To make potassium nitrate, dissolve 40 grams of ammonium nitrate in 100 milliliters of water, add 37 grams of potassium chloride and gently heat the solution. Pour the solution through a coffee filter, and then freeze it until crystals of potassium nitrate form. Pour off any remaining ammonium chloride liquid and dry the crystals. Grocery stores sell potassium chloride as sodium-free salt, and ammonium nitrate is found in instant cold packs, which can typically be purchased at a drugstore.
A solution of ammonium nitrate and potassium chloride undergoes a chemical reaction. The two chemicals exchange ions to form potassium nitrate and ammonium chloride. Since ammonium chloride is much more water soluble than potassium nitrate, the potassium nitrate crystallizes as it cools and falls out of solution.
Potassium nitrate is commonly known as saltpeter. In the Middle Ages, people used the substance to preserve meat. West Africans use the substance as a thickening agent in dishes such as okra soup and to reduce cooking time when boiling beans and tough meat. Manufacturers use potassium nitrate to make fertilizers, gunpowder and fireworks. Scientists and students also use the chemical for experiments.
Bat guano deposits, which people collected from caves, were once the main source of potassium nitrate. Urine or dung may also be converted to potassium nitrate by filtering it through potash.