Measuring Acidity

The California citrus industry depends on chemistry. The acidity of the soil and water influences the fruit trees. Chemists developed the insecticides and fungicides used by growers. Ripeness was determined by the percentage of citric acid in the juice. And finally, large quantities of inferior fruit were processed, by chemical means, into pectin and citric acid. Clearly, the citrus industry needed an accurate gauge of acidity, the problem that sent Glen Joseph to seek the advice of his old friend Arnold Beckman.

Chemists measured acidity by several methods. The most prominent was the colorimetric method familiar to most high school chemistry students. Slips of paper were coated with litmus - a water-soluble powder derived from lichens - then dipped into the solution under examination. If the litmus-treated paper turned red, the solution was acidic; if it turned blue, alkaline. Color-coded charts helped the examiner determine the acidity of the solution. A cheap test, but it is unusable in the citrus industry since the sulfur dioxide that was added to citrus juice as a preservative bleaches out the litmus.

Chemists also used electrochemical methods which rely on ionic theory to calibrate acidity. In this theory acidity is defined by the concentration of hydrogen ions: the greater the concentration, the more acidic a solution. The measure used is the pH - potential of hydrogen - scale, developed in 1909 by Søren Sørenson, a Danish biochemist. The pH scale has values ranging from 0 (extremely acidic) to 14 (extremely alkaline). Joseph used hydrogen electrodes in trying to determine the acidity of lemon juice, but again sulfur dioxide interfered with the test.

Joseph next tried electrodes which use glass as the contact rather than a metal wire or plate common in the hydrogen electrode. The glass electrode provided reliable readings - as long as the equipment did not break, which happened frequently because of the need to use very thin glass to reduce resistance to the electrical currents that were being measured. And if the electrode worked, invariably the galvanometer Joseph used to read the current failed. It was this problem that finally drove Joseph to call on Beckman, who immediately concluded that Joseph was using the wrong equipment.

The galvanometer was wrong, Beckman told Joseph, because it "just requires too much current. Use a vacuum tube voltmeter."1 Beckman was familiar with vacuum tubes from his work a decade earlier at Bell Labs, and he knew that vacuum tubes were effective amplifiers of weak electrical signals. And he realized that the solution to Joseph's problem was amplification: that is, Beckman understood that Joseph needed to make the current stronger. To do that, he needed sturdier glass electrodes, but unfortunately glass is a poor conductor. The conclusion to Beckman was obvious: make a strong amplifier to increase current and combine it with a sturdy glass electrode. As he later said, "the electronic amplifier would also have greater sensitivity than the galvanometer, so thicker-walled, more rugged glass electrodes could be used."2

A vacuum tube is a sealed glass bulb containing a cathode and an anode which transmit and receive electrical currents. Between the two poles is a grid - a small metal mesh - which controls the flow of electricity. Small changes in the electrical current, which came from an outside source such as a battery, lead to greater changes - an amplification - in the current flowing between the cathode and anode. Beckman thought that by putting the pH measurement into the grid circuit of the vacuum tube the resulting reading would be amplified and could easily be read by am ammeter, a device for measuring currents.

Joseph left Beckman's office armed with a sketch using two vacuum tubes to amplify and then re-amplify the signal. But he soon returned saying that the device did not work. Beckman decided he would build the instrument himself, and this time it not only worked, it worked so well that Joseph was soon asking Beckman whether he could make another one just like the first. Joseph's second request changed Arnold Beckman's life for it launched Beckman on a new career, turning an assistant professor of history into an inventor, entrepreneur, and noted philanthropist.

1 Arnold O. Beckman, Interview by Jeffrey L. Sturchio and Arnold Thackray at University of Pennsylvania, 23 April 1985 (Philadelphia: Chemical Heritage Foundation, Oral History Transcript #0014A), p. 33.

2 Arnold O. Beckman, Speech before the Newcomen Society, Los Angeles, November 10, 1975, printed as Arnold O. Beckman, Beckman Instruments, Inc. (New York, The Newcomen Society in North America, 1976), p. 11


next | back


Measuring acidity | Significance of the pH meter | Beckman starts a business | Arnold O. Beckman: The early years
Businessman and philanthropist | Landmark designation and acknowledgments

Copyright ©2004 American Chemical Society. All Rights Reserved. 1155 16th Street NW, Washington DC 20036
202-872-4600, 800-227-5558