Glauber's nitric acid synthesis

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Main article: Nitric acid

Glauber's nitric acid synthesis is a classic reaction discovered back in the days of alchemy. This is how concentrated nitric acid ("aqua fortis") was first prepared. Today it is still a mainstay of amateur chemists.

The reaction

Reacting sulfuric acid with a nitrate, such as potassium nitrate yields nitric acid:

H2SO4 + KNO3 → KHSO4 + HNO3

It is simple as that. It is not recommended to use excess KNO3, because bisulfate is a more useful side product than sulfate: it can be used to prepare oleum. Sodium nitrate (the saltpeter of Chile) can also be used instead of potassium nitrate. When this reaction was important industrially it was taken to a mix of mostly bisulfate with only a little neutral sulfate, which could be poured molten. Taking it further makes better use of the sulfuric acid, but the end of the reaction requires a higher temperature, which decomposes more of the nitric acid produced and the sulfate product needs to be chipped out afterwards.

The setup

Glauber's original setup was a retort. Some amateur chemists, such as the author of this article, still use retorts for this, since for this reaction a retort is somewhat practical and cheaper than a modern ground-glass distillation setup. But if you have such a setup, it is even better.

DO NOT use any rubber parts in your setup! You can use PTFE tape.

The reagents

First, you need a nitrate, or saltpeter. Potassium or sodium nitrates will be just fine. Ammonium nitrate is undesirable; it is possible to prepare nitric acid using it, but it will be of lower grade due to side reactions. Calcium nitrate (the saltpeter of Norway) can be used to make nitric acid, but this salt has its own peculiarities that make the reaction with it different from Glauber's classic synthesis.

Second, you need sulphuric acid, with a concentration no less than 60% (chamber acid) if you want to prepare azeotropic nitric acid, or highly concentrated (95-98%) if you want to prepare red fuming nitric acid (RFNA). If you do not have sulfuric acid of this strength, concentrated phosphoric acid (80-85%) will do in a pinch, this will produce azeotropic nitric acid (but remember that this synthesis involves heating, and hot phosphoric acid slowly attacks glassware).

Step by step guide

  1. Measure the stoichiometric amounts of reagents.
  2. Put the saltpeter in your boiling flask or retort.
  3. Pour the acid into your boiling flask or retort.
  4. Assemble the setup, put the receiving flask on its place.
  5. Start to heat the boiling flask or retort gently.
  6. Soon you will see bubbling in the reacting mixture and brown vapors of nitrogen dioxide above it. If everything is done right, nitrogen dioxide will soon disappear. If it does not want to disappear and emerges from your apparatus, you are probably overheating the boiling flask.
  7. Azeotropic acid should emerge only slightly yellowish, RFNA pronouncedly yellow. If your azeotropic acid comes out too yellow, or your RFNA comes out brown, you are overheating the boiling flask.
  8. After bubbling ceases, turn off the heating, cool down and disassemble the apparatus. Proceed to clean out the "goat" of bisulphate or dihydrophosphate from your boiling flask or retort.

De-NOxing your acid

Yellow azeotropic nitric acid can be turned white, or RFNA converted to WFNA, by using oxygen. Oxygen oxidizes NO2 to N2O5, which combines with the residual water in your acid to produce nearly pure nitric acid.

Oxygen can be introduced with two methods. One is directly bubbling oxygen gas through your acid. This is the preferable method, since it produces no water and increases the concentration and purity of your acid. The other is adding hydrogen peroxide into the acid. A useful de-NOxing agent is urea peroxide, finely ground in a mortar: the peroxide oxidizes the oxides of nitrogen, and the urea forms the insoluble salt urea nitrate, which precipitates.

Vacuum distillation

If you have a vacuum distillation setup, you can produce WFNA (only slightly yellowish or completely colorless) directly from distillation, with no extra steps.


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