Introduction

The pH meter was never intended to be the mainstay of chemical analysis it is today. Based on a projected total market of 600 instruments, the pH meter began as a tool to measure the acidity of citrus fruit. From this basic task came the first chemical instrument to make extensive use of electronics, changing the way chemists began to think about how instruments could work and function.

The pH, or potential of hydrogen, scale was devised by Soren P. Sorensen in 1909 as a way to report the hydrogen ion concentration , or acidity, of aqueous solutions. Defined as the negative log of the hydrogen (or hydronium) ion concentration, pH values typically range from 0 (very acidic; [H3O+] = 1 M) to 14 (very alkaline; [H3O+] = 10-14 M). The standard tool to measure pH before commercial instrumentation was the hydrogen electrode / reference electrode cell. The standard hydrogen electrode is still the accepted technique for determining standard reduction potentials, but preparing and using a hydrogen electrode is a tedious, painstaking process.

The creation of the commercial pH meter began with a request from the citrus industry. Citrus growers have long known that citrus trees are highly sensitive to the acidity of soil and water, and even the ripeness of fruit was determined using a formula involving the percentages of citric acid and solids in juice. (Thackray and Myers, 2000) An accurate assessment of pH seemed the obvious answer to standardizing fruit quality.

Glen Joseph of the California Citrus Grower's Exchange (later Sunkist corporation) was working on a way to reliably determine the acidity of citrus fruit when he was referred to Arnold Beckman then a young professor at the California Institute of Technology. The problem Joseph faced was that his current was too small. Joseph had been using a conventional glass electrode apparatus, and was unable to get enough current for economical measurement without making the electrode too fragile for routine use. Beckman recommended Joseph stop trying to make new electrodes and try amplifying the current with a two vacuum tube amplifiers in series. Beckman's system would make the current large enough to read without the expensive, delicate electronics Joseph was trying to avoid. Beckman sketched a circuit diagram of his "acidimeter" and told Joseph how to assemble it. A few days later, Joseph returned, saying the instrument did not work. Beckman thought the parts had been assembled improperly and proved it by building a new acidimeter himself that worked perfectly.

Three months after their first meeting, Joseph returned to Beckman to ask for a second acidimeter. The original had become so popular around the lab that Joseph never had time to use it! The potential market demand was obvious, and Beckman set to work designing two more instruments: one to sell to Joseph and the other to test the market. Beckman remembered thinking, "Gee, if he could use two of those things in that little laboratory he has, maybe there's a market for them" (Sturchio and Thackray, 1985).

Most instrumentation of the era was assimilated as a conglomeration of chemical systems and few electronics. Beckman saw the opportunity for an instrument with electronics at its heart that was small, portable, and self-contained. In Beckman's own words, "Maybe you want to call it entrepreneurship or invention, I don't know. But anyway, I thought, well, heck, let's make a complete instrument then, get rid of all the stuff spread on the desktop and make it a compact unit." (Sturchio and Thackray, 1985)

After receiving a positive response at the 1936 American Chemical Society national meeting and finding a distributor, Beckman and National Technical Laboratories began production of the Model G acidimeter, later renamed the Model G pH meter. Ed Patterson, Beckman's initial distributor, had estimated a total global demand of 600 pH meters over 10 years before the market would be completely saturated. In 1936, the first year of production, 444 Model G pH meters were sold: 75% of Patterson's predicted global demand for a decade!

The original Beckman Model G was a walnut box 12" wide by 8" deep by 9" high, weighing approximately 15 pounds and equipped with a leather carrying handle. The sample was placed in a small beaker attached to a door that swung out from a porcelain compartment set into the front of the instrument, as shown in Figures 1 and 1a. The glass and reference electrodes were normally fastened to the door, but could be removed for washing, remote measurement, and sample changes as necessary. The controls, galvanometer, and readout were located beneath a wooden lid. The lid protected the instrument from dust, gave the model G a compact, clean appearance, and cleverly switched off power and sealed the sample door as the lid was closed via metal levers in the casing.

The controls of the Beckman G, shown in Figure 2, featured a prominent "pH Dial" at left with a button at its center and a galvanometer gauge on the right. Above the pH dial was a small window displaying pH and voltage. There were also controls to compensate for temperature, zero adjustment, and specify output in terms of pH, +MV, or -MV [sic].

Compared to the litmus paper, the advertised accuracy of ± 0.02 pH units was amazing, and in contrast to the complex apparatus used by Joseph, the model G was the essence of simplicity. No longer was it necessary to prepare a hydrogen electrode or wrestle with delicate galvanometers; the pH displayed by the instrument was the pH of the sample.

The need for design refinement became clear in mid 1937. A report from Stanford University was published claiming that glass electrodes measured the depth of immersion rather than pH. Beckman described the news of the depth error as "devastating to us; we finally looked into it and found out that, sure enough, it was true," but he was determined to find a solution (Sturchio and Thackray, 1985). The problem occurred because the glass bulb of the electrode was open to the rest of the apparatus. Thus if the depth of immersion changed, the surface area of the conducting glass membrane also changed, resulting in inconsistent current across the glass membrane and incorrect pH values. The physical design of the Model G minimized these errors because of the relatively constant depth at which the probes were held in the sample chamber, but this was just coincidence, and irrelevant for any measurement using remote probes. (Thackray and Myers, 2000)

Beckman and Howard Cary tried many different fixes for the electrode before deciding to redesign the entire probe from scratch. In late 1937 a working prototype of a sealed glass electrode was produced, and NTL became was the nation's sole supplier of the most accurate, precise electrodes in the world. The design of the sealed glass electrode has been the standard pH electrode since, and Beckman's patent and manufacturing rights "really gave us a stranglehold on the glass electrode business" (Sturchio and Thackray, 1985).

Sales flyers and brochures for the Beckman G are very interesting in the choice of features promoted and length of explanation of some features now taken for granted. Ease of use is the strongest pitch in early advertisements for the "Beckman Acidimeter," followed closely by the advantages and benefits offered by the amplification system. The sales brochure for a Model G acidimeter, shown in Figure 3, can be dated to pre-1939 by a paragraph on the last page (not shown) stating, "TheÉglass-electrode Acidimeter was developed by the National Technical Laboratories under the direct supervision of Dr. Arnold O. Beckman of the California Institute of Technology" (National Technical Laboratories, 1936-1938). Beckman did not leave Caltech to become the president of NTL until 1939.

This sales brochure is also interesting because the model portrayed has several characteristics not found in later models, notably the designation "Beckman Acidimeter" under the pH dial (this is hard tell in the picture shown) rather than "Beckman pH Meter" found on instruments produced after approximately 1938. The directions inside of the lid are different from later models, as is the shape of the galvanometer gauge. The instrument shown does not have a zero adjuster in the top left of the control panel, but does have a metal guide on the bottom of the sample door and a lever of unknown purpose connecting the sample door to something inside not found on later models.

The cover of one other particularly interesting sales brochure is reproduced in Figure 4. This painting seems to hint at the revolution in instrumentation for which the Model G would become known. Light radiates from the instrument, illuminating the dark laboratory and the face of the chemist bent over his experiment. It is almost as if the Model G is casting its glow on all around and casting older technology into the shadows.

The spirit of entrepreneurial research that characterized National Technical Laboratories in the late 1930s has persisted through the installment of Beckman as president of NTL in 1939, the transition from NTL to Beckman Instruments, Inc., and to present-day Beckman Coulter, Inc. After developing the high-pH electrode, many other electrodes were developed such as temperature-specific electrodes, micro-electrodes for samples as small as 15 µL, and a complete range of ion-selective electrodes. The quality of Beckman electrodes even gave rise to an advertising slogan, "This is Beckman pH," used in promotional materials from the 1960s and 1970s.

Modern pH meters are very similar to their predecessor, the Beckman Model G. The glass electrode is still the most common method for pH measurement, though separate glass and reference electrodes have been replaced by the glass combination electrode.

The greatest change in modern pH meters has been in internal electronics. The elimination of vacuum tubes has permitted the design of handheld pH meters and overcome the reliance on large, high-output batteries that gave the Model G much of its considerable heft. Null-balanced galvanometers have largely been replaced by automatic circuitry, though null-balance meters were still used in high-end Beckman pH meters at least into the 1970s. Ironically, low-cost student pH meters often employ an amplified voltmeter reading directly in mV or pH, making modern student pH meters more akin to Beckman's hand-sketched design than their modern counterparts.

The degree to which electronics have changed can be seen dramatically in the circuit diagram from Beckman's original patent for the acidimeter, issued in 1938, as shown in Figure 5 (Beckman and Fracker, 1936). The design is relatively straightforward to follow with minimal experience in electronics. Modern instruments are filled with electronics, computer chips, and digital readouts that make their design much more complicated and difficult for a beginner to understand.

Despite the vastly superior electronics available today, the sensitivity of modern pH meters is comparable to that of the Beckman G. This is a result of the unavoidable junction potential of any glass electrode, and is a testament to the quality of the much-promoted vacuum tube amplification system.

The Beckman Model G represented a shift in the way of thinking toward chemical analysis and instrumentation. Before the advent of the Model G, chemical instrumentation was primarily chemistry-based, with only rare applications of electronic components. What instrumentation was available was sold as a set of components to be assembled by the user; idiosyncrasies and extensive calibrations had to be worked out before routine analysis could begin.

The Beckman Model G made electronics the heart of a chemical instrument, resulting in an instrument that was rugged, self-contained, and needed little initial calibration before use. Today, electronics are crucial to almost every form of analytical instrumentation, and it is expected that most instruments will work directly from the factory with a minimum of trouble. The Beckman Model G represented a revolution in pH measurement, but its greater impact has been on our intuitive expectation of what a chemical instrument is and how it should perform.

References

Thackray, A.; Myers, M. Jr. Arnold O. Beckman: One Hundred Years of Excellence. Chemical Heritage Foundation: Philadelphia, PA, 2000.

Sturchio, J. L.; Thackray, A. Center for History of Chemistry, Philadelphia, PA. Unpublished Interview of Arnold O. Beckman. (Conducted at the University of Pennsylvania, April 23, 1985.), 1985.

Beckman, A. O.; Fracker, H. E. Apparatus for Testing Acidity. U. S. Patent 2,058,761, October 27, 1936.

National Technical Laboratories. "Now: The new portable, direct-reading Glass Electrode Beckman Acidimeter" (sales brochure). National Technical Laboratories: Pasadena, CA. 1936-1938.