Electrochemical measuring cell

The invention is directed to an electrochemical measuring cell 1 for detecting different gas components. The electrochemical measuring cell includes several measuring electrodes (8, 12, 13), a common counter electrode 21 and a common reference electrode 20 in an aqueous electrolyte 3. The measuring electrodes each include a diffusion membrane (7, 11) and individual diaphragms (9, 14) limiting the inflow of gas to the individual measuring electrodes (8, 12). The diffusion membranes (7, 11) cover the measuring electrodes (8, 12, 13). The measuring cell 1 is so improved that the selectivity of the detection of different gas components is improved. An electrolyte barrier 19 is provided at least between one of the measuring electrodes 8 and the remaining measuring electrodes (12, 13). The electrolyte barrier 19 prevents lateral diffusion within the diffusion membrane (7, 11).

BACKGROUND OF THE INVENTION 
An arrangement for simultaneously detecting different gas components is 
disclosed in German patent publication 4,136,779. The arrangement includes 
a plurality of measuring electrodes, a common counter electrode and a 
common reference electrode in an aqueous electrolyte. The measuring 
electrodes are mounted behind a diffusion membrane and are subjected to 
the gas components to be detected via individual diaphragms forward of the 
diffusion membrane. These diaphragms limit the inflow of the gas. The 
formation of the measurement value takes place with the aid of a 
potentiostatic evaluating circuit which also controls the potentials on 
the measuring electrodes and also determines these potentials. 
It is disadvantageous in this known arrangement that cross sensitivities 
occur because of diffusion of the gas components within the diffusion 
membrane. These cross sensitivities affect the selectivity of the 
detecting reaction. The cross sensitivity can be reduced by selecting 
specific work potentials. However, the sensitivity of the gas detection is 
affected during specific applications. On the other hand, there are 
detecting reactions wherein a lateral diffusion between two measuring 
electrodes is required, such as in the determination of carbon monoxide. 
European patent publication 0,126,623 discloses an electrochemical 
measuring cell for detecting carbon monoxide. This measuring cell has two 
measuring electrodes and a gas path between the measuring electrodes. The 
known measuring cell has two platinum measuring electrodes as well as a 
platinum reference electrode and a platinum counter electrode. The 
measuring electrodes are mounted one behind the other and a partially 
hydrophobic and partially hydrophilic matrix lies between the measuring 
electrodes. The first measuring electrode is disposed on the upper end of 
the measuring cell and is subjected directly to the gas to be measured. 
The entire carbon monoxide portion is oxidized on this measuring 
electrode. Hydrogen is present in addition to carbon monoxide and is 
partially oxidized also on the first measuring electrode and then diffuses 
through the hydrophobic matrix to the second measuring electrode where it 
is completely converted. 
It is disadvantageous with this known measuring cell that a detection of 
additional gases is not possible. 
SUMMARY OF THE INVENTION 
It is an object of the invention to provide a measuring cell of the kind 
described above which is improved in that the selectivity of the different 
gas components to be detected is improved. 
The electrochemical measuring cell of the invention is for detecting 
different gas components of a gas sample and includes: a housing having at 
least two openings directed toward the gas sample and defining an 
electrolyte chamber; an aqueous electrolyte contained in the chamber; a 
plurality of measuring electrodes disposed in the aqueous electrolyte; 
diffusion membrane means covering the measuring electrodes; a reference 
electrode common to the measuring electrodes; a counter electrode common 
to the measuring electrodes; the reference electrode and the counter 
electrode being disposed in the electrolyte so as to be in spaced 
relationship to each other and to the measuring electrodes; first and 
second individual aperture means for limiting the flow of the gas sample 
to individual ones of the measuring electrodes; and, electrolyte barrier 
means for defining an electrolyte barrier between at least one of the 
measuring electrodes and the remaining ones of the measuring electrodes to 
prevent a lateral diffusion within the diffusion membrane means. 
The advantage of the invention is primarily seen in that a lateral 
diffusion between individual measuring electrodes is effectively prevented 
by an electrolyte barrier within the diffusion membrane. The electrolyte 
barrier can be configured in such a manner that the diffusion membrane is 
interrupted between the measuring electrodes and is welded to a carrier at 
the separating location. 
The diffusion membrane is advantageously configured as a first diffusion 
membrane and a second diffusion membrane. The electrolyte barrier is a gap 
between the first diffusion membrane and the second diffusion membrane 
which is filled with electrolyte. 
In an advantageous manner and to detect a first gas component, a first 
electrode is provided with a first diaphragm and, to detect a second 
component, a second measuring electrode is provided with a second 
diaphragm which limits the inflow of gas to the second measuring 
electrode. A third measuring electrode is connected to the second 
measuring electrode via the diffusion membrane functioning as a gas path. 
The electrolyte barrier is disposed in the diffusion membrane between the 
first measuring electrode on the one hand, and the second and third 
measuring electrodes on the other hand. 
Such a measuring cell is especially advantageously suited to detect a 
mixture of oxygen, carbon monoxide and hydrogen. The oxygen is reduced at 
the first measuring electrode. Carbon monoxide and a portion of the 
hydrogen are converted at the second measuring electrode. The portion of 
the hydrogen, which is not oxidized at the second measuring electrode, 
reaches the third measuring electrode via the diffusion path and is 
completely converted at this third measuring electrode. A lateral 
diffusion of oxygen from the second measuring electrode to the first 
measuring electrode is prevented by the electrolyte barrier in the 
diffusion membrane between the first measuring electrode on the one hand, 
and the second and third measuring electrodes on the other hand.

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION 
The measuring cell 1 includes a measuring cell housing 2 which encloses an 
electrolyte chamber 4 filled with an aqueous electrolyte 3. A first 
breakthrough 5 and a second breakthrough 6 are provided at the upper end 
of the measuring cell housing 2. The first breakthrough 5 is closed by a 
first diffusion membrane 7 and a first measuring electrode 8. The first 
measuring electrode 8 is applied to the first diffusion membrane 7 at the 
side thereof facing toward the electrolyte chamber 4. The entry of gas to 
the first diffusion membrane 7 is limited by a first diaphragm 9 having a 
capillary 10. The second breakthrough 6 is closed by a second diffusion 
membrane 11. In the region of the second breakthrough 6, a second 
measuring electrode 12 is mounted on the side facing toward the 
electrolyte chamber 4 and a third measuring electrode 13 is mounted 
outside of the overlap region of the second breakthrough 6. 
The third measuring electrode 13 is connected via the second diffusion 
membrane 11 (functioning as a gas path) to the second breakthrough 6 and 
the second measuring electrode 12. The gas entry to the second diffusion 
membrane 11 is limited by a second diaphragm 14 disposed forward of the 
second breakthrough 6. A glass fiber mat 15, which is impregnated with the 
electrolyte 3, is pressed upon the measuring electrodes 8, 12 and 13. The 
glass fiber mat 15 communicates with the aqueous electrolyte 3 via a 
porous T-shaped member 17 and a porous intermediate piece 18. 
A gap 19 is located between the first diffusion membrane 7 and the second 
diffusion membrane 11 and is filled with electrolyte 3. The gap 19 
functions as an electrolyte barrier which prevents the diffusion of gas 
between the diffusion membranes 7 and 11. A reference electrode 20 and a 
counter electrode 21 are mounted within the electrolyte chamber 4 as 
common electrodes of the measuring electrodes 8, 12 and 13. 
The measuring cell 1 of the invention is especially suitable for 
investigating a gas mixture comprising oxygen, carbon monoxide and 
hydrogen such as in the analysis of flue gas. The measuring electrodes (8, 
12, 13), the reference electrode 20 and the counter electrode 21 are 
connected to a triple potentiostat (not shown) as known per se. The 
potential at the first measuring electrode 8 is so adjusted that all 
oxygen located in the breakthrough 5 is reduced at the first measuring 
electrode 8. The potentials of the second measuring electrode 12 and the 
third measuring electrode 13 are adjusted to approximately equal values in 
a manner that the carbon monoxide, which diffuses in via the second 
diaphragm 14 into the second breakthrough 6, is completely oxidized at the 
second measuring electrode 12 and that also the hydrogen is partially 
oxidized and that the excess hydrogen reaches the third measuring 
electrode 13 through the second diffusion membrane 11 and is completely 
converted at the third measuring electrode 13. A direct diffusion of 
oxygen from the region of the second measuring electrode 12 into the 
region of the first measuring electrode 8 is prevented by the electrolyte 
barrier 19 between the first diffusion membrane 7 and the second diffusion 
membrane 11. 
It is understood that the foregoing description is that of the preferred 
embodiments of the invention and that various changes and modifications 
may be made thereto without departing from the spirit and scope of the 
invention as defined in the appended claims.