16. Oxygen Measurement Flashcards
How may the concentration
of oxygen in a gas mixture be
measured?
x7
The different methods are: > Clark polarographic electrode > Galvanic fuel cell > Paramagnetic O2 analyser > Mass spectrometer > Photoacoustic spectroscope > Raman spectroscope > Chemical (e.g. Haldane apparatus).
Clark polarographic electrode:
Fig. 65.1 The Clark electrode
overall rxn
4Ag O2 2H2O 4 Cl 4 AgCl 4OH
> Silver/silver chloride (Ag/AgCl) anode
and platinum (Pt) cathode are
suspended within a potassium chloride (KCl) solution. Both are covered
by an oxygen permeable membrane.
> Voltage of 0.6 V is applied across
the electrodes to allow linearity
between the current measured
and the oxygen concentration in the sample.
> Flow of current is measured.
> Anode reaction: electrons (e–) are
generated by the reaction of Ag+ with
the Cl– ions from the KCl solution.
> Cathode reaction: O2 combines with e– and water to generate hydroxyl (OH–) ions. O2 + 4e– + 2H2O → 4OH–
> The greater the amount of O2 available,
the greater the rate of electron
uptake at the cathode and
hence the greater the flow of current.
> Flow of current is therefore proportional to
the O2 tension at the cathode.
> Halothane may cause falsely high
O2 readings but this problem is
overcome by the use of a
halothane-impermeable membrane.
> The cathode has a lifespan of around
3 years because it becomes coated
in protein.
The membrane covering it helps to
reduce protein deposition,
but increases the
response time of the electrode.
> The silver chloride anode
will eventually be consumed.
Galvanic fuel cell:
> Similar in principle to the
Clark polarographic electrode.
> Gold (Au) mesh cathode and
lead (Pb) anode are suspended within a
potassium hydroxide (KOH) solution.
> Anode reaction:
electrons are generated from
the reaction between OH–
from KOH and the Pb anode.
Pb + 2OH– → PbO + H2O + 2e–
> Cathode reaction:
O2 combines with electrons
and water to generate hydroxyl ions
(i.e. same reaction for the Clark electrode).
O2 + 4e– + 2H2O → 4OH–
> Unlike the Clark electrode,
no battery is required as the
fuel cell generates
its own voltage.
> Response time of the system is
slow at approximately 30 s and therefore
not suitable for breath-to-breath measurements.
> Reagents are consumed limiting
equipment lifespan to 6–12 months.
> The redox reaction at the
cathode is temperature sensitive.
Hence, temperature compensation is
achieved using a thermistor.
> Gas mixtures containing N2O
may damage the fuel cell.
N2O reacts at the lead anode,
generating N2,
which alters the pressure within the cell
potentially causing damage.
Paramagnetic oxygen analyser:
> O2 is a paramagnetic gas,
which means
that it is attracted
towards a
magnetic field
because it has unpaired electrons
in its outer shell.
Most other gases (e.g. N2)
are diamagnetic and are
repelled from magnetic fields.
> Analyser is composed of
two nitrogen-filled glass spheres
connected in a dumbell arrangement,
suspended from a filament within a gas-tight
chamber. A mirror is attached to the dumbell.
> Glass spheres are subjected to a
non-uniform magnetic field.
> If O2 is added to the chamber,
it is attracted towards the magnetic field,
causing rotation of the glass spheres.
> The degree of rotation of the
glass spheres can be measured using a
simple light beam deflection principle.
A beam of light passing to the
mirror gets deflected as the mirror rotates.
This deflected beam is sensed by a photodetector, which is calibrated to match the degree of rotation of the system to the oxygen concentration within the chamber.
> Newer versions use the
null deflection principle.
Instead of the glass
spheres rotating,
a current is supplied to oppose
the movement of the spheres.
The amount of current required to
keep the spheres in their
resting position is calibrated
to O2 concentration.
Mass spectrometer:
> Gas mixture is drawn into an
ionising chamber where it is
bombarded by electrons.
As the electrons collide,
they can knock off electrons from the
gas molecules causing some
of them to become charged.
> These charged particles are then accelerated through a strong magnetic field that deflects them to varying degrees depending on their mass and charge.
The ion streams are measured at a
detector plate, the number of plates
determining how many gases can be measured.
> Compounds of identical molecular weight
(MW) are distinguished by
identifying their breakdown products,
e.g. N2O and CO2 both have MW 44,
so N2O is identified from
its smaller nitric oxide fragment (MW 30).
> A rapid response times of less than
0.1 s means mass spectrometry can
be used for continuous gas analysis.
> It can measure a variety of gases within a mixture.
> Water vapour can interfere with the apparatus.
> It is a very bulky and an expensive piece of
equipment.
Photoacoustic spectroscopy:
> Based on the photoacoustic effect,
which was discovered by Alexander Graham Bell
in his search for a means of wireless communication.
> Photoacoustic effect is the conversion
between light and sound waves.
> Materials exposed to non-visible portions
of the light spectrum
(i.e. infrared and ultraviolet light)
can produce acoustic waves.
> By measuring the sound at
different wavelengths,
a photoacoustic spectrum of a gas sample
can be recorded and used to identify the
components within that sample.
> This effect can be used to
study solids, liquids and gases.
Raman spectroscopy:
> When light interacts with a
gas molecule the
rotational and vibrational energy of the
molecule is altered during the interaction.
The resulting transfer of energy
to or from the light changes
its wavelength by amounts
characteristic of the molecule concerned.
Monochromatic radiation is therefore changed during its passage through a gas sample chamber into a spectrum of wavelengths, which depends on the structure of the individual gas molecules.
Thus the type of molecules present and their
concentrations in the gas sample can be determined.
> Raman effect is the scattering of a
photon in a gas.
Raman scattering can occur with a change in vibrational, rotational or electronic energy of a molecule.
> Raman spectroscope is composed of a
helium–neon laser
as its radiation source,
a gas sample cell and a set of eight detectors,
each with a specific radiation wavelength filter.
> Filters are manufactured to measure
O2, N2, CO2, N2O
and certain volatile agents.
Haldane apparatus:
> This is utilised as an instrument for
estimating the proportion of oxygen in
expired gases.
> It consists of a burette containing a volume of gas.
> The gas is then exposed to a solution of
pyrogallol (a powerful reducing
agent able to absorb oxygen).
> The volume of the remaining gas is then measured.
> The reduction in gas volume
indicates the quantity of oxygen absorbed
by the pyrogallol.
> This system can also be used to measure
CO2, but here potassium
hydroxide solution is used instead of pyrogallol.
