Just as a warning, this experiment deals with highly corrosive and dangerous chemicals.
This experiment must only be carried out in a fume hood or outdoors.
Because toxic nitrogen dioxide gases are produced and nitric acid is quite
volatile and is a very strong oxidizer. Proper safety gear is absolutely
required. Commercially available nitric acid is usually around 68%
whereas fuming nitric acid is greater than 86% nitric acid.
The uses of fuming nitric acid are rather limited compared to the conventional
68% nitric acid.
One interesting use is it used as an oxidizer in liquid-fueled rockets.
However in organic chemistry fuming nitric acid has the main use of
producing highly nitrated compounds. In the production of many high explosives
such as TNT, which stands for trinitrotoluene. The toluene molecule must be nitrated
three times.
This is very difficult to do with the more dilute nitric acid and the highly
concentrated fuming nitric acid is required to get that third nitration
onto the toluene molecule. The same concept applies to the production of
many other high explosives. For this experiment you only need two chemicals
really, 110 grams of potassium nitrate and 60 milliliters of
concentrated sulfuric acid. The potassium nitrate can be swapped out for another
nitrate salts such as sodium nitrate.
However you should not use ammonium nitrate because one of the products
produced will be ammonia gas which can neutralize the nitric acid that you produce.
First, add about 110 grams of potassium nitrate to a round
bottom flask.
Next, add 60 milliliters of concentrated sulfuric acid to the potassium nitrate
in the round bottom. And finally add some boiling stones to prevent bumping.
This is just to show you what the entire distillation apparatus looked like.
The distillation should not be done in direct sunlight and you can cover it
with aluminum foil if you like to prevent the breakdown of some of the gases.
I leave the round bottom uncovered and I actually take off the aluminum foil
later on
for the sake of the video. You can put the receiving flask in an ice bath
to keep the volatility of the distilled nitric acid as low as possible, but this
is optional. In the round bottom flask the boiling sulfuric acid reacts with
the potassium nitrate according to the reaction shown above. The KHS04 remains
as a solid salt in the flask
where is the HNO3 can be distilled off. It is very important to keep the
temperature at around 83 degrees Celsius, which is the point that nitric acid boils at.
When it first starts coming over it might have a brown color like this.
The brown gas is nitrogen dioxide gas and it taints the distillate a little, so the
distillate comes over as a yellow color. Good control of the temperature and
keeping the temperature at or below 83 degrees Celsius will limit the amount of
nitrogen dioxide gas that is produced. The nitrogen dioxide gas is mostly
formed by decomposition of the nitric acid due to excessive heating.
After distilling for a while you might notice that your distillate clears up and becomes
much more clear and is no longer tainted by a yellow color. It is fine if it
doesn't clear up but it likely means that your temperature is too high.
Soon the flask will look like it doesn't have much liquid left in it and most of
the sulfuric acid has been react with the nitrate salt. At this point you
should remove the flask from heat and let it cool to room temperature before
disposing of it.
The final product is about 30 milliliters of highly-concentrated
yellow nitric acid. In the distillation flask you are left with a big clump of
hard sulfate salt.
This highly concentrated nitric acid is referred to as fuming nitric acid.
Is called fuming nitric acid because at concentrations above
86% nitric acid it fumes when you shoot air into it.
To test the nitric acid
I pour it on a couple of copper pennies. Nitric acid is one of the very few acids
that will actually dissolve copper.
However when the nitric acid is added to the copper you'll notice that there
isn't a vigorous reaction occurring. When copper reacts with highly concentrated
nitric acid it forms a protective copper oxide layer which prevents
nitric acid from reacting with any more of the copper.
This is called passivation. When a small amount of water is added to reduce the
concentration of nitric acid, the passivating effect disappears.
When the concentration of nitric acid is lowered, it quickly and vigorously reacts with the copper.
Next is a demonstration on what fuming nitric acid does to typical lab gloves.
First is about two milliliters a fuming nitric acid on nitrile gloves.
And surprisingly the nitrile gloves quickly burst into flames.
Adding two milliliters to now latex gloves has a similar result.
This is a good demonstration to show that lab gloves are not perfect and they
are susceptible to chemicals. When dealing with fuming nitric acid if all
you have is latex gloves or nitrile gloves,
it is recommended to not wear any gloves at all. From what I've read the only
gloves that will protect you from a spill of fuming nitric acid are butyl
rubber gloves.
This is just a comparison to show you what regular lab grade concentrated
nitric acid does to a latex glove.
Nothing appears to happen and there's no vigorous reaction or fire,
however the nitric acid is capable of penetrating through the latex glove.
Even though nothing visible is happening if you were to get nitric acid on your
hands and you are wearing a latex glove,
it's quite possible that you would still suffer from nitric acid burns.
Latex gloves offer little to no protection against nitric acid.