Rochelle salt crystals are easy to grow crystals that display piezoelectricity and ferroelectricity. The clear crystals are non-toxic, as Rochelle salt is an antioxidant food additive (E337) that contributes a salty, cooling flavor to food. Here are simple instructions for growing the crystals and a look at how to observe their piezoelectricity.
Materials
You only need two ingredients to grow Rochelle salt crystals:
- Rochelle salt
- Water
The key ingredient is Rochelle salt (KNaC4H4O6∙4H2O). Rochelle salt goes by several other names: sodium potassium L(+)-tartrate tetrahydrate, potassium sodium tetrahydrate, E337, and Seignette’s salt. French apothecary Pierre Seignette, of La Rochelle, France, was the first to prepare the chemical. It’s used in Biuret’s reagent and Fehling’s solution, so if you work in a lab, you may have access to it. Otherwise, you can get it online or you can make it from baking soda (sodium bicarbonate) or washing soda (sodium carbonate) and cream of tartar (potassium bitartrate).
- To make it, first turn baking soda into washing soda. Spread out some baking soda on a cookie sheet or pan and bake it in the oven for half an hour. Ordinary baking temperature is fine.
- Heat 1 cup (about 200 ml) of water to boiling.
- Stir in cream of tartar (about 4 oz or 145 g).
- Slowly add the washing soda from the first step. Bubbles released by each addition indicate the two chemicals are reacting. Continue adding washing soda until it doesn’t produce bubbles. There may be some undissolved material in the bottom of the container.
- Run the liquid through a coffee filter to separate the solids from the liquid.
- Place the filtered liquid in an undisturbed location and watch the crystals grow!
Alternatively, start with Rochelle salt from a chemical supply house or online retailer. It does not need to be reagent grade, but you do want clear crystal or a white powder that is free of any other chemicals. Make sure the chemical name includes “tetrahydrate,” as sodium potassium tartrate lacking the water in its formula won’t work.
Grow Rochelle Salt Crystals
Basically, growing Rochelle salt crystals is just like growing table salt crystals.
- Stir Rochelle salt into boiling water until no more dissolves. Add Rochelle salt until a bit of undissolved material remains at the bottom of the container. This makes a saturated solution.
- Filter out the undissolved material by pouring the liquid through a coffee filter or filter paper.
- Set the container of liquid in a place where it won’t be disturbed or exposed to extreme temperature changes.
- Crystals form as the solution cools and evaporates. Slow cooling and evaporation yields large crystals while fast cooling and evaporation quickly produces tiny crystals.
The most common problem is that the Rochelle salt solution isn’t concentrated. If you don’t see crystals after a few days, refrigerate the container (because Rochelle salt is more soluble at a higher temperature) or increase water evaporation. You can increase the evaporation rate by blowing a fan in the room.
For a large single crystal:
- Remove a small crystal from the container. This is a seed crystal.
- Tie a bit of nylon fishing line around the crystal.
- Pour the liquid the crystal grew in into a new clean container.
- Hang the seed crystal in this liquid. Make sure it isn’t touching the sides or bottom of the container.
- Remove any other crystals that grow in the container because they will compete with your seed crystal.
- When you are satisfied with the crystal, remove it from the liquid and place it on a paper towel to dry.
If you want a very large crystal, you’ll need to add more saturated Rochelle salt solution from time to time. Be careful that the fresh solution actually is saturated, or else it will dissolve your seed crystal. Test it by adding an unwanted crystal or small amount of undissolved Rochelle salt. If the crystal/undissolved matter remain after a few minutes, it’s safe to add the solution. But, if Rochelle salt dissolves in the liquid, don’t use it until enough water evaporates so that the liquid passes the test.
Keeping Rochelle Salt Crystals
Rochelle salt crystals are quite beautiful! They are stable, but you do need to take care to protect them from moisture and heat. The crystals decompose if the temperature exceeds 55° C (131° F). You’ll want to keep the crystal surfaces exposed for piezoelectricity experiments, but if you just want to keep a crystal for your collection or use in decorative projects, you can seal it with resin or clear nail polish. Don’t seal it with a product that contains water or else the exposed part of the crystal will dissolve.
Rochelle Salt Crystal Piezoelectricity
Rochelle salt crystals are piezoelectric. What this means is that they generate electricity in response to mechanical stress, such as pressure or twisting. This is how Rochelle salt crystals work in microphones, phonograph pick-ups, and earpieces. Other piezoelectric materials include table sugar (sucrose) and quartz.
You can easily demonstrate Rochelle salt piezoelectricity using everyday materials. Connect two small pieces of aluminum foil to two wires. Connect the wires to an LED or other object that requires a small amount of electricity to work. Now, either wrap the two pieces of foil around the Rochelle salt crystal (but don’t let the foil pieces touch each other) or set the crystal on one piece of foil and place the other piece on top of it. Gently press on the crystal to induce piezoelectricity. You do not need to press hard.
Are you interested in a more advanced piezoelectricity project using Rochelle salt crystals? You can make a homemade speaker:
References
- Brewster, David (1824). “Observations of the pyro-electricity of minerals”. The Edinburgh Journal of Science. 1: 208–215.
- Kassaian, Jean-Maurice (2007). “Tartaric Acid.” Ullmann’s Encyclopedia of Industrial Chemistry (7th ed.). Wiley. doi:10.1002/14356007.a26_163
- Newnham, R.E.; Cross, L. Eric (November 2005). “Ferroelectricity: The Foundation of a Field from Form to Function”. MRS Bulletin. 30: 845–846. doi:10.1557/mrs2005.272