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.
A pH meter is actually a voltmeter that uses a hydronium ionselective 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 precipitated 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.
Modern automated titrators do an even better job than manual titration, 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 twopoint 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 a nd 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 while ease of calibration is also important.
HQd and IntelliCAL™ Benchtop Meter and ISE Probe
- HQd meter permits easy switching and set-up of multiple smart probes
- Electrode’s large reference junction provides more surface area for sample contact
- HQd system can test ammonia, pH, dissolved oxygen, conductivity, BOD, ORP and sodium
- System automatically recognizes testing parameters, calibration history and method settings
ARC Sensor System
- For pH, dissolved oxygen and conductivity measurements
- Each sensor features a built-in microprocessor that can communicate with both analog (4 – 20 mA) and digital modbus interfaces
- ARC sensors can be precalibrated right in the lab, reducing costs associated with installation and downtime
HI 99161N Food and Dairy
- Available with waterproof casing
- pH probe features built-in temperature sensor
- Easy to clean probe resists clogging
- Offers dual button operation
- Delivers 1000 hour battery life with low battery warning
pHASE pH Electrodes
- Provide highly accurate and stable pH measurement changes in dynamic process environments under wide ranging variable temperature conditions
- Available in two designs: the costsaving, user-friendly sealed design and the Rapid-Renew refillable design, ideal for dirty samples
- Easy- to-use and low maintenance sealed design