Potassium sodium tartrate tetrahydrate, also known as Rochelle salt, is a double salt of tartaric acid first prepared by Pierre Seignette, of La Rochelle, France. Potassium sodium tartrate and monopotassium phosphate were the first materials discovered to exhibit piezoelectricity. Formula: KNaC4H4O6·4H2O Molar mass: 282.1 g/mol Melting point: 167°F (75°C) Solubility in water: 26 g / 100 mL (0 °C); 66 g / 100 mL (26 °C) Appearance: large colorless monoclinic needles Boiling point: 220 °C (428 °F; 493 K) anhydrous at 130 °C; decomposes at 220 °C