I have been pondering over a way to improve the yield of chromyl chloride synthesised out of HCl gas (itself made by mixing concentrated sulphuric
acid and table salt) and CrO₃. I tried it the usual way, i.e. mixing crushed NaCl and CrO₃ and then pouring concentrated sulphuric acid (c.s.a) on
the resulting powder, but I got a bad yield (20%?). I think the culprit is the 95% sulphuric acid I have, which I suspect is not concentrated enough,
and maybe my glassware wasn't dry enough either.
So I'm planning to redo the experiment this way: use my 95% c.s.a. to generate HCl gas. Feed the HCl gas to a round bottom flash and bubble it through
a solution of CrO₃ dissolved in c.s.a. That way, the c.s.a should pull out the water produced as a by-product of the reaction CrO₃ + HCl →
CrO₂Cl₂ + H₂O.
So, question 1: if the reaction produces, say, 1 mol of water, how many moles of c.s.a should I use to efficiently remove all water and avoid it
interfering with the chromyl chloride?
Question 2: it is said that CrO₃ is soluble in c.s.a, but I didn't find any reference substantiating that claim, nor have I found any solubility
data so far. Does anyone know how soluble is CrO₃ in c.s.a?
I was also wondering if I could use oleum. I'm about to order a special quartz test tube with a ground glass joint to pyrolyse copper sulphate into
copper oxide and sulphur trioxide that I could then bubble into my c.s.a to get 100%+ concentration. That oleum could be mixed with the chromium
trioxide to really pull out all the water.
Question 3: what kind of (flexible) tubing resists to SO₃? Silicone?
Also, question 4: should I distill the chromyl chloride as soon as it forms, or let the reaction proceed and only then distil CrO₂Cl₂ from the
c.s.a?
Thanks a bunch!wg48temp9 - 3-9-2019 at 13:25
You will probably be interest in this patent titled Process for producing chromyl chloride: https://patentimages.storage.googleapis.com/57/27/72/cb715e7... It talks about phase seperation of the chloride and the water sulphuric acid
mixture and other info.
S.C. Wack - 3-9-2019 at 15:40
If one looked at my scans they might find some educational material. The dry method variations in Brauer and Cumming do not use CrO3 directly; this
might be important. Heating with any water would be bad, yes; they do not use 95% acid. Brauer also has the wet method with CrO3 but this is much less
detailed than the Inorganic Syntheses preparation giving some 50-60% yield of pure product.
CrO3 precipitates readily from sulfuric acid at higher concentrations. Sometimes at lower concentrations the crystals are bigger and nicer.
[Edited on 3-9-2019 by S.C. Wack]Keras - 4-9-2019 at 04:58
Brauer also has the wet method with CrO3 but this is much less detailed than the Inorganic Syntheses preparation giving some 50-60% yield of pure
product.
Thanks! I didn't know I could find anything about chromyl chloride in Inorganic Syntheses. The method described therein seems simple and baffling: it
uses CrO₃ in aqueous solution, whereas I always heard that chromyl chloride was destroyed by any water (that also what NileRed claims in his video).
One must assume that around 0°C the hydrolysis is so slow that it doesn't impact the yield negatively…
