pH drift can affect laboratory results, product quality, process control, and regulatory compliance. It is common in applications such as pharmaceutical testing, food and beverage analysis, water treatment, chemical manufacturing, and environmental monitoring. However, in most cases, pH drift can be reduced by identifying its cause and following good measurement practices.

Measuring pH with a pH sensor: accurate technique and proper electrode care are essential for identifying and correcting pH drift. Source: Mettler Toledo

What is pH drift?

pH drift is the gradual change in the pH value of a sample over time. For example, a solution that initially measures pH 7.00 may read pH 7.15 after sitting for several minutes or hours.

This change can happen for two different reasons:

• The sample has genuinely changed. Its chemical composition may be affected by air, temperature, evaporation, contamination, or an ongoing reaction.
• The measurement system is creating the change. The sensor may be dirty, poorly calibrated, damaged, or unsuitable for the sample.

Not all drifting pH readings indicate that the sample is changing. Sample pH drift results from genuine chemical changes in the sample, whereas electrode drift originates from the sensor or measurement system itself. Distinguishing between these two sources is essential for effective troubleshooting and reliable pH measurements.

Knowing which type of drift is occurring is the first step toward solving the problem.

Why does pH drift occur?

Exposure to air

Air exposure is a common cause of pH drift. Carbon dioxide from the atmosphere can dissolve into a solution and form carbonic acid, gradually lowering the pH. This is particularly noticeable when measuring pure water or alkaline solutions.

To reduce this effect, keep samples covered whenever possible and avoid leaving containers open longer than necessary.

Temperature changes

pH is influenced by temperature. A sample measured at room temperature may produce a different result when measured at a higher or lower temperature. Temperature can also affect the response of the pH sensor. Automatic Temperature Compensation (ATC) can improve measurement accuracy, but it does not stop the sample itself from changing. For consistent results, measure samples at a controlled and consistent temperature.

Contamination

Even a small amount of contamination can affect a pH reading. Residues from previous samples, cleaning agents, dirty glassware, or cross-contamination from another solution may all cause unexpected changes.

This is especially important when working with complex samples such as food products, pharmaceutical formulations, wastewater, and biological materials.

Evaporation and chemical reactions

When water evaporates, the remaining components in the sample become more concentrated. Depending on the sample, this may cause the pH to increase or decrease.

Some samples also continue to react after preparation. Oxidation, hydrolysis, fermentation, and biological activity can all cause genuine pH changes over time.

Sensor or calibration problems

Sometimes the sample is stable, but the instrument is not. A clogged reference junction, dirty glass membrane, dried-out sensor, damaged cable, or incorrect calibration can lead to unstable or drifting readings.

Expired or contaminated calibration buffers are another common cause of unreliable results.

Ways to fix and prevent pH drift

Check whether the drift is real

Before adjusting the sample, check the measurement system. Measure the same sample using a second, properly calibrated pH meter and sensor. If both instruments show a similar change, the sample is likely drifting. If only one instrument shows the change, inspect the sensor and calibration.

Use fresh calibration solutions

Reliable calibration starts with the right buffer solutions. Use fresh, certified buffers that cover the expected pH range of the sample.

Mettler Toledo offers technical buffer solutions in several pH values, including convenient single-use sachets. These sachets help minimize contamination because fresh buffer can be used for each calibration. Always check the expiration date and make sure the buffer group selected in the meter matches the buffers being used.

pH buffer solutions used for calibrating and verifying the performance of pH electrodes. Source: Mettler Toledo

Calibration frequency depends on the application. Samples that are difficult to measure or require high accuracy may need calibration more frequently than simple, stable solutions.

Keep the sensor clean and properly stored

Rinse the sensor with deionized water between measurements and gently blot it dry. Avoid wiping the glass membrane, as this can damage the surface or create static charges.

The sensor should also be stored in the correct electrode storage solution. Storing it in distilled or deionized water can shorten its service life and affect its performance. Cleaning should be tailored to the sample; for example, protein, oil, and solid residues may require different cleaning procedures.

Mettler Toledo solutions

Choosing the right sensor can make a significant difference, particularly when working with difficult samples. A sensor designed for clear aqueous solutions may not be suitable for viscous, oily, solid, or semi-solid materials.

Portfolio of sensors used for monitoring and measuring various parameters across laboratory, industrial, and process applications. Source: Mettler Toledo

Conclusion

pH drift can reduce accuracy, compromise product quality, and create costly process problems. The best solution combines proper sample handling, temperature control, fresh calibration buffers, suitable cleaning, correct sensor storage, and regular instrument verification.

Mettler Toledo pH meters, sensors and certified calibration solutions can help laboratories overcome many common measurement challenges. By selecting the right sensor and following a consistent calibration and maintenance routine, users can achieve more stable, accurate, and dependable pH results.

If you would like to learn more about pH calibration and practical ways to avoid measurement errors, explore pH Calibration: How-to Video