Separating sodium sulfate from chromium trioxide solution

Question

I was looking at the industrial process for the production of chromium trioxide by treating sodium chromate or the corresponding sodium dichromate with sulfuric acid. This results in the formation of chromium trioxide as well as sodium sulfate. I think both of them would be mixed as a solution; so what way is employed to separate them? I read about the process in Wikipedia.

I thought of using methods which I know like distillation, but I don't know how can I check if they work practically and are employed industrially. I searched the web as well and couldn't find anything satisfactory.

I am in 12th grade and I haven't really studied this topic, so I didn't know how to approach this question.

Answer 1

The overall reaction is:

$$\ce{H2SO4 + Na2Cr2O7 → 2 CrO3 + Na2SO4 + H2O}$$

How to separate chromium trioxide and sodium sulfate from solution?

Since chromium trioxide is soluble, it exist as chromic acid in the solution. So, in the reaction pot, there are compounds like $\ce{H2CrO4, H2Cr2O7}$ along with our target compound $\ce{CrO3}$, all in equilibrium. But with the addition of a large excess of concentrated sulfuric acid, water is withdrawn from the hydrated forms and the chromium trioxide separates from the solution in the shape of red needles. Successively, the reaction pot is made to stand overnight for the chromium trioxide to precipitate slowly in form of needles and then filtered off from the solution using Buchner filter. The synthesis procedure of chromium trioxide can be found in prepchem.com:

100 grams of sodium dichromate are dissolved in 250 ml of water and the obtained solution is filtered. 200 ml of concentrated sulfuric acid are dropwise added with constant stirring until a slight permanent precipitate of chromium trioxide is formed. The mixture is cooled for half an hour or longer, then slowly, while stirring, 200 ml of concentrated sulfuric acid are added dropwise. The reaction mixture is left to stand over night in order that the crystal meal may become somewhat coarser. The larger crystals (and purer) of chromium trioxide could be obtained by heating the solution with crystal meal with stirring to $\pu{100 ^{\circ}C}$ and allowing to cool slowly. If this process is repeated once or twice, a more satisfactory product will be obtained. The obtained crystals of chromium trioxide are filtered with a Buchner funnel with a sintered glass disc instead of the usual paper filter (chromium trioxide is a strong oxidizing agent and will oxidize organic material). After draining the crystals completely and pressing the surface and treating with 15 ml of concentrated (65-70%) nitric acid. The nitric acid is removed by suction and the operation is repeated twice with 10 ml of nitric acid each time. Finally, the red crystals of chromium trioxide are dried by suction as much as possible, transferred in a dry evaporating dish and by gentle heating the remaining nitric acid is removed. Dried product is stored in a stoppered bottle. Yield is 60%.

Note: chromium trioxide is toxic, corrosive, a powerful oxidizer and a potent carcinogenic. Sulfuric acid is corrosive and can burn your skin if you are careless. Handle those chemicals with care. Perform this synthesis only after you become a qualified chemist. This answer is only for educational purpose only.