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 E0 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. |
