Last weekend I produced aqueous nitric acid by the direct acidification of ammonium nitrate by an equimolar amount of sulfuric acid. If I remember
correctly is a process which had been proposed here before, but no one was quite sure how well it would work. I decided to find out, and it does.
The next step will be to determine if fuming nitric acid can be produced in a similar process under vacuum distillation.
I used an equal amount of water by volume to the concentrated sulfuric acid, which the acid was added into prior to adding the ammonium nitrate. This
would be equivalent to about 60% H2SO4 by weight or 90%w/v, which should be easily obtainable by concentrating battery acid if someone should want to
go that route. The negative heat of solution of the ammonium nitrate helped to absorb the positive heat of solution of the sulfuric acid upon
addition, although both additions were done quite gradually and I have not calculated the total enthalpy.
I distilled it until the reaction liquor in the flask became frothy with a volume about 1.5 times the original volume, yielding a little over 500mL of
acid per liter of raw reaction liquor. The main portion of the distillate came over at 113°C (evidently the boiling point of the azeotrope at my
altitude) after a brief fraction at 94°C (the boiling point of water at my altitude). I did not drain the water prior to the main fraction, and the
total product had a specific gravity of 1.36. There were no signs of ammonia (which I feared could be produced as decomposition product) at any stage
of the distillation, which would have given a smoky appearance to the vapor. Once the distillation stabilized, the acid came over with a slight
yellow tint and some NO2 color was visible inside the condenser. This was cleared up by bubbling pure oxygen through the obtained acid for a few
minutes, although air could have been used with reduced efficiency.
I am in the process of dissolving the spent reaction liquor, which is a solid mass upon cooling and consists primarily of ammonium bisulfate with a
small excess of sulfuric and nitric acids. Once this is done I intend to the react the solution with an excess of either zinc oxide or copper oxide,
which should proceed (probably slowly) according to the reaction CuO + NH4HSO4 --> CuSO4 + NH3 + H2O. The resulting sulfate will then be suitable
for thermal decomposition to yield SO3 after preliminary heating has removed all the left over nitrate from the system in the form of NO2 by
decomposition of copper nitrate. Zinc sulfate will give a lower yield of SO3, giving instead a mixture of SO3 and SO2. The latter could be used to
produce a bisulfite for unrelated uses, or could be catalytically oxidized to yield SO3. I am under the impression, however, that CuSO4 decomposes
pretty selectively to SO3 and CuO; correct me if I am wrong. Zinc oxide/sulfate would certainly be a cheaper way to go in terms of how much material
is tied up in this cyclic process, but the decomposition is not as favorable. Are there any other oxides that would be good candidates for this?
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and then heated the gell to form black Copper
oxide. After decantation 6M HCl was added to form clear green solution and an aluminum wire was dropped in. Went home and came back next day to find
clear solution and salmon colored copper deposits with a little debris from the aluminum.
