Johannes Nicolaus Brønsted born in Varde (February 22, 1879 - December 17, 1947) was a Danish physical chemist.
He received a degree in chemical engineering in 1899 and his Ph. D. in 1908 from the University of Copenhagen. He was immediately appointed professor of inorganic and physical chemistry at Copenhagen.
In 1906 he published his first of many papers on electron affinity. In 1923 he introduced the protonic theory of acid-base reactions, simultaneously with the English chemist Thomas Martin Lowry. The same year, the electronic theory was proposed by Gilbert N. Lewis, but both theories are commonly used.
He became known as an authority on catalysis by acids and bases. He has the Brønsted catalysis equation named after him. He also came up with the highly used theory of the proton donor along with Lowry. Brønsted theorised that as a hydrogen atom (always found in an acid) is ionised once dissolved in water, it loses its electron and becomes a proton donor. The hydroxide ion, which occurs when an alkali is formed when a substance is dissolved in water is called a proton receiver. This leads to a neutralization reaction where the ions combine creating hydrogen hydroxide, otherwise known as water. The pH scale may be interpreted as "power of hydrogen", and the definition is based on the work of Brønsted and Lowry.
In World War II he opposed the Nazis, and in consequence he was elected to the Danish parliament in 1947