Litmus paper. pH indicators. Titrations.
Measuring pH without the convenience of modern pH probes is either time-consuming or not very precise. Sure, color changes on litmus paper can be pretty and weirdly satisfying, but measurement precision on the order of 0.5 pH unit just isn’t sensitive enough for research in the 21st century. “Large-scale titrations” is simply an oxymoron.
Today, scientists can take all the inner workings of pH meters for granted and focus on the implications of pH measurements rather than the time and effort associated with generating the measurements themselves. However, researchers who have been doing the work for long enough eventually run into questions they realize they don’t always have the answers to.
How do pH meters work?
pH meters have one job: to measure pH. Traditional pH indicator dyes do this through a visual color change that occurs in response to changes in pH. In contrast, pH meters do this by measuring voltage.
pH is simply a measurement of the concentration of free hydrogen ions (H+) in a water-based, or aqueous, solution. Electronic pH meters can measure the concentration of H+ in a solution by measuring the potential, or separation of H+ across a glass pH-probe barrier, as voltage.
One of the special characteristics of glass pH probes is the glass itself: pH sensors use a special lithium-silicate glass. This expensive lithium-silicate pH glass allows H+ to reversibly bind to both sides of the glass, creating a separation of charge between a reference solution inside a pH sensor and the sample solution being measured (Figure 1).
Figure 1. Separation of H+ across the lithium glass of a pH probe. https://elscolab.com/en-nl/blog/unbreakable-cip-resistant-ph-electrode
In a lithium-glass pH sensor, the inside of the pH sensor is filled with a neutral (pH 7.0) potassium chloride (KCl) reference solution with a constant concentration of H+. The concentration of H+ in the aqueous sample being measured will vary depending on the pH of the sample:
- If the amount of H+ binding to the pH sensor glass from the sample solutions equals the amount of H+ binding on the opposite side of the glass from the reference solution, there is no difference in potential. The voltage, or potential difference, measured by the pH meter is 0.
Figure 2. Adapted from: https://www.youtube.com/watch?app=desktop&v=zJTQLce-WC8
- If the amount of H+ binding to the pH sensor glass from the sample solutions is different from the amount of H+ binding on the opposite side of the glass from the reference solution, there is a difference in potential. The voltage, or potential difference, measured by the pH meter is not 0.
Figure 3. Adapted from: https://www.youtube.com/watch?app=desktop&v=zJTQLce-WC8
How is pH calculated from voltage?
Fortunately, the voltage measured from the pH sensor is linearly proportional to the pH of the solution being measured. In fact, at 25°C, one pH unit is equal to a change of approximately 57.14 mV. This all goes back to the Nernst equation but is more simply described as:
This is why calibration of pH meters is so important during an experiment and why at least two reference points are required, preferably flanking the pH range to be measured. Depending on the temperature of the solutions, the slope of the equation, or mV change per pH unit, can change (Figure 2).
Figure 4. Change in voltage vs. change in pH (slope) varies depending on temperature. https://www.phionics.com/2021/09/07/how-temperature-affects-ph-measurements/
pH meters, therefore, measure the potential difference between the reference solution and the solution being measured as a voltage and calculate pH based on the equation above. Calibrating the meter will refine the slope of the equation to account for variation due to temperature.
How can pH be measured faster?
Waiting for pH meter readings to stabilize can be time-consuming, especially on a large scale. While using thinner lithium glass can facilitate more rapid pH measurements, it also increases the likelihood that the sensor glass will break.
One of the best ways labs can improve pH measurement efficiency is by automating the process. Hudson created one of the few standalone automated pH meters on the market, Rapid_pH, allowing labs to delegate pH measurements to a machine designed specifically for the job. Once a run is set and started, the machine functions entirely autonomously, freeing up laboratory staff to start the next experiment or analyze the results of the last run.
While large numbers of samples can be measured by student volunteers or other lab staff, repetitive and time-consuming measurements are prime opportunities to introduce data variability, inaccuracies, and equipment misuse. Invariably, a technician will be interrupted only to lose track of which sample they were measuring or to leave the sensor out to dry. Automating pH measurement eliminates these often costly mistakes.
To learn more about the Rapid_pH, click here or contact a Hudson rep. To learn more about pH measurement, check out some frequently asked pH questions below.
FA(pH)Qs
Why can’t pH sensors dry out?
- Gel layers form between lithium-silicate pH glass and aqueous solutions. This layer is critical for the efficient movement of ions between the glass and the solution and for accurate pH measurement. Reference junctions in pH sensors, which aren’t discussed here, also need to stay hydrated. If an electrode stays dry for 24 to 48 hours, it can have a significant effect on the speed of the sensor. Contact the manufacturer of your pH meter probe to inquire about rehydration if your probe has dried out.
Why is KCl used for pH sensor storage and reference solutions?
- Gel layers form between lithium-silicate pH glass and aqueous solutions. This layer is critical for the efficient movement of ions between the glass and the solution and for accurate pH measurement. Reference junctions in pH sensors, which aren’t discussed here, also need to stay hydrated. If an electrode stays dry for 24 to 48 hours, it can have a significant effect on the speed of the sensor. Contact the manufacturer of your pH meter probe to inquire about rehydration if your probe has dried out.
Do pH sensors wear out?
- Yes. Over time, the lithium glass of a pH sensor wears out and isn’t as sensitive. Depending on the digital pH tester, calibration slopes within a specific range indicate that the sensor is working properly. Some of the best indicators that your probe may be too old or failing are drifting pH values and long equilibration times. The length of time a sensor lasts depends on the brand, maintenance, and the amount of use. Fortunately, most pH testers offer replacement sensors if the current sensor wears out or breaks.
How do I maintain my pH measurement device?
- Glass pH sensors get contaminated from sample matrices and need to be cleaned. Manufacturers generally recommend NOT wiping pH probes to keep the glass hydrated and reduce the likelihood of damage. Between samples, be sure to rinse the sensor with deionized water. More in-depth cleanings may require specialized cleaning solutions, depending on the application. Contact the manufacturer of your pH reader for more information.
