Erica Tennenhouse, PhD
Measuring the acidity or alkalinity of an aqueous solution is a simple task for most labs. But other sample types can pose unique challenges that require special pH measurement considerations.
“One of the bigger issues we come across is measuring the pH of soil,” says Dave Masulli, senior applications engineer at Hanna Instruments (Woonsocket, RI). Soil can be tricky to deal with because there are a number of ways one can prepare their sample, and a high risk of the electrode becoming contaminated. Measuring pH in foods such as meat or cheese, says Masulli, can also be challenging because of the fats and oils present.
According to Pamela Millett from HORIBA Scientific (Irvine, CA), the most difficult samples for measuring pH are those that are very small, viscous, or acidic. To that list, Beth Britt from Mettler Toledo (Columbus, OH) adds samples containing Tris buffers—because these buffers are reactive with one of the components of a pH electrode, one must specially design an electrode to ensure that interactions do not take place, she explains.
“Pretty much anything that’s a simple aqueous solution is very easy to measure, but as you start to add more components to that aqueous solution, those components can have unanticipated effects,” says Britt.
Anatomy of pH measurement
pH measurement systems consist of three main components: the meter, which measures electrode voltages and displays the results; the electrode, which interacts with the sample; and the reference electrode, which delivers a constant output regardless of the activity of the hydrogen ion. “The most important piece of it for matching the application is really the electrode,” says Britt, who notes that electrodes can cross over into different meters.
The right electrode
Some electrodes are designed with tougher sample types in mind. For instance, because oily samples make it hard to get decent electrical contact between the pH electrode and the solution, they require a special kind of junction that lets out a lot of electrolyte solution, says Britt.
Masulli would also opt for an open electrode junction system when analyzing soil samples, to avoid having the electrode junction clog up. “Other things we can do [for soil analysis] include using a pointed electrode tip to allow for direct soil measurements, which definitely cuts down a lot of the sample prep and makes it easier for customers to get the job done,” he says.
For food analysis, Hanna Instruments offers electrodes that have stainless steel blades at the end. “They allow you to take a direct measurement without having to blend a piece of cheese and then filter and measure the supernatant,” says Masulli. “It makes it really easy for people who may not have technical expertise or the equipment to process samples.”
Then there are electrodes that are ideally suited to more viscous liquids, says Millett. These “allow more of the internal solution of the pH electrodes to bleed into the sample, giving it the ionic activity it needs for a good measurement of viscous fluid.”
As an example, Millett describes HORIBA’s flex sensor technology, which can handle a wide range of sample types, including very small samples, liquid samples, and solid samples.
The recent focus for Hanna Instruments has been on expanding its HALO Bluetooth electrodes—a line of electrodes that hook up to Android or iOS devices, allowing the user’s phone to function as a full-fledged pH meter. They offer a variety of these electrodes for specific applications, including food, wine, and agriculture, says Masulli.
For additional resources on pH meters, including useful articles and a list of manufacturers, visit www.labmanager.com/ph-meters