vanadium

For a class of 24 (or 48) with 12 (or 24) groups of two…

Equipment needed:
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

  • 10.12 g vandal sulfate hydrate (80 millimoles)
  • 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.  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.