A century ago, chemist Søren Sørensen invented what would become a crucial diagnostic tool: the pH scale.

Since its founding in 1876 by beer magnate J.C. Jacobsen, the Carlsberg Laboratory in Copenhagen has been a center of biochemical discovery. At the turn of the twentieth century, its scientists synthesized several of the amino acids essential to human health and analyzed the chemistry of proteins. And exactly 100 years ago, Søren Sørensen, Carlsberg’s director of chemistry, devised a vital diagnostic tool for measuring acidity, thus advancing the detection of digestive, respiratory and metabolic disorders.

Sørensen’s invention was the pH scale. (The term pH refers simply to “the power of hydrogen,” and it gave utility to nineteenth-century findings that acids release hydrogen ions when combined with water, whereas alkaline compounds combine with the hydrogen ions in water.) During the 1890s, the Latvian chemist Wilhelm Ostwald and others had invented electrical conductivity equipment that could measure the quantity of hydrogen ions in a solution, but Sørensen succeeded in expressing those measurements in an elegant formula and placing the results on a simple scale. The solutions he tested received pH values running from 0 (the most acidic) to 14 (the most alkaline).

Sørensen found that enzymes that hasten biochemical reactions work well in certain pH environments and poorly in others—pepsin, an ingredient of gastric juice, loves acid, but lipase, found in the pancreas, requires alkalinity—so aberrant pH levels of bodily fluids can signify health problems. Human blood, for instance, normally tests within a narrow range of pH 7.35 to 7.45, near the scale’s neutral midpoint of 7. Higher or lower blood pH values (indicating alkalosis or acidosis) can help diagnose metabolic and respiratory problems. Acidosis points to pulmonary malfunctions, kidney failure or an inability to excrete acids; and alkalosis can signal hyperventilation, dehydration or liver failure, among other problems.

Because of the seriousness of these health threats, pH measurement has become routine in blood analysis. Urine is also commonly analyzed for pH to help detect such problems as diabetes (high acidity) and urinary tract infections and blockages (high alkalinity).

Though Sørensen won a number of awards and honors for his work, the Nobel Prize eluded him. Perhaps he found succor in drafts of beer, of which he was reportedly a skilled brewer. No question he knew its pH: 4.5.