How to Make Rochelle Salt From Kitchen Ingredients

Rochelle salt or potassium sodium tartrate is an interesting chemical that is used to grow large single crystals, which are attractive and interesting, but also may be used as transducers in microphones and gramophone pickups. The chemical is used as a food additive to contribute a salty, cooling taste. It is an ingredient in useful chemistry reagents, such as Fehling's solution and Biuret reagent. Unless you work in a lab, you probably don't have this chemical lying around, but you can make it yourself in your own kitchen.

Rochelle Salt Ingredients

• Cream of Tartar
• Washing Soda or Sodium Carbonate (which you can get by heating baking soda or sodium bicarbonate in a 275°F oven for an hour)

Instructions

1. Heat a mixture of about 80 grams cream of tartar in 100 milliliters of water to a boil in a saucepan.
2. Slowly stir in sodium carbonate. The solution will bubble after each addition. Continue adding sodium carbonate until no more bubbles form.
3. Chill this solution in the refrigerator. Crystalline Rochelle salt will form on the bottom of the pan.
4. Remove the Rochelle salt. If you redissolve it in a small amount of clean water, you can use this material to grow single crystals. The key to growing Rochelle salt crystals is to use the minimum amount of water needed to dissolve the solid. Use boiling water to increase the solubility of the salt. You may wish to use a seed crystal to stimulate growth on a single crystal rather than throughout the container.

Commercial Preparation of Rochelle Salt

Commercial preparation of Rochelle salt is similar to how it's made at home or in a small lab, but pH is carefully controlled and impurities are removed to ensure product purity. The process starts with potassium hydrogen tartrate (cream of tartar) that has a tartaric acid content of at least 68 percent. The solid is either dissolved in liquid from a previous batch or in water. Hot caustic soda is introduced to attain a pH value of 8, which also causes a saponification reaction. The resulting solution is decolorized using activated charcoal. Purification involves mechanical filtration and centrifugation. The salt is heated in a furnace to drive off any water before being packaged.

Persons interested in preparing their own Rochelle salt and using it for crystal growth may wish to adopt some of the purification methods used in commercial production. This is because cream of tartar sold as a kitchen ingredient may contain other compounds (e.g., to prevent caking). Passing the liquid through a filter medium, such as filter paper or even a coffee filter, should remove most of the impurities and allow for good crystal growth.

Rochelle Salt Chemical Data

• 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

Rochelle Salt and Piezoelectricity

Sir David Brewster demonstrated piezoelectricity using Rochelle salt in 1824. He named the effect pyroelectricity. Pyroelectricity is a property of some crystals characterized by natural electrical polarization. In other words, a pyroelectric material can generate a temporary voltage when heated or cooled. While Brewster named the effect, it was first referenced by the Greek philosopher Theophrastus (c. 314 BC) in reference to the ability of tourmaline to attract straw or sawdust when heated.

Sources

• 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