First introduced by Beckman in 1936, pH meters have become a mainstay of academic, industrial, and manufacturing laboratories. pH meters determine the acidity or alkalinity of a solution by measuring the concentration of hydrogen ions, H+, or more accurately the hydronium ion, H3O+. pH is defined as the negative log of the hydronium ion concentration, or in mathematical terms.
pH = - log10 [H+]
A pH meter is actually a voltmeter that uses a hydronium ion-selective glass electrode to measure ion concentrations in the vicinity of the probe. Meters tend to be small, about the size of a shoe box. Most are benchtop units, but numerous vendors now sell battery-powered portable meters for field use.
pH meters are used in many industries: chemical, biological, environmental, forensics, consumer products, foods, and many others for which acidity measurements are warranted. A significant application is monitoring titration, a lab method that quantifies the concentration of an analyte in solution. Acid-base titrations measure concentrations of an acidic or alkaline substance. From the quantity of neutralizing species (acid or base) added at the point where pH is 7, or neutral, one can calculate the original concentration of the acid or base in the solution.
Titrations are also employed to adjust the pH of products or, in the case of biology, of buffers or standard solutions.
As George Porter, titration product manager at Metrohm (Riverview, FL) notes, most pH meters may be used as solution voltmeters, which greatly expands their utility beyond acidity and alkalinity. Meters can perform salt concentration measurements by noting the dramatic drop in electrical potential when, say, a halide is precipita ted with silver ion. “pH is a derived value calculated from a voltage. With salt concentration measurements you’re titrating to a set millivolt point,” Mr. Porter observes.
Specialized applications include salt concentration in foods, free amines in chemical reagents or preparations, and Karl-Fischer titration for water in nonaqueous media.
Chemists and biologists reading this article have probably conducted manual titrations, but modern automated titrators do an even better job, without the possibility of user error, by delivering precise quantities of acid or base. Without accurate pH metering, this application would not be possible.
Because the performance of the hydrogen ion electrode shifts and degrades over time, pH meters must be calibrated regularly. Manufacturers suggest performing a two-point calibration using two standard pH solutions daily for routine work, and before every measurement for sensitive measurements.
William McGlynn, Ph.D., a food scientist at the Oklahoma Cooperative Extension Service, recommends that users base pH meter purchase decisions on the instrument’s resolution and accuracy, probe type (detachable or integrated), electrode type (sealed or longer-lived refillable), and auto-calibration with temperature compensation.
Integrated probes are easier to use but more difficult to replace when they wear out or break. Auto-calibration and temperature compensation are desirable features for sensitive protocols that require frequent calibration.
Metrohm’s George Porter advises purchasers to consider how the meter is constructed, particularly if the circuits are galvanically separated. “You’re looking for an isolated signal,” he says. Another factor is how the instrument handles data. High-throughput labs, particularly those using automated titrators, require meters that upload data directly to a storage device or network.
Purchasers should take a hard look at ease of calibration, says David Minsk, president of Hanna Instruments (Woonsocket, RI), because “there’s a wide spectrum of technical astuteness among users.” Some methods, particularly in regulated industries or engineering, may call for a three-, four-, or five-point calibration; others are conducted in extreme environments. “You need an instrument you can calibrate easily within your application’s operating range.”
Solid state versus electrode probes
Recently, a team at Oxford University in the U.K. patented a revolutionary type of pH meter. The technology, exclusively licensed to Senova Systems (Sunnyvale, CA), has been touted as the first significant departure from the ubiquitous electrode-based meter.
The Senova product replaces sensitive glass electrodes with a solid-state sensor that is insensitive to temperature changes and may be sterilized. According to the company, the device provides robustness and miniaturization and, perhaps best of all, it never requires calibration.
Over the years lab workers have come to expect more from instrumentation, and pH meters are no exception. Mr. Minsk observes that processor-based features such as calibration data logging, automated uploading, and operational modes for specific industries or situations are highly desirable. “Users are also looking for broader functionality such as conductivity and ion selectivity in one instrument.”
Angelo DePalma holds a Ph.D. in organic chemistry and has worked in the pharmaceutical industry. You can reach him at firstname.lastname@example.org.
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