It’s easy to make Rochelle salt using two common kitchen ingredients. Rochelle salt is sodium potassium tartrate tetrahydrate or potassium sodium tetrahydrate (KNaC4H4O6·4H2O). Rochelle salt yields large piezoelectric crystals, used for science experiments and as transducers in microphones and gramophone pickups. The chemical is a food additive that imparts a cooling, salty flavor. It’s also an ingredient in chemistry reagents, including Biuret reagent and Fehling’s solution. You can purchase Rochelle salt, but it’s fairly expensive. Why not try making it yourself?
Rochelle Salt Ingredients
You need washing soda (sodium carbonate), cream of tartar (potassium bitartrate or potassium hydrogen tartrate), and water. If you can get washing soda, great! Use it. Most people don’t have ready access to washing soda, but can get baking soda (sodium bicarbonate). All you need to do to convert baking soda into washing soda is apply gentle heat.
- Washing soda or baking soda (1 box, about 500 g)
- Cream of tartar (roughly 4 oz or 145 g)
- Water (preferably distilled)
You’ll need a decent amount of cream of tartar to get enough Rochelle salt to grow crystals. A good ratio is two small containers of cream of tartar to one regular box of baking soda.
Got pure chemicals and a scale? If you start with 500 grams of baking soda or washing soda and 200 grams of cream of tartar, you should collect around 210 grams of Rochelle salt.
To convert baking soda into washing soda, spread baking soda on a baking sheet and heat it in a 275 °F ( 135 °C) for 30 minutes to 1 hour. Neither the temperature nor the time is particularly critical (you can’t “burn” baking soda or washing soda). You’ll know the conversion is complete when the powder stops bubbling/steaming and its texture changes. Collect the dry powder and seal it to prevent moisture from converting the washing soda back into baking soda.
Make Rochelle Salt
- Make a slurry by stirring cream of tartar (two containers, about 4 oz or 145 g) in 200 ml hot water (just below boiling).
- Add the washing soda, one small scoop at a time. Stir after each addition.
- Stop adding washing soda when it no longer causes bubbling. The liquid should appear translucent. It’s okay if you over-shoot slightly and there is a bit of undissolved washing soda in the container.
- Filter out any solids using filter paper or a coffee filter.
- Optional: Pour the liquid through an activated charcoal filter to decolorize it.
- Cover the container with a paper towel to keep out dust, but allow evaporation. Leave the container at room temperature, out of direct sunlight. As the liquid cools, Rochelle salt crystals form. Allow crystallization to proceed until you don’t see further growth. If you’re interested in quickly collecting the chemical rather than growing large crystals, refrigerate the liquid for faster results.
- Collect these crystals. They are the Rochelle salt.
- If desired, perform a recrystallization to further purify the salt. To do this, dissolve the crystals in a small amount of hot, distilled water. Collect the crystals that grow from this solution.
The Chemistry of Making Rochelle Salt
If you start with baking soda, the first step is the conversion to washing soda:
2NaHCO3 → Na2CO3 + CO2 + H2O
The reaction between cream of tartar (sodium bitartrate) and sodium carbonate in water yields Rochelle salt (sodium potassium tartrate tetrahydrate):
KHC4H4O6 + Na2CO3→ C4H4O6KNa·4H2O
Commercial Rochelle Salt Preparation
Commercial Rochelle salt preparation starts with potassium hydrogen tartrate (cream of tartar) containing at least 60% tartaric acid. After dissolving the cream of tartar in water or liquid from a previous bath, hot caustic soda additions adjust the pH to around 8 and also cause a saponification reaction. Passing the solution across activated charcoal decolorizes it. Mechanical filtration and centrifugation remove most remaining impurities. Finally, a furnace drives off water prior to packaging. This last step typically produces sodium potassium tartrate (not the tetrahydrate). So, readers intending to purchase Rochelle salt for crystal growing should check a product’s chemical formula.
Rochelle Salt Facts
- IUPAC Name: Sodium potassium L(+)-tartrate tetrahydrate
- Also Known As: Rochelle salt, Seignette’s salt, E337
- CAS Number: 304-59-6
- Chemical Formula: KNaC4H4O6·4H2O
- Molar Mass: 282.1 g/mol
- Appearance: Colorless, odorless monoclinic needles
- Density: 1.79 g/cm³
- Melting Point: 75 °C (167 °F; 348 K)
- Boiling Point: 220 °C (428 °F; 493 K)
- Solubility: 26 g / 100 mL (0 ℃); 66 g / 100 mL (26 ℃)
- Crystal Structure: Orthorhombic
The Greek philosopher Theophrastus (c. 314 BC) observed tourmaline’s ability to attract sawdust and straw when heated, but he didn’t name the effect. Sir David Brewster used Rochelle salt to demonstrate pyroelectricity and piezoelectricity in 1824.
A pyroelectric material generates a temporary voltage upon heating or cooling. Pyroelectric crystals are naturally electrically polarized. Temperature changes shift atom positions within the crystal, changing polarization and generating voltage. A piezoelectric material generates a voltage under the application of mechanical stress. Piezoelectric solids contain electric dipole moments. Normally, the dipoles orient randomly, but mechanical stress reorients the dipoles. In some materials, the opposite effect occurs, where an applied electrical field mechanically deforms the crystal.
Pierre and Jacques Curie expanded upon Brewster’s experiments. They found that quartz and Rochelle salt display the strongest piezoelectricity, but cane sugar, topaz, and tourmaline also exhibit the effect.
- Brewster, David (1824). “Observations of the pyro-electricity of minerals”. The Edinburgh Journal of Science. 1: 208–215.
- Fieser, L. F.; Fieser, M. (1967). Reagents for Organic Synthesis, Vol.1. Wiley: New York. p. 983.
- Kassaian, Jean-Maurice (2007). “Tartaric Acid.” Ullmann’s Encyclopedia of Industrial Chemistry (7th ed.). Wiley. doi:10.1002/14356007.a26_163
- Lide, David R., ed. (2010). CRC Handbook of Chemistry and Physics (90th ed.). CRC Press, pp. 4–83.
- 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