Development of the Beckman pH Meter

National Historic Chemical Landmark

Dedicated March 24, 2004, at Beckman Institute at the California Institute of Technology in Pasadena, California.

Commemorative Booklet (PDF)

When Arnold Beckman, a professor of analytical chemistry at the California Institute of Technology, was asked to devise a way to measure acidity in citrus fruit, the resulting “acidometer” revolutionized chemical instrumentation. The innovative features of the pH meter, including its use of integrated electronic technology and all-in-one design, were the basis for subsequent modern instrumentation developed by Beckman and his company.


Arnold Beckman’s Challenge: Measuring Acidity in Citrus

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 an 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.

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Significance of Beckman’s pH Meter

Glen Joseph's request for a second unit led Arnold Beckman to conclude that if Joseph "could use two of these [amplifiers to measure pH] in that little laboratory he has, maybe there's a market for them."3 Beckman immediately built two more devices, one for Joseph and one to see if it could be marketed. But more importantly, Beckman began to rethink his approach, soon concluding that the amplifier should not be a separate device but part of an integrated instrument to measure pH. As Beckman later recounted, his initial patent, filed in 1934, was not for a pH meter but rather for an amplifier. "It was later on," he said, "that we put the thing all together in a little walnut case."4 Initially called an "acidimeter," it included a vacuum tube amplifier, a measuring electrode and a data meter.

Chemists were just beginning to make use of electrical instruments in their research. This was usually done to meet a specific need and consisted of linking various devices together, which were then spread out on the workbench in a laboratory. Beckman changed this: Not only did he invent an amplifier that was innovative because of its sensitivity, but he also built an integrated instrument. In other words, Beckman not only figured out how to measure pH accurately; he also revolutionized instrumentation by building the first chemical instrument in one compact unit that utilized electronic technology and which was portable. This simplified research as a chemist no longer had to assemble various components to test data. Now the chemist could purchase the instrument, provide a power source and immediately begin collecting data. It was no longer necessary to assemble the requisite components and the chemist did not require much knowledge of the electronics. This rather basic but innovative approach to instrument design provided the basis for the subsequent development of modern instrumentation by Beckman and others.

Beckman's new instrument forced him to rethink his career. He was still an assistant professor at the California Institute of Technology, an institution that frowned on faculty that linked research with commercial endeavors. Beckman later said he "was happy there, doing research and teaching chemistry."5 He had devoted time to the problem of the pH meter only as a favor to an old college friend. At the same time, Beckman realized the significance of the design of the pH meter and believed there was a market for what he called an "acidimeter," an instrument which chemists and technicians could take into the field to measure acidity. His dilemma was simple: How could he, a Caltech professor, market his instrument?

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Early History of Beckman Instruments

The answer to Beckman's quandary lay in making the acidimeter part of National Inking Appliance, a small business with which Beckman was already involved. Beckman's interest in National Inking began when I. H. (Buzz) Lyons, the president of the National Postal Meter Company, visited him. National Postal Meter, based in Los Angeles, manufactured postage meters, and while the major supplier of the meters was Pitney Bowes, in the 1930s National Postal Meter was a competitor. But Lyons had a problem that threatened sales: His machines kept clogging.

Beckman quickly concluded that pigment settling caused the clogging problem. Beckman told Lyons the solution was simple: Use butyric acid in the formula for making the ink. Unfortunately, butyric acid has an awful smell, and Lyons could not get any major ink producers to manufacture Beckman's formula. Beckman then said he would produce the ink himself as a side project since he was still an assistant professor at the California Institute of Technology. A better ink was not Beckman's only involvement with National Postal Meter. Beckman also began collaborating with an inventor at the company, Hector Jewell, on ways to constantly apply ink to typewriter ribbons. Beckman came up with two methods to apply the ink, both resulting in patents. Armed with a non-clogging ink and two re-inking devices, National Postal Meter created National Inking Appliance on November 26, 1934, with Beckman as vice president and manager of the subsidiary. This was the beginning of Beckman's career as an entrepreneur, although he would protest for the rest of his life that "really, I'm not business-oriented… I'm concerned more about solving the technical problems and coming up with something that's useful for the advancement of science."6

Beckman found space for National Inking Appliance by renting, for five dollars a month, a ten-foot by twenty five-foot area in the back of a garage in East Pasadena. He equipped it with some laboratory glassware, Bunsen burners and an exhaust hood, and he hired two Caltech students, Robert Barton and Henry Fracker, to work part-time along with him producing ink for National Postal Meter. But the patented inking pads failed when secretaries refused to use them, a failure that in the end proved a blessing for Beckman, since he now had a facility ready to produce the acidimeter. The failure also taught Beckman that inventions had to be ingenious and commercial to be successful.

On April 28, 1935, Beckman engineered a name change: National Inking Appliance Company became National Technical Laboratories. The change indicated that Beckman was altering his focus; instead of producing one product as a sideline, he now was beginning to see a future based on a program to produce and sell sophisticated scientific instruments. Moreover, National Technical Laboratories, or NTL, was an independent corporation, not a subsidiary of National Postal Meter. NTL did take $9,000 from National Postal Meter as seed money in exchange for ninety percent of stock. Beckman kept the remaining ten percent of the stock and he drew a modest salary.

At its start, NTL had only one product: postage meter ink, which it was manufacturing for National Postal Meter. But development of the acidimeter continued in the garage in East Pasadena and by September 1935, Beckman and his assistants had a marketable instrument housed in a wooden box with a handle and a latch, just in time for the fall national meeting of the American Chemical Society in San Francisco. Beckman took the acidimeter with him "and showed it to several chemists. In particular, I asked some of my former professors, whether, in their opinion, there would be a market for such an instrument. To put things in perspective, I should point out that our instrument was priced at $195, and it would be competing to some extent with litmus paper which cost only a few cents a vial."7

Beckman's professors suggested he show the instrument to laboratory apparatus dealers. "Their most optimistic estimate," he says, "was that 600 might be sold over a ten-year period before the market would be saturated. Not a very great sales potential, but I decided to go ahead. After all, it was only a spare-time activity."8 To help boost sales, the name was changed from "acidimeter" to "pH meter" to emphasize the scale that measures acidity and alkalinity. At the same time, "with reckless disregard for overhead costs,"9 Beckman moved production from the garage in East Pasadena to a vacant store building at 3330 East Colorado Street in Pasadena with a rent of $50 a month, ten times what he had been paying.

In 1936, the first full year of sales, NTL sold 444 pH meters, with a gross income of $60,000 and a net profit of $2,358. By 1939, 1,995 pH meters had been sold and the profit for that year was $22,160. Success meant that soon the "Beckman Glass Electrode pH Meter" was featured in the catalogs of all the major instrument dealers in the United States. The growth occurred even though initially Beckman's two assistants built each meter by hand. Soon additional staff was hired, including a one-person sales force to coordinate orders that came in from dealers in chemical instruments. The decision to use component parts rather than manufacturing them achieved economies of production. For example, costs were kept down by purchasing vacuum tubes. The enamel boxes that housed the pH meter's electrodes came from the Gaffers and Sattler stove company, which produced them as containers for salt and pepper. Business was so good that in 1937 NTL introduced the Model G pH meter, eventually selling thousands of this version.

By 1939 Beckman had to make a major decision about his career. For four years, he had been running NTL but had only the formal title of vice president. On May 11, 1939, the board of directors created the new full-time position of president, with a salary of $10,000 and stock as remuneration for running the company that had sales of $140,000 a year. Beckman understood that NTL had grown to the point where "somebody had to run the show full-time. It was a case of whether I tried to do that or whether I went out and hired a professional to do it."10 So "with great reluctance," Beckman resigned from Caltech to become president of NTL.11

The decision to leave Caltech was a difficult one for Beckman, but he sensed that not only did the company need a full-time president, but also that his role as an entrepreneur conflicted with the aura of pure science cultivated at Caltech. As he later said, "I enjoyed my association with Caltech and also I had a feeling, which is an extension of this attitude of the pure scientist, that anybody engaging in commercialism was somehow a second-class citizen… I thought also, am I prostituting my scientific training by leaving academic and going industry?" Beckman concluded that he was not "prostituting" himself, eventually coming to agree with "many friends [who] say that I've done more for science by making thousands of instruments available for others then I would have done with my own two hands in a laboratory. I hope they're right. I don't argue with them too strongly on that."12

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Biography and Legacy of Arnold O. Beckman

Arnold O. Beckman: Early Years

Arnold O. Beckman was born on April 10, 1900, in Cullom, Illinois, a town of about 500 people. His father, George Beckman, was a blacksmith. As a nine-year-old, Arnold stumbled on a book that once belonged to an aunt, J. Dorman Steele's Fourteen Weeks in Chemistry. Steele was a teacher who loved developing new scientific lectures and experiments. Aiming to encourage inquisitiveness in his readers, Steele related his lessons to household objects and materials. Steele probably thought his audience would be curious adults and high school students, not nine-year-olds.

Beckman devoured Steele's lessons, which left him with a life-long love of chemistry. He was especially enamored of the illustrations and descriptions of laboratory experiments, encouraging the youngster to try his own hand at them. For his tenth birthday his father gave Arnold a small "shop," an 8-by-10 foot shed that stood behind the family home, in which he could conduct his "experiments." For chemicals, Arnold scoured his mother's pantry and shopped at the local druggist for "vinegar and baking soda—sodium bicarbonate, lye—ordinary things of that sort."13 And soon his older brother Roland, who worked in Chicago, obtained for Arnold chemical equipment such as glassware, test tubes and a mortar and pestle. In later years, when the family moved to Normal and Bloomington, Illinois, Beckman always commandeered laboratory space in which to do his chemical experiments.

The family moves allowed Arnold to attend University High School, the teaching school affiliated with Illinois State University in Normal. As a freshman, Beckman convinced the school principal that since he was planning on studying chemistry in college, he had no need for Latin and should be allowed to take chemistry in his first year. Soon, Beckman found the chemistry laboratories at Illinois State; then he found a mentor, Professor Howard Adams, who agreed to allow the high school student to take college chemistry courses. Adams also drove the youngster fifty miles every Saturday to Urbana so Arnold could use the more sophisticated equipment in the chemical laboratories at the University of Illinois. Beckman later called Adams "a great friend of mine."14

Adams helped the seventeen-year-old land a client, the local gas company, so that Arnold could get practical experience as a consultant in analytical chemistry. Arnold had business cards printed and the little laboratory in his home became the "Bloomington Research Laboratories" with him as "Chief Scientist." Beckman's consulting work for Union Gas and Electric was to run analyses of wood chips soaked in ferric chloride to determine if the concentration was high enough to remove the noxious smell that came from burning Illinois coal.

Beckman aided the American war effort in the First World War when Adams helped him land a post at the Keystone Steel and Iron Company in Pekin, Illinois. There was a great need for steel during the war, and Beckman was allowed to leave University High several months early so that he could analyze steel samples for Keystone to determine their carbon content. The quality of the manufactured steel depended on its carbon content. "The procedure," Beckman later said, "was that the chemist would go over to the open hearth furnace with a little mold, reach in and get a sample of the molten iron, put in the mold, and then with tongs carry it back to the lab while it was still hot." Beckman had to make four different tests, and "as I recall, I got to the speed where I could run all four analyses in 30 minutes."15

Despite leaving high school two months early, Beckman was the valedictorian of his class. After graduation, Beckman entered the Marines, but never served overseas. He did make it as far as the Brooklyn Navy Yard, where Mabel Meinzer, the future Mrs. Beckman, served him Thanksgiving dinner in November 1918. The following fall, Beckman enrolled at the University of Illinois at Urbana-Champaign.

Since Beckman already had completed two-and-half years of college chemistry, he skipped the introductory courses, enrolling instead in advanced courses offered by the chemistry department. He also came into contact with Carl "Speed" Marvel, then a budding young chemist who would go on to a brilliant career as an organic chemist. Marvel "commanded the respect of everybody, first of all from his knowledge and competence, and also from his general behavior."16 Marvel assigned Beckman a research project involving mercury compounds. While the dangers of mercury poisoning were not unknown in 1920, Beckman and Marvel took risks in their experiments that would not be permitted years later. Both men soon began to show symptoms of mercury poisoning, which for Beckman led to a change in career emphasis, as he switched from organic to physical chemistry.

Armed with a B.S. degree in chemical engineering and a master's in physical chemistry, Beckman entered the new California Institute of Technology in Pasadena to pursue a Ph.D. After a year at Caltech, Beckman headed east, to be near Mabel. He took a job with Bell Labs in New York, where vacuum tubes were being perfected and which Beckman made good use of ten years later. In June, 1925, Arnold and Mabel were married, and a year later the young couple journeyed to California so that Beckman could finish his degree at Caltech.

As a graduate student, Beckman won his first patent, for a signaling device that rang when a car exceeded a certain speed. "You might draw the conclusion from that," Beckman says, "that my interest in that was derived from the number of tickets I was getting for fast driving."17 After receiving his Ph.D. in 1928 for research on the photochemical decomposition of hydrazine, Beckman was invited to join the Caltech faculty. Beckman easily settled down to life as a university assistant professor, continuing his photochemical research and teaching freshman chemistry. But he had other pursuits as well. The chemistry department at Caltech prided itself on pure research, while Beckman had an interest in applied science, served as a private consultant for industry, and remained intrigued by industrial innovation and instrumentation. And then in 1934 Glen Joseph came to visit.

Arnold O. Beckman: Businessman and Philanthropist

Glen Joseph's visit in 1934 led to the development of the pH meter, which in the late 1930s became a commercial success. When Beckman decided to leave academia and devote himself fulltime to business, National Technical Laboratories began to manufacture other instruments in addition to the pH meter. Because business was good and because NTL was branching out, the company moved in 1940 into its new 12,000-square-foot facility at 820 Mission Street in South Pasadena.

NTL's next major advance was the DU spectrophotometer, which used some of the technology first applied in the pH meter. Both instruments, for example, shared the same amplifier. Both devices lived up to Beckman's motto: "Simplicity for the user is the keynote in the design of all our instruments."18 The DU spectrophotometer simplified laboratory procedures for making and recording the ultraviolet spectra of compounds. By helping to determine their molecular structure, the DU has assisted in the development of drugs and foods. For example, during the Second World War the instrument helped researchers to understand the chemical structure of penicillin.

National Technical Laboratories aided the war effort in other ways. Beckman was asked to assist in the development of radar, specifically to develop accurate and reliable control knobs. After some trial and error Beckman developed the Helipot—short for "helical potentiometer"—which precisely fit the military's needs and soon NTL was deluged with orders, forcing Beckman to create a subsidiary of NTL known as the Helipot Corporation to produce the components. Helipot was the first company owned directly by Beckman and it also signified that Beckman was interested in producing components as well as instruments.

NTL grew rapidly throughout the 1940s. In 1948 Beckman was able to purchase enough stock to gain control of the company. In 1950 the company changed its name to Beckman Instruments and two years later it became a publicly traded corporation. Sale of stock brought an infusion of capital that Beckman Instruments used to buy new, expensive technology and to pursue costly lines of research.

As his business grew, Beckman became more involved in community issues. Southern California was growing rapidly in these years and by mid-century reliance on the automobile had made smog a regional problem. Beckman was a member of a Los Angeles Chamber of Commerce scientific advisory group and he encouraged research that identified ozone as the likely cause of smog. He chaired the Special Committee on Air Pollution; its findings, released in 1953, led to enactment of city and state pollution control measures. In 1970 he was named to the Federal Air Quality Board.

In 1953 Beckman Instruments broke ground on a huge complex in Fullerton, California, a facility still used by the company's successor, Beckman Coulter, Inc. In 1965 Beckman retired as president of Beckman Instruments, becoming chairman of the board. This change did not slow the company's growth. In 1969 Beckman Instruments introduced the glucose analyzer and two years later the blood urea nitrogen analyzer. These were cutting-edge instruments that allowed doctors to make rapid diagnoses. Beckman researchers, using techniques developed in the company's early years, continued to make medical instruments both more sophisticated but also simpler to use. Beckman Instruments soon began marketing the STAT Lab, a series of instruments linked to a central computer that performed quick diagnoses in the emergency room.

Beckman Instruments diversified throughout the last decades of the twentieth century, developing into a leader in the manufacturing of instruments used in medicine and industry and in research, even space exploration. In 1981 Beckman Instruments merged with SmithKline Corporation, the huge pharmaceutical company. The new corporation became known as SmithKline Beckman. In 1989 SmithKline, deciding that the two companies did not fit well, spun off Beckman Instruments. In 1997 Beckman Instruments became Beckman Coulter, a multinational company with offices in 130 countries and sales in 2002 in excess of two billion dollars. In 2011, Beckman Coulter was acquired by Danaher Corporation.

Having made a fortune, Beckman decided to give it away. Beckman's interest in philanthropy intensified as he was surrendering control of the business he built. In 1977 he and his wife created the Beckman Foundation with a mission to support basic scientific research with an emphasis on chemistry. The Beckmans were the staff and the office was their dining room table. In addition to a number of small grants, the Beckman Foundation gave substantial gifts to the Scripps Clinic and to the University of Illinois.

In the 1980s the Beckmans made major donations to create five Beckman Institutes devoted to cutting-edge research in the molecular sciences. The first was the Beckman Research Institute at the City of Hope National Medical Center in Duarte, California. Shortly after came the Beckman Laser Institute at the University of California at Irvine. In 1989 the Beckman Center, part of Stanford University's Medical School, opened to foster research in molecular and genetic medicine. There were also Beckman Institutes at the universities with which Beckman had been associated: the University of Illinois and the California Institute of Technology. In addition to the five institutes, Beckman gave two million dollars to the Chemical Heritage Foundation in Philadelphia for the Beckman Center for the History of Chemistry.

Arnold Beckman died in 2004, at age 104.

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Research Notes and Further Reading

Research Notes

  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
  3. Arnold O. Beckman, Interview by Mary Terrell, 16 October and 4 December 1978 (California Institute of Technology, Oral History Project, Caltech Archives, 1981), p. 28.
  4. 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. 31.
  5. 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
  6. Arnold O. Beckman, Interview by Jeffrey L. Sturchio and Arnold Thackray at University of Pennsylvania, 23 July 1985 (Philadelphia: Chemical Heritage Foundation, Oral History Transcript #0014B), p. 11.
  7. 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. 13.
  8. Ibid.
  9. Ibid., p.14
  10. Beckman, Interview by Sturchio and Thackray, 23 July 1985, p. 13.
  11. Beckman, Speech, p. 15.
  12. Beckman, Interview by Sturchio and Thackray, 23 July 1985, p. 13.
  13. Arnold O. Beckman, Interview by Mary Terrell, 16 October and 4 December 1978 (California Institute of Technology, Oral History Project, Caltech Archives, 1981), p. 2.
  14. 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. 1. Beckman also says "I've always been indebted to him for the encouragement I got." Beckman, Interview by Terrell, p. 4.
  15. Beckman, Interview by Sturchio and Thackray, 23 April, 1985, p. 2.
  16. Ibid., p. 5.
  17. Ibid., p. 27.
  18. 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. 13

Further Reading

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Landmark Designation and Acknowledgments

Landmark Designation and Acknowledgments

Landmark Designation

The American Chemical Society designated the development of the Beckman pH meter as a National Historic Chemical Landmark at the Beckman Institute of the California Institute of Technology on March 24, 2004. The plaque commemorating the development reads:

Arnold O. Beckman developed the first commercially successful electronic pH meter while a member of the faculty of the California Institute of Technology. This rugged and portable "acidimeter," which had all necessary components housed in a single unit, allowed scientists to measure acidity accurately and rapidly. It immediately met an important need of the California citrus industry: how to measure the pH of lemon juice. The innovative features of the pH meter, including an early use of integrated electronic technology, were the basis for subsequent modern instrumentation developed by Beckman and Beckman Instruments.


Adapted for the internet from “The Development of the Beckman pH Meter,” produced by the National Historic Chemical Landmarks program of the American Chemical Society in 2004.

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Cite this Page

ACS Style

American Chemical Society National Historic Chemical Landmarks. Development of the Beckman pH Meter. (accessed Month Day, Year).

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“The Development of the Beckman pH Meter” commemorative booklet
“The Development of the Beckman pH Meter” commemorative booklet produced by the National Historic Chemical Landmarks program of the American Chemical Society in 2004 (PDF).