S. P. L. Sørensen

Søren Peter Lauritz Sørensen was born on 9 January 1868 in Denmark. A chemist by training, he introduced the concept of pH while working to quantify acidity and alkalinity — a measurement problem that had bedeviled labs for decades. The scale he devised became the universal standard, compact enough to fit on a test strip and precise enough to matter in blood, soil, and fermentation tanks. He died on 12 February 1939, seventy-one years old, having left chemistry a tool it uses thousands of times a day.