Claude Louis Berthollet (9 December 1748 – 6 November 1822) was a Savoyard-French chemist who became vice president of the French Senate in 1804.
Claude Louis Berthollet was born in Talloires, near Annecy, then part of the Duchy of Savoy, in 1749.
Berthollet, along with Antoine Lavoisier and others, devised a chemical nomenclature, or a system of names, which serves as the basis of the modern system of naming chemical compounds. He also carried out research into dyes and bleaches (introducing the use of chlorine as a bleach) and determined the composition of ammonia. Berthollet was one of the first chemists to recognize the characteristics of a reverse reaction, and hence, chemical equilibrium. Potassium chlorate (KClO3), a strong oxidizer, is known as Berthollet's Salt.
Berthollet was engaged in a long-term battle with another French chemist Joseph Proust on the validity of the law of definite proportions. While Proust believed that chemical compounds are composed of a fixed ratio of their constituent elements irrespective of the methods of production, Berthollet believed that this ratio can change according to the ratio of the reactants initially taken. Although Proust proved his theory by accurate measurements, his theory was not immediately accepted partially due to Berthollet's authority. His law was finally accepted when Berzelius confirmed it in 1811. But it was found later that Berthollet was not completely wrong because there exists a class of compounds that do not obey the law of definite proportions. These non-stoichiometric compounds are also named berthollides in his honor.
Berthollet was one of several scientists who went with Napoleon to Egypt, and was a member of the physics and natural history section of the Institut d'Égypte.
In April, 1789 he was elected a Fellow of the Royal Society of London.  In 1801, he was elected a foreign member of the Royal Swedish Academy of Sciences.
He died in Arcueil, France in 1822.
Satish, Kapoor (1970–80). "Berthollet, Claude Louis". Dictionary of Scientific Biography. 2. New York: Charles Scribner's Sons. pp. 73–82. ISBN 0684101149.
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