Experiment 3: Oxidation States of Vanadium

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Background
It is known that vanadium in extremely small amounts is a nutritional requirement for many types of organisms, possibly including higher animals. The longest know example of vanadium in biochemistry is the accumulation of vanadium by certain lower marine organisms called
tunicates. They concentrate vanadium by a factor of ~106 over its level in the surrounding marine environment.

Vanadium exhibits an extended redox chemistry and can exist in several different oxidation states, each with a characteristic color.

Materials:

Mercury zinc amalgam
Ammonium metavanadate solution

SAFETY

Mercury zinc amalgam is toxic, keep under aqueous solution.

Procedure:

Place 5 grams of zinc amalgam in a 125mL Erlenmeyer flask.
Add 20 mL of vanadium solution. The solution should be yellow. Pour a small amount in a
small testtube as a reference.
Stopper the flask and gently swirl the solution. Record any color changes. Pour a small amount in a small testtube as a reference. (Hint: yellow + blue = green)
Swirl the flask again and record any color changes. Save a small amount in a small testtube as a reference.
Swirl the flask vigorously until another color change is observed.
Compare all the colored solutions. Have the instructor or TA check the final color.
Optional: Collect UV-VIS spectra on the colored solutions.

Clean-up: The chemical waste should be disposed in the designated containers.

Discussion:

Write the balanced chemical equations for each reduction step. Identify the colored species.
Provide the oxidation state and the d electron count for each vanadium species.
Arrange the vanadium species in order of increasing crystal field splitting.
Can V5+ and V4+ exist free in aqueous solution? Why?