1909

pH and Indicators

Svante Arrhenius (1859–1927), Søren Peder Lauritz Sørensen (1868–1939), Hans Friedenthal (1870–1943), Pál Szily (1878–1945)

Swedish scientist Svante Arrhenius helped establish, in 1887, that acidic and basic solutions could be defined as having an excess or shortage of hydrogen ions, respectively. Water, he proposed, had a very small amount of H+ (hydrogen ion) and OH− (hydroxyl ion) in it, and disturbing this equilibrium was what made an acid or a base. In 1909, Danish chemist Søren Sørensen, acting on a suggestion by German scientist Hans Friedenthal, invented what we now know as the pH scale, which ranges from 0 to 14, from extremely acidic to extremely basic.

Mathematically, pH is the logarithm of 1 over the hydrogen ion activity in the solution. Plain water is pH 7 (neutral), and human blood is pH 7.4. (Anyone who tries to sell you on making your blood more acidic or alkaline to treat disease is peddling nonsense, by the way—the body works hard to keep your blood’s pH right at 7.4.) Many chemicals will change color depending on whether they’re in an acidic or basic solution. Their two ionized forms—meaning whether they’ve received or given up a hydrogen ion—are different enough to absorb different colors of light, and various compounds will cross from acidic to basic at various points along the pH scale.

Friedenthal and Hungarian chemist Pál Szily worked out a series of compounds that could determine pH, and Sørensen extended the list even further. One of the most well known is phenolphthalein. In acid solution, and out to a mildly basic pH of 8.2, it is colorless. But when it finally loses a proton, the new anionic (negatively charged) species is a brilliant purple-pink—a startling change that is always fun to watch. For many years, chemical analysis depended on such colorimetric techniques, using standardized solutions to carefully titrate a test sample until a visible change took place. Modern instruments, starting with the electrochemical pH meter, have moved most of these entertaining but now antiquated procedures solely into textbooks. The idea of measuring the strength of acids and bases, though, is still fundamental to chemistry, medicine, and biology.

This wide-range pH indicator paper has apparently just been used to test something that is very alkaline indeed.