Nearly 110 years ago, while running experiments with beer at the world-renowned Carlsberg research lab in Copenhagen, Danish chemist Søren Peter Lauritz Sørensen developed the simple yet enduring pH scale, which measures whether a substance is acidic or basic. Sørensen’s landmark invention is celebrated in Tuesday’s interactive Google Doodle, which lets you sort sour and bitter foods on different sides of the pH scale to find out how it works.
Many of us already have an intuitive grasp of which side of the scale tomatoes or broccoli fall on thanks to our own built-in pH tester, our tongues. Slightly bitter-tasting foods like leafy greens and legumes have a pH higher than 7, marking them as alkaline, or basic. Sour foods like lemons have a pH lower than 7, making them acidic. Pure water, which is neutral, sits right at 7.
Born to a farming family on January 9, 1868, in a tiny town near the coast of Denmark, Sørensen studied science and started his early career consulting for the Danish navy. He earned his doctorate for his research on cobalt oxalates, complex inorganic structures that have applications in nanotechnology.
At the age of 33, he was appointed as the head of chemistry at the Carlsberg Laboratory in Copenhagen, an institution founded to answer this question: How do you brew the best beer of the highest quality?
The laboratory was already famous for being the first place to cultivate pure yeast and for developing the Kjeldahl method, a technique for measuring the nitrogen content in food and beverages that’s still in use today.
Sørensen soon added more jewels to the lab’s crown. He was primarily studying fermentation, as one does when one works at a lab supported by a brewing company. In particular, he studied the formation of amino acids, the building blocks of proteins.
He also studied the enzymes made from proteins and quickly realized that hydrogen ion concentrations were important to how to these enzymes performed their functions. He developed the pH scale as a way to keep track of these conditions in a solution.
But what does the scale actually measure?
The term pH means “potential of hydrogen,” and the scale is the negative base 10 logarithm of the concentration of positively charged hydrogen in a solution. Let’s break that down: The concentration of hydrogen ions, a.k.a. protons, in a liquid determines how acidic or basic it is, but this amount can vary drastically, which is why scientists use a logarithmic scale, where each unit changes by a factor of 10. And since the scale is negative, the smaller the number, the more concentrated the protons.
That means that a substance with a pH of 4 is 10 times more acidic than one with a pH of 5 and 100 times more acidic than a pH of 6.
This scale, which runs from 0 to 14, takes a complicated chemical phenomenon and distills it into an easy-to-grasp metric.
It’s now used widely throughout the sciences in applications ranging from designing batteries to diagnosing blood disorders to measuring humanity’s impact on the ocean, and remains Sørensen’s most famous accomplishment.
The Carlsberg laboratory was Sørensen’s scientific base for the rest of his life. His accomplishments earned him memberships in scientific societies around the world, and his colleagues remembered him as a genial educator. “He was kindly, courteous, ever-willing to listen to those who had not his fund of knowledge and always ready and glad to impart something from his vast store of learning,” wrote A.J. Curtin Cosbie in the journal Nature in an obituary for Sørensen, who died February 12, 1939.
Cosbie also wrote, “Sørensen’s classic work on hydrogen ion concentration will remain as a permanent monument among those who know little of his other work.”
So raise a glass of your favorite drink to toast Sørensen, and perhaps check its pH before you take a sip.