The simple pH scale with a far-reaching impact

S. P. L. Sørensen in the chemical department of the Carlsberg Research Laboratory, where he developed the methods that led to the introduction of the pH scale. Photo: The Carlsberg Museum. Scan from Holter and Møller (eds.): The Carlsberg Laboratory (1976)

From school chemistry class, many will recall that the acidity of a liquid is measured on the pH scale and that it can be tested using a strip of paper that changes colour to indicate the amount of acid or alkali. What fewer know is that it was a Dane who invented the scale and introduced the term pH.

With the pH scale, the chemist Søren Peter Lauritz Sørensen (1868–1939) gave the world a new language for understanding acids and alkalis. As head of the chemical section of the Carlsberg Research Laboratory from 1901 to 1938, his work involved both the chemical aspects of brewing and basic research topics. His studies of proteins and enzymes showed that their activity was extremely sensitive to even minor variations in acidity. However, there was no instrument for measuring the level with a high degree of precision.

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This chapter is an excerpt from the book 'From Yeast to Universe – Danish Basic Research Supported by the Carlsberg Foundation: 150 Highlights from 150 Years', published by Strandberg Publishing to mark the Carlsberg Foundation’s 150th anniversary. The book offers a kaleidoscopic insight into 150 examples of significant and memorable Danish basic research activities supported by the Carlsberg Foundation over a century and a half. The 150 examples have been selected by 25 Danish researchers.

This inspired Sørensen to undertake a series of painstaking experiments measuring hydrogen concentrations, developing buffer solutions and testing the use of indicators, such as litmus and methyl red. Step by step, he arrived at the principles that laid the foundation for the pH scale. The humble pH number is based on an impressive amount of electrochemical measurements, tests, failures, adjustments and scientific patience.

Sørensen’s work was based on rapid developments in the field of chemistry. Danish chemistry research was experiencing a scientific golden age, with chemists such as Niels Bjerrum (1879–1958), Johannes Nicolaus Brønsted (1879–1947) and Sørensen as leading figures. Danish chemistry laboratories were internationally acclaimed and contributing with theories and methods that set new standards for chemistry as a modern science.

Sørensen’s work was based on the Swedish chemist Svante Arrhenius’s (1859–1927) theory on electrolytic dissociation, presented during the late 19th century. Arrhenius proposed that acids are chemical compounds that release hydrogen ions in water, while alkalis release hydroxide ions. This was the first basic explanation of what happens in an acidic or alkali solution and why they can eat into materials or burn your skin.

However, something was missing. While chemists knew what acids and alkalis do, they had no way to measure acidity precisely. The chemists spoke of ‘strong’ or ‘weak’ acids, but there was no standardised quantifiable scale. Sørensen changed this when he introduced the pH scale: a single number, typically in between 0 and 14, with 7 being neutral, lower numbers indicating acidity and higher numbers indicating an alkali substance. Now it was possible to compare measurements across laboratories, countries and disciplines.

Sørensen also developed buffer solutions: liquids that maintain an almost stable pH level even if small amounts of acid or alkali are added. His chemical composition of these solutions, based on phosphates, glycine and boric acid, allowed researchers to work under stable and controlled pH conditions and is still in use in research today.

Grant

Grant years: 1901–1938 (first and latest) Purpose: Laboratory research 1901–1938 (annual support for the Carlsberg Research Laboratory)

For Carlsberg as a brewery, the pH scale had a big impact. High-precision pH measurements enabled the brewers to control the acidity of the mash and fermentation – thus avoiding mistakes that influenced the conversion of starch to sugar – the fermentation process and the subsequent generation of alcohol. This helped improve the taste, clarity and shelf life of the beer, and Sørensen’s method soon became a regular element of the brewery’s quality assurance protocol.

Today, most people first encounter the pH scale at school, when a small strip of indicator paper changes colour in a liquid. However, this simple demonstration is based on one of Danish chemistry’s most significant contributions: a universal tool that laboratories, breweries, doctors, farmers, researchers and many others still use every day. The pH scale was created in the Carlsberg Research Laboratory and ultimately became one of the most widespread chemical measurement methods in the world.

The chapter is written by Kristian Sjøgren.