OxygenOxygen solubility is low in aqueous solutions, namely 36 mg l−^1 bar−^1 at 30°C in pure
water. Mass transfer is, therefore, determinant whether a culture suffers from oxygen
limitation or not.
Several attempts to measure pO 2 have been made in the past. Generally, oxygen is
reduced by means of a cathode operated at a polarising potential of 600–750 mV which is
generated either externally (polarographic method) or internally (galvanic method). A
membrane separates the electrolyte from the medium to create some selectivity for
diffusible substances rather than non-diffusible materials. The membrane is responsible
for the dynamic sensor characteristics which are diffusion controlled.
Measurements of oxygen in the gas phase are based on its paramagnetic properties.
Any change of the mass concentration of O 2 affects the density of a magnetic field and
thus the forces on any (dia-or para)magnetic material in this field. These forces on, for
instance, an electro-balance, can be compensated electrically and the current can be
converted into mass concentrations: further conversion into a molar ratio, e.g. % O 2 ,
requires the knowledge of total pressure.
Carbon dioxideCO 2 affects microbial growth in various ways according to its appearance in catabolism
as well as in anabolism. Morphological changes and variations of growth and metabolic
rates in response to pCO 2 have been demonstrated.
pCO 2 can be measured indirectly: the pH value of a bicarbonate buffer, separated from
the medium by a gas-permeable membrane, drops whenever CO 2 diffuses into this
compartment and vice versa; pH depends on the logarithm of pCO 2. Either a glass
electrode or optical principles can be used for pH determination.
CO 2 in the gas phase can be determined by means of its significant infrared
absorbance at wave lengths λ<15μm, particularly at 4.3μm; or by acoustic means. The
integrated photoacoustic spectroscopy and magnetoacoustics (PAS/MA) technology for
combined CO 2 and CO 2 analysis has a rapid response time and a small sample volume is
sufficient. The acoustic methods are accurate, stable over long periods and very simple to
use.
Redox potentialBioprocess media and culture liquids contain many different components which can exist
in a reduced and an oxidised form as redox couples. The resulting redox potential, as
measured by a redox electrode, is related to an “overall availability of electrons” rather
than to a specific compound. The extracellular redox measurement is very instructive,
specifically under micro aerobic conditions where the pO 2 sensor signal becomes
inaccurate. The signal generation is faster than that of pO 2 because the diffusion step is
omitted.
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