Claude Louis Berthollet

Claude Louis Berthollet was a Savoyard-French chemist who served as vice president of the French Senate in 1804. His scientific work advanced the understanding of chemical equilibria through the concept of reverse chemical reactions, and he helped shape modern chemical nomenclature. On the practical side, he showed that chlorine gas could bleach materials and created the first modern bleaching agent—a solution of sodium hypochlorite.

Born in Talloires, near Annecy, in the Duchy of Savoy, Berthollet began his education in Chambéry and later earned a medical degree in Turin. His rapid progress in chemistry led to his election to the Academy of Sciences in 1780. Alongside Antoine Lavoisier and others, he developed a system for naming chemical compounds that underlies today’s nomenclature. He researched dyes and bleaches, introducing chlorine gas as a commercial bleach in 1785. In 1789, at his Paris laboratory on the quay Javel, he produced a weak sodium hypochlorite solution, called “Eau de Javel,” by passing chlorine gas through sodium carbonate. He also first made potassium chlorate, a strong oxidant and bleach now known as Berthollet’s Salt. In 1785, he determined the elemental composition of ammonia. Berthollet was among the first chemists to recognize reverse reactions and chemical equilibrium.

He engaged in a long dispute with Joseph Proust over the law of definite proportions. Proust argued that compounds have fixed elemental ratios regardless of how they are made; Berthollet thought the ratio could vary based on the initial reactants. Proust’s accurate measurements eventually proved his theory, but Berthollet’s authority delayed its acceptance until Jöns Jacob Berzelius confirmed it in 1811. Later, non-stoichiometric compounds—which do not obey the law—were named berthollides in Berthollet’s honor. He also accompanied Napoleon to Egypt as part of the Institut d’Égypte’s physics and natural history section.

Berthollet was elected a Fellow of the Royal Society of London in April 1789, a foreign member of the Royal Swedish Academy of Sciences in 1801, and an associate member of the Royal Institute of the Netherlands in 1809. He became an Honorary Fellow of the Royal Society of Edinburgh in 1820 and a Foreign Honorary Member of the American Academy of Arts and Sciences in 1822. His 1788 publication *Méthode de Nomenclature Chimique*, co-authored with Lavoisier, Guyton de Morveau, and Fourcroy, received a Citation for Chemical Breakthrough Award from the American Chemical Society in 2015. A French high school in Annecy bears his name.

Berthollet married Marie Marguerite Baur in 1788. Their son, Amédée-Barthélémy Berthollet, died in 1811 from carbon monoxide poisoning after a charcoal-burning suicide, during which he recorded his physiological and psychological experiences as a final scientific contribution. Berthollet was accused of being an atheist. He died in Arcueil, France, in 1822.