S.P.L. Sørensen, the pH concept and its early history

Although the two-letter symbol pH is extremely common in chemistry and elsewhere, its origin and early dissemination has only received scant attention among chemists and historians of science. Introduced as a convenient symbol for ‘hydrogen ion exponent’ by the Danish biochemist S.P.L. Sørensen in 1909, after a decade or two pH won broad acceptance in the fields of physiology, biochemistry, medical research, and industrial chemistry in particular. Apart from detailing how pH and related concepts were initially received, this paper examines the language and nomenclature associated with the pH scale until about 1930. What is today written as pH was in the past symbolized in a variety of other ways. Although Sørensen never became a Nobel laureate, he was nominated many times and the evaluations of the Nobel committees throw further light on how his innovation was conceived by contemporary chemists and physiologists. The paper also discusses, if in less detail, how the original definition pH = - log[ H⁺ ] pH = - log H + was revised in the early 1920s into the currently accepted form involving hydrogen ion activities rather than concentrations. The paper is essentially but not strictly limited to the period from about 1905 to the mid-1930s.