Page 396 - Instrumentation Reference Book 3E

Common electrochemical analyzers 379

will be burned or consumed on the outer elec- The main limitation of this type of oxygen
trode and in doing so use the stoichiometric sensor is the rate of diffusion of oxygen across
amount of oxygen required for combustion. the membrane; this determines the speed of
response and, at low oxygen partial pressure, this
may become unacceptably slow. However, to
17.8.4 Fuel cell oxygen-measuring instruments overcome this, one type of fuel cell oxygen sensor
Galvanic or fuel cells differ from polarographic has a completely exposed cathode, i.e., not
cells and the high temperature ceramic sensors in covered with a PTFE membrane.
that they are power devices in their own right, In common with ail membrane cells, the
that is, they require no external source of power response of the micro-fuel cell is independent of
to drive them. One manufacturer’s version is sample flow rate but the cell has a positive tem-
shown in Figure 17.37. perature-dependence. This is accommodated by
A lead anode is made in that geometric form incorporating negative temperature coefficient
that maximizes the amount of metal available for thermistors in the measuring circuit. These fuel
reaction with a convex disc as the cathode. Per- cells have sufficient elec:rical output to drive
forations in the cathode facilitate continued wet- readout meters without amplification. However,
ting of the upper surface with electrolyte and where dual- or multi-range facilities are required
ensure minimum internal resistance during the some amplification may be necessary.
oxygen sensing reaction. The surfaces of the cath-
ode are plated with gold and then covered with a
PTFE membrane. Both electrodes are immersed 17.8.5 Nersch cell for oxygen measurement
in aqueous potassium hydroxide electrolyte. Dif- This galvanic cell differs from fuel cells in that a
fusion of oxygen through the membrane enables third electrode is added to the cell and a potential
the following reactions to take place: applied to provide anodic protection to the
anode. In one manufacturer’s cell (Figure 17.38)
Cathode 4e- + 02 + 2H2O + 4OH- the cathode is silver and the anode cadmium. The
Anode third electrode is platinum. The anodic protection
Pb + 2OW- i PbO + H2O + 2e
Overall limits the cadmium current to a few micro-
amperes and extends the life of the cadmium.
cell reaction 2Pb + 02 --j PbO However, this arrangement gives an electrical
The electrical output of the cell can be related to output from the cell, which is non-linear with
the partial pressure of oxygen on the gas side of the oxygen partial pressure, and it is necessary for
membrane in a manner analogous to that the signal to be passed through a “shaping” circuit
described for membrane-covered polarographic for the readout to be given in concentration units.
cells. In this instance, however, because there is Calibration is carried out by generating a pre-
no applied potential and no resultant hydrolysis determined concentration of oxygen in a sample
of the electrolyte, absence of oxygen in the sample by electrolysis, and electrodes for this are incorp-
corresponds to zero electrical output from the cell. orated in the cell. When dry gas samples are
There is a linear response to partial pressure of being used they must be humidified to prevent
oxygen, and a single point calibration, e.g., on the water-based electrolyte in the cell from
air, is sufficient for most purposes. drying out.

17.8.6 Sensor for oxygen dissolved in water
PTFE membrane
Electrochemical sensors with membranes for oxy-
gen determination can be applied to measuring
oxygen dissolved in water; both polarographic
and galvanic sensors can be used.
Electrolyte
A most popular type of sensor is the galvanic
Mackereth electrode. The cathode is a perforated
silver cylinder surrounding a lead anode with
an aqueous electrolyte of potassium bicarbonate
(Figure 17.39). The electrolyte is confined by a
silicone rubber membrane, which is permeable to
oxygen but not to water and interfering ions.
Lead anode The oxygen which diffuses through the mem-
brane is reduced at the cathode to give a current
Figure 17.37 Diagrammatic micro-fuel cell oxygen
sensor. CourtesyAnalysis Automation. proportional to the oxygen partial pressure.