THE TIME EVOLUTION OF THE HARD AND SOFT ACIDS AND BASES THEORY - Theoretical and Computational Research in the 21st Century - page 4

1880s, leading to a picture where acids and bases release hydrogen and
hydroxide ions, respectively, and their interaction is responsible for the
acid-base reactions. In the twentieth century, the acid and base defi nitions
met considerable conceptual sublimation, paralleling the newly emerging
quantum theory of atoms and molecules. This was achieved in three steps,
however not necessarily chronologically.
It was in the year 1923, within several months of each other, that the
famous Dutch scientist Johannes Nicolaus Brønsted and the British scien-
tist Thomas Martin Lowry published [7(a), (b)] independently the same
theory about the behavior of acids and bases.
Brønsted [7(b)] used the quotation to describe acid and base:
… acids and bases are substances that are capable of splitting off or
taking up hydrogen ions, respectively.
One of the most important contributions of Lowry's theory is that he
put forward the defi nition of acids and bases with the state of the hydrogen
ion in solution, whereas Brønsted used free H + ion. Lowry, in his letter to
the editor used the hydronium ion H 3 O + that is commonly used today. Here
is the quotation of Lowry [7(a)]:
It is a remarkable fact that strong acidity is apparently developed only
in mixtures and never in pure compounds. Even hydrogen chloride only
becomes an acid when mixed with water. This can be explained by the
extreme reluctance of a hydrogen nucleus to lead an isolated existence
…. The effect of mixing hydrogen chloride with water is probably to
provide an acceptor for the hydrogen nucleus so that the ionization of
the acid only involves the transfer of a proton from one octet to another.
As such, the foreground theory belongs to Lowry and Brønsted theory,
[7(a), (b)] which assumes the proton as the particle, never free, which
intermediates between an acid (the donor) and a base (the acceptor) com-
pounds during chemical reactions. Within this framework, the new acid-
base interaction paradigm looks as follows:
acid 1 + base 2 ↔ acid 2 + base 1
But the Lowry and Brønsted theory [7(a), (b)] excessively enhanced the
role of proton; fortunately, due to the Lewis' intuition, the electron pair
was soon recognized as a more general conceptual entity in defining acids,
bases, and their chemical interaction.