Bioreactors used in pharmaceutical processes normally require autoclavation or SIP (Sterilize In Place) prior to the start of a batch. pH sensors are in contact with the process so are exposed to elevated heat and fluctuating pressure during this process. Over repeated sterilizations the end-user may experience drift in the pH measurement. This phenomenon is explained below.
The pH Glass Membrane
The measurement half-cell of the pH electrodes is comprised of three main parts: an Ag/AgCl element, an internal buffer at 7pH, and a pH sensitive glass membrane. Alkali metal salts embedded within the membrane glass undergo an ion exchange reaction when exposed to hydrogen ions in the measured liquid. This ion exchange produces a voltage potential which fluctuates based on changing pH. These changes in voltage are the raw sensor output used by the controller or transmitter to display pH values.
The Effects of Autoclave or SIP Cycles on pH Sensors
In autoclave / SIP cycles, the prolonged exposure to elevated temperatures causes some alkali metal salts within the membrane glass to release into the internal buffer solution. The excess alkali hydroxides cause a shift from 7pH. This change the electrode buffer solution alters the sensor output thus resulting in a shift in the mV output of the sensor.