Potassium sodium tartrate tetrahydrate, also known as Rochelle salt, is a double salt of tartaric acid first prepared (in about 1675) by an apothecary, , of La Rochelle, France. Potassium sodium tartrate and monopotassium phosphate were some of the early materials discovered to exhibit piezoelectricity. This property led to its extensive use in crystal phonograph cartridges, microphones and earpieces during the post-World War II consumer electronics boom of the mid-20th century. Such transducers had an exceptionally high output with typical pick-up cartridge outputs as much as 2 volts or more. Rochelle salt is deliquescent so any transducers based on the material deteriorated if stored in damp conditions.
It has been used medicinally as a laxative. It has also been used in the process of silvering mirrors. It is an ingredient of Fehling's solution (reagent for reducing sugars). It is used in electroplating, in electronics and piezoelectricity, and as a combustion accelerator in cigarette paper (similar to an oxidizer in pyrotechnics). Sodium potassium tartrate is also important in the food industry.
It is a common precipitant in protein crystallography and is also an ingredient in the Biuret reagent which is used to measure protein concentration. This ingredient maintains cupric ions in solution at an alkaline pH.
Preparation
thumb|left|Large Rochelle salt crystal grown aboard [[Skylab]]
Larger crystals of Rochelle salt have been grown under conditions of reduced gravity and convection on board Skylab.
Rochelle salt crystals will begin to dehydrate when the relative humidity drops to about 30% and will begin to dissolve at relative humidities above 84%.
Piezoelectricity
In 1824, Sir David Brewster demonstrated piezoelectric effects using Rochelle salts, which led to him naming the effect pyroelectricity.
In 1919, Alexander McLean Nicolson worked with Rochelle salt, developing audio-related inventions like microphones and speakers at Bell Labs.
Current applications
Rochelle salt-based composites have gained renewed interest for their applications in impact energy absorption and smart sensing technologies.
Recent research has demonstrated the growth of Rochelle salt crystals within 3D-printed cuttlebone-inspired structures, resulting in multifunctional composites that combine mechanical robustness with piezoelectric properties. The chambered microstructure inspired by cuttlefish bone provides high stiffness and energy absorption capacity, making these composites suitable for protective equipment and structural health monitoring.
The developed composites exhibit remarkable mechanical performance, with enhanced fracture toughness and resistance to impact. Under cyclic loading, they maintain consistent piezoelectric output for up to 7000 cycles. Impact tests show voltage outputs peaking at approximately 8 V, and a piezoelectric coefficient (d33) around 30 pC/N.