Chemistry Laboratory #6: Preparation of Ammonium Decavandate and Standardization of KMnO₄

*Adapted from "Exploring Chemical Analysis", D. Harris, Freeman, 1997.

February 13, 2001

Introduction: Vanadium is a transition metal (sitting between titanium and chromium on the Periodic Table), the compounds of which have a rich variety of colors. For this reason it was named after Vanadis, the scandinavian goddess of beauty, by its "re-discoverer" N. G. Sefström in 1830. Like many transition metals, its ions can have a variety of charges and, one way of analyzing for the amount of vanadium in a given material is to measure how much of a known reagent is required to change the charge on the vanadium from one known value to another. This is an example of a redox titration and is the subject of this two-week laboratory. The first week will be spent preparing a vanadium compound, ammonium decavanadate, as well as preparing and standardizing a solution of an oxidizing agent, potassium permanganate, KMnO₄. Next week, you will use the potassium permanganate to analyze your prepared vanadium to ascertain if its composition is consistent with the expected formula, (NH₄)₆V₁₀O₂₈ • 6H₂O.
 

Standardization of KMnO₄ Solution:

1) Prepare 1 L of 0.9 M H₂SO₄. Dilute enough concentrated H₂SO₄ (caution! "Do as you otter, add acid to water!") in deionized water to make the desired concentration. Prepare the solution in a large beaker; after mixing well, place on a hot plate and gently boil. As soon as the solution begins to boil, place in an ice bath and cool to room temperature.
 

2) Prepare 500 mL of 0.02 M KMnO₄ by diluting 50 mL of the 0.2 M KMnO₄. Use a graduated cylinder may be used to measure the amount of stock solution necessary and add to an amber bottle. Add enough deionized water to make the total volume approximately 500 mL. Mix thoroughly.
 

3) Weigh (accurately) a 0.20 - 0.25 g sample of sodium oxalate, Na₂C₂O₄, and add to a 400 mL Erlenmeyer flask or 500 mL beaker. Add 250 mL of 0.9 M H₂SO₄. Place on a magnetic stir plate and begin stirring at a moderate rate. Using a buret, deliver enough KMnO₄ to completely react with the sodium oxalate; the equivalence point will be indicated when the purple color of the permanganate solution persists for 30 seconds after the addition. The first few milliliters will not react quickly; once the reaction starts however, the rate increases considerably. Record the volume delivered.
 

The reaction taking place is:

4) Based on the mass of sodium oxalate used, the volume of KMnO₄ used and the above stoichiometry, calculate the molarity of KMnO₄.

5) Weigh out three additional sample of sodium oxalate and titrate as above. Calculate the molarity of the KMnO₄ based on your final three titrations and calculate the mean, standard deviation and 95% confidence interval.
 

Preparation of Ammonium Decavanadate, (NH₄)₆V₁₀O₂₈^.6H₂O:
 

1) Heat 3.0 g of ammonium metavanadate (NH₄VO₃) in 100 mL of water with constant stirring (do not boil) until all of the solid has dissolved. Add 4 mL of 50 volume 50% acetic acid with stirring.
 

2) Add 150 mL of 95% ethanol with stirring and then cool the solution in an ice bath for 15 minutes.
 

3) Filter the orange product and wash with two 15 mL portions of ice-cold ethanol.
 

4) Store in a dry environment (free from dust) for two weeks when you will analyze the compound for vanadium content.