Upsilon - 5-11-2015 at 09:00
I really can't find any good ways of making clean chromic acid anywhere. Most sources only show production of "cleaning grade" chromic acid by
reaction of a chromate with sulfuric acid:
K2CrO4 + H2SO4 <--> H2CrO4 + K2SO4
I don't imagine yield is very good with this method, as it seems to me to be only a tepid equilibrium, since all species involved are very
water-soluble and stable. It probably favors the right side only because sulfuric acid is stronger than chromic acid, but again this is probably not
enough to get a good yield. Not to mention actually trying to separate the chromic acid from that mess, which is an even bigger problem.
So I did some thinking, and I believe I have something that might work. Starting with common potassium chromate solution, calcium chloride is added.
Upon heating close to boiling, calcium chromate becomes very poorly soluble while potassium chloride becomes insanely soluble. This difference can be
abused to filter off the calcium chromate in very high yield. After cooling, the calcium chromate becomes quite soluble again. Combining this solution
with phosphoric acid solution, soluble chromic acid is formed while solid calcium phosphates precipitate out.
Amos - 5-11-2015 at 12:45
Why is it that you want to make pure chromic acid? If you really must, there is a way even with just dichromate and sulfuric acid.
Simply make chromium trioxide as seen here and combine with the proper amount of water.