In theory, the potential difference across the glass membrane of the measurement electrode should be 0 mV if both the inner buffer solution and the measured liquid solution possess an equal pH value (normally pH 7). In practice however a potential difference of a few millivolts, the asymmetry potential, is measured across the membrane.
The difference in age of the inner and outer membrane glass gel layer is partly responsible for the asymmetry potential. The inner gel layer starts developing from the first day after the glass membrane is filled with the inner buffer solution (following manufacture) and will hardly change afterwards. The outer gel layer is continuously attacked through chemical reaction with the measured liquid solution and in certain cases even by abrasion from particulates in the process.
The asymmetry potential can also be attributed to small imperfections in the manufacture of the glass membrane. During normal use, exposure of the glass membrane to strong acids or strong alkaline solutions alters the external surface of the glass membrane to the extent that the response of the membrane to the presence of hydrogen ions gradually changes. Due to these concerns, Hamilton has several different types of pH membrane glass depending on the application.
The asymmetry potential should not be larger than +/- 47 mV (≈ +/- pH 0.8) at pH 7 and can be compensated for during the calibration process.