How to Make Sodium Nitrate Crystals


Sodium nitrate crystals form from a solution of sodium nitrate dissolved in water.
Sodium nitrate crystals form from a solution of sodium nitrate dissolved in water. (photo: Vadim Sedov)

It’s easy to make sodium nitrate crystals. Sodium nitrate (NaNO3) is also known as Chilean saltpeter, to distinguish it from potassium nitrate or saltpeter. It’s a safe chemical to work with, as sodium nitrate is used as a food additive. It’s also used in fertilizers, smoke bombs, and as a rocket propellant. Here are instructions for making sodium nitrate crystals.

Materials

  • Sodium nitrate
  • Distilled water

You can buy sodium nitrate from Amazon or other websites. It may be sold as a pure chemical or as Chilean saltpeter. Further down, I have instructions for how to make it yourself, if you wish.

The solubility of sodium nitrate, like other salts, greatly depends on temperature:

  • 73 g/100 mL (0 °C)
  • 91.2 g/100 mL (25 °C)
  • 180 g/100 mL (100 °C)

But, it isn’t as easy to grow the crystals using temperature to control crystallization as many other recipes because dissolving sodium nitrate in water is endothermic. When you stir the salt into hot water, it will lower the temperature.

Grow Crystals

Sodium nitrate crystals grow Hopper (step-ladder) faces in the trigonal system.
Sodium nitrate crystals grow Hopper (step-ladder) faces in the trigonal system. (Al`fred Gabdullin, VKontakte)

You can grow crystals directly from a solution of sodium nitrate in water or you can grow a single large crystal from a seed crystal.

  1. Dissolve 110 grams sodium nitrate per 100 ml hot water. This will be a saturated solution. Remember, the water cools as the sodium nitrate dissolves. Heat the solution if all of the sodium nitrate doesn’t dissolve. One method of growing crystals is to allow this solution to sit in an undisturbed location and allow it to produce crystals as the liquid evaporates. You can place a paper towel or coffee filter over the container to keep the liquid clean.

Growing a crystal from a seed crystal involves making a solution, pouring off a bit of it, and then resuming growth in a supersaturated solution:

  1. Prepare the aforementioned solution and let it reach room temperature.
  2. Place a few grains of sodium nitrate in a shallow dish and pour a small amount of liquid over the. As the liquid evaporates, small seed crystals will form. Select a crystal or two for further growth.
  3. To prepare the supersaturated growing solution, to your existing solution add 3 grams of sodium nitrate per 100 ml of water in the original solution. So, if you prepared 300 ml of solution, you would add an extra 9 grams of sodium nitrate.
  4. Carefully add your seed crystal to this liquid. You can suspend the crystal from a nylon monofilament. A nylon monofilament or wire is used because it won’t wick up the solution.
  5. Seal the jar and allow the crystals to grow at a constant temperature, someplace they won’t be disturbed. Sodium nitrate is very sensitive to temperature changes, so maintaining a constant temperature is important. If you have difficulty maintaining a temperature, you can place the sealed jar inside a water bath. If you don’t see crystal growth after a few days, try lowering the temperature slightly.

Sodium Nitrate Crystal Properties

Sodium nitrate crystals display birefringence. (Stanislav Popov, VKontakte)

Sodium nitrate forms clear crystals that display the optical phenomenon of birefringence, which is also seen in calcite crystals. The crystals show double refraction–a property easily observed by setting the crystal on a book page. Sodium nitrate crystallizes in the trigonal or rhombohedral shape, depending on conditions. Crystals typically resemble squished boxes with Hopper faces. The Hopper shape is also seen in bismuth crystals. Other properties displayed by sodium nitrate crystals are cleavage and glide.

How to Make Sodium Nitrate

Many reactions produce sodium nitrate.

The reaction between ammonium nitrate and sodium chloride yields a crystalline precipitate perfect for crystal-growing:

NH4NO3 + NaCl → NaNO3 + NH4Cl

To get 100 grams sodium nitrate, mix 68.76 grams sodium chloride and 94.17 grams hot ammonium nitrate solution. Sodium nitrate crystals fall out of solution upon cooling. Filter the solid (the sodium nitrate) and rinse with either very cold water or else acetone.

It’s also easy to make by reacting nitric acid with sodium bicarbonate (washing soda), sodium bicarbonate (baking soda), or sodium hydroxide (lye):

Na2CO3 + 2HNO3 → 2NaNO3 + 2H2O + CO2
HNO3 + NaHCO3 → NaNO3 + H2O + CO2
NaOH + HNO3 → NaNO3 + H2O

Be aware, the reaction between sodium hydroxide and nitric acid is highly exothermic, so that reaction should only be performed over an ice bath!

Another method reacting ammonium nitrate with either sodium hydroxide or sodium bicarbonate:

NH4NO3 + NaOH → NaNO3 + H2O + NH3
NH4NO3 + Na2CO3 → 2NaNO3 + 2H2O + 2NH3 + CO2

To make 100 grams of sodium nitrate, mix 94.17 grams ammonium nitrate with either 47.06 grams of sodium hydroxide or 62.35 grams sodium bicarbonate. Heat the mixture in a fume hood, because it releases ammonia gas.

How to Get Dry Crystals of Sodium Nitrate

Sodium nitrate does not form a hydrate, so once crystals form, they are easy to keep as mineral specimens. When you remove a crystal from water, you can set it on a paper towel to dry. Alternatively, you can rinse the crystal with acetone. Sodium nitrate is insoluble in acetone.

Similarly, use evaporation and rinsing with acetone to dry sodium nitrate prepared from a chemical reaction. The easiest way to dry the crystals is using vacuum filtration, washed with acetone.

References

  • Born, M.; Wolf, E. (2002). Principles of Optics (7th ed.). Cambridge University Press.
  • PubChem (2020). “Sodium nitrate (Compound).” National Library of Medicine.
  • Shu, Yidan; Li, Yang., et al. (2018). “A multi-component mass transfer rate based model for simulation of non-equilibrium crystal growth.” Computer Aided Chemical Engineering 44, 142901434. doi:10.1016/B978-0-444-64241-7.50233-0
  • Zumdahl, Steven S. (2009). Chemical Principles (6th ed.). Houghton Mifflin Company. ISBN 978-0-618-94690-7.

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