Rhombo
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Nitric Acid KNO3 in excess?
First post. Great trepidation. Why do most procedures I read or watch on YT use an excess of KNO3 in the reaction with H2SO4 instead of a
stoiciometric amount? When I use this process I really want to save KHSO4 as a secondary product so I am interested in a process which also optimizes
it's purity. Thanks. Please no kill I.
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imidazole
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For many people, Sulfuric acid is an expensive reagent compared to nitrate salts. There isn't really a reason you can't stop at the bisulfate, but
many prefer to get the additional nitric acid.
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Sulaiman
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unless you want RFNA or WFNA
combine stoichiometric quantities of sulphuric scid, nitrate salt and water,
to produce azeotropic nitric acid (20%) by distillation
leaving bisulphate solution in the boiling pot.
PS For various reasons (purity of reactants, measurement accuracy,) it is not possible to make a perfectly stoichiometric mixture
and
even f you could make a perfectly stoichiometric mixture
as the reaction proceeds the concentration of the original reactants decreases,
slowing the reaction rate.
in theory you would end up with the last ions of each reactant randomly moving around,
eventually bumping into a 'partner' ion until the last pair have almost no chance of meeting.
So
normally we choose to use excess of one or more reactants to
. make full use of a valuable reactant
. ensure that certain byproducts are not produced
. minimise contamination of the product
. maintain a useful production rate
. probably more but that will do for now
so there are options for the nitric acid production that range from
excess dilute sulphuric acid to produce bisulphate solution plus dilute or azeotropic nitric acid
or
conc. sulphuric plus nitrate to produce RFNA and bisulphate
to
use the bisulphate to make more nitric acid in-situ.
So, you will always have imperfect stoichiometry
but you can to a large extent choose the type and degree of imperfection.
EDIT: See the next post
Sorry about the brain fart, az. nitric acid 68% not 20%
[Edited on 17-4-2025 by Sulaiman]
CAUTION : Hobby Chemist, not Professional or even Amateur
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woelen
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Azeotropic HNO3 is not 20% by weight, but 68% or so by weight.
Adding a little water to the mix indeed works fine, it also makes the procedure somewhat easier (less caking, more liquid). But don't add too much
water, because in that case, you initially first only get very dilute acid and then the azeotrope.
Recently, I also did the distillation and my acid is much more concentrated than azeotropic (it is yellow, gives off fumes and above the acid there is
a brown color of NO2). I only have 30 ml or so of this acid, and for most experiments, I simply dilute this with a small amount of water.
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Rhombo
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Fantastic. Thank you for a great explanation.
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