Mettler Toledo's guidelines for pH measurement

Measurement of pH is a frequent task in a lab, the results of which are often dependent on the performance of the instruments used.

While there are regular check-ups and calibration possibilities for pH meters, measurements can be influenced by external factors, such as the surrounding temperature or the age of the buffer solutions.

Technique directly affects the success and repeatability of experiments and so Mettler Toledo has provided a few tips for being more accurate.

Below are a selection of its recommendations:Diaphragm clogging: TRIS-based buffers can damage standard pH equipment, as the reference junction on conventional pH electrodes can clog when TRIS reacts with silver ions in the filling solution.

This may also occur with protein in the buffer, such as BSA, eventually causing slow or fluctuating readings, or even wrong results.

To ensure reliable results and accurate buffer values choose an electrode specifically designed to be compatible with TRIS-based buffers.

Automatic and manual temperature compensation: Automatic temperature compensation (ATC) is most effective with sample volumes greater than 10 ml.

Sensors with an integrated temperature probe allow ATC by the pH meter, whereas sensors without an integrated temperature probe require a separate probe.

Manual temperature compensation is extremely accurate, but time-consuming.

Always enter temperatures in the measurement settings of the pH meter, and adjust with every temperature change.

Store sample and sensor at the same temperature: Store the sensor with the samples, as the greatest accuracy is achieved when the pH membrane, reference system and sample are at the same temperature.

Measure the sample, not the sensor: With very small samples, the sensor can take so long to reach equilibrium that its temperature is wrongly interpreted as the sample temperature.

Take the time necessary to ensure that you actually measure the sample temperature.

Use fresh buffers: Unopened buffers typically have a shelf life of two years, opened buffers three to six months, and alkaline buffers (pH 10 or higher) one month, as their pH changes noticeably through contact with carbon dioxide in the air.

Always use fresh buffer solution, store buffers in closed bottles and never use them after the expiry date.