Low Drift in pH Electrodes

Drift is one of the principle problems with pH measurement, and the need for stability and repeatable standards of accuracy in this area is constant. Detailed below is how one pharmaceutical company puts a popular series of electrodes to the test in the search for a more reliable method of pH measurement.

If you were asked which are the three most commonly taken measurements in the world, what would you say? You might correctly guess that temperature is one and that weight is another, but would you guess that pH is, in fact, the third?

The theory of pH is well understood and pH electrode/meter systems have been available for many years. It is now very straightforward to obtain readings, but accuracy and repeatability are not so easy to achieve, especially in a multi-user environment with different samples and matrices.

The precision of pH measurement in the pharmaceutical industry is vital as it directly affects production, stability and analysis of the of the active ingredients.

Achieving accuracy and repeatability across the different departments at a large Pharmaceutical company like Pfizer at Sandwich falls to dedicated professionals in the guise of departmental metrologists.

"We had recognised the need for high-performance pH electrodes to give quick, accurate readings in a variety of samples. However we found that, from day to day, calibration readings were showing a marked drift, which led to a short lifetime for the electrode", explained one such postholder.

Pfizer approached Thermo Electron during an in-house exhibition run by Fisher Scientific (UK), to help to resolve these problems. After consideration of the many different pH applications that Pfizer undertake, it was decided that Thermo Electron would provide one of their new K-series electrodes on trial. These unique pH electrodes employ a patented cross-linked polymer as the reference electrolyte. The polymer itself is exceptionally stable and contains no silver ions - a frequent source of problems when they react with samples. Benefits for the user are excellent speed of response, high accuracy and an unrivalled, low drift.

Results

Calibration is performed daily when electrodes are in use. Millivolt values and electrode slope were recorded over a two month period. Millivolt values in pH 7 buffer show the E 0 value, and electrode slope indicates how Nernstian the response of the electrode is. Together these two values give a goods idea of the general health of the electrode. The results from Pfizer were certainly impressive:

Electrode 1 was in regular use and always calibrated against pH 7 and pH 4 buffers. Electrode 2 was in occasional use and calibrated with pH 7 and either pH 4 or pH 10 buffers. This accounts for the higher deviation in slope values (there is usually a difference in slope between alkaline and acidic values). Most importantly there was no sign of drift in the readings. A common cause of electrode failure is a gradual drift to the point where readings are outside the values permitted by the auto-calibration systems on most modern pH meters.

The K-series electrodes proved to be more accurate day-in, day-out that other electrodes that had been used in this department. Thermo Electron's experience shows that the excellent stability results in a fast speed of response, not only in buffers but also in samples. Lack of drift means that longer electrode lifetime can be expected with due care and maintenance.

Pfizer now use a variety of K-series pH electrodes at the Sandwich site: a standard K Series electrode for verifying buffers and routine research, a semi-micro version for small samples, an Ultra-K version for pure waters, and in addition to these, a micro mersion is being designed that can be used in NMR tubes and for samples of 100µL.

De-ionised and distilled water had been a particular problem area due to the paucity of ions present. Slow, drifting readings were the result. The Ultra-K electrodes that were supplied tackle these problems by utilising the stability of the K- polymer to improve the speed of response. In addition the polymer is sufficiently firm that no physical junction (e.g. a ceramic frit) is required. A simple drilled hole in the electrode body allows the reference material to be in direct contact with the sample over a much larger surface area. This reduces the large junction potentials that are present in conventional systems, making accurate readings easier and faster.

In the past, attempts at solving these types of application problems have resulted in the use of toxic mercury or light sensitive iodine in the reference system of pH electrodes. The K series electrodes use a safe, stable, non-toxic polymer that performs better and has a broader range of applications.