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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.
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.
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.
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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.
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 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:
Download sizes are approximately 500 kByte, 850 kByte and 400 kByte. |
Discussion of results
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.
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