Concept of pH, introduced in 1909 by Søren Sørensen, who was the head of Carlsberg Laboratory, as a convenient way of expressing acidity and the negative logarithm of hydrogen ion concentration.
Sørensen (1868–1939), who held a Ph.D. from the University of Copenhagen, directed the chemical department of the Carlsberg Laboratory, which was supported by the beer company of the same name, brewing is 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 the concept 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 technique, 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.