Potassium sodium tartrate tetrahydrate, also known as Rochelle salt, is a remarkable double salt of tartaric acid. Its discovery dates back to the 17th century when Pierre Seignette, an apothecary from La Rochelle, France, first prepared it. Since then, this compound has gained recognition for its unique properties and wide range of applications.
A History of Piezoelectricity
Potassium sodium tartrate, along with monopotassium phosphate, was the first material to exhibit piezoelectricity. This fascinating property propelled its extensive use in crystal phonograph cartridges, microphones, and earpieces during the post-World War II consumer electronics boom. These transducers boasted an exceptional high output, with typical pick-up cartridge outputs reaching 2 volts or more. However, Rochelle salt is deliquescent, making the transducers susceptible to deterioration in damp conditions.
Various Applications
Apart from its role in electronics, Rochelle salt has found uses in other fields as well. Medicinally, it has been used as a laxative, while in the process of silvering mirrors, it plays a significant role. Moreover, it is an essential ingredient in Fehling's solution, a reagent used to detect and reduce sugars. In the realm of organic synthesis, it helps break up emulsions during aqueous workups, especially for reactions involving aluminium-based hydride reagents. The food industry also relies on sodium potassium tartrate for its various applications.
Protein Crystallography and Beyond
In protein crystallography, Rochelle salt acts as a common precipitant. It is also a crucial ingredient in the Biuret reagent, utilized to measure protein concentration. This ingredient aids in maintaining cupric ions in solution at an alkaline pH, enabling accurate protein analysis.
Preparation and Growth
To obtain Rochelle salt, the starting material is tartar with a minimum tartaric acid content of 68%. This is dissolved in water or the mother liquor of a previous batch, followed by basification with hot saturated sodium hydroxide solution to pH 8. The solution is then decolorized and chemically purified before being filtered. The filtrate is evaporated and crystallizes on slow cooling. After separation from the mother liquor, washing, drying, and sieving, it is ready for commercial use. Larger crystals have even been grown aboard Skylab under unique conditions of reduced gravity and convection.
Contributions to Science and Audio Technology
Rochelle salt holds a significant place in the history of science and audio technology. In 1824, Sir David Brewster demonstrated the piezoelectric effects of Rochelle salts, leading him to name the phenomena pyroelectricity. In 1919, Alexander McLean Nicolson made notable contributions to audio-related inventions like microphones and speakers at Bell Labs.
Potassium sodium tartrate tetrahydrate, or Rochelle salt, continues to captivate researchers and innovators alike due to its exceptional properties and diverse applications. Its fascinating journey from its discovery in the 17th century to its role in modern technology serves as a testament to the endless possibilities of scientific exploration.
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