All of us know how essential is oxygen for our living…
As this is what happens without oxygen
Instrumentation engineers too realized the
importance of oxygen thus they made an instrument “OXYGEN ANALYZER” for
measuring the proportion of oxygen and displaying the oxygen content.
One among the primarily used oxygen analyzer is
discussed
Zirconia
Oxygen Analyzer (Direct Method):
This method gives the amount of oxygen
using the concepts of electrochemistry. Here there is an electrolyte; 2
electrodes and a generated EMF (tells the oxygen concentration).
Let us have more insight of each component:
1. Zirconia
sensor:
2. Reference and Measured
Gas:
The reference
gas is usually taken as air which has a fixed amount of oxygen in it (20.9%)
and measured gas is the one whose oxygen concentration has to be measured.
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3 . Platinum electrodes:
It comprises of two
electronically conducting, chemically inert electrodes attached to either side
of a solid electrolyte tube. The plates provide a catalytic surface for the
change in oxygen molecules.
4. Electric Oven,
Thermocouple and Temperature Controller:
In Zirconia
sensor the temperature has to be above 4500C as only after that the
zirconia become active as an electrolyte. Thus a thermocouple measures the
temperature and controller gives the desired o/p to the electric heater and the
desired temperature is maintained.
Principle of Measurement
There is a
difference in concentration of oxygen (consequently a difference in the partial
pressure of oxygen) on either side of the measured gas and reference gas which drives
the oxygen to move from higher concentration to lower concentration.
Depending on the
concentration on either side following reactions occur on each electrode:
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The oxygen ions
formed flow through the electrolyte and the electrons through the outer wire as
happens in the galvanic cell producing an EMF.
Just a
small review of class 8
A galvanic
cell is an electrochemical cell which converts
chemical energy to electrical energy through a spontaneous chemical reaction.
In a galvanic cell there are two half-cells. Each half-cell contains an
electrode in an electrolyte. The separation is necessary to prevent direct
chemical contact of the oxidation and reduction reactions, creating a potential
difference. The electrons released in the oxidation reaction travel through an
external circuit before being used by the reduction reaction.
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In oxygen
analyzer we don’t need to have separate half cells as here zirconia is a solid
acting as an electrolyte so no contact of the reaction takes place.
Let us
move to high school and review some further concepts
The EMF
generated by the galvanic cell in non-standard condition is given by the Nernst
Equation:
Where, E cell 0 is the
cell potential under standard conditions (25oC, 1.0 M solution
concentrations, 1.0 atm gas pressures), E
cell is the
cell potential under nonstandard conditions, R is the gas constant 8.314 J/mol-K, T is the temperature in Kelvin, F is Faraday's constant (96,485 coulombs/mol), n is the number of electrons transferred in the balanced
oxidation/reduction reaction, and Q
is the reaction quotient ([products]/[reactants]).
Applying
the Nernst Equation in Zirconia Oxygen Analyzer
The
potential difference across the cell is given by Nernst equation:
Here, E= E cell 0 -E cell
all the terms represent the same terms as illustrated above and P1& P2 are
the partial pressures of the oxygen on either side of the zirconia tube.
The Nernst
equation can therefore be reduced to:
Thus, if the oxygen partial pressure at one of the electrodes is known
(reference gas-Air) and the temperature of the sensor is controlled (constant
temperature for zirconia to act as a conducting electrolyte) and then
measurement of the potential difference between the two electrodes enables the
unknown partial pressure to be calculated using above equation. This partial
pressure refers to the unknown oxygen concentration.
The following 3
cases can be there:
If P1>P2;
ions flow from P1 to P2 i.e. a positive EMF
If P1<P2;
ions flow from P2 to P1 i.e. a negative EMF
If P1=P2; no net
ion flow i.e. a zero EMF
The analyzer
electronically calculates the oxygen partial pressure, and therefore oxygen
concentration, of a sample gas with unknown oxygen concentration. This is
accomplished by measuring the potential, E, produced across the zirconium cell
electrodes, substituting for E in the Nernst equation and anti-logging to
obtain P1 (sample).
The zirconia
oxygen analyzer may be used for the measurement of oxygen at any level between
0-100% in gas mixtures.
But the Zirconia Oxygen Analyzer has a few
drawbacks
1.
Incapable of measuring
flammable gas mixtures due to the high temperature that needs to be maintained
for the conductivity of zirconia.
2.
As there is a sensor that comes
in direct contact with the reference gas, so can’t be used for corrosive
gases.
For
overcoming these drawbacks another oxygen analyzer was introduced for knowing
more about it stay tuned for the next article…
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