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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.

What glass type best handles autoclave / SIP exposure?

Hamilton has done extensive research on electrolytes and pH membrane glass formulations to limit drift due to autoclave / SIP exposure. For these applications the PHI glass formulation is recommended. PHI glass is highly resistant to drift thus provides accurate pH measurement over multiple sterilization cycles.

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