowly and with constant stirring, to
800cm3 of distilled water and make up to
1000cm3
HAZARD WARNING – Concentrated sulphuric acid is corrosive and reacts violently with water.
Therefore you must:
 Wear safety spectacles and gloves
 When diluting, add acid to water, not water to acid
 Mop up small spillages with excess water
Name:
Date:
Experiment – Illustrating the oxidation states of vanadium
Aim
The purpose of this experiment is to illustrate the presence of several different oxidation states for
vanadium, and to show how it is possible to change from one oxidation state to another.
Introduction
Starting with a solution containing vanadium (V) in acid conditions, you use powdered zinc as a
reducing agent. Colour changes indicate the formation of other oxidation states. After this first
series of reactions you perform some changes between oxidation states by using a variety of
oxidising and reducing agents.
Assessment and Timing
You are not being assessed on this experiment but it will count for 1.5 hours of practical time
Requirements
Safety glasses
Disposable gloves
Conical flask, 100cm3
Spatula
Ammonium polytioxovanadate(V), NH4VO3 (ammonium metavanadate)
Measuring cylinder, 25cm3
Sulphuric acid, dilute, 1 mol dm -3 H2SO4
Sulphuric acid, concentrated, H2SO4
5 test tubes and holder
Test tube rack
Zinc powder, Zn
Bunsen burner, tripod and gauze
Heat proof mat
Filter funnel and paper
Potassium manganate (VII) solution, 0.02 mol dm -3 KMnO4 (permanganate)
Sodium sulphite, Na2SO3
Potassium iodide solution, 0.05mol dm -3 KI
Sodium thiosulphate solution, 0.1mol dm -3 Na2S2O3
Wash-bottle of distilled water
HAZARD WARNING – Concentrated sulphuric acid is corrosive and reacts violently with water.
Therefore you must:
 Wear safety spectacles and gloves
 When diluting, add acid to water, not water to acid
 Mop up small spillages with excess water
Procedure
1. First complete the table below to assist you in identifying the different oxidation states of
vanadium.
Ion (hydrated)
*VO2+
VO2+
V3+
V2+
Colour
Yellow
Blue
Green
Violet
Dioxovanadium (V)
Oxovandaium (IV)
Vanadium (III)
Vanadium (II)
Oxidation state
Name
*The VO3- ion in the ammonium salt is converted to VO2+ by acid:
VO3-(aq) + 2H+(aq)   VO2+(aq) + H2O(l)
2. Place about 0.25g (one spatula measure) of ammonium trioxovanadate (V) in a conical
flask and add about 25cm 3 of dilute sulphuric acid. Carefully add about 5cm 3 of
concentrated sulphuric acid and swirl the flask until you obtain a clear yellow solution.
3. Pour about 2cm 3 of this vanadium (V) solution into each of two test tubes ready for later
tests.
4. To the conical flask add 1 – 2 g (one spatula measure) of zinc powder, a little at a time.
Swirl the flask at intervals and record any observed colour changes in the results table.
5. When the solution has become violet (you may need to heat the flask for this final
change), filter about 2cm 3 into each of three test tubes.
6. To one of the three tubes, add a little at a time, an excess of acidified potassium
manganate (VII) solution, shaking after each addition, until no further change is observed.
7. Keep the other test tubes of solutions for later tests. Answer the first two questions and
complete the results table as far as you can before doing further tests.
8. To one of the tubes containing vanadium (V) add a little sodium sulphite and shake. Filter
if cloudy. Now boil carefully (in a fume cupboard, if possible) to remove excess sulphur
dioxide and add about the same volume of vanadium (II) solution. Record your
observations.
9. To the second of the tubes containing vanadium (V), add about 2cm 3 potassium iodide
solution and mix. Then add about 2cm 3 of sodium thiosulphate solution. Record your
observations.
Results Table
Test
Observations
Summary of Reaction
Ammonium vanadate + acid
White solid turned red and
dissolved to a yellow solution
VO3-  VO2+
+5  +5
Vanadium (V) + zinc
Vanadium (II) + manganate
(VII)
Vanadium (V) + sulphite
Add vanadium (II)
Vanadium (V) + iodide +
thiosulphate
Questions
1. How do you explain the first appearance of a green colour in the solution?
2. What are the subsequent changes in colour and why do these changes occur?