Unique to most other species on the periodic table, vanadium has four stable oxidation states in addition to its metallic form! Changing oxidation states involves removing and/or adding electrons, and this means that vanadium ions can be used to store electrical energy!
What is fun about vanadium is that each oxidation state is a different color, and the act of changing oxidation states is both aesthetically and conceptually beautiful chemistry. In this lab we give students V2+, V4+, and V5+ (V3+ is the other stable one). Each of these is a different color, but we don’t tell them which is which. Students have to figure it out by titrating in a solution of potassium permanganate (also a very beautiful magenta color), and use the amount of permanganate added and colors along the way to determine which is which.
Doesn’t that sound more fun than a colorimetric titration? Then again, if you want to impress someone sitting next to you on an airplane by telling them what you do for a living, then “colorimetric titrations” might be a great place to start.
In the future, we’ll post more details about the experiment itself, but for the time being, we’ve got the details of the chemicals and solutions that need to be made up prior to starting this lab. If you want more details, contact Grimm right away!
For a class of 24 (or 48) with 12 (or 24) groups of two…
12 (24) Burets (25 mL or 50 mL are fine)
12 (24) Buret clamps and ringstands
12 (24) Stirplates
12 (24) Small stirbars
12 (24) 50 mL beakers
We assume a concentration of 100 mM vanadium solutions and 20 mM potassium permanganate solutions. The managanese undergoes a five-electron reduction from Mn7+ to Mn2+, and 20 mM of permanganate is still really bold in color! Since the permanganate is 1/5th as concentrated, it will take equal volumes of the permanganate solution to titrate through each vanadium oxidation state. For instance, if we had 10 mL of V4+, it will take 10 mL of the permanganate solution to reach complete conversion to V5+. If we had 10 mL of V3+ it would take 10 mL of the permanganate to reach V4+ and another 10 mL of the permanganate to reach V5+…
If each group is taking 10 mL of each vanadium solution and we assume that we need 50% extra to give some groups the opportunity to redo if they need/want to, then we need an absolute minimum 180 mL for 12 groups (360 mL for 24 groups). Again, let’s round up and call that 200 mL for 12 groups (400 mL for 24 groups).
Here’s the rub: V5+ and V4+ are air stable. V3+ and V2+ are stable, but not air stable, so they have to be made at class time or kept in a Schlenk vial under inert atmosphere which is not super practical for a class lab. When we run this lab, we make twice as much V5+, which is cheaper than V4+, and we prepare double the amount of V5+ and reduce that to V2+ using zinc granules. Zinc powder is not necessarily a good idea since it contains a large fraction of zinc oxide. That’s not a problem per se, but that means that you’ll need a significant amount of zinc. In this way we don’t have V3+ but in the way we run this lab, this becomes a feature not a bug: by providing only three of the four oxidation states, we ask the students to figure out which is which, and which one is missing.
Preparation of the V5+ solution (enough for forty 10 mL samples of the V5+ and forty 10 mL sample for conversion to a V2+ solution) is as follows. As of summer 2018, 250 g of 99% pure ammonium metavanadate NH4VO3 is $104 (205559-250G at Sigma-Aldrich) or 50 g of ACS-grade is $57.50 (10028-25G at Sigma-Aldrich). The solution will need to be stirred under mild heating to completely dissolve all of the ammonium metavanadate. This can be made days or weeks in advance.
- Preparation 800 mL of the 0.1 M V5+ solution
- 9.35 g ammonium metavanadate (80 millimoles)
- 80 mL conc. HCl
- ~720 mL DI water (fill to 800 mL)
Preparation of the V4+ solution (enough for forty 10 mL samples of the V4+ solution) is as follows. As of summer 2018, 25 g of 97% pure vanadium(IV) oxide sulfate hydrate, VOSO4 • xH2O, is $68.10 (233706-25G at Sigma-Aldrich) or 100 g for $146 (233706-100G at Sigma-Aldrich). Vanadium(IV) oxide sulfate also goes by vandal sulfate. The amount of hydrate x, is usually less than 6 and often about 5 or so. The anhydrous compound has a formula mass of 163 g/mol, and if we assume x = 5, then the formula mass is 253.1 g/mol. The solution will need to be stirred under mild heating to completely dissolve all of the vanadium(IV) oxide sulfate hydrate. This can be made days or weeks in advance.
- Preparation 400 mL of the 0.1 M V4+ solution
- 7.96 g vandal sulfate hydrate (40 millimoles, assume 2 waters on the arbitrary hydrate for a FW of 163 + 36 g/mol)
- 40 mL conc. HCl
- 360 mL DI water (fill to 400 mL)
Preparation of the V2+ solution (enough for forty 10 mL samples of the V2+ solution) is as follows. For context, V2+ and V3+ are stable, but they’re not air stable. That is to say, you can make V2+ in a glovebox and it would stick around for decades. The best analogy is with the Li0 in batteries: it’s stable, but it’s not air stable. Because V2+ is not air stable, this solution must remain stirring over zinc for the duration of its need. We usually make this solution up concurrently with its use. We use zinc granules. As of the summer of 2018, 100 g of 20-30 mesh zinc granules (0.5-1 mm diameter) are $28.10 (243469-100G at Sigma-Aldrich).
- Preparation 400 mL of the 0.1 M V2+ solution
- 400 mL of the V5+ solution
- a few grams of zinc granules.