U.S. patent number RE31,916 [Application Number 06/258,752] was granted by the patent office on 1985-06-18 for electrochemical detection cell.
This patent grant is currently assigned to Becton Dickinson & Company. Invention is credited to Keith F. Blurton, Harry G. Oswin.
United States Patent |
RE31,916 |
Oswin , et al. |
June 18, 1985 |
Electrochemical detection cell
Abstract
An electrochemical cell comprising an anode, a cathode and a
reference electrode operating in an aqueous electrolyte is utilized
for detection of noxious gases in air. The gas is oxidized at the
anode and detection thereof occurs as a result of the current
generated by the reaction. A fixed potential difference is
maintained between the anode and the reference electrode to avoid
generation of undesired current from reactions involving an
oxygen-water redox couple within the cell which would invalidate
anode-cathode current for gas detection purposes. The fixed
potential is chosen from within the range of about 0.9 to 1.5
volts.
Inventors: |
Oswin; Harry G. (Chauncey,
NY), Blurton; Keith F. (Ossining, NY) |
Assignee: |
Becton Dickinson & Company
(Paramus, NJ)
|
Family
ID: |
26778484 |
Appl.
No.: |
06/258,752 |
Filed: |
April 29, 1981 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
Reissue of: |
088267 |
Nov 10, 1970 |
03776832 |
Dec 4, 1973 |
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Current U.S.
Class: |
204/412;
204/432 |
Current CPC
Class: |
G01N
27/4045 (20130101) |
Current International
Class: |
G01N
27/49 (20060101); G01N 027/46 () |
Field of
Search: |
;204/1T,1N,1S,1K,195P,195R |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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1153551 |
|
Aug 1963 |
|
DE |
|
1163576 |
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Feb 1964 |
|
DE |
|
1202595 |
|
Aug 1970 |
|
GB |
|
Other References
Ives et al., "Reference Electrodes", 1961, pp. 360-378. .
K. A. Gresch et al., "Die Coulometrische Analyse", p. 83, Verlay
Chermie, Weinhein, (1961). .
W. Vielstick, "Brennstoff Element", p. 41, Verlag Chermie,
Weinheim, (1965). .
J. E. Harrar et al., Anal. Chem., pp. 1148-1156, vol. 38, No. 9,
Aug. 1966. .
J. S. Mayell et al., J. Electrochem. Soc., pp. 438-446, Apr. 1964.
.
W. T. Grubb et al., J. Electrochem. Soc., pp. 477-478, Apr. 1964.
.
Giichi Muto et al., Analysis and Instruments, vol. 6, No. 5, pp.
287-291, (1968)..
|
Primary Examiner: Tung; T.
Attorney, Agent or Firm: Larson and Taylor
Claims
What is claimed is:
1. An electrochemical cell for quantitatively detecting a
.Iadd.noxious .Iaddend.gas .Iadd.in the presence of air
.Iaddend.comprising an anode having catalyst bonded to
polytetrafluoroethylene to provide a diffusion electrode, a
cathode, a reference electrode .Iadd.at which substantially no
current flow.Iaddend., and an aqueous electrolyte .[.in contact
with.]. .Iadd.within an electrolyte container, each of said anode,
cathode and reference electrode having a catalytic surface
contacting .Iaddend.said .[.anode, cathode and reference
electrode;.]. .Iadd.electrolyte, and said reference electrode
contacting air, .Iaddend.means for exposing said anode to a
.[.gaseous substance.]. .Iadd.gas containing a noxious gas
.Iaddend.to be detected; .Iadd.said anode being arranged in said
cell so that one surface of said anode contacts said gas exposing
means and the opposite surface contacts said electrolyte whereby
the noxious gas to be detected migrates from the surface contacting
said gas exposing means to the internal surface of said anode to
interface with the electrolyte for reaction in the presence of the
catalyst, .Iaddend.means connecting said anode and reference
electrode, said latter means being a potentiostat .[.for.].
maintaining a fixed potential on said anode relative to said
reference electrode of from about 0.9 to 1.5 volts with respect to
the potential of the reversible hydrogen couple in the electrolyte
of said cell which potential is independent of the concentration of
the .Iadd.noxious .Iaddend.gas to be detected and means for
measuring said current flowing between sad anode and cathode of
said cell, said measured current being a measure of the
concentration of the gas being detected.
2. The electrochemical cell of claim 1 wherein the anode comprises
a material selected from the group consisting of platinum, rhodium,
iridium, ruthenium, palladium, osmium, tungsten oxide, tungsten
carbide, molybdenum oxide, molybdenum sulphide, gold, and alloys or
mixtures thereof.
3. The electrochemical cell of claim 2 wherein the anode includes
platinum.
4. The electrochemical cell of claim 2 wherein the anode includes
gold.
5. The electrochemical cell of claim 1 wherein the electrolyte is
potassium hydroxide.
6. The electrochemical cell of claim 1 wherein the electrolyte is
phosphoric acid.
7. The electrochemical cell of claim 1 wherein the electrolyte is
sulfuric acid.
8. The electrochemical cell of claim 1 wherein the electrolyte is
contained in a matrix.
9. The electrochemical cell of claim 1 wherein the electrolyte is
free flowing. .[.10. An electrochemical cell for quantitatively
detecting a gas comprising an anode, a cathode, a reference
electrode, and an aqueous electrolyte in contact with said anode,
cathode and reference electrode; means for exposing said anode to a
gaseous substance to be detected including a chamber behind said
anode, said chamber defining a labyrinthine path; means connecting
said anode and reference electrode, said latter means being a
potentiostat for maintaining a fixed potential on said anode
relative to said reference electrode of from about 0.9 to 1.5 volts
with respect to the potential of the reversible hydrogen couple in
the electrolyte of said cell which potential is independent of the
concentration of the gas to be detected, and means for measuring
said current flowing between said anode and cathode of said cell,
said measured current being a measure of the concentration of the
gas being detected..].
Description
BACKGROUND OF THE INVENTION
Field of the invention
The present invention relates to electrochemical cells, and
particularly to the structure and arrangement of a cell especially
suitable for detection and measurement of noxious gases in the
atmosphere.
Discussion of the prior art
In recent times, great awareness has developed regarding the
dangers of air pollution, particularly in urban or industrialized
areas. As the level of noxious elements in the atmosphere
increases, a greater need arises for equipment to detect and
measure the quantity of such elements so that their presence in the
atmospheres can be reduced or eliminated. In order to meet needs
arising in connection with pollution control, extensive activity
has been devoted to development and production of equipment useful
in solving this problem. For the successful development of such
equipment, primary consideration must be accorded to the
requirements of commercial and operational feasibility. Although
systems may exist which may be considered functionally successful,
actual utilization in practical applications has quite often been
thwarted due to the cost or complexity of such equipment.
Therefore, in many cases where beneficial reduction of air
pollution has been an important desideratum, its achievement has
been rendered impractical by the inordinately complex or costly
aspects of the means proposed therefor.
Accordingly, there exists an urgent present need for air pollution
control equipment which is both effective in operation and which
can be practically utilized in widespread commercial applications
without incurrence of excessive cost. This requirement exists in
connection with equipment for the detection and measurement for
polluting materials, as well as for equipment whereby the
quantities of such materials may be controlled or reduced.
The general criteria applied to measuring and testing equipment
such as the cell of the present invention include requisites for
portability, non-prohibitive cost and accuracy in measuring the
quantity of the gas detected. In the prior art, it has been found
difficult to simultaneously fulfill all of these requirements.
Increasing the accuracy of measuring equipment has inherently
involved an increase in either the size or the complexity of such
equipment thereby disadvantageously affecting either cost or
portability or both. Quite often, problems related to the
simultaneous provision of these features have been decisive in
obstructing the practical development and utilization of particular
detection apparatus.
It is, therefore, considered of significant importance and a
valuable contribution to the art of pollution control equipment to
provide detection apparatus capable of accurately measuring gas
quantity which is also of a relatively convenient size enabling
portability, and which does not involve prohibitive cost for its
manufacture and practical utilization.
SUMMARY OF THE INVENTION
Briefly, the present invention may be described as an
electrochemical cell for the detection of noxious gases, said cell
comprising an anode, a cathode, an aqueous electrolyte, means for
exposing the anode to a substance to be detected, means defining a
reference potential, and means for maintaining a fixed potential
upon said anode relative to said reference potential, said fixed
relative potential being from within a range wherein an
oxygen-water redox couple within the cell is ineffective to
generate current at a level which is discernible relative to the
level of current produced therein by a reaction involving the
substance to be detected.
By a more specific aspect of the invention, the fixed relative
potential is chosen from within the range of about 0.9 to 1.5 volts
anodic relative to the hydrogen couple as a zero base.
By another specific aspect of the invention, the cell is
constructed to comprise an anode chamber defining a labyrinthine
path through which the air is pased to appropriately expose to the
anode the substance to be detected. Alternatively, the anode
chamber may comprise propeller means for effecting such appropriate
exposure.
BRIEF DESCRIPTION OF THE DRAWINGS
A better understanding of the present invention may be had by
reference to the following detailed description of the preferred
embodiments thereof taken in connection with the accompanying
drawings wherein:
FIG. 1 is a view in perspective of a cell embodying the principles
of the present invention;
FIG. 2 is a sectional view taken along the line 2--2 of FIG. 1;
FIG. 3 is a side elevation partially broken away of an interior
portion of the cell of FIG. 1 depicting in better detail the anode
chamber of the cell;
FIG. 4 is a side elevation partially broken away showing the
cathode structure of the cell which, in FIG. 1, is out of view on
the back side thereof;
FIG. 5 is a partial view in perspective of an alternative
embodiment for the anode chamber of the cell of the invention;
FIG. 6 is a chart derived from the Electromotive Series of Elements
indicating for exemplary redox couples theoretical relative
electrode potentials determining whether a couple will undergo an
oxidation or a reduction reaction;
FIG. 7 is a curve depicting the nature of the relationship between
current which may be developed in a cell due to an oxygen-water
redox couple and applied electrode potential; and
FIG. 8 is a schematic diagram of a potentiostat circuit for
controlling operation of the cell and particularly as applied in
maintaining a fixed relative potential difference between the cell
anode and a reference electrode.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT
Referring now in detail to the drawings, there is shown in FIG. 1
an electrochemical detection cell embodying the principles of the
present invention which comprises an electrolyte container 10
having a liquid electrolyte 12 therein with an electrolyte matrix
14 extending from within the electrolyte to between the electrodes
of the cell. The matrix 14 is formed from fibrous glass material
and operates as a wick having the electrolyte absorbed therein in a
liquid phase resulting from its continued immersion in the
reservoir 12.
A pair of support members 16 and 18 mounted on opposite sides of
the electrolyte matrix 14 at the upper end thereof operate to
retain, respectively, an anode 20 and a cathode 22 in contact with
the electrolyte matrix 14. The anode 20 may be mounted or embedded
on the support member 16 in any known manner, with the cathode 22
being similarly mounted upon the support 18. The supports 16 and 18
operate to provide structural mounting for the electrodes 20 and 22
and to maintain the electrodes in operable electrochemical
relationship with regard to the electrolyte contained within the
matrix 14.
A third or reference electrode 24 is also mounted upon the support
member 16, in the same manner as the anode 20 but spaced slightly
therebelow, in contact with the electrolyte matrix 14.
The support member 16 is structured to define a labyrinthine path
on the interior side thereof upon which the anode 20 is supported.
The labyrinthine path is generally designated by the numeral 36 and
is defined within a generally rectangular cavity formed by inner
walls 28, 30, 32 and 34. A plurality of horizontal spacer members
38, 40 and 42 extend from the sidewalls 30 and 34 in alternating
fashion to vertically space the channels of the labyrinthine path
36. An inlet conduit 44 extends through the support member 16 to
establish gas flow communication into the labyrinthine path 36 and
an outlet conduit 46 permits exit gas to flow out of the
labyrinthine path 36.
At the lower end of the support member 16, there is provided a
generally rectangular opening 48 defined by inner walls 48a, 48b,
48c and 48d extending completely through the support member 16 to
permit the reference electrode 24 to be exposed to the surrounding
atmosphere. Similarly, and as best shown in FIGS. 2 and 4, the
support member 18 has defined therethrough a generally rectangular
opening 50 bounded by inner walls 50a, 50b, 50c and 50d, which
permits exposure of the cathode 22 to the ambient atmosphere.
In the operation of the electrochemical cell of the present
invention, atmospheric air containing a noxious impurity such as
carbon monoxide which is to be detected is introduced at a metered
rate into the anode chamber defined by the labyrinthine path 36
through the inlet conduit 44. As the air is passed over the anode
20, the electrochemical reaction which occurs by virtue of the
exposure of the impurity to the anode 20 will produce a current in
the external circuit of the cell thereby enabling detection and
measurement of the impurity. Passage through the labyrinthine
chamber 36 of air samples containing an impurity to be detected
will effect adequate exposure to the anode 20 of the impurity in a
manner providing an appropriate reaction rate of the impurity as a
result of the fact that the air will be exposed over a relatively
constant area of the anode 20.
The requirement that the impurity to be detected be exposed to the
anode over a relatively constant anode surface area relates to the
obvious necessity for insuring that any change which takes place in
the measurement reading of the output circuit occurs as the result
of changes in impurity concentration, and not as a result of
phenomenon which bears no relationship to the measurements desired.
If the change occurs as a result of other factors, not related to
or indicative of impurity concentration, the cell will be rendered
less accurate or inoperative. Accordingly, it will be understood
that exposure of the impurity of the anode in a sporadic or
uncontrolled manner which would unpredictably vary the level of
anode area exposed, will effect variations in the impurity reaction
occurring at the anode thereby adversely affecting the validity of
the output reading.
It will also be appreciated that variation in the rate of flow of
the air sample through the anode chamber defined by the
labyrinthine path 36 can also produce undesired variations in the
output reading. Therefore, in the specific embodiment depicted in
FIGS. 1-4, the air sample is passed through the labyrinthine path
36 at a metered rate to insure that changes in the output reading
are the result of changes in impurity concentration and not caused
by unpredictable flow rate variations.
Accordingly, it will be understood that, as a general rule, the
cell of the present invention must be operated in a manner to
insure that changes in the output reading are a valid and accurate
representation of changes in the concentration of the impurity to
be measured. Any other factors, such as exposed anode area or air
sample flow rate, must either be maintained constant or controlled
and counterbalanced in order to negate the net effect thereof, in a
manner which will be apparent to those skilled in the art, to
insure valid operation of the cell of the present invention.
After having been scrubbed by reaction at the anode 20 of the
detected impurity, the air sample is passed out of the anode
chamber through the outlet conduit 46.
The basic principles underlying the present invention may be more
readily explained by reference in the description of a preferred
embodiment to operation of the cell in connection with detection of
a particular substance or impurity. Although the principles of the
invention may be utilized in cells appropriate for detecting any
one of a variety of noxious substances, for the purposes of
facilitating the present description it will be assumed that the
cell depicted in FIGS. 1-4 is constructed to react and detect
carbon monoxide from an air sample passed through the labyrinthine
path 36.
Accordingly, in the cell of FIGS. 1-4 arranged for detection of CO,
the electrolyte 12 is an aqueous solution of sulfuric acid and is
maintained at ambient room temperature. The material chosen for the
anode 20 is platinum black, and the electrode shown is a
Teflon-bonded diffusion electrode, well known to those skilled in
the art both as to composition of material and physical structure.
Although a variety of other materials may be chosen from within the
knowledge of those skilled in the art for the cathode 22 and the
reference electrode 24, in the preferred embodiment of FIGS. 1-4,
these electrodes are also selected to be Teflon-bonded platinum
black diffusion electrodes similar to the anode 20.
An important requirement of the present invention is the
maintenance of a fixed relative potential between the anode 20 and
the reference electrode 24. The circuitry whereby this is
accomplished is shown in FIG. 8, and a more detailed description of
the arrangement and operation thereof will be provided hereinafter.
FIG. 8 depicts a potentiostat circuit which is generally arranged
in accordance with conventional principles within the knowledge of
those skilled in the art and which enables the maintenance of the
fixed relative potential between the anode 20 and the reference
electrode 24 without development of current flow therebetween. The
circuit also operates to enable appropriate current flow in the
external circuit between the anode 20 and the cathode 22 when an
impurity such as carbon monoxide is reacted within the cell of the
invention.
The significance of the fixed relative potential which is
maintained between the anode 20 and the reference electrode 24 may
be best understood from the chart of FIG. 6 which depicts the
relationship between theoretical reversible electrode potential and
its effect upon reactions which will take place when certain
elements are exposed within a cell to an electrode with an applied
potential. The chart of FIG. 6 is derived from the Electromotive
Series of Elements, and sets forth by way of example certain
electrochemical couples and their theoretical reversible electrode
potentials. The electrochemical redox couples are set forth on the
right side of a vertical scale with related theoretical electrode
potentials indicated therealong on the left side.
The value of the theoretical reversible electrode potential of a
redox couple will determine whether the couple will undergo
oxidation or reduction as a result of exposure to an electrode
having a potential applied thereto. Thus, if a potential is applied
to an electrode such that it is more anodic than the reversible
electrode potential of the couple, the reduced species of this
couple will be oxidized. Conversely, if a potential is applied to
an electrode such that it is more cathodic than the reversible
electrode potential of the couple, then the oxidized species of
this couple will be reduced.
As previously stated, the preferred embodiment of the present
invention described herein is arranged to accomplish detection of
carbon monoxide in an oxygen containing atmosphere. As shown on the
scale of FIG. 6, the CO.sub.2 /CO redox couple is indicated as
occupying a theoretical level at about -0.12 volt relative to the
theoretical levels of other couples on the scale. Accordingly, if
the cell of FIGS. 1-4 is arranged such that the potential
difference which is maintained between the anode 20 and the
reference electrode 24 is more anodic than -0.12 to a sufficient
degree, carbon monoxide exposed at the anode 20 will undergo an
oxidation reaction and the CO.sub.2 /CO couple will go in the
direction indicated by the following formula:
Of course, it will be understood that the degree to which an
actually applied potential level must be more anodic or more
cathodic than the theoretical reversible electrode potentials will
vary depending upon specific circumstances. However, it should be
understood that the specific level differences required under
practical circumstances will be within the knowledge of those
skilled in the art.
One of the problems which may be encountered in the utilization of
measuring equipment such as the cell of the present invention
relates to the fact that more than one impurity may be exposed to
the working electrode of the cell. Accordingly, the reactions which
occur at the working electrode would produce current indicative of
more than one impurity and it would be impossible to distinguish
the presence and quantity of a particular impurity. In most
practical applications relating to atmospheric air, the level of
carbon monoxide in the air far exceeds the level of other
impurities such as nitric oxide and hydrocarbons. Accordingly, in
most ordinary situations involving atmospheric air it will not be
necessary to make provisions to distinguish between the elements in
the air since any readings which are generated in the output
circuit will be predominantly the result of carbon monoxide
presence. Of course, if a very high degree of accuracy is required,
or if the level of nitric oxide or hydrocarbons is sufficiently
high to invalidate the accuracy of the cell output, provisions must
be made to enable the various impurities to be distinctly detected
and measured. A manner for accomplishing this would involve passing
of an air sample through a series of individual cells, with each of
the cells being constructed to react one only of a plurality of
impurities with the cell environment being inert to the other
impurities. This can be accomplished by appropriate selection of
anode and/or cathode materials, the substance for the electrolyte,
and the temperature at which the electrolyte is maintained.
Thus, in a situation involving atmospheric air, a series of three
cells may be individually structured and appropriately arranged to
have an air sample passed therethrough in series in a manner
whereby each cell will detect a single impurity. The sequential
arrangement of the cells will be important since in some cases a
preceding cell will scrub or react all of the impurity contained in
an air sample thereby making that impurity unavailable for reaction
in a subsequent cell. A more detailed description of an arrangement
whereby the individual impurities contained in atmospheric air may
be separately detected will be provided hereinafter.
Another more significant problem with regard to generation of
undesired unauthentic current in the external circuit, and the
problem with which the present invention is primarily concerned,
relates to the fact that an oxygen-water redox couple will be
potentially available within the cell to produce, in the external
circuit, current which is not derived from reaction of the impurity
to the detected. Such a redox couple results from oxygen contained
in the incoming atmospheric air and water contained in the
electrolyte. For example, under certain circumstances water may
become oxidized at one of the electrodes of the cell thereby
generating current in the external circuit that would not be
distinguishable from the current generated by the impurity
reaction. Likewise, oxygen may undergo reduction within the cell
thereby similarly generating undesired current.
In accordance with a basic principle of the present invention, an
electrochemical cell may be arranged to effectively abate current
which might result from reactions of an oxygen-water couple within
the cell.
The mechanism of the present invention enabling control within the
cell of the oxygen-water couple is the maintenance of a fixed
potential upon the anode relative to the reference electrode
creating a condition whereby the oxygen-water couple produces in
the external circuit no discernible current relative to the current
produced by reaction of the impurity. In accordance with the
principles of the invention, it has been found that a fixed
potential selected from within the range between 0.9 and 1.5 volts
relative to the reversible electrode potential of the hydrogen
couple will enable achievement of the benefits of the
invention.
Referring now to FIG. 6 in order to more clearly understand the
specific selection of preferred potential ranges, it will be seen
that the oxygen-water redox couple is referenced upon the scale at
+1.23 volts. This indicates that at an electrode having a potential
more cathodic than +1.23 volts there would occur a reaction
involving reduction of oxygen. If the potential of the electrode
was chosen to be more anodic than +1.23 volts then oxidation of
water would occur at the electrode. Of course, with an electrode
potential established in the region between 0.9 and 1.50 volts, any
couple in a region more cathodic thereto would undergo oxidation.
For example, with a potential in this region, oxidation of CO would
invariably occur due to the fact that the CO.sub.2 /CO couple is
referenced on the scale at -0.12 volt, which is a level
significantly more cathodic than the level at which the electrode
potential would be established.
The curve of FIG. 7 is intended to depict the general relationship
which exists between current developed in a cell due to an
oxygen-water redox couple and the level of potential applied to the
electrode at which the reactions occur. It will be noted that in
curve A, at a level of +1.23 volts no current will be generated as
a result of reaction at an electrode charged at that level. As the
level of charge upon the electrode is varied, anodic or cathodic
current will commence to be generated depending upon the direction
in which the potential level is varied. However, it is important to
note that in order for there to be generated current of any
consequence, it will be necessary that the potential upon the
electrode be at a level which varies to a substantial degree from
the +1.23 volts level.
The curve labeled A generally depicts the situation which would be
developed with an electrode comprising platinum. As seen in FIG. 7,
as the potential on such an electrode is varied in a direction
either more anodic or more cathodic than +1.23 volts, little or no
current will be developed for a range of potential variation
between the points labeled x and y. Only when the potential upon
the electrode of curve A becomes more anodic than the y potential
level will there commence to be developed discernible anodic
current. Similarly, no discernible cathodic current will be
developed until after the potential applied to the electrode of
curve A becomes more cathodic than the x potential level. Thus, it
will be understood that if the electrode of curve A has a potential
applied thereto which is maintained between the limits x and y, no
discernible current will be generated at the electrode as a result
of the oxygen-water redox couple.
The shape and nature of the curve of FIG. 7 will depend primarily
upon the choice of electrode material involved. For different
electrode materials, curves having basically the same shape as
curve A may be developed except that the range of applied
potentials across which no discernible oxygen-water redox couple
current will be developed may be across wider limits. Additionally,
the type of electrolyte involved will likewise affect the specific
nature of the curve. For the purpose of the present disclosure, it
is not deemed necessary to set forth with great accuracy and detail
curves for specific cells since the development of such curves will
be within the knowledge of those skilled in the art. However, it is
deemed useful to depict the general shape of such curves so that
there may be developed a better understanding of the fact that a
range of potential level exists within which no discernible
oxygen-water redox couple current will be generated.
For the curve labeled A, the levels of x and y may be very
approximately assumed to be 1.0 and 1.7 volts for a platinum
electrode. The necessity for such a high degree of approximation
arises due to the fact that much more than the material of the
electrode must be known in order to more accurately establish the
value of these levels. Accordingly, the figures set forth are not
deemed of significance other than as a general indication of the
voltage levels which may be involved.
A second example of the types of curves which may be generated is
the curve labeled B, wherein there is depicted the conditions which
would exist with a gold electrode in an acid electrolyte. Again,
very approximate levels for the potentials q and r would be,
respectively, 0.7 and 1.8 volts. Within this range, no discernible
oxygen-water redox couple current would be generated at an
electrode having the indicated potential maintained thereupon.
With regard to the selection of electrode material, especially for
the working electrode which in the present case is the anode, the
material chosen should be such that it will operate effectively
within the cell to oxidize the particular impurity to be detected.
Obviously, one important requirement of the electrode material will
be that it is stable in the cell electrolyte. This and other
similar conventional requirements for an electrode material will be
apparent to those skilled in the art. Of more pertinence with
regard to the application of a fixed relative potential is the
characteristic of the electrode material to effect reaction, i.e.,
oxidation, of the impurity to be detected when the electrode is
charged at the fixed potential of the present invention. It will be
found that different electrode materials will produce differing
results to react a given impurity when maintained at a particular
electrode potential. This behavior is especially pertinent in
connection with the current level generated as a result of the
reactivity of the impurity at a particular electrode. In some
cases, as a result of the particular choice of electrode material
and of the level of potential applied thereto, the reaction of a
particular impurity at such an electrode may not proceed at a
sufficiently high rate to generate a level of current which will
permit an adequate reading in the output circuit to detect and
measure the impurity reacted. Accordingly, the selection should be
made so that for a given impurity an electrode potential level may
be chosen within the limits of the present invention to produce
maximum current for a fixed amount of impurity to be reacted. In
this manner, the oxygen-water redox couple current may be
eliminated by virtue of the fixed electrode potential chosen from
within the limits of the invention, i.e., 0.9 to 1.5 volts, with
simultaneous enhancement of detection current being provided by
virtue of selection of the electrode material which will operate
most effectively to react the impurity to be detected at the
particular fixed electrode potential utilized.
In the cell of the preferred embodiment of the invention which is
depicted in FIG. 1 and which is intended for the purpose of
detecting and measuring carbon monoxide, a range of between 1.07
and 1.13 volts is preferred for the anode fixed relative potential.
The specific preferred fixed relative potential is 1.1 volts. As
previously stated, the preferred material for the anode 20 of this
cell is platinum or platinum black and the electrolyte 12 is chosen
to be an aqueous solution of sulfuric acid. With these parameters,
it will be found that for carbon monoxide there will be generated
maximum current when the relative electrode potential is maintained
fixed at 1.1 volts. As the fixed relative potential deviates
substantially above or below this level the current generated for a
fixed quantity of CO reacted will be significantly reduced.
Additionally, at 1.1 volts, any slight deviation in the constancy
of this fixed relative potential level will produce relatively less
variation in current than would be produced if the potential were
to be maintained at some other level.
Accordingly, it will be seen that although undesired current from
an oxygen-water redox couple may be avoided by maintenance of the
fixed relative electrode potential of the present invention, cell
accuracy and performance may be enhanced by appropriate selection
of other parameters having in mind the criteria set forth
herein.
Although platinum and platinum black are set forth as preferred
materials for the CO detection cell of FIGS. 1-4, other materials
may be suitably utilized. Other materials which would be suitable
for utilization in a cell constructed in accordance with the
principles of the present invention to detect and measure carbon
monoxide may be chosen from the group consisting of platinum,
rhodium, iridium, ruthenium, palladium, osmium, tungsten oxide,
tungsten carbide, molybdenum oxide, molybdenum sulfide, and alloys
or mixtures thereof. In general and as indicated by the
aforementioned grouping, it will be found that anode materials for
a CO-detection cell may be appropriately selected from the noble
metals.
The particular structure and arrangement of a cell formulated
within the scope of the present invention may deviate from the
specific structure set forth in connection with FIGS. 1-4. The
material utilized for the matrix 14 need not necessarily be glass
but may be formed to comprise either silica, zirconia, or various
polymers. Additionally, the electrolyte need not be immobilized by
absorption in a matrix but may be provided in the form of a "free"
electrolyte. The important consideration in this connection is,
however, that the cell be arranged so that the impurity to be
detected and reacted at the working electrode be permitted to
migrate to the interface of the electrolyte and the surface of
working electrode in order to insure reaction of the impurity
thereat. Thus, it would be appropriate, for example, to construct
the cell of the present invention having a free electrolyte in
contact with one side of the working electrode and exposing the
opposite side of the working electrode to the substance, e.g.,
carbon monoxide, to be detected. The particular structure of the
electrodes set forth in the cell of FIGS. 1-4 comprises a
Teflon-bonded porous electrode, and as a result of the porosity of
the electrode the gas to be detected will migrate from the external
surface of the electrode, i.e., the surface of anode 20 exposed to
the labyrinthine path 36, to the internal surface thereof which is
interfaced with the electrolyte, i.e., the opposite surface of
anode 20 which is in abutment with the matrix 14. Replacement of
the matrix 14 with a free electrolyte arrangement would not impede
the reactivity of the cell.
The selection of materials for the cathode and for the reference
electrode of a cell constructed within the scope of the present
invention may be made within the knowledge of those skilled in the
art. Criteria for selection of these materials will relate to
commonly known principles of electrochemistry and should be
conventionally achievable. For example, the cathode material
should, of course, be electronically conducting and have low
solubility in the electrolyte. Since, as is true in any
electrochemical cell of the type described herein, the reaction
occurring at one electrode must be Faradaically equivalent to the
opposite redox reaction occurring at the other electrode, it will
be understood that to complete the electrolytic cell described
herein, a reduction process must occur at the cathode which will be
Faradaically equivalent to the oxidation process occurring at the
anode. Thus, in an example of the specific cell described in
connection with FIGS. 1-4, the material for the cathode should be
chosen such that this electrode will be capable of catalyzing water
reduction or oxygen reduction or reduction of the appropriate redox
couple having its oxidation counterpart occurring at the anode, as
for example by the oxidation of CO at the anode 20.
Since the reference electrode is a nonpolarized electrode and does
not actively participate in the electrolytic process of the cell,
criteria for selection of materials for this electrode would relate
primarily to its ability to cooperate in maintaining the fixed
potential relative to the anode and to its general adaptability to
the cell environment including, for example, low solubility in the
electrolyte.
As shown in the drawings of FIGS. 1-4, both the reference electrode
24 and the cathode 22 are supported in abutment with the
electrolyte matrix 14 in a manner whereby their opposite sides are
exposed to the ambient atmosphere. With regard to the reference
electrode 24, an alternative possibility to this arrangement would
be to expose the electrode 24 only to air which has been previously
passed through the anode chamber and from which all or a
substantial portion of the carbon monoxide has been removed by
reaction within the cell. An advantage in using this approach is
that it would avoid polarization of the reference electrode 24
which occurs as a result of oxidation of carbon monoxide in the
ambient air to which the reference electrode 24 is exposed.
Although such polarization occurs in the cell of FIGS. 1-4, it is
relatively small due to the fact that there is no appreciable gas
flow across the surface of the reference electrode 14 and,
accordingly, the effects of oxidation of CO at this electrode are
relatively minor and tolerable. The net effect of such polarization
is to cause the anode to become more cathodic resulting in lower
readings of CO presence in the air sample passed through the anode
chamber. Accordingly, the choice of whether to avoid such
polarization will depend upon the degree of percision required from
the cell and the practicality of incurring the expenditure involved
in the achievement of such precision.
Another important aspect of the cell of the present invention
relates to the arrangement of the anode chamber through which the
substance to be detected is passed for exposure to the anode
surface. As previously stated, it is important that conditions of
the cell irrelevant to changes in impurity concentration be
maintained such that the output reading of the cell is no
invalidated thereby.
As previously stated, the anode chamber arrangement is significant
in effecting appropriate exposure to the surface of the anode of
the oxidizable substance to be detected. In FIGS. 2 and 3, the
labyrinthine channel 36 directs the flow path of the CO-bearing air
in contact with the anode 20. The shape and configuration of the
channel 36 insures that a substantially constant anode area is
exposed to the CO, and assuming an appropriate flow rate, the
operation of the cell will be such that no changes in the output
circuit will occur as a result of sporadic variations in the anode
area contacted. Although the labyrinthine channel 36 is considered
an appropriate and preferred approach for effecting appropriate
exposure of the CO within the anode chamber, alternative
arrangements are possible within the scope of the present
invention.
One such alternative arrangement is depicted in FIG. 5 which shows
a portion of an electrolytic cell constructed in accordance with
the present invention and particularly depicting the anode chamber
thereof. The cell depicted in FIG. 5 comprises an anode 20a
abutting an electrolyte matrix 14a in an identical manner as
depicted in FIGS. 1-4. A support member 60 has the anode 20a
mounted therein in a manner similar to the mounting of anode 20
upon support member 16. The support member 60, instead of providing
the labyrinthine channel 36, defines as an alternative thereto an
anode chamber 62 which is generally circular in its configuration
and which is fully exposed to the surface of the anode 20a. A
propeller mechanism 64 driven to rotate by an appropriate means
(not shown) which will be within the knowledge of those skilled in
the art, operates to swirl air within the anode chamber 62 thereby
to enhance the scrubbing effect produced upon the surface of the
anode 20a. The inlet means whereby air is introduced into the anode
chamber 62 comprise a centrally located conduit 66 defined
internally of the shaft of propeller 64 in flow communication with
the anode chamber 62. An exit conduit 68 extending in flow
communication from the sidewall of chamber 62 permits the contents
of chamber 62 to pass therefrom in a direction tangentially of said
sidewall. Thus, air entering the anode chamber 62 through the
central conduit 66 will be swirled about the chamber by the action
of the propeller 64 thereby effecting oxidation of CO as a result
of contact with the anode surface. Subsequently, the air will be
emitted through the conduit 68 by virtue of the swirling motion
imparted thereto by the propeller 64.
The operation of a cel structured in accordance with FIG. 5 is
again effective to achieve an appropriate distribution across the
surface of the anode 20a of CO contained in the air introduced into
the anode chamber 62. A valid current reading may be obtained in
the output circuit of the cell of FIG. 5 due to the fact that the
CO is uniformly distributed over a fixed anode surface area. Such
fixed uniform distribution operates in essentially the same manner
as the labyrinthine channel 36 of FIGS. 1-4 to insure that changes
in exposed anode surface area do not operate to produce erroneous
indication of CO concentration in the output circuit readings. The
specific embodiment of FIG. 5 is considered especially appropriate
for utilization with applications involving relatively lower CO
concentrations since it operates to increase current levels for a
given CO concentration thereby enhancing the effectiveness of the
external circuit readings as indications of the presence and
quantity of CO.
The maintenance of constant potential between the anode and the
reference electrode of the cell of the invention is accomplished by
a potentiostat circuit, connected to the cell in the manner
depicted in FIG. 8, which is conventional and within the knowledge
of those skilled in the art. The potentiostat circuit of FIG. 8
operates to maintain a constant relative potential between the
anode and the reference electrode.
In FIG. 8, the electrochemical cell of the invention is shown
schematically as comprising an anode 70, a cathode 72, and a
reference electrode 74, with the anode connected through a switch
76 to ground potential 78. The circuit basically comprises an
operational amplifier 80 having both the reference electrode 74 and
the cathode 72 connected thereto. A DC power supply 82 having a
connection 84 to ground potential 78 is connected to the amplifier
80 through leads 86 and 88 with resistors 90, 92, and 94 connected
thereacross in parallel between the power supply 82 and the
amplifier 80. Resistor 92 comprises a rheostat and is connected to
the amplifier 80 through a lead 96 whereby adjustment of the
resistor 92 enables adjustment of the fixed relative potential
which is to be maintained between the reference electrode 74 and
the anode 70. The cathode 72 is conected to the amplifier 80
through a resistor 98 having a voltmeter 100 connected thereacross.
The reference electrode 74 is connected to the operational
amplifier 80 through a lead 102 and as the relative potential
between the reference electrode 74 and the anode 70 develops a
tendency to vary from the fixed level established by adjustment of
rheostat 92, the amplifier 80 operates through a negative feedback
to maintain constant the relative potential between the anode 70
and the reference electrode 74. The factor creating the tendency to
alter the anode-reference electrode fixed relative potential is
developed as a result of reaction at the anode 70 of the impurity
to be detected, i.e., oxidation of CO contained within the air
sample flowing across the face of the anode 70 as indicated by the
arrow 104. The output current of the operational amplifier 80 will
pass through the resistor 98 and will be a result of and related to
the level of oxidation of CO occurring at the anode 70. Therefore,
the reading taken at the voltmeter 100 will be representative of
the oxidation reaction occurring at the anode 70 and of the
quantity of material oxidized. The voltmeter 100 may be readily
calibrated in a known manner to provide determination of the
quantity of CO occurring in the air sample taken, and if the
conditions in the anode chamber are in accordance with the
teachings previously set forth, appropriate readings may be
generated pursuant to the principles of operation provided.
Both the potentiostat circuit of FIG. 8, and the operational
amplifier 80 included therein, are considered fully conventional
and within the knowledge of a skilled artisan.
It should be understood that any deviation which might occur in the
relative potential difference between the anode and the reference
electrode will affect the accuracy and precision of the cell.
Accordingly, the extent of deviation which may be tolerated will
depend upon the degree of precision required for a particular
application. The potentiostat circuit of FIG. 8 is considered to
provide a degree of constancy for the relative electrode potential
difference which will be adequate for most applications in
connection with atmospheric air. Where a higher degree of precision
may be required circuitry other than that of FIG. 8, which may be
more precisely constructed to insure greater accuracy, may be
used.
Furthermore, it should be appreciated that although the invention
is importantly characterized by the maintenance of a constant or
fixed relative potential difference, deviations in said fixed
relative potential may occur within the concepts of the present
invention and without departure from the scope and purview
thereof.
As previously stated, impurities other than carbon monoxide may be
measured and detected by cells constructed in accordance with the
present invention. For example, by providing certain modifications
which may relate to either the material of the electrode, the
electrolyte composition, or the temperature of the electrolyte, and
appropriately adjusting the fixed relative potential between the
anode and the reference electrode, a cell may be adapted to oxidize
a specific impurity in a manner whereby other impurities
simultaneously contained in an air sample will be inert to the cell
environment. Inasmuch as nitric oxide and hydrocarbons are the two
most significant elements, in addition to carbon monoxide, which
may be usually present in atmospheric air, it is considered
appropriate to describe, as examples of cell modifications,
arrangements whereby these elements may be measured, detected and
removed from an air sample.
Accordingly, assuming a system wherein it was desired to measure
and detect all three of the more significant impurities present in
atmospheric air, i.e., carbon monoxide, nitric oxide and
hydrocarbons, this could be accomplished by a three-cell
arrangement comprising a separate cell to individually detect and
react each of these impurities. Normally, it would be most
appropriate to pass the air sample first through a cell for
detection of the nitric oxide. Such a cell should preferably
comprise a gold anode and a sulfuric acid electrolyte maintained at
room temperature. In this cell, the fixed relative potential to be
maintained between the anode and the reference electrode should
preferably be from within the range between 1.0 and 1.3 volts. As a
result of passage through the anode chamber of such a cell, the air
sample would have removed therefrom all or most of the nitric oxide
contained therein by oxidation at the anode of the cell. The
current developed in the external circuit of the cell as a result
of such oxidation would operate in the same manner as previously
described in connection with FIGS. 1-4 for detection of carbon
monoxide, and, accordingly, detection and measuring of the nitric
oxide, as well as removal of all or of a substantial portion
thereof from the air sample, could be accomplished.
Subsequent to passage through the nitric oxide detection cell, the
air sample would be passed to the cell for detection and oxidation
of carbon monoxide. Such a cell, comprising a platinum electrode
and an electrolyte consisting of sulfuric acid at ambient
temperature, may be structured in accordance with the description
previously set forth in connection with FIGS. 1-4.
A third cell for the detection and measurement of hydrocarbons in
the air sample should preferably comprise a platinum black
electrode and an electrolyte consisting of phosphoric acid at a
temperature within the range between 100.degree. C. and 200.degree.
C. The fixed potential maintained between the anode and the
reference electrode should be preferably from within the range
between 1.05 and 1.15 volts. The air emitted from the CO-detection
cell should be introduced into the third cell for detection of
hydrocarbons, with oxidation of the hydrocarbons occurring at the
anode in a manner similar to that previously described, to produced
external current indicating hydrocarbon presence and the amount
thereof.
Of course, each of the three cells described should include a
potentiostat circuit to maintain the fixed relative potential
between the anode and the reference electrode, in the manner
previously described. In each case, undesired current produced by
an oxygen-water couple would be problematic and cold be dealt with
and avoided in accordance with the principles of the present
invention from the description set forth herein.
As has been stated, because of the specific structure and
arrangement of each individual cell, no problems will arise in any
one of the cells from undesired detection current caused by
presence and reaction of an impurity which is not to be detected by
that particular cell. For instance, in the foregoing arrangement
utilizing three cells, the air samples are first passed through the
nitric oxide detection cell. The presence in this cell of CO and
hydrocarbons will not adversely affect the validity of the current
in the external circuit as a measurement of NO presence due to the
fact that neither carbon monoxide nor hydrocarbons will be oxidized
in this cell since these elements are inert to the gold anode of
the cell. Similarly, the air passed through the carbon monoxide
cell will not involve oxidation of either nitric oxide or
hydrocarbons. Nitric oxide would normally be reactive in the CO
cell, but since this element has either been removed or reduced to
insignificant amounts as a result of passage through the first NO
detection cell, no problem arises. The hydrocarbons require an
electrolyte other than sulfuric acid at ambient temperature for
oxidation to occur and, accordingly, their presence in the CO cell
will not effect a reaction. Therefore, it will be seen that the
problem of plural impurities in an air sample which could obstruct
the accurate detection of a single impurity, is readily dealt with
in the manner described by appropriate selection of cell
conditions, i.e., fixed relative potential and anode and
electrolyte characteristics, and by appropriate sequential
arrangement of the cells. The problem of undesired current
generated as a result of the oxygen-water couple which has also
been problematic, will also be readily avoided by the application
of the appropriate fixed relative potential in accordance with the
principles of the present invention in the manner herein
described.
From the foregoing it should be apparent that the principles of the
present invention will have broad application in cells utilized in
a variety of environments for various purposes. Although the
foregoing description has been limited to the detection and
measuring of impurities in atmospheric air, it should be understood
that the invention need not be so limited although this will
probably be its most important area of application.
Other areas of application for the present invention could be in
connection with industrial equipment, for example, process plants
which require detection and measurement of certain gaseous
substances. In connection with this type of application, it is
important to note that the substance to be detected may be exposed
to the surface of the working electrode of the cell without oxygen
presence. This would not adversely affect the operation of the cell
in detecting a particular impurity or gaseous substance. Since the
cell would comprise an aqueous electrolyte, the impurity exposed at
the interface of the electrolyte and the working electrode would be
oxidized thereby generating detection current. It will be clear
that exposure to the anode of the impurity alone or of the impurity
without oxygen, will not impede occurrence of a detection reaction.
Furthermore, removal or absence of oxygen from the impurity-bearing
environment would operate to obviate the necessity for the lower
limit of 0.9 volt in the establishment of the fixed relative
potential between the working electrode and the reference
electrode. It will be understood that this lower limit is
established to insure avoidance of oxygen reduction in the cell
which would generate undesired current. Since in the exemplary
industrial application referred to no oxygen may be available to
effect this reaction, the problem will not arise and the
requirement for the lower limit is removed. However, the
requirement for the upper limit of 1.5 volts would remain due to
the fact that oxidation of the water in the electrolyte would be a
possibility to be avoided. Accordingly, it will be clear that where
the impurity to be detected is not exposed to the working electrode
in an oxygen-containing environment, the limits of the present
invention may be defined by a fixed relative potential between the
working electrode and the reference electrode which is not more
anodic than +1.50 volts.
Another specific embodiment of the present invention may have
application in the detection and measurement of the level of
alcohol in a person's breath. Such a cell would be primarily
arranged to measure and detect ethanol although methanol would also
be detectable with such a cell. In the specific embodiment of a
cell for the detection of ethanol/methanol, sulfuric acid in an
aqueous solution would be the prefered electrolyte and the range of
fixed relative potential between the anode and the reference
electrode would be preferably between 1.05 and 1.13 volts.
Although in the foregoing description the present invention has
been described by reference to specific preferred embodiments
thereof, it is to be understood that modifications and alterations
in the structure and arrangement of the invention, other than those
set forth herein, may be achieved within the knowledge skilled in
the art and that such modifications and alterations are to be
considered as within the scope and purview of the invention.
* * * * *