While no two labs are exactly the same, there are some elements of lab research that feel universal. One of the most common and essential measurements done in a lab is pH measurement. Changes in pH or incorrect pH can have significant impacts on research and can lead to inaccurate results or delays in timelines.
A pH measurement system has three parts: a pH meter, a pH buffer or calibration solution and a pH electrode. While the pH meter processes the signals from the electrode and displays the pH measurement, it is the pH electrode that senses the hydrogen ion activity in the sample and controls the precision and reproducibility of the pH results. It is the most pivotal part of the entire pH system, so choosing the right electrode is critical to an accurate pH measurement. Selecting the right pH electrode, however, is only part of the equation. Labs can make several mistakes when using and storing a pH electrode that can compromise pH measurements. Treating a pH electrode correctly can help ensure accurate, repeatable pH analysis.
Selecting the right pH electrode
It may seem obvious, but one of the most essential factors in selecting the right pH electrode is the sample. For example, many modern pH electrodes contain silver ions as part of their reference system. But some samples, like proteins, tris buffer, enzymes, and sulfide, may react with silver, which can compromise the sample and cause the electrode to malfunction. Labs can avoid this issue by selecting a pH electrode with a double junction system, which prevents contact between the sample and the silver ions. The consistency of the sample can also impact the kind of junction best suited for an application. Electrodes with open, capillary, renewable or flushable junctions work best for samples containing particulates or thick and sticky materials, such as cosmetics, foods and environmental waste.
Additionally, the ionic strength, or levels of all ions in a sample, can affect hydrogen ion activity and pH readings. Most electrodes are designed for mid-range ionic strength samples, which makes it challenging to get accurate readings for low- and high-ionic-strength samples. Labs should ensure the pH electrodes they’re using are designed to allow good electrolyte flow, resist blockage, and are easy to clean.
Sample containers can also impact the kind of electrode used. If measuring pH in large containers, labs should consider choosing a long-reach electrode. Thin-stemmed electrodes are best suited for narrow tubes, and semi-micro and microelectrodes are best for small containers. The design of the electrode can also improve the electrode’s performance in a specific sample or application. Durable epoxy body electrodes with bulb guards to protect the glass pH bulb are good for use in the field or on the production floor. Glass electrodes are resistant to most chemicals, so they are a good choice for petrochemical or solvent samples. The qualities of the sample, sample container, and specific application can all have an impact on how pH electrodes will perform, so it’s critical to understand these factors when selecting the right pH electrode.
Common mistakes & how to fix them
Once labs have selected the right pH electrode for their research, the next step is to properly care for the electrode to ensure it keeps functioning correctly. Labs can make several common mistakes regarding pH electrodes that can impact pH measurements.
Incorrect electrode storage: Incorrect storage can damage an electrode and interfere with its performance. A common mistake is leaving an electrode in something like water, pH buffer, or the last sample tested. Other mistakes include leaving an electrode dry, leaving the fill hole open, having little to no fill solution left, or letting crystals form inside. For proper electrode storage, always use a specialized pH electrode storage solution. The fill hole should be closed before storing, the fill solution should be topped up, and no crystals should be formed inside.
Storage bottle handling: Incorrectly handling the electrode and its storage bottle can also impact how the electrode will perform. Pulling an electrode out of the storage bottle can create a vacuum and pull the fill solution out of the electrode. Pushing the electrode back into the bottle can create pressure that can cause unstable readings and electrode failure. When extracting the electrode from the storage bottle, it is crucial to unscrew the cap first and then remove the bottle to avoid a vacuum. When putting the electrode back into storage, push the cap onto the electrode first, place the electrode into the bottle, and tighten the cap to avoid introducing air bubbles.
Fill solution neglect: Issues with fill solution can reduce hydraulic pressure and slow the electrolyte fill solution outflow rate, harming the electrode’s performance. Common issues include low fill solution, old crystallized or discolored fill solution, or a closed fill hole. To keep the pH electrode working correctly, always open the fill hole during testing, top up the fill solution, and drain and refill it periodically based on the specific application.
Using degraded pH calibration buffers: Reusing buffers and using expired or improperly stored buffers can lead to invalid calibrations. Improperly stored buffers can become contaminated, evaporate, pick up gases and particles from the air and even grow mold and bacteria, leading to incorrect pH calibration. It is essential that buffers be poured fresh into clean beakers, disposed of at the end of the day, and always used within the expiration date.
Incorrect electrode rinsing: Improperly rinsing an electrode can interfere with pH analysis. This can include failing to rinse the electrode before moving to a different sample, not using enough water, or using tap water when rinsing. All of these mistakes can also lead to contamination of samples, so it is especially important to rinse an electrode correctly. Use a generous amount of purified water when rinsing and rinse everything that touches the solution.
Failure to clean the electrode: Without cleaning the electrode, its junction can clog, and the bulb can become dirty and coated. As a result, pH readings can drift and become slow or erratic. To clean the electrode properly, rinse it before storage or soak it in a particular electrode cleaning solution.
Overlooking temperature correction: Failure to correct temperature can make pH readings inaccurate. The farther the actual sample temperature is from the manually set temperature, the more severe this error can be. Consider using an Automatic Temperature Compensation probe or triode electrode with an integrated temperature sensor. These probes will allow the correct pH value to be assigned during calibration and correct the calibrated slope for temperature effects for samples not at room temperature.
Although these are common mistakes that labs can make when handling and storing a pH electrode, there are relatively easy ways to avoid them, which is critical to keeping a pH electrode functioning correctly.
Every pH measurement relies on a pH electrode, and every pH measurement needs to be as accurate and reproducible as possible to ensure your work goes as expected, whether process control, quality control, research, or sample characterization. Selecting the right pH electrode for the specific application and treating it carefully while using and storing it can help labs achieve the best possible results.
About the author:
Gayle Gleichauf is the Applications Lab Manager for Thermo Fisher Scientific Water Analysis Instruments in Chelmsford, Mass. With degrees in chemistry and environmental science, she has been working in the lab and in the field for over 25 years. In her current role, she helps match customers to the right equipment they need to make our world healthier, cleaner and safer.
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