Fig. 8.11. Valve opening and pH changes with proportional plus derivative control (Shinskey, 1973).
is a measure of the oxidation-reduction potential of a biological system and can be determined as a voltage (mV), the value in any system depending on the equilibrium of:
Reduced form ^ Oxidized form + electron(s)
(negative value) (positive value)
The measuring electrode consists of gold, platinum or iridium which is welded to a copper lead. The interpretation of results presents difficulties and is confusing (Hailing, 1990). The culture is not at redox equilibrium until possibly the end of a growth cycle. During the cycle, although some redox half-reactions may be in equilibrium they cannot all be in a dynamic system. The micro-organisms can also be at a different redox potential from the broth. It has been speculated that the probe signal is indicating something about the relative concentrations of uncertain and probably varying chemical species! This is far from the ideal for a sensor which should only be measuring a specific factor. If the broth contains traces of oxygen this will probably dominate the signal. It is the ability to detect low concentrations of oxygen in media (1 ppm) where redox electrodes may have a good application for determining oxygen availability in anaerobic or micro-aerophilic processes operated on a small scale (Kjaergaard and Joergensen, 1979).
Hailing (1990) gives further details on routine handling, sterilization, testing and calibration.
Aspects of redox potential have been reviewed by Jacob (1970), Kjaergaard (1977) and Hailing (1990). It
The measurement of dissolved carbon dioxide is possible with an electrode, since a pH or voltage change can be detected as the gas goes into solution. The first available electrode consisted of a combined pH electrode with a bicarbonate buffer (pH 5) surrounding the bulb and ceramic plug, with the solution being retained by a PTFE membrane held by an O-ring. Unfortunately, this electrode was not steam sterilizable. This basic design has been modified so that dissolved C02 from the sample permeates a stainless-steel reinforced silicone bi-layer membrane and dissolves in the internal bicarbonate electrolyte of the electrode. The subsequent pH shift is determined by an internal pH element. This system is extremely sensitive to shifts in the pH element as one pH unit change represents a tenfold increase in pC02, but precautions have been taken in the design to ensure minimal drift of the sensor. It is possible to calibrate the electrode on-line using specially formulated buffer solutions. This new version can be steam sterilized (Gary et al., 1988).
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