Ammonium halates -- unstable compounds

Ammonium bromate and ammonium iodate both are unstable and rather energetic compounds, especially the bromate salt is very energetic. They can easily be prepared.

This is a fairly safe experiment, despite the fact that an explosive compound is made. The only important thing is that this experiment may not be scaled up. It is also important not to store ammonium bromate.

Required chemicals:

• iodic acid

• ammonia (12% or better,  it is important that the ammonia is clean and not full of detergents or other additives)

• sodium bromate (potassium bromate is not suitable)

• ammonium nitrate

Required equipment:

• petri dish
• test tube
• small burner, used for heating a test tube

Safety:

• Ammonia at a concentration of 12% is quite noxious. It has a strong pungent smell. Use adequate ventilation.
• Iodic acid is a strong and corrosive oxidizer, avoid contact with the skin.
• Sodium bromate is a strong oxidizer. Most likely, sodium bromate is a carcinogen as well, so avoid exposure to this chemical!

Disposal:

• Let the test tubes with the decomposed material cool down and then rinse them with a dilute solution of sodium hydroxide, don't use ammonia for cleaning them! The waste may be flushed down the drain.

Preparation of ammonium bromate

Ammonium bromate can be prepared from sodium bromate and ammonium nitrate. Both solids dissolve in water very well, while ammonium bromate only dissolves moderately well. In a two-step crystallize/recrystallize process one can obtain ammonium bromate of good purity.

Unfortunately this process only works with sodium bromate. Potassium bromate does not dissolve very well in water and hence it is not suitable for making ammonium bromate.

Take 1 ml of water and dissolve a gram or so of ammonium nitrate in the water, such that a concentrated solution is obtained.

Take another 1 ml of water and dissolve a spatula full of sodium bromate in the water (approximately 300 mg).

Mix the two solutions. If a white solid is formed, then heat the liquid, until the liquid is clear again.

Let the liquid cool down. After a few hours there will be a thick layer of crystalline solid at the bottom. This is impure ammonium bromate.

Decant the clear liquid above the solid and put the solid on a piece of folded paper tissue. This can be done by tapping the test tube on the paper tissue, until all of the solid is on the paper tissue. Due to capillary effects, the paper absorbs most of the liquid, which still sticks to the solid.

Carefully scrape the solid from the paper tissue, assuring that no paper is scraped off. Put the solid in a test tube and add half a ml of water. Heat the liquid, until most of the solid has dissolved. Add water drop by drop, in the meantime keeping the liquid hot, until all of the solid has dissolved.

Let the liquid cool down again. After a few hours again a crop of crystals will have formed at the bottom. This is almost pure ammonium bromate.

Again decant the liquid and put the solid on a piece of paper tissue. Once most of the liquid is absorbed by the paper, carefully scrape the solid from the paper and put it on a petri dish and spread a little bit. Put the solid in a dry warm place (e.g. on a heating radiator). After a few hours a perfectly dry white crystalline solid is obtained. The yield will be a few hundreds of mg of solid.

Scrape the solid from the glass and put it in a small tightly closed container for further experimenting. According to some literature it may decompose spontaneously, so do not keep this solid around for a long time, use all of it in subsequent experiments within one day of preparation and in the meantime store the small vial with the ammonium bromate in a glass jar with a loosely tightened metal screw cap, just to be sure that in the case of accidental premature decomposition nothing serious can happen.

Heating of the solid ammonium bromate

Put a small amount of the ammonium bromate in a test tube and heat the test tube above the flame of an alcohol burner or a small bunsen burner. After a few seconds a fairly loud pop/whoop sound is produced, a brief flash of orange light is produced and then the solid has decomposed, one of the decomposition products being bromine. It is not a true explosion, but it is close to it. Most remarkable is that the entire test tube is filled with the light of an orange flash for a short time (at most 30 ms). Below, 4 pictures show the sequence of events, just before the decomposition, the flash of orange light and just after the decomposition. Time between two pictures is 33 ms. It also is nice to see that immediately after the flash of light the resulting bromine is higher up in the test tube and more dilute than 33 ms later. This is due to the heat, in which the gas mix expands and which quickly contracts on cooling down.

The same experiment was repeated in a dimly lit room, just to get a better impression of the flash of light. The result of this experiment is shown in the four pictures below, which again were taken at a frame rate of 30 frames per second.

The four pictures nicely show that the entire test tube is filled with a fairly bright orange light. Most remarkable is that the light is spread throughout the entire test tube.

Another observation made in the experiments with ammonium bromate is that no really high temperature is needed for ignition of the solid. Just a few seconds after the flame is put below the test tube, the solid decomposes and only a few seconds after the decomposition one can touch the test tube again without getting burnt skin. The precise ignition temperature was not measured, but it is expected not to be far above 100 °C.

Immediately after the decomposition, one can also observe a very thin white smoke above the test tube, and the smell of bromine can be observed. The white smoke most likely is ammonium bromide, possibly contaminated with some unreacted ammonium bromate.

Preparation of ammonium iodate

Preparation of ammonium iodate is very simple, when one has access to iodic acid. One simply has to add excess ammonia to a solution of iodic acid.

Dissolve a spatula full of iodic acid in half a ml of water.

Add one ml of 12% clean ammonia. Assure that excess ammonia is added. After mixing the liquid should still have a smell of ammonia.

When the ammonia is added, then a very fine crystalline precipitate is formed. For convenience, it is best to add the liquids to each other in a petri dish and not in a test tube. The precipitate has a tendency to stick to the glass and this is rather annoying, when this is inside a test tube. Mix the liquids with each other with a small plastic spatula (don't use metal) or a glass rod.

After mixing, spread the liquid with the precipitate over the petri dish and put the petri dish in a dry and warm place (e.g. on a heat radiator at 50 ºC or so).

After one day, the material is dry and has no smell of ammonia anymore.

Heating of the solid ammonium iodate

Put a small amount of the ammonium iodate in a test tube and heat the test tube above the flame of an alcohol burner or a small bunsen burner. For a while, nothing seems to happen, but at a certain moment a strong hissing noise is produced and the solid decomposes. Black/grey crystals of iodine are deposited on the cooler parts of the test tube and purple vapor of iodine can be observed.

The decomposition of ammonium iodate is not as violent as the decomposition of ammonium bromate, but still it is quite violent. No light is produced in the decomposition reaction and it also takes longer before the reaction sets off. The temperature of ignition is higher. The four pictures below show the onset of decomposition and the total decomposition. Complete decomposition is reached in 60 ms or so.

No gas, smoke or vapor escapes the test tube, all of the iodine settles at the glass as a black/grey crystalline solid.

Videos of decomposition reactions

Videos with audio were made of both decomposition reactions. The bromate reaction is filmed in diffuse daylight and in a dimly lit room, the iodate reaction is filmed in daylight. All three videos can be downloaded here:

• decomposition of ammonium bromate, filmed in daylight
• decomposition of ammonium bromate, filmed in dimly lit room
• decomposition of ammonium iodate, filmed in daylight

Download sizes are approximately 500 kByte, 850 kByte and 400 kByte.

Discussion of results

Although ammonium ion is very reluctant to oxidation in aqueous solution, it is more easily oxidized in the solid state at elevated temperature. Both ammonium bromate and ammonium iodate easily exhibit an internal oxidation/reduction reaction. They have their own fuel and own oxidizer in one compound. The ammonium ion serves as fuel, the bromate (iodate) ion serves as oxidizer.

When the solids decompose, the following main reactions occurs (without doubt there will be numerous side reactions):

   2NH₄BrO₃(s) → N₂(g) + 4H₂O(g) + Br₂(g) + O₂(g)

   2NH₄IO₃(s) → N₂(g) + 4H₂O(g) + I₂(s, g) + O₂(g)

Especially in the experiment with the ammonium bromate, one can also nicely see that water is formed. After the reaction, the inside of the test tube is wet. This cannot easily be seen on the pictures and videos, but if the experiment is done and the test tube is inspected after the reaction, then one can nicely see that it is wet inside.

One possible side reaction is the reaction in which halide ion is formed. This most likely explains the presence of a thin white smoke in the decomposition of ammonium bromate. The bromide ion can form NH₄Br which appears as thin white smoke. This side reaction, however, does not occur at a large extent. The white smoke is very thin (hardly visible) and there is a lot of bromine vapor in the test tube.

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