Søren Sørensen. Courtesy Oesper Collection, University of Cincinnati.
Industry laboratories, while intended to advance practical industrial processes, can also be the birthplace of theoretical concepts. Such was the case with the concept of pH, which was introduced in 1909 by the Danish chemist Søren Sørensen (1868–1939) as a convenient way of expressing acidity—the negative logarithm of hydrogen ion concentration.
A Ph.D. from the University of Copenhagen, Sørensen was the director of the chemical department of the Carlsberg Laboratory, which was supported by the beer company of the same name, brewing being one of the oldest chemical industries. At the time, he was working on the effect of ion concentration in the analysis of proteins. Sørensen subsequently became a leader in the application of thermodynamics to protein chemistry, and in this work he was assisted by his wife, Margrethe Høyrup Sørensen.
The context for the introduction of pH was the slow changeover from the old color-change tests for indicating the degree of acidity or basicity to electrical methods. In the latter, the current generated in an electrochemical cell by ions migrating to oppositely charged electrodes was measured, using a highly sensitive (and delicate) galvanometer. Until Sørensen developed the pH scale, there was no widely accepted way of expressing hydrogen ion concentrations.