A pH sensor determines the alkalinity or acidity of a solution and in downstream bioprocess operations, it is an important parameter to measure because large biomolecules may only be stable within a certain pH range. To measure the pH there are two electrodes (one pH sensitive electrode and one a reference electrode) used in an electrochemical pH probe and the measured potential between the two electrodes defines the number of hydrogen ions in the solution, which by definition gives the pH value of the solution.
In downstream processing product purification operations, most often, liquid is flowing dynamically through a tube and ideally it is desirable for pH probe to provide real-time measurements of pH in a wide range. Due to the fact that in some processes, there may be some rapid shifts in pH due to valve position changes or process changes, the electrochemical pH measurement method offers ideal accuracy and rapid response that is required to capture these changes.
With the necessity to capture real-time, in-line measurements a flow cell is required to both securely hold the probe in place and also attach to the tube in which the liquid is flowing. Additionally, a flow cell should have a qualified pressure rating.
Electrochemical pH probes initially should have a 2-point calibration. Calibration values consist of both the zero point, the point where the pH electrode delivers zero potential, and the slope of the calibration line. Therefore the pH electrode has to be calibrated with accurately defined buffer solutions. In order to make an ideal calibration, the zero point of the measurement at pH 7, is completed by placing the probe in a buffer solutions of pH=7.0. The 2nd calibration point determines the slope of the calibration line with the 2nd point having a value ideally in the measuring range and at least 2 pH units different than pH of 7. In both cases if the slope remains stable, the pH 7 values can be checked and an offset entered into the software via either the PC software with eth control systems or the monitor menu.
PendoTECH offers two products for in-line pH measurements. The PendoTECH Process Development In-line pH Probe & Flow Cell is designed for process development that can be re-used or can be disposed of to eliminate concerns of cross contamination. This product requires calibration that can be completed by the graphical user interface software wizard if used with a PendoTECH Control System or if plugged directly into a pH monitor, the monitor menus are accessed to perform a 2 point calibration. This product does not have the complete biocompatibility profiles for use in GMP operations. The PendoTECH Single Use In-line pH Sensor is designed for GMP operations or can also be used in process development applications. This product comes pre-calibrated with values for the pH 7 and the slope. And it has fully traceability on materials and qualification of biocompatibility.