Søren Peter Lauritz Sørensen (9 January 1868 – 12 February 1939) was a Danish chemist, famous for the introduction of the concept of pH, a scale for measuring acidity and alkalinity. Personal life Sørensen was born in Havrebjerg, Denmark in 1868 as the son of a farmer. He began his studies at the University of Copenhagen at the age of 18. He wanted to make a career in medicine, but under the influence of chemist Sophus Mads Jørgensen decided to change to chemistry. While studying for his doctorate he worked as assistant in chemistry at the laboratory of the Technical University of Denmark, assisted in a geological survey of Denmark, and also worked as a consultant for the Royal Navy Dockyard. Sørensen was married twice. His second wife was Margrethe Høyrup Sørensen, who collaborated with him in his studies. Work From 1901 to 1938, Sørensen was head of the prestigious Carlsberg Laboratory, Copenhagen. While working at the Carlsberg Laboratory he studied the effect of ion concentration on proteins and, because the concentration of hydrogen ions was particularly important, he introduced the pH-scale as a simple way of expressing it in 1909. The article in which he introduced the scale (using the notation ) was published in French and Danish as well as in German described two methods for measuring acidity which Sørensen and his students had refined. The first method was based on electrodes, whereas the second involved comparing the colours of samples and a preselected set of indicators. (Sørensen, 1909). From p. 134: "Die Größe der Wasserstoffionenkonzentration … und die Bezeichnung für den numerischen Wert des Exponent dieser Potenz benütze." (The magnitude of the hydrogen ion concentration is accordingly expressed by the normality factor of the solution concerned, based on the hydrogen ions, and this factor is written in the form of a negative power of 10. By the way, as I refer [to it] in a following section (see p. 159), I just want to point out here that I use the name "hydrogen ion exponent" and the notation for the numerical value of the exponent of this power.) From pp. 159–160: "Für die Zahl p schlage ich den Namen "Wasserstoffionenexponent" … Normalitätsfaktors der Lösung verstanden." (For the number p I suggest the name "hydrogen ion exponent" and the notation . By the hydrogen ion exponent () of a solution is thus understood the Briggsian logarithm of the