How to prepare potassium nitrate from salt substitute and cold compresses

Potassium nitrate is a salt that has many uses, especially for home experiments. It is one of the main components of black powder, and is also frequently used in various types of pyrotechnics to produce light purple or lilac flames.

Due to the very fact that it is an ingredient in gunpowder and other explosives, potassium nitrate is difficult to obtain in some countries. In fact, in some countries its sale to the general public is prohibited. For this reason, in this article we show how to obtain this substance from homemade materials that are easy to obtain. At the same time, we will take the opportunity to highlight some important chemical concepts related to chemical reactions, solubility, and stoichiometry.

What chemical reaction occurs?

The reaction that occurs in this experiment is a double displacement or double decomposition reaction between ammonium nitrate and potassium chloride to give potassium nitrate and ammonium chloride. The chemical equation is presented below:

In this reaction, potassium displaces ammonium, while ammonium displaces potassium. The reaction products are all initially in the aqueous state, that is, dissolved in water. However, upon cooling the solution, the solubility of potassium nitrate decreases abruptly, so it precipitates, while the others remain in solution.

At 0°C, the solubility of potassium nitrate is 13.3 g per 100 g of water, while that of ammonium chloride is 29.4 g per 100 g of water. For this reason, potassium nitrate begins to precipitate before ammonium chloride, and the former can be obtained with acceptable purity when the solution is cooled. However, some ammonium chloride may end up precipitating along with potassium nitrate at low temperatures, so a purification process must be carried out on potassium nitrate to improve its quality and make it useful for various experiments.

Reagents and Materials Needed

reagents

The present experiment requires the following starting reagents:

• 150 g of salt substitute or sodium-free salt. This is a salt generally prescribed to hypertensive patients who cannot consume sodium in the diet. It is actually potassium chloride (KCl).
• 2 cold compresses. Most contain ammonium nitrate (NH ₄ NO ₃ ), others contain calcium ammonium nitrate (CAN). Note should be taken of which of the two compounds it contains since the amounts vary depending on the case.
• Pure water. Filtered water can be used, but for best results, it is preferable to use battery water (it is deionized or ultrapure water).

materials and instruments

• Scissors.
• Plastic container of approx. 1L capacity.
• Microwave oven.
• Thermal kitchen gloves.
• 4 glass jars of 500 mL or 1 L (very clean).
• 2 large spoons.
• Kitchen scale, preferably a digital pastry scale that has an appreciation of 0.1g.
• From 2 to 4 coffee filters.
• 2 leagues or rubber bands.

experimental procedure

Part 1: Synthesis of potassium nitrate

To synthesize potassium nitrate, the following steps must be followed:

1. Using scissors, cut the cold packs and extract their content by adding it to a plastic container.
2. Place one of the glass jars on the balance and tare (reset to zero).
3. Weigh into the bottle 108 g of ammonium nitrate (or 290 g if it is CAN).
4. In a second flask weigh 200 g of pure water. To do this, place the empty bottle on the balance, tare (reset to zero), and then slowly add water to 200 g. This amount doesn’t need to be exact, you can go by a couple of grams and it won’t appreciably affect the result.
5. Mix the heavy ammonium nitrate into the water by stirring constantly with a spoon. The container will cool down considerably because the dissolution reaction is strongly endothermic.
6. Heat this solution slightly in the microwave oven. The solution will likely appear cloudy and there may be a fine powdery residue at the bottom of the bottle. This is an anti-caking agent usually made from silica and is an inert substance, but we must filter it out.
7. Place one or two coffee filters in the third jar attached with the rubber band and filter the hot ammonium nitrate solution. Once the filtering is finished, discard the filters and the solid they contain (it can be thrown away without problems).
8. In the fourth bottle, weigh 100 g potassium chloride (salt substitute) following the same procedure as before (place on balance, tare, then weigh).
9. Transfer the 100 g of freshly weighed KCl to the flask containing the filtered solution and stir vigorously.
10. Microwave the solution to or just below boiling.
11. While heating, clean the first jar and fit a coffee filter in the mouth with the rubber band.
12. Once hot, using oven mitts to avoid burning yourself, filter the hot solution to remove as much turbidity as possible and any undissolved KCl crystals.
13. Let the solution cool until lukewarm, then put it in the freezer and let it sit until it is almost freezing (the freezing point of this solution is much lower than that of pure water, so it will not freeze in a refrigerator domestic).
14. On cooling, a good deal of white, needle-like crystals of potassium nitrate should have formed. The next step is to filter the product, this time keeping the solid and discarding the solution.
15. Let the crystals dry on absorbent paper or on a tray in the air. Once dry, weigh the product.

Part 2: Purification of potassium nitrate

At this point we already have potassium nitrate. However, the ammonium chloride is heavily contaminated, especially with the second reaction product.

If all went well and there were no significant losses during the experiment, we should have about 120 g of impure potassium nitrate of which about 13 g are impurities. This should be able to dissolve completely in 100 g of boiling water, since its solubility of this compound at 100 °C is 118.3 g/100 g H 2 _O. Once dissolved, recrystallization can be carried out to obtain the pure product.

For the recrystallization we will need 2 very clean jars (it does not matter that they are not dry) and two additional coffee filters.

The procedure is the next:

1. 100 g of water are weighed into a flask and heated to a boil in the microwave.
2. Meanwhile, all of the newly synthesized impure potassium nitrate is transferred to the first flask. If you want, you can weigh the solid to determine the recrystallization yield, but it is not necessary.
3. Add the hot water to the solid potassium nitrate and stir until dissolved. It is important that the solution is not cooled, nor is the first flask in which the water was heated. If necessary, heat everything again just before filtering.
4. Place the coffee filter in the empty container secured with the rubber band and quickly filter the hot solution. A completely colorless and transparent solution, free of turbidity, should be obtained.
5. Allow the potassium nitrate solution to cool down as slowly as possible. The slower it cools, the better the recrystallization process will be, resulting in larger and purer crystals. To make the process even slower, the jar can be placed in a water bath with boiling water and removed from the burner. The larger amount of water will slow down the cooling.
6. After a couple of hours, put the bottle in the refrigerator to reduce the temperature even more and thus obtain a greater quantity of crystals.
7. Finally, using another coffee filter, filter the cold solution and recover the crystalline solid. It can be left on a tray covered with absorbent paper for a few hours to allow it to air dry, and when dry to the touch the tray can be placed in an oven at around 50-60°C for a further hour or two to complete the drying process.
8. Finally, we weigh our final product and bottle it in a glass jar with an airtight seal.

After this recrystallization, the vast majority of the ammonium chloride and any other impurities should have been removed. The solid before recrystallization contained approximately 13 g of ammonium chloride as impurities. Since the solid dissolved in 100 g of water and the solubility of ammonium chloride at 0°C is 29 g per 100 g of water, those 13 grams of impurity will remain dissolved while the potassium nitrate crystallizes.

Thanks to the above, a very pure and high quality potassium nitrate can be obtained for any experiment that is desired to be carried out.

References

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ChemEurope. (nd). potassium_chloride . Little lights. https://www.chemeurope.com/en/encyclopedia/Potassium_chloride.html

Dr Lithium. (2010, January 4). How To: Make potassium nitrate at home . WonderHowTo. https://science.wonderhowto.com/how-to/make-potassium-nitrate-home-293015/

The effect of temperature on solubility – Solubility – GCSE Chemistry (Single Science) Revision – WJEC . (nd). BBC Bitesize. https://www.bbc.co.uk/bitesize/guides/z4s48mn/revision/2

Gomez Garcia, MN, Dobrosz-Gomez, I., & Ojeda Toro, JC (2020). Potassium Nitrate Solubility in (Water + Ethanol) Mixed Solvents at Different Temperatures and Hydrochloric Acid Concentrations. Experimental Study and Modeling Using the Extended UNIQUAC Model. Journal of Chemical & Engineering Data , 65 (2), 567–576. https://pubs.acs.org/doi/10.1021/acs.jced.9b00753

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The Canadian Chemist. (2016, July 26). How to make Potassium nitrate (household materials) [Video]. Youtube. https://www.youtube.com/watch?v=NPdeSNyguQ0