[three winged hawk]

How would one turn a solution of Cr(OH)3 into solid Na2Cr2O7?  It seems necessary to add concentrated acid, but if HCl was added, how would one seperate the sodium bichromate from the NaCl?

[woelen]

How would one turn a solution of Cr(OH)3 into solid Na2Cr2O7?  It seems necessary to add concentrated acid, but if HCl was added, how would one seperate the sodium bichromate from the NaCl?

From Cr(OH)3 you don't get Na2Cr2O7 by simply dissolving the hydroxide. The hydroxide contains chromium in the +3 oxidation state, while the dichromate contains chromium in the +6 oxidation state.

If you really want dichromate from Cr(OH)3 then you need to oxidize the stuff. This can be done by dissolving it in a solution of NaOH (use as little as possible of NaOH, just enough to dissolve the Cr(OH)3). Now you get a green solution of sodium chromate (III), aka sodium chromite. To this add some H2O2 (3% will do, but it gives you dilute solutions) and heat the liquid (do not boil it, that is not needed and is quite dangerous with H2O2). The liquid will turn yellow slowly. Now you have Na2CrO4 dissolved in water (sodium chromate). If the liquid is totally yellow, then you need to add a pinch of MnO2. This causes all excess H2O2 to decompose. That really is necessary. Let all MnO2 settle during a day or so and decant the clear yellow solution. Add some acid to this (preferrably dilute H2SO4, but HCl also will do). This causes the liquid to change from yellow to orange. Evaporate away some water to concentrate the solution. Evaporating to dryness is not a good idea. Na2Cr2O7 is very hygroscopic and very soluble, so it will be really hard to get the solid.

If you don't have MnO2, then heating the liquid for a longer time, just below boiling may also destroy all H2O2. Adding in some crystals of KMnO4 also works. This is converted to KOH and MnO2 at once. Only use a very small amount of MnO2 or KMnO4, it is just needed for catalytic decomposition of the H2O2.

If you use KOH instead of NaOH, then you might be able to get some solid K2Cr2O7. This is much less soluble and not hygroscopic. It crystallizes well from water.

Wilco

PS: Know what you are doing. Chromate and dichromate are no children's toys  
They contain chromium in the +6 oxidation state and such compounds are believed to be human carcinogens. So, be sure that you are not exposed to the liquid or its fumes.
KOH, NaOH are very bad for the skin and even worse for the eye (instant destruction). Concentrated H2O2 also is quite corrosive to the skin.

[Grafter]

Sound advice.

[three winged hawk]

Wow, thanks for the great info!  How would one know with any accuracy the amount of dichromate in solution?

So dichromate from chromium(III) seems like more trouble than is necessary for a homemade oxidizer capable of cleaving an alkene's double bond.  Is it possible to easily and cheaply build an ozone generator?  How could one determine the precise amount of ozone to pass through the reaction mixture?  How would one avoid overoxidation?

Much thanks

[Grafter]

Go for a bit of silent electrical discharge (eg photocopier) in an O₂ atmosphere.

[Borek]

Wow, thanks for the great info!  How would one know with any accuracy the amount of dichromate in solution?

Google chromianometry. But if there is some oxidizer left in the solution you will have problems. You may also try to precipitate barium choromate and weigh it. These are classical quantitative methods for analysis.

[woelen]

Wow, thanks for the great info!  How would one know with any accuracy the amount of dichromate in solution?

If you know the amount of Cr(OH)3 in your solution, then you can compute the amount of dichromate. For each two Cr(OH)3 you get one Cr2O7(2-).

So dichromate from chromium(III) seems like more trouble than is necessary for a homemade oxidizer capable of cleaving an alkene's double bond.  Is it possible to easily and cheaply build an ozone generator?  How could one determine the precise amount of ozone to pass through the reaction mixture?  How would one avoid overoxidation?

Probably the use of an ozone generator brings in even more trouble. How do you think you can bubble the gas through the reaction mixture. You need a high-oxygen content atmosphere to start with.

Much thanks