Accurate pH measurement is essential in a wide range of fields, including biology and pharmaceuticals, environmental and agriculture, food and beverage sciences, and more. Inaccurate pH readings can have costly consequences such as the loss of bacterial cultures, failure of chemical reactions and corrosion of equipment or infrastructure, to name a few. Therefore, ensuring that pH is measured with the utmost care and precision is crucial in any laboratory.
The most common electrodes used in modern pH measurement instruments are combination electrodes incorporating a glass bulb measuring electrode and silver/silver chloride (Ag/AgCl) reference electrode into one sensor. These electrodes have a suitable pH measurement range for most routine applications, and are favored for their relative affordability and high sensitivity when used correctly. However, these electrodes are susceptible to damage, matrix interferences and other problems that can impact the accuracy of measurements, requiring proper handling, maintenance and storage to avoid compromising performance. Here are a few tips to help you properly care for and operate your pH meter:
1. Keep It Clean
One of the most common causes of pH measurement problems, such as inaccurate readings and slow response times, is inadequate or improper cleaning of the sensor. The exact cleaning frequency and solution may vary depending on the application and frequency of use, but cleaning should be done periodically as well as when performance issues are noticed or when contaminants are visible upon inspection. General cleaning may include soaking or gentle swirling in a diluted detergent or hydrochloric acid (HCl) solution (ex. 0.1 M HCl) for approximately 15 minutes, followed by a rinse in deionized (DI) water, refilling of the electrolyte and soaking of the electrode in storage solution for at least one hour.
Proteins and oils tend to coat the glass bulb and are not easily removed by water or general cleaning solutions, thus specific solutions are typically used to remove these contaminants from the electrode. For protein contamination, pepsin in HCl is used to break down and remove the proteins. A typical preparation is 1% pepsin in 0.1 M HCl, in which the electrode may be soaked for anywhere from 15 minutes to an hour. For oils, a mild, non-ionic detergent, methanol or ethanol can be used for cleaning. If you are regularly measuring protein-rich and oily samples, these methods should be a part of your routine cleaning.
Other contaminants that require special cleaning include silver sulfide precipitation and bacteria. When measuring samples containing sulfides, such as wastewater, the sulfides can react with silver ions from the Ag/AgCl reference electrode and cause silver sulfide precipitates to clog the junction. This will be visible as a black substance inside the sensor. These clogs may be removed by cleaning with a thiourea solution. Bacteria and other microbial growth can also be cleaned with thiourea or diluted bleach.
Carryover and contamination between samples and reagents are also a concern. The electrode should always be gently rinsed with DI water between immersions in sample or calibration solutions. The glass sensor should never be wiped with a tissue, cloth or similar implement to clean it; this can not only damage and dehydrate the electrode but also generate static electricity, which can throw off measurements. If excess droplets of liquid need to be removed, gentle blotting a lint-free wipe will do.
2. Keep It Calibrated
Calibration of the pH meter using buffers of known pH ensures measurement accuracy by compensating for error caused by electrode aging and drift. Calibration frequency will also depend on use frequency, application and the level of precision needed for each experiment. Therefore, calibration schedules can range from weekly, to daily, to before each use for the most sensitive applications. Calibration should be performed both periodically and after certain events such as cleaning, electrode replacement, long-term storage, measurement of a particularly strong solution (such as a strong acid with a pH less than 2) or when error is expected (ex. after an unexpected result). The meter should also be calibrated before measuring samples with a different expected pH range and temperature than the previous calibration.
Two-point calibration (using two calibration buffers) is necessary to ensure highly sensitive measurements—calibration using three or more buffers, while more time consuming, further increases precision. For two-point calibration, select buffers with pH values surrounding the expected sample pH, and which are at least 3 pH units apart from each other. Commonly, pH 7 buffer is used as the first buffer, to establish a zero point, while the second buffer is pH 4 if the sample is expected to be acidic and pH 10 if the sample is expected to be basic. The sensor should be rinsed with DI water between each calibration buffer to prevent carryover and contamination.
Crucial to successful calibration is the use of fresh (non-expired), uncontaminated and properly stored buffers—use of expired or tainted buffers for calibration is another common cause of error in pH measurements. Any buffer that is removed from the container should be discarded after calibration and never poured back into the container. Note that alkaline calibration buffers have a shorter shelf life than neutral or acidic buffers due to their tendency to absorb carbon dioxide, which lowers their pH—do not assume that buffers purchased at the same time will also expire at the same time.
Another important consideration for calibration is using similar conditions to how the subsequent measurement will take place. For example, if the sample will be stirred during measurement, stir the calibration buffer in a similar fashion. Temperature also impacts pH measurements, so calibrations should take place at the same temperature as the samples, and instruments should be recalibrated before being used at a different temperature.
3. Keep It Hydrated
In order to function properly, pH sensors should not be allowed to dry out. Appropriate storage of the sensor when not in use is essential to ensure accurate measurements and longer electrode lifetime. The sensor should ideally be stored in a commercially available storage solution or the reference electrolyte solution, which for most probes is 3 M or 4 M potassium chloride (KCl). If neither is available, the electrode can be stored in a ph 4 buffer, or ph 7 buffer as a last resort. Electrodes should never be stored in DI water, as this will cause ions to leach out of the sensor, degrading its performance and shortening its lifetime. The cap of the sensor can be filled with storage solution and sealed on while it is not in use.
A preservative, such as 4% sodium benzoate, can be included in KCl storage solution to prevent mold growth during long-term storage (i.e. several weeks of storage). When storing the electrode for an extended period, you may want to check on it periodically to ensure there is still enough storage solution immersing the electrode. Placing a small sponge, dampened with storage solution, inside the sensor cap can help retain moisture for longer periods of time. The electrode should always be conditioned and recalibrated after a long period of storage.
If the electrode dries out, there is a chance that it can be renewed by soaking it in storage solution overnight. However, if you are unable to get a reading, or if the error is too great when attempting calibration, the electrode should be replaced.
4. Follow Best Practices During Measurement
Dipping a probe into a liquid sample may seem like a straightforward task, but some technique and handling errors during measurement can impact measurement accuracy. Firstly, the sensor should always be handled with care – do not allow the glass bulb to come in contact with skin, clothes or other surfaces such as the walls or bottom of the sample container. As mentioned previously, be sure to rinse the probe gently with DI water before, between and after measurements. The fill hole cap (on refillable electrodes) should be loosened or opened during measurement, which enables the electrolyte to flow more quickly through the junction, speeding up stabilization. The reference solution should also be replenished as needed—follow manufacturer’s instructions to determine how close to the fill hole the solution level should be.
It is recommended to slowly stir the sample solution during measurement, as this not only ensures homogeneity but also speeds up response time due to the continuous flow of sample across the electrode. Ideally a magnetic stirrer should be used at a low setting, ensuring a consistent stirring speed for each sample (and calibration), with care taken to avoid contact between the probe and the stir bar. One should avoid using the probe itself to stir the solution, as this could risk damaging the probe and is less consistent. Stirring too fast can also impact pH readings, as vortexes can be created that pull air into the sample container. Avoid performing measurements in plastic containers, especially when using a stir bar; compared with glass, plastic is more susceptible to generating static electricity that can interfere with the probe.
Lastly, make sure that both the glass bulb and the reference junction are fully submerged in the sample solution. You will not be able to get a proper measurement if the junction is not in contact with the sample, so ensure there is enough sample in the container to submerge both components.