Automated Measurement and Monitoring of Bioprocesses: Key Elements of the M3C Strategy - Measurement, Monitoring, Modelling, and Control of Bioprocesses - page 5

The classical electrodes for pH, redox, and pCO 2 monitoring measure an
electrical potential difference and, therefore, require good cabling and amplifiers
with high input impedance. The classical electrodes for pO 2 monitoring measure
an electrical current which is quite low; therefore, the demands with respect to
electrical components are also high. Temperature compensation of the raw signals
is often implemented in combination electrodes and the amplifiers. New versions
of electrodes based on optical principles do not suffer from these electrical
constraints.
2.2.1 pH
Many currently used pH electrodes are constructed from special glass. Today, such
electrodes no longer suffer from a significant alkali error, they show very little drift
over time, and their lifetime is usually limited by (unintended) mechanical damage.
The electrodes are constructed as combination electrodes; i.e., they also contain the
reference electrode in a single rod. The reference is usually an Ag/AgCl electrode
dipped into a reference electrolyte and must be in electrical contact with the mea-
suring solution. If the reference electrolyte is a low-viscosity liquid, it must be
separated from the measuring solution by a diaphragm in order to minimize
exchange of the two liquids but satisfy the electrical connection. These diaphragms
are built of porous glass or ceramics and can clog, especially when proteins adsorb or
salts precipitate. The combination electrode will then drift (significantly) due to the
increasing electrical resistance of the diaphragm junction. This is sometimes
counterbalanced by pressurization of the reference electrolyte chamber, thus forcing
a small electrolyte efflux through and ''cleaning'' the diaphragm. Newer types of pH
electrode have a hydrogel-solidified reference electrolyte and no longer need a
diaphragm. Such electrodes are, in our experience, much more stable and show
practically no drift over their (mechanically limited) lifetime.
These classical pH electrodes are potentiometric electrodes: they determine a
potential difference. Hence, the amplifier must have very high input impedance
and the cable connecting the electrode to the amplifier should be as short as
possible and must be well shielded. Newer versions have the amplifier [plus some
further intelligence such as calibration data, therefore also being called intelligent
sensor management (ISM)] integrated on a built-in chip and transmit data in digital
form, in some cases also via a wireless path. This is certainly a timely improve-
ment compared with the critical transmission of analog raw data.
Specifically from food biotechnologists came the need to substitute glass—if a
glass electrode breaks in a production batch, they had to dump the whole batch;
imagine, if you were to eat a yoghurt containing broken glass. Suppliers were able to
come up with an alternative, i.e., the proton-selective field-effect transistor
(pH-FET), which is a derivative of the usual metal-oxide-semiconductor FET
(MOSFET). The gate electrode made, e.g., of Ta 2 O 5 is not electrically charged but
exposed to the measuring solution, and so controls the current from source to drain.