Fireworks season is coming up, so before I get into the new fireworks projects, I wanted to cover the synthesis of a common chemical used for pyrotechnics: ammonium nitrate. Another fun project to try with ammonium nitrate is to make an endothermic reaction. You can buy ammonium nitrate as a pure chemical or you can collect it from instant cold packs or some fertilizers. You can make ammonium nitrate by reacting nitric acid with ammonia, but if you don't have access to nitric acid (or don't want to mess with it), you can make ammonium nitrate from readily available home chemicals.
You will need:
- 138 g sodium bisulfate (found with pool chemicals, used to lower pH)
- 1 mole equivalent of a nitrate salt… any of the following
- 85 g sodium nitrate (common food preservative)
- 101 g potassium nitrate (which you can buy or make yourself)
- 118 g calcium nitrate (tetrahydrate)
- ammonia (common household cleaner)
- methanol (optional, which may be found as HEET fuel treatment)
- Dissolve the sodium bisulfate in the mininum amount of water (about 300 ml).
- Dissolve your nitrate salt in the minimum amount of water (amount depends on the salt).
- Mix the two solutions.
- Next you want to neutralize the solution, which is quite acidic. Stir in ammonia until the pH of the mixture is 7 or higher. Use a pH meter (or pH paper). Reacting ammonia, sodium bisulfate, and nitrates will give you sodium sulfate and ammonium nitrate.
- Sodium sulfate and ammonium nitrate have different solubilities in water, so boil the solution to get the sodium sulfate to crystallize. Remove the liquid from heat when crystals of sodium sulfate form in the bottom of the pan.
- Chill the solution in the freezer to get as much of the sodium sulfate as possible to drop out of the solution.
- Run the solution through a filter (coffee filter or paper towels) to separate the solid sodium sulfate from the ammonium nitrate solution.
- Allow the ammonium nitrate solution to evaporate, which will give you ammonium nitrate, with some sodium sulfate impurity. This is 'good enough' for most chemistry projects.
- If you want to further purify the ammonium nitrate, dissolve it in about 500 ml of methanol. The ammonium nitrate is soluble in methanol, while the sodium sulfate is not.
- Run the solution through a filter, which will give you sodium sulfate on the filter and a solution of ammonium nitrate.
- Allow the methanol to evaporate from the solution to obtain crystalline ammonium nitrate.
The chemicals used in this project are smelly and corrosive, so this project should be performed under a fume hood or outdoors. As always, wear gloves, eye protection, and appropriate clothing. Some of the reagents and the final product are flammable or are oxidizers, so keep the chemicals away from open flames.