Oxidation States of Vanadium: Reduction of V5+ to V2+

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Oxidation States of Vanadium: Reduction of V⁵⁺ to V²⁺
This demo is available on the ACS video "Close-Up on Chemistry". Video is a nice alternative to the waste from this demo (mercury and zinc). Plus the demo works very well and is visible on the video.

Description: A flask containing a NH₄VO₃ solution and zinc amalgam is gently swirled and the color of the solution turns from yellow to blue. A second swirl turns the solution from blue to green. A third swirl brings the solution to a violet color.
Concept: Transition metal compounds are often colored.
Materials:

Zinc amalgam-

In a 500 mL Erlenmeyer dissolve 2 grams of mercuric chloride in 300 mL

of water. Add 2 mL of nitric acid. Add 250 grams of zinc (20 - 30 mesh works best). Stopper flask and shake for a few minutes. Pour off liquid and wash the amalgam with water several times. Store the amalgam under water in a sealed jar.
Vanadium solution-

In a large beaker, dissolve 6 - 8 grams of NaOH in 200 mL of water. Add 10 grams of ammonium vanadate. Warm the beaker to completely dissolve the compound. Stir constantly. Add 500 mL of sulfuric acid solution (50 mL of H₂SO₄ in 450 mL of water). Dilute to 1 Liter. Store well stoppered.

Safety: Mercuric chloride is highly toxic. Fatal poison. Wear gloves and safety goggles. Wear mask when measuring zinc powder. Do not breath in zinc powder. Damage to mucous membranes could result.
Procedure:

Place 50 grams of zinc amalgam in a 1 Liter Florence Flask

Add 200 mL of vanadium solution. Stopper the flask. The solution should be yellow. Pour a small amount in a beaker as a reference.

Stopper the flask and gently swirl the solution once. The solution should become blue. Save a small amount in a beaker.

Swirl the flask again and the color should become green. Save.

Swirl the flask vigorously until the solution is a violet color.

Compare all the colored solutions.
Clean-up: Collect the leftover chemicals as hazardous chemical waste for EH&S.

Background:

1. Oxidation state form ⁺5 (yellow) to ⁺4 (blue):

Zn(s) + 2(VO₃) ^1- (aq) + 8 H₃O⁺ (aq)

2 (VO)²⁺ (aq) + Zn²⁺ (aq) + 12 H₂O (l)

VO₃ ^1- (yellow) VO²⁺ (blue)

2. Oxidation state from ⁺4 (blue) to ⁺3 (blue-green):

Zn(s) + 2 (VO)²⁺ (aq) + 4 H₃O⁺ (aq)

2 V³⁺ (aq) + Zn²⁺ (aq) + 6 H₂O (l)

VO²⁺ (blue) V³⁺ (aqua-blue)

3. Oxidation state from ⁺3 (blue-green) to ⁺2 (violet)

Zn(s) + 2 V³⁺ (aq) 2 V²⁺ (aq) + Zn²⁺ (aq)

V³⁺ (green) V²⁺ (violet)
Notes: This demo has been done in C105.
To oxidize V²⁺ back to V⁵⁺, add 0.1 M ceric sulfate drop by drop back through the color changes until solution is yellow again. To make the ceric sulfate, add 33.2 g of Ce(SO₄)₂ to 100 mL of 1.0 m H₂SO₄.

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