Production of Fuming Nitric Acid

Introduction

Nitric Acid is an extremely useful chemical and one of the three most used mineral acids in any common lab (the others being Sulfuric Acid and Hydrochloric Acid). It can be used to produce nitrate salts and to nitrate organic compounds. It has some nice properties which are illustrated in another post.

It is usually available as a 68% solution (azeotropic concentration). Such solutions are usually referred to as Azeotropic Nitric Acid. Solutions of Nitric Acid with concentrations greater than 86% are referred to as Fuming Nitric Acid. The name comes from the fact that said solutions fumigate when an air stream is applied.

There are also two other common designations for specific Nitric Acid solutions. WFNA (White Fuming Nitric Acid) is Nitric Acid that has 2% or less of water and less than 0.5% of Nitrogen Oxides dissolved. RFNA (Red Fuming Nitric Acid) is Nitric Acid that has 2% or less of Water and about 13% Dinitrogen Tetroxide dissolved.

In this procedure, Nitric Acid will be produced with a water content below 2%. However, the final product will have Nitrogen Oxides dissolved. It can’t be classified as WFNA (because of the Nitrogen Oxides dissolved) nor as RFNA (because the procedure isn’t adapted to produce a specific percentage of Nitrogen Oxides). As such, the Nitric Acid produced by this process will be referred to as Fuming Nitric Acid. It can be easily converted to WFNA, which will be further explained.

Reagents

• Sodium Nitrate (Potassium Nitrate can also be used);
• Sulfuric Acid 98%.

Dangers

This procedure is dangerous. Fuming Nitric Acid is very corrosive and is a powerful oxidizer. It is infamous by its capability of setting latex gloves on fire. It is also permeable in most types of gloves. It releases toxic and corrosive Nitrogen Oxide gases. It can react violently with a lot of organic matter, specially solvents. A big spill of Fuming Nitric Acid may end monstrously.

This reaction should be done in a Fume Hood or in a very well ventilated area. Protective googles should be used. Gloves are NOT recommended.

• Sodium Nitrate doesn’t pose any significant health hazard.
• Sulfuric Acid is highly corrosive. Very hot Sulfuric Acid is going to be handled which is even more corrosive than the cold version.

Procedure

Start by weighting 85 grams of Sodium Nitrate and by measuring 70 mL of Sulfuric Acid (98%).

Mount a three-neck flask. A two-neck flask can also be used.

Prepare the thermometer adapter.

Warning: Regular thermometer adapters should not be used because Fuming Nitric Acid easily attacks rubber and most types of plastic. However, PTFE (Teflon) can be used. I prepare my thermometer by using the glass body of a regular adapter. However, instead of using the o-ring and the plastic cap, I use PTFE. I wrap it around a section of the thermometer until it makes a good seal with the glass body, as seen in the image above.

Prepare and insulate (from light) a distillation apparatus.

Note: Insulating the apparatus isn’t quite mandatory, it’s actually a bit optional. This will be explained further in the theory section.

Prepare the thermometer adapter in the flask and add a curve (where the distillation apparatus is going to connect). The Sodium Nitrate can already be charged into the reactor.

Here is how it’s supposed to look:

Add the Sulfuric Acid to the reactor.

Add a glass stopper to stop the third neck of the flask and finish insulating everything.

Start heating. The heating process is pretty standard. The initial heating should be done slowly. I’ve found that maintaining a mixture temperature of about 110ºC makes the distillation somewhat faster than keeping a lower temperature and doesn’t visibly affect yield.

Once the temperature reaches 80ºC the first bubbles appear and the reactor starts changing color.

Throughout the distillation the color of the flask should be paid attention to. It is a better indicator than the thermometer to know if the reactor is being properly heated. If it is being properly heated, the color in the reactor should be more yellowish than brownish. Here is how it looks:

If it starts looking like the following photo, temperature should be decreased a little. It doesn’t affect yield that much though.

And here is what happens if the temperature is too high. This is to be avoided as it affects the final yield.

Throughout the distillation the receiving flask should look like this:

Once the distillation rate drops and the temperature stars rising too much the distillation is stopped. The end point isn’t always evident. Another way of detecting the end point is when around 40 mL of Nitric Acid have distilled over.

Here is the final product:

My final yield was 38 mL of Fuming Nitric Acid.

Disposal and Clean Up

Disassembling the apparatus may be problematic. There will be leftovers of Nitric Acid everywhere within it. The method recommended is to disassemble it piece by piece.

Prepare a sink and get the water running. Remove a piece (with a flask underneath to catch any acid that drips) and immediately take it to the sink and flood it with water.

In the end the flask will have a gunk of Sulfuric Acid soaked in Sodium Bisulfate.

This can easily be washed. Water should be added (very slowly) to the flask. Don’t freak about breaking the precious add-acid-to-water rule.

If everything is done with caution, spills can easily be avoided. And they should. My only spill (a single drop) did a number to my hot plate:

Theory

Sulfuric Acid reacts with Sodium Nitrate to produce Nitric Acid and Sodium Bisulfate (Sodium Hydrogen Sulfate), as follows:

   (1)

Sodium Bisulfate can also react with Sodium Nitrate. This reaction produces Sodium Sulfate and more Nitric Acid:

   (2)

Nitric Acid, especially when concentrated, decomposes (very slowly):

   (3)

Stoichiometry

Sulfuric Acid – 98,08 g/mol – 100,1 g/mol (98%) – 54,4 mL (98%)

Sodium Nitrate – 84,99 g/mol

Sodium Bisulfate – 120,06 g/mol

Nitric Acid – 63,01 g/mol

Reaction (3) rate is increased by exposure to light or by heating Nitric Acid past its boiling point (83 ºC).

If reactions (1) and (2) are combined, one would expect that 2 mol of Nitric Acid are produced from 1 mol of Sulfuric Acid. However, reaction (2) is to be avoided. Bellow are two of the main reasons reaction (2) is avoided:

• Reaction (2) tends to occur at temperatures well above 110ºC. At those temperatures Nitric Acid decomposition rate increases a lot, affecting yield;
• Reaction (1) is the reaction between a liquid and a solid while reaction (2) is the reaction between two solids. As such, formation of hotspots in reaction (1) is not problematic while it is in reaction (2). It is also easier to smoothly heat a liquid mixture than a solid mixture.

Taking into consideration that reaction (2) needs to be avoided it is concluded that 1 mol of Sulfuric Acid and 1 mol of Sodium Nitrate should be used. There is, what so ever, one final consideration: there should be an excess of Sulfuric Acid. There are two main reasons for this:

• By using excess Sulfuric Acid all Sodium Nitrate gets used in the reaction and none of it is wasted;
• Towards the end of the reaction, there will be a lot of Sodium Bisulfate (solid) and less Sulfuric Acid (liquid). If an excess of Sulfuric Acid isn’t used, near the end, the reaction mixture won’t be submerged in liquid anymore and formation of hotspots becomes problematic, affecting the yield. As already said, it is also easier to smoothly heat a liquid mixture than a solid mixture.

After everything is considered, the best quantities to use are 70 mL of Sulfuric Acid and 85 g of Sodium Nitrate. These quantities can easily be scaled up or down.

The reason why the apparatus needs to be protected from exposure to light should be clear by now. Light catalyzes reaction (3), especially in the vapor phase. I’ve found that, however, the yield doesn’t significantly decrease if the apparatus isn’t insulated.

Insulating the reactor, though, is a good ideas as it allows for even heating and more temperature control, decreasing chances of hotspot formation.

My final yield was 38 mL (57 grams). Titration indicated a 97% (m/m) concentration. The theoretical yield is 63 grams (100% Nitric Acid). This accounts for a 88% yield. Most losses occurred on the clean up, as some Nitric Acid was still trapped in the joints.

Finale Notes

This is an easy to perform reaction that produces a very useful and interesting chemical. A post where some of its uses are shown can be found here.

The product obtained here is Fuming Nitric Acid. It can be easily converted to WFNA by simply bubbling oxygen through it. This will be covered in a future post. It can also be easily converted to regular Nitric Acid by dilution.