Brönsted, Johannes Nicolaus (1879-1947) was a Danish physical chemist whose work in solution chemistry, particularly electrolytes, resulted in a new theory of acids and bases. Brönsted was born on 22 February 1879 in Varde, Jutland, the son of a civil engineer. He was educated at local schools before going to study chemical engineering at the Technical Institute of the University of Copenhagen in 1897. He graduated two years later and then turned to chemistry, in which he qualified in 1902. After a short time in industry, he was appointed an assistant in the university's chemical laboratory in 1905, becoming Professor of Physical and Inorganic Chemistry in 1908. In his later years he turned to politics, being elected to the Danish parliament in 1947. He died on 17 December in that year, before he could take his seat. Brönsted's early work was wide ranging, particularly in the fields of electrochemistry, the measurement of hydrogen ion concentrations, amphoteric electrolytes, and the behaviour of indicators. He discovered a method of eliminating potentials in the measurement of hydrogen ion concentrations, and devised a simple equation that connects the activity and osmotic coefficients of an electrolyte, and another that relates activity coefficients to reaction velocities. From the absorption spectra of chromic - chromium (III) - salts he concluded that strong electrolytes are completely dissociated, and that the changes of molecular conductivity and freezing point that accompany changes in concentration are caused by the electrical forces between ions in solution. He related the stages of ionization of polybasic acids to their molecular structure, and the specific heat capacities of steam and carbon dioxide to their band spectra. In 1912 he published work with Herman Nernst on the specific heat capacities of steam and carbon dioxide at high temperatures. Two years later he laid the foundations of the theory of the infra- red spectra of polyatomic molecules by introducing the so-called valency force-field. Brönsted also applied the newly developed quantum theory of specific heat capacities to gases, and published papers about the factors that determine the pH and fertility of soils. In 1887 Svante Arrhenius had proposed a theory of acidity that explained its nature on an atomic level. He defined an acid as a compound that could generate hydrogen ions in aqueous solution, and an alkali as a compound that could generate hydroxyl ions. A strong acid is completely ionized (dissociated) and produces many hydrogen ions, whereas a weak acid is only partly dissociated and produces few hydrogen ions. Conductivity measurements confirm the theory, as long as the solutions are not too concentrated. In 1923 Brönsted published (simultaneously with Thomas Lowry in Britain) a new theory of acidity which has certain important advantages over that of Arrhenius. Brönsted defined an acid as a proton donor and a base as a proton acceptor. The definition applies to all solvents, not just water. It also explains the different behaviour of pure acids and acids in solution. Pure dry liquid sulphuric acid or acetic (ethanoic) acid does not change the colour of indicators nor react with carbonates or metals. But as soon as water is added, all of these reactions occur. In Brönsted's scheme, every acid is related to a conjugate base, and every base to a conjugate acid. When hydrogen chloride dissolves in water, for example, a reaction takes place and an equilibrium is established: HCl + H(subscript2)O ‹-› H(subscript3)O(to the power of +) + Cl(to the power of -) (Acid 1 + Base 2 ‹-› Acid 2 + Base 1) HCl is an acid for the forward reaction, but the hydroxonium ion (H(subscript3)O(to the power of +)) is an acid in the reverse reaction; it is the conjugate acid (acid 2) of water (base 2). Similarly, the chloride ion (Cl(to the power of -), base 1) accepts protons in the reverse reaction to form its conjugate acid (HCl, acid 1). In this theory acids are not confined to neutral species or positive ions. For example, the negatively charged hydrogen sulphate ion can behave as an acid: HSO(to the power of -)(subscript4)(aq) + H(subscript2)O(1) ‹- › H(subscript3)O(to the power of +) + SO(to the power of 2)(subscript4)(to the power of -)(aq) It donates a proton to form the hydroxonium ion. Author not available, Br?ed, Johannes Nicolaus (1879-1947). , The Hutchinson Dictionary of Scientific Biography, 01-01-1998. |