Mature product lines such as pH meters are fundamentally limited in underlying technology, so they evolve through adding features and functionality.
High-end pH meters often come with USB or Bluetooth communications and networking capability. Beckman Coulter (Brea, CA) has sold “Bluetoothed” meters for two years. Along with the company’s SmartLogger software, users can capture pH data in real time, “and it doesn’t need to be hardwired,” says support engineer Tom Dawkins.
Companies are moving away from handwritten records, particularly with numeric data, for several reasons. Human error in copying multi-digit readouts is the most obvious. Increasingly, firms prefer to maintain data in repositories from which it may be shared, referred to from outside the lab, and presented to auditors. Software can track readings, usage, user identity, calibration history, etc. Some pH software packages, including SmartLogger, comply with the U.S. Food and Drug Administration’s 21 CFR Part 11 regulation on electronic records and signatures.
“The interesting thing about the pH marketplace is we have very sophisticated users and those who are just looking for a number,” says Mr. Dawkins. The first group is composed of researchers who perform kinetics experiments and advanced titrations, while the latter are found on production lines. “Having to serve both ends of the market with similar products makes designing new meters quite challenging.”
Beckman’s pH meters can separately measure pH, dissolved oxygen, conductivity, and (through ionselective electrodes) various ionic species. Mr. Dawkins recognizes the “clamor” for multifunction meters that measure two or more parameters simultaneously, “but this adds complexity to the measurement, the probe, the meter, and the data.”
Overcoming wear and tear
Electrode life is one of the most controversial issues in the total cost picture for pH meters. Probes are subjected to chemical, mechanical, and temperature-related damage. “How long an electrode lasts depends on so many factors, including who is using it,” Mr. Dawkins says. “If you don’t take care of the electrode during and between measurements, it will go bad a lot faster than if you follow the manufacturer’s directions.”
In some instances, the answer may be ruggedized electrodes, which respond more slowly than conventional designs but are more resilient to everyday use and abuse.
Another possibility is the use of maintenance-free pH electrodes suitable for pH measurements or titration. Metrohm’s (Riverview, FL) model uses a potassium chloride gel instead of conventional liquid electrolyte and is suitable for both pH measurement and titration.
“Users don’t have to worry about fill solutions or closing the electrolyte vent at the end of the day,” explains George Porter III, titration product manager. The gel self-indicates, by turning from opaque to clear or translucent, when its useful life is depleted. Users must then replace the electrode, but Mr. Porter offers assurance that “they are priced to be easily replaceable.”
Stretching the boundaries
Hanna Instruments (Woonsocket, RI) has become something of a specialist for pH measurements in the food industry. Company president David Minsk explains that standard pH electrodes are only reliable when used in dilute aqueous solutions, but most foods are complex, viscous mixtures. Hanna has developed a probe sporting a “conic” tip that allows it to penetrate thick sauces, even cheese. Another model employs a blade that clears the way for the probe to enter solid meat products. Several of these designs use electrodes made of viscolene, a nontoxic material that does not leach into samples. “Unless you take samples, a standard electrode could theoretically leak silver chloride into your batch of ketchup,” says Mr. Minsk.
Food laboratories share a regulatory requirement with pharmaceutical R&D: data traceability and consistency under GLP (Good Laboratory Practices). Hanna’s GLP-certified pH meters allow up to three-point calibration with five standard or three custom buffers, a calibration reminder, and storage/retrieval of calibration history and performance.
“Top pH meters all have GLP features,” Mr. Minsk says. “If you’re a food or pharmaceutical lab, you want to be able to show an auditor verifiable proof that your meter has been calibrated to specifications.”
Metrohm’s George Porter also notes that demand for high-end pH meters has picked up, particularly from pharmaceutical firms.
Metrohm and Hanna Instruments have both noted increased demand for multiple functionality in pH meters: conductivity, alkalinity, and ion selectivity in addition to pH. Hanna Instruments has even bundled a pH meter with global positioning functionality, for automatically recording pH measurements as a function of location, say, on a large lake.
Titrations are frequently pH-based, so pH meters often serve in that capacity as well. “Having more functionality in a single form factor adds value,” says Mr. Minsk.
Users are particularly fond of meters with ion-selective electrodes. Newer models, Mr. Porter explains, respond rapidly—and comparably to individual stand-alone electrodes. In Metrohm’s calcium-specific electrode, the reference function is built in so the electrode is compact and connects via a single wire. “If you’re using it in combination with other probes, it doesn’t take up a lot of space in the measurement vessel.” Metrohm targets this ruggedized electrode to environmental markets.
Metrohm also manufactures a solidstate electrode containing no liquid junctures, so drying out is never an issue. The “brain” is a sensor chip printed onto the electrode itself. Useful life is “well over 1,000 measurements,” Mr. Porter says. Operators simply swap out the chip, which is significantly cheaper than replacing the entire electrode. “Because they’re only replacing the chip, customers will be much less prone to push the usable life of the probe and much more likely to replace it when it’s time.”
“pH meters as instruments have come a long way since their invention in the 1930s,” notes Beckman Coulter’s Tom Dawkins. “Today’s meters sport digital displays, data recording, and advanced user interfaces. But while electrodes and housings have become sophisticated, it’s basically the same device on the inside as 80 years ago.”
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
For additional resources on pH meters, including useful articles and a list of manufacturers, visit visit www.labmanager.com/pH-meters