CA2059949A1 - Gas sensor - Google Patents
Gas sensorInfo
- Publication number
- CA2059949A1 CA2059949A1 CA002059949A CA2059949A CA2059949A1 CA 2059949 A1 CA2059949 A1 CA 2059949A1 CA 002059949 A CA002059949 A CA 002059949A CA 2059949 A CA2059949 A CA 2059949A CA 2059949 A1 CA2059949 A1 CA 2059949A1
- Authority
- CA
- Canada
- Prior art keywords
- iodine
- gas
- sensor
- sensor according
- cell
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Abandoned
Links
Classifications
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
- G01N27/26—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating electrochemical variables; by using electrolysis or electrophoresis
- G01N27/403—Cells and electrode assemblies
- G01N27/404—Cells with anode, cathode and cell electrolyte on the same side of a permeable membrane which separates them from the sample fluid, e.g. Clark-type oxygen sensors
- G01N27/4045—Cells with anode, cathode and cell electrolyte on the same side of a permeable membrane which separates them from the sample fluid, e.g. Clark-type oxygen sensors for gases other than oxygen
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N33/00—Investigating or analysing materials by specific methods not covered by groups G01N1/00 - G01N31/00
- G01N33/0004—Gaseous mixtures, e.g. polluted air
- G01N33/0009—General constructional details of gas analysers, e.g. portable test equipment
- G01N33/0027—General constructional details of gas analysers, e.g. portable test equipment concerning the detector
- G01N33/0036—Specially adapted to detect a particular component
- G01N33/0054—Specially adapted to detect a particular component for ammonia
-
- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02A—TECHNOLOGIES FOR ADAPTATION TO CLIMATE CHANGE
- Y02A50/00—TECHNOLOGIES FOR ADAPTATION TO CLIMATE CHANGE in human health protection, e.g. against extreme weather
- Y02A50/20—Air quality improvement or preservation, e.g. vehicle emission control or emission reduction by using catalytic converters
Abstract
ABSTRACT OF THE DISCLOSURE
An electrochemical gas sensor for sensing an alkaline gas such as ammonia. The sensor comprises at least sensing and counter electrodes provided in a cell containing an aqueous electrolyte, the cell further including a diffusion barrier to restrict the access of gas to the cell, and a chemical species with which the gas reacts in use to form a product which is more electrochemically active than the gas. The chemical species is one of iodine, Nesslers reagent and a solution of manganous and silver nitrates.
An electrochemical gas sensor for sensing an alkaline gas such as ammonia. The sensor comprises at least sensing and counter electrodes provided in a cell containing an aqueous electrolyte, the cell further including a diffusion barrier to restrict the access of gas to the cell, and a chemical species with which the gas reacts in use to form a product which is more electrochemically active than the gas. The chemical species is one of iodine, Nesslers reagent and a solution of manganous and silver nitrates.
Description
~0~9~9 GAS SENSOR
FIELD OF THE INVENTION
The invention relates to electrochemical gas sensors.
DESCRIPTION OF THE PRIOR ART
Amperometric, electrochemical sensors have been widely adopted to meet a growing demand for suitable measuring devices in safety and process control applications. These sen:sors operate on fuel cell and battery principles utilising the direct electrochemical oxidation or reduction of the gas to be measured at a gas diffusion electrode, in combination with a gaseous diffusion barrier, to produce an electrical signal which is directly related to the concentration of gas being measured.
A paper entitled "A Versatile Electrochemical Monitor For Air-Quality Measurements" by Miller et al, Journal of the Air Pollution Control Association, Vol. 21, No. 7 (July 1971), pages 414-417 describes an electrochemical instrument for measuring acid gases. However, this technique, which involves reacting a gas to be sensed with an aqueous mixture of iodate and iodide has never been considered suitable for sensing alkaline gases such as ammonia.
US-A-3821090 (and also US-A-3774269) describes a cell for measuring the acid gas NO2. These work on similar principles to those described above in the Miller et al article and are not readily adaptable to the detection of alkaline gases.
In principle, ammonia gas could be detected by means of -an amperometric gas sensor, using the direct anodic oxidation reaction:
2NH3 = N2 + 6}~ ~ 6e (1) In practice ammonia forms the ammonium ion (NH4 ) in the aqueous solution comprising the sensor electrolyte:
NH3 + H20 = NH4~ + OH ~2) This ion is particularly stable and does not readily undergo electrochemical reaction. In acid solutions NH4 is the predominant species and no response can be obtained , ..
., . . ~, .
.
.
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from sensors by direct oxidation, even when utilising very active electrocatalysts such as platinum, at extreme anodic overpotentials, at which the oxygen evolution reaction occurs to a significant extent and imposes an unacceptable background current. In electrolyte solutions of higher pH
the equilibrium of equation (2) lies further to the left hand side and some direct anodic oxidation response can be achieved. However, even in strong alkali, the NH3 oxidation signals suffer undesirable effects such as slow response, hysteresis on removal of the test gas and signal decay and drift. Furthermore, since active electrode catalysts are required such as platinum, the sensors suffer from cross interferences from other gases, such as carbon monoxide, hydrogen, etc. which may co-exist with ammonia in the atmosphere being monitored. An example of a sensor which directly detects the oxidation of ammonia is described in EP-A-0395927 published on 14 April 1990 (and thus not a prior publication). GB-A-2225859 describes a measuring cell for determining ammonia in which the electrolyte contains a soluble non-oxidizable reagent which reacts completely with ammonia to form an oxidizable product which is able to be converted by oxidation into a non-oxidizable, soluble and, chemically and electrochemically inert secondary product. The preferred reagent is an organic ammonium salt which reacts with ammonia to form an amine. Commercial products incorporating this idea ha~e a significant size which makes them generally undesirable and have a poor low temperature performance.
3 O SUMMARY OF ~FHE INVENTION
In accordance with the present invention, an electrochemical gas sensor for sensing an alkaline gas comprises at least sensing and counter electrodes provided in a cell containing an aqueous electrolyte, the cell 35 further ~ncluding a diffusion barrier to restrict the access of gas to the cell, and a chemical species with which the gas reacts in use to form a product which is more . . ' ~
-. ., ~o~9~
electrochemically active than the gas, wherein the chemical species is one of:
a) iodine;
b) Nesslers reagent; and c) a solution of manganous and silver nitrates.
The invention is particularly suitable for the detection of ammonia and the preferred chemical reagent is iodine. Ammonia dissolves readily in water to produce alkali in accordance with reaction (2) above and iodine reacts in the alkaline conditions formed producing iodide and iodate ions according to the equation:
60H + 3I2 = 5I + I03 + 3H20 (3) The iodide ion ~I) readily undergoes electrochemical oxidation at the sensing electrode to provide a current which is directly related to the ammonia concentration and regenerates iodine in part for further reaction:
2I = I2 + 2e (4) The overall sensor reaction, providing a measure of the ammonia concentration is then the combination of eguations ~2), (3) and (4), namely:
12NH3 ~ 6H20 + I2 = 2I03 + 12NH4 + lOe (S) Any gas producing an alkaline reaction in water would produce a response in a sensor based on reaction (5) above.
Alkaline gases include hydrazines and organic amines.
However, for most applications ammonia is the only gas likely to be present producing an alkaline reaction. Since the iodide/iodine reaction (4) proceeds readily on moderately active catalysts, interferences from likely co-existing gases with ammonia such as carbon monoxide, hydrogen, etc. can be avoided. Acidic gases such as carbon dioxide, sulphur dioxide, etc. will not re2ct with the iodine in ~olution and will not therefore produce a response from the sensor provided they do not themselves undergo direct electrochemical reaction at the electrode catalyst.
The sensor electrolyte solution should have low pH
buffer capacity to provide optimum sensitivity to the :
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'' . . ~ ' ', , ' :
, ~ .
FIELD OF THE INVENTION
The invention relates to electrochemical gas sensors.
DESCRIPTION OF THE PRIOR ART
Amperometric, electrochemical sensors have been widely adopted to meet a growing demand for suitable measuring devices in safety and process control applications. These sen:sors operate on fuel cell and battery principles utilising the direct electrochemical oxidation or reduction of the gas to be measured at a gas diffusion electrode, in combination with a gaseous diffusion barrier, to produce an electrical signal which is directly related to the concentration of gas being measured.
A paper entitled "A Versatile Electrochemical Monitor For Air-Quality Measurements" by Miller et al, Journal of the Air Pollution Control Association, Vol. 21, No. 7 (July 1971), pages 414-417 describes an electrochemical instrument for measuring acid gases. However, this technique, which involves reacting a gas to be sensed with an aqueous mixture of iodate and iodide has never been considered suitable for sensing alkaline gases such as ammonia.
US-A-3821090 (and also US-A-3774269) describes a cell for measuring the acid gas NO2. These work on similar principles to those described above in the Miller et al article and are not readily adaptable to the detection of alkaline gases.
In principle, ammonia gas could be detected by means of -an amperometric gas sensor, using the direct anodic oxidation reaction:
2NH3 = N2 + 6}~ ~ 6e (1) In practice ammonia forms the ammonium ion (NH4 ) in the aqueous solution comprising the sensor electrolyte:
NH3 + H20 = NH4~ + OH ~2) This ion is particularly stable and does not readily undergo electrochemical reaction. In acid solutions NH4 is the predominant species and no response can be obtained , ..
., . . ~, .
.
.
20~9~4~
from sensors by direct oxidation, even when utilising very active electrocatalysts such as platinum, at extreme anodic overpotentials, at which the oxygen evolution reaction occurs to a significant extent and imposes an unacceptable background current. In electrolyte solutions of higher pH
the equilibrium of equation (2) lies further to the left hand side and some direct anodic oxidation response can be achieved. However, even in strong alkali, the NH3 oxidation signals suffer undesirable effects such as slow response, hysteresis on removal of the test gas and signal decay and drift. Furthermore, since active electrode catalysts are required such as platinum, the sensors suffer from cross interferences from other gases, such as carbon monoxide, hydrogen, etc. which may co-exist with ammonia in the atmosphere being monitored. An example of a sensor which directly detects the oxidation of ammonia is described in EP-A-0395927 published on 14 April 1990 (and thus not a prior publication). GB-A-2225859 describes a measuring cell for determining ammonia in which the electrolyte contains a soluble non-oxidizable reagent which reacts completely with ammonia to form an oxidizable product which is able to be converted by oxidation into a non-oxidizable, soluble and, chemically and electrochemically inert secondary product. The preferred reagent is an organic ammonium salt which reacts with ammonia to form an amine. Commercial products incorporating this idea ha~e a significant size which makes them generally undesirable and have a poor low temperature performance.
3 O SUMMARY OF ~FHE INVENTION
In accordance with the present invention, an electrochemical gas sensor for sensing an alkaline gas comprises at least sensing and counter electrodes provided in a cell containing an aqueous electrolyte, the cell 35 further ~ncluding a diffusion barrier to restrict the access of gas to the cell, and a chemical species with which the gas reacts in use to form a product which is more . . ' ~
-. ., ~o~9~
electrochemically active than the gas, wherein the chemical species is one of:
a) iodine;
b) Nesslers reagent; and c) a solution of manganous and silver nitrates.
The invention is particularly suitable for the detection of ammonia and the preferred chemical reagent is iodine. Ammonia dissolves readily in water to produce alkali in accordance with reaction (2) above and iodine reacts in the alkaline conditions formed producing iodide and iodate ions according to the equation:
60H + 3I2 = 5I + I03 + 3H20 (3) The iodide ion ~I) readily undergoes electrochemical oxidation at the sensing electrode to provide a current which is directly related to the ammonia concentration and regenerates iodine in part for further reaction:
2I = I2 + 2e (4) The overall sensor reaction, providing a measure of the ammonia concentration is then the combination of eguations ~2), (3) and (4), namely:
12NH3 ~ 6H20 + I2 = 2I03 + 12NH4 + lOe (S) Any gas producing an alkaline reaction in water would produce a response in a sensor based on reaction (5) above.
Alkaline gases include hydrazines and organic amines.
However, for most applications ammonia is the only gas likely to be present producing an alkaline reaction. Since the iodide/iodine reaction (4) proceeds readily on moderately active catalysts, interferences from likely co-existing gases with ammonia such as carbon monoxide, hydrogen, etc. can be avoided. Acidic gases such as carbon dioxide, sulphur dioxide, etc. will not re2ct with the iodine in ~olution and will not therefore produce a response from the sensor provided they do not themselves undergo direct electrochemical reaction at the electrode catalyst.
The sensor electrolyte solution should have low pH
buffer capacity to provide optimum sensitivity to the :
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'' . . ~ ' ', , ' :
, ~ .
2 ~
dissolving ammonia gas; it should also contain an ionically conducting supporting electrolyte. To meet these requirements salts of strong acids and strong bases are dissolved in the solution containing iodine. Examples of '5 suitable electrolytes are salts of the alkali and alkaline earth metals such as the chlorides of lithium, sodium, potassium, calcium etc. Salts such as lithium and calcium chlorides are hygroscopic and provide the additional benefit of controlling the sensor water balance, preventing the sensor from drying out.
Although some of the reacted iodine is regenerated by electrochemical oxidation of the iodide product (equation 4) there is a net consumption of iodine according to equation 5 of 1 mole per 12 moles of ammonia. The sensor life will therefore be determined by the volume of electrolyte and iodine concentration contained in the sensor. Iodine only has a limited solubility in water but measures can be taken to increase the iodine capacity of the sensor, without the need to carry a large volume of electrolyte. Free solid iodine contained within the electrolyte reservoir would ensure that the solut~ion remains saturated with iodine; however, solid iodine has a measurable vapour pressure and slowly diffuses out of the sensor. In addition to lost iodine capacity, the volatile iodine can cause problems of corrosion to metal surfaces external to the cell.
In one approach, the iodine is bound chemically to another compound, such as starch, which releases iodine reversibly as free iodine in equilibrium with the complex.
In this way sufficient iodine is available to satisfy the sensing electrode reactions for detecting ammonia, but at a sufficiently low concentration to reduce iodine loss by volatilisation to an extremely low level. The starch complex also allows a considerable iodine capacity to be obtained in a relatively small electrolyte volume. An alternative approach would be to implant a permeation device inside the sensor, containing an iodine source and . .
' ~, '"
.
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.
20~3~9 which is designed to have suitably low iodine diffusion rate so as to provide a controlled release of iodine into the electrolyte to maintain the concentration at the appropriate level. The iodine source may be for example, solid iodine, aqueous or non-aqueous iodine solutions or any compound which may release elemental iodine which can then diffuse at a controlled rate fro~ the capsule, into the body of the sensor and thence into its electrolyte.
In one alternative, but less preferred, arrangement the electrolyte contains Nesslers Reagent. This comprises a solution of mercuric iodide dissolved in excess potassium iodide solution in which the complex mercuric-iodide ions HgI3 and HgI4 are formed. These complexes do not precipitate mercuric oxide on adding alkali but do give a yellow-brown coloration with NH3 forming the amino component Hg20INH2, the iodide of "Millons Base" (Hg2 O(OH)NH2). This compound can then undergo anodic oxidation of the amino group to produce the signal of the sensor.
This reagent is not a preferred material since the mercury salt would impose pro~lems with disposal of the sensors at end of their life.
In a further arrangement the electrolyte can contain a solution of manganous and silver nitrates which reacts with hydroxide liberated by the dissolution of ammonia gas according to the equation:
Mn + 2Ag + 4OH = MnO2 ~ 2Ag + 2H20. (6) Following this reaction, either the MnO2 can be cathodically reduced or the Ag anodically oxidised to produce the sensor signal related to the NH3 concentration.
30The diffusion barrier can be of any conventional type including a gas phase diffusion barrier, a Xnudsen barrier, a solid barrier or a combination of two or more of these.
Although the sensor can have just sensing and counter electrodes, in general a third, rèference electrode is provided to keep the sensing electrode at the correct, working potential.
-.
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It has been found that sensors according to the invention are of small size and have good low temperature performance~
BRIEFL DESCRIPTION OF THE DRAWINGS
~i Some examples of sensors in accordance with the invention and a comparative example will now be described with reference to the accompanying drawings, in which:-Figure lA is an exploded view of an example of a sensor according to the invention;
Figure lB illustrates the connections to the electrodes in the Figure lA example;
Figure lC is an exploded view of the top plate of Figure lA;
Figure 2 illustrates the response of a conventional sensor after one week;
Figures 3 and 4 illustrate the response of two sensors according to the invention after one week;
Figure 5 illustrates the response of the conventional sensor after two weeks; and, Figure 6 illustrates the sensitivity of a sensor according to the invention over a period of 11 weeks as well as that of a conventional sensor over 2 weeks.
DETAILED DESCRIPTION OF THE EMBODIMENTS
The sensor shown in Figures lA-lC is generally of conventional form and will not be described in detail.
Briefly, the sensor comprises a composite top plate 1 shown in more detail in Figure lC which is mounted in use to a base plate 2 having an outwardly facing, annular flange 3 defining an electrode well 4. Within the electrode well 4 are provided a counter electrode S comprising PTFE tape and a catalyst layer connected to a current collector 6.
The counter electrode 5 is provided in a sandwich with a separator 7, a further current collector 8, a reference electrode 9 (comprising PTFE layer and catalyst), a separator lO, a current collector 11, and a sensing electrode 12 (again of PTFE and catalyst). Finally, an o-ring seal 13 is provided between the sensing electrode .
- . , :
2~5~
and the top plate 1. A wick 14 extends from the separator 7 through apertures in the counter electrode 5 and base plate 2 into an electrolyte reservoir 15 containing the electrolyte. The reservoir 15 is sealed by an end plate/seal 16.
The arrangement of the current collectors 6, 8, and 11 is shown in more detail in Figure lB.
A gas phase diffusion barrier is provided in the top plate 1 in the form of a capilliary 17. ~he construction of the top plate 1 is shown in more detail in Figure lC and comprises a capilliary plate 18 containing the capilliary 17, the plate having three sections 18A, 18B, and 18C of progressively increasing diameter. A Mupor tape filter 19 is fitted into the section 18B of the capilliary plate 18 while a capilliary plate mask 20 containing six equi-angularly spaced, drilled holes of l.lmm diameter is snap fitted into the section 18C.
Three sensors were constructed to this established commercial design, (described also in G~ Patent 2,094,005), incorporating a capillary diffusion barrier of six 1.1 mm diameter holes, a bonded, gas diffusion sensing electrode comprising a carbon based electrocatalyst, a silver/silver chloride reference electrode and a silver/silver iodate counter electrode. The sensors were primed with different electrolytes (described below) and operated in an electrical control circuit according to Blazhenov et al ~GB
Patent 1,101,101, (1968)) with a +soomv bias potential on the sensing electrode relative to the reference electrode.
Sensor 1. Primed with electrolyte consisting of sN
LiCl, 3.lM NaCl.
;- Sensor 2. Primed with electrolyte consisting of sM
; LiCl, 2.8M NaCl, saturated with I2.
Sensor 3. Primed with electrolyte consisting of 7.7M
LiCl, 2.8M NaCl, 1% starch, saturated with I2.
Each sensor was allowed to settle for a week on the electrical control circuit, and the steady baseline ~zero-gas response) noted. Responses to ammonia were then `.
.
- , :
.
- 20~9~
measured by exposing the sensors to a 41.3 ppm NH3 (sensors 1 and 2) and 48.9 ppm NH3 (sensor 3) in air test gas at a flow rate of 200ml minl. Responses to the gases CO,H2, S02 and CO2 were similarly measured after the NH3 exposure.
Results of these tests are given in Table 1 below.
.
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Table 1. Charaderistics of Ammonia Sensors.
SENSOR TEST GAS B~SELINE NH, RESPONSE Cross . (ppm NH, eciuiv) (n~ ppm ') Sensitivity 1. LiCI/NaCI NHJair 7.6 63 ~C9O/Paem -0.2 5% CO,/air zero i1 i8J3a3irPm zero.
5O4~pNpm 196 2. LiCi/NaCi/l~ 41 3ppm 2 . 6 9 0 C9OlPaiPrm Q1 5% CO,/air zero 2HO,/7aplrpm -0.1 194ppm 105 SO~/~ir 3. LiCl/NaCI/ 4 8.9ppm 5 . 5 9 0 I,/starch NHJair ~C9C5~/PaiPrm Zero 5% CO,/air zero 1 B8ppm H,/air 194ppm 1~5 .
2 ~ S ~
The (conventional) iodine-free electrolyte ~sensor l) produced a rather unstable response initially (Figure 2) which suffered more hysteresis on removal of the test gas than either of the other two sensors (Figures 3 and 4).
Furthermore, within 2 weeks the response of sensor l to NH3 had reduced to about one third that of its initial response (Figure 5). Both sensors 2 and 3, containing iodine produced stable NH3 responses with lower hysteresis than sensor 1 and which remained virtually unchanged with time over a test period of 3 or 4 weeks. As can be seen in Figure 6 (line 30), sensor 3 when exposed to 50ppm NH3 in air at a flow rate of 200ml/min exhibited a very slow decline in response over a period of lO weeks, in contrast to sensor 1 (line 31).
All 3 sensors had very low cross interferences to C0, H2 and C02. Sulphur dioxide gave a significant response due to direct electrochemical oxidation in sensor 1 (about 200%
NH3 equivalent), but this was somewhat reduced with the I2 systems at about 100%.
.
. " ' .
.
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dissolving ammonia gas; it should also contain an ionically conducting supporting electrolyte. To meet these requirements salts of strong acids and strong bases are dissolved in the solution containing iodine. Examples of '5 suitable electrolytes are salts of the alkali and alkaline earth metals such as the chlorides of lithium, sodium, potassium, calcium etc. Salts such as lithium and calcium chlorides are hygroscopic and provide the additional benefit of controlling the sensor water balance, preventing the sensor from drying out.
Although some of the reacted iodine is regenerated by electrochemical oxidation of the iodide product (equation 4) there is a net consumption of iodine according to equation 5 of 1 mole per 12 moles of ammonia. The sensor life will therefore be determined by the volume of electrolyte and iodine concentration contained in the sensor. Iodine only has a limited solubility in water but measures can be taken to increase the iodine capacity of the sensor, without the need to carry a large volume of electrolyte. Free solid iodine contained within the electrolyte reservoir would ensure that the solut~ion remains saturated with iodine; however, solid iodine has a measurable vapour pressure and slowly diffuses out of the sensor. In addition to lost iodine capacity, the volatile iodine can cause problems of corrosion to metal surfaces external to the cell.
In one approach, the iodine is bound chemically to another compound, such as starch, which releases iodine reversibly as free iodine in equilibrium with the complex.
In this way sufficient iodine is available to satisfy the sensing electrode reactions for detecting ammonia, but at a sufficiently low concentration to reduce iodine loss by volatilisation to an extremely low level. The starch complex also allows a considerable iodine capacity to be obtained in a relatively small electrolyte volume. An alternative approach would be to implant a permeation device inside the sensor, containing an iodine source and . .
' ~, '"
.
.- . , - .
.
20~3~9 which is designed to have suitably low iodine diffusion rate so as to provide a controlled release of iodine into the electrolyte to maintain the concentration at the appropriate level. The iodine source may be for example, solid iodine, aqueous or non-aqueous iodine solutions or any compound which may release elemental iodine which can then diffuse at a controlled rate fro~ the capsule, into the body of the sensor and thence into its electrolyte.
In one alternative, but less preferred, arrangement the electrolyte contains Nesslers Reagent. This comprises a solution of mercuric iodide dissolved in excess potassium iodide solution in which the complex mercuric-iodide ions HgI3 and HgI4 are formed. These complexes do not precipitate mercuric oxide on adding alkali but do give a yellow-brown coloration with NH3 forming the amino component Hg20INH2, the iodide of "Millons Base" (Hg2 O(OH)NH2). This compound can then undergo anodic oxidation of the amino group to produce the signal of the sensor.
This reagent is not a preferred material since the mercury salt would impose pro~lems with disposal of the sensors at end of their life.
In a further arrangement the electrolyte can contain a solution of manganous and silver nitrates which reacts with hydroxide liberated by the dissolution of ammonia gas according to the equation:
Mn + 2Ag + 4OH = MnO2 ~ 2Ag + 2H20. (6) Following this reaction, either the MnO2 can be cathodically reduced or the Ag anodically oxidised to produce the sensor signal related to the NH3 concentration.
30The diffusion barrier can be of any conventional type including a gas phase diffusion barrier, a Xnudsen barrier, a solid barrier or a combination of two or more of these.
Although the sensor can have just sensing and counter electrodes, in general a third, rèference electrode is provided to keep the sensing electrode at the correct, working potential.
-.
' " . '' ' '' ~' , ' . . ~ ' ~ ' :
- , , ~
2 0 ~
It has been found that sensors according to the invention are of small size and have good low temperature performance~
BRIEFL DESCRIPTION OF THE DRAWINGS
~i Some examples of sensors in accordance with the invention and a comparative example will now be described with reference to the accompanying drawings, in which:-Figure lA is an exploded view of an example of a sensor according to the invention;
Figure lB illustrates the connections to the electrodes in the Figure lA example;
Figure lC is an exploded view of the top plate of Figure lA;
Figure 2 illustrates the response of a conventional sensor after one week;
Figures 3 and 4 illustrate the response of two sensors according to the invention after one week;
Figure 5 illustrates the response of the conventional sensor after two weeks; and, Figure 6 illustrates the sensitivity of a sensor according to the invention over a period of 11 weeks as well as that of a conventional sensor over 2 weeks.
DETAILED DESCRIPTION OF THE EMBODIMENTS
The sensor shown in Figures lA-lC is generally of conventional form and will not be described in detail.
Briefly, the sensor comprises a composite top plate 1 shown in more detail in Figure lC which is mounted in use to a base plate 2 having an outwardly facing, annular flange 3 defining an electrode well 4. Within the electrode well 4 are provided a counter electrode S comprising PTFE tape and a catalyst layer connected to a current collector 6.
The counter electrode 5 is provided in a sandwich with a separator 7, a further current collector 8, a reference electrode 9 (comprising PTFE layer and catalyst), a separator lO, a current collector 11, and a sensing electrode 12 (again of PTFE and catalyst). Finally, an o-ring seal 13 is provided between the sensing electrode .
- . , :
2~5~
and the top plate 1. A wick 14 extends from the separator 7 through apertures in the counter electrode 5 and base plate 2 into an electrolyte reservoir 15 containing the electrolyte. The reservoir 15 is sealed by an end plate/seal 16.
The arrangement of the current collectors 6, 8, and 11 is shown in more detail in Figure lB.
A gas phase diffusion barrier is provided in the top plate 1 in the form of a capilliary 17. ~he construction of the top plate 1 is shown in more detail in Figure lC and comprises a capilliary plate 18 containing the capilliary 17, the plate having three sections 18A, 18B, and 18C of progressively increasing diameter. A Mupor tape filter 19 is fitted into the section 18B of the capilliary plate 18 while a capilliary plate mask 20 containing six equi-angularly spaced, drilled holes of l.lmm diameter is snap fitted into the section 18C.
Three sensors were constructed to this established commercial design, (described also in G~ Patent 2,094,005), incorporating a capillary diffusion barrier of six 1.1 mm diameter holes, a bonded, gas diffusion sensing electrode comprising a carbon based electrocatalyst, a silver/silver chloride reference electrode and a silver/silver iodate counter electrode. The sensors were primed with different electrolytes (described below) and operated in an electrical control circuit according to Blazhenov et al ~GB
Patent 1,101,101, (1968)) with a +soomv bias potential on the sensing electrode relative to the reference electrode.
Sensor 1. Primed with electrolyte consisting of sN
LiCl, 3.lM NaCl.
;- Sensor 2. Primed with electrolyte consisting of sM
; LiCl, 2.8M NaCl, saturated with I2.
Sensor 3. Primed with electrolyte consisting of 7.7M
LiCl, 2.8M NaCl, 1% starch, saturated with I2.
Each sensor was allowed to settle for a week on the electrical control circuit, and the steady baseline ~zero-gas response) noted. Responses to ammonia were then `.
.
- , :
.
- 20~9~
measured by exposing the sensors to a 41.3 ppm NH3 (sensors 1 and 2) and 48.9 ppm NH3 (sensor 3) in air test gas at a flow rate of 200ml minl. Responses to the gases CO,H2, S02 and CO2 were similarly measured after the NH3 exposure.
Results of these tests are given in Table 1 below.
.
:
' ' : ` ' ' -:
`` ' ' ' :
~0~4~
Table 1. Charaderistics of Ammonia Sensors.
SENSOR TEST GAS B~SELINE NH, RESPONSE Cross . (ppm NH, eciuiv) (n~ ppm ') Sensitivity 1. LiCI/NaCI NHJair 7.6 63 ~C9O/Paem -0.2 5% CO,/air zero i1 i8J3a3irPm zero.
5O4~pNpm 196 2. LiCi/NaCi/l~ 41 3ppm 2 . 6 9 0 C9OlPaiPrm Q1 5% CO,/air zero 2HO,/7aplrpm -0.1 194ppm 105 SO~/~ir 3. LiCl/NaCI/ 4 8.9ppm 5 . 5 9 0 I,/starch NHJair ~C9C5~/PaiPrm Zero 5% CO,/air zero 1 B8ppm H,/air 194ppm 1~5 .
2 ~ S ~
The (conventional) iodine-free electrolyte ~sensor l) produced a rather unstable response initially (Figure 2) which suffered more hysteresis on removal of the test gas than either of the other two sensors (Figures 3 and 4).
Furthermore, within 2 weeks the response of sensor l to NH3 had reduced to about one third that of its initial response (Figure 5). Both sensors 2 and 3, containing iodine produced stable NH3 responses with lower hysteresis than sensor 1 and which remained virtually unchanged with time over a test period of 3 or 4 weeks. As can be seen in Figure 6 (line 30), sensor 3 when exposed to 50ppm NH3 in air at a flow rate of 200ml/min exhibited a very slow decline in response over a period of lO weeks, in contrast to sensor 1 (line 31).
All 3 sensors had very low cross interferences to C0, H2 and C02. Sulphur dioxide gave a significant response due to direct electrochemical oxidation in sensor 1 (about 200%
NH3 equivalent), but this was somewhat reduced with the I2 systems at about 100%.
.
. " ' .
.
, , ' ' "~ -~
Claims (8)
1. An electrochemical gas sensor for sensing an alkaline gas, the sensor comprising at least sensing and counter electrodes provided in a cell containing an aqueous electrolyte, the cell further including a diffusion barrier to restrict the access of gas to the cell, and a chemical species with which the gas reacts in use to form a product which is more electrochemically active than the gas, wherein the chemical species is one of:
a) iodine;
b) Nesslers reagent; and c) a solution of manganous and silver nitrates.
a) iodine;
b) Nesslers reagent; and c) a solution of manganous and silver nitrates.
2. A sensor according to claim 1 for detecting ammonia, wherein said chemical species is iodine.
3. A sensor according to claim 2, wherein said iodine is bound to another compound to reduce its volatility.
4. A sensor according to claim 3, wherein said iodine is bound chemically to starch.
5. A sensor according to claim 1, wherein said sensor further comprises a permeation device containing an iodine source so as to provide a controlled release of iodine into the electrolyte.
6. A sensor according to claim 5, wherein said iodine source comprises solid iodine.
7. A sensor according to any of claims 1 to 3, wherein the diffusion barrier is one or a combination of a gas phase diffusion barrier and a Knudsen barrier.
8. A sensor according to any of claims 1 to 3, further comprising a reference electrode.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB9101643.6 | 1991-01-25 | ||
| GB919101643A GB9101643D0 (en) | 1991-01-25 | 1991-01-25 | Gas sensor |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2059949A1 true CA2059949A1 (en) | 1992-07-26 |
Family
ID=10689000
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002059949A Abandoned CA2059949A1 (en) | 1991-01-25 | 1992-01-23 | Gas sensor |
Country Status (6)
| Country | Link |
|---|---|
| US (1) | US5234567A (en) |
| EP (1) | EP0496527B1 (en) |
| JP (1) | JPH05196597A (en) |
| CA (1) | CA2059949A1 (en) |
| DE (1) | DE69201653T2 (en) |
| GB (1) | GB9101643D0 (en) |
Families Citing this family (30)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| EP0546291A1 (en) * | 1991-12-11 | 1993-06-16 | Drägerwerk Aktiengesellschaft | Electrochemical measuring cell for the determination of ammonia or hydrazine in a sample |
| US5338429A (en) * | 1993-03-05 | 1994-08-16 | Mine Safety Appliances Company | Electrochemical toxic gas sensor |
| CH686981A5 (en) * | 1994-02-09 | 1996-08-15 | Christian Dr Huggenberger | Ammonia sensor. |
| GB2292804A (en) * | 1994-08-31 | 1996-03-06 | Transducer Research Inc | Electrochemical gas sensor with electrodes deposited on a wick |
| WO1996033404A1 (en) * | 1995-04-21 | 1996-10-24 | Mine Safety Appliances Company | Electrochemical gas sensor for the detection of nitrogen dioxide |
| GB9513158D0 (en) * | 1995-06-28 | 1995-08-30 | Wallace & Tiernan Ltd | Sensor |
| GB9601604D0 (en) * | 1996-01-26 | 1996-03-27 | Ciba Geigy Ag | Pigment compositions |
| DE19639311C2 (en) * | 1996-09-25 | 1998-07-16 | Draegerwerk Ag | Electrochemical measuring cell for the detection of oxidizing gases |
| DE19726453C2 (en) * | 1997-06-21 | 2000-08-10 | Draegerwerk Ag | Electrochemical oxygen sensor |
| US6731976B2 (en) | 1997-09-03 | 2004-05-04 | Medtronic, Inc. | Device and method to measure and communicate body parameters |
| US6126848A (en) | 1998-05-06 | 2000-10-03 | International Business Machines Corporation | Indirect endpoint detection by chemical reaction and chemiluminescence |
| US6228280B1 (en) | 1998-05-06 | 2001-05-08 | International Business Machines Corporation | Endpoint detection by chemical reaction and reagent |
| US6194230B1 (en) | 1998-05-06 | 2001-02-27 | International Business Machines Corporation | Endpoint detection by chemical reaction and light scattering |
| US6066564A (en) * | 1998-05-06 | 2000-05-23 | International Business Machines Corporation | Indirect endpoint detection by chemical reaction |
| US6228769B1 (en) | 1998-05-06 | 2001-05-08 | International Business Machines Corporation | Endpoint detection by chemical reaction and photoionization |
| US6180422B1 (en) | 1998-05-06 | 2001-01-30 | International Business Machines Corporation | Endpoint detection by chemical reaction |
| US6251784B1 (en) | 1998-12-08 | 2001-06-26 | International Business Machines Corporation | Real-time control of chemical-mechanical polishing processing by monitoring ionization current |
| US6176765B1 (en) | 1999-02-16 | 2001-01-23 | International Business Machines Corporation | Accumulator for slurry sampling |
| US6248224B1 (en) * | 1999-05-12 | 2001-06-19 | Mst Analytics Inc. | Toxic sensor and method of manufacture |
| US20020151770A1 (en) | 2001-01-04 | 2002-10-17 | Noll Austin F. | Implantable medical device with sensor |
| GB2374419B (en) * | 2001-03-09 | 2004-12-29 | Zellweger Analytics Ltd | Electrochemical gas sensor |
| GB0319043D0 (en) * | 2003-08-13 | 2003-09-17 | Anglo Europ Design Engineers L | Electrochemical gas sensors |
| TW200538723A (en) * | 2004-03-03 | 2005-12-01 | Zellweger Analytics Agch | Electrochemical gas sensor |
| CH697047A5 (en) * | 2004-05-27 | 2008-03-31 | Membrapor Ag | Ammonia sensor. |
| US20120241319A1 (en) * | 2011-03-25 | 2012-09-27 | Life Safety Distribution Ag | Gas Detector Having Bipolar Counter/Reference Electrode |
| KR101869811B1 (en) * | 2011-12-13 | 2018-06-22 | 삼성전자주식회사 | device for detecting ammonia gas and management system of semiconductor manufacturing factory used the same |
| JP6208435B2 (en) * | 2013-02-27 | 2017-10-04 | 学校法人東海大学 | pH measurement electrode |
| KR101786967B1 (en) | 2013-08-01 | 2017-10-18 | 삼성전자주식회사 | Gas sensor module, refrigerator having the same and control method for the refrigerator |
| DE102014002502B4 (en) | 2014-02-21 | 2021-08-05 | Dräger Safety AG & Co. KGaA | Electrochemical gas sensor and use of an electrochemical gas sensor |
| TWI706126B (en) * | 2018-12-28 | 2020-10-01 | 財團法人工業技術研究院 | Gas sensing device and gas sensing system |
Family Cites Families (8)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US3764269A (en) * | 1971-12-28 | 1973-10-09 | North American Rockwell | Sensor for fluid components |
| US3821090A (en) * | 1973-06-14 | 1974-06-28 | Rockwell International Corp | Method for directly measuring no2 in air |
| DE2624266A1 (en) * | 1976-05-29 | 1977-12-08 | Hartmann & Braun Ag | Electrochemical measuring cells for gas-analytical measurements - uses addition of iodides to doping substance to cover immission and emission ranges |
| US4474648A (en) * | 1980-04-30 | 1984-10-02 | City Technology Limited | Gas sensor |
| US4568445A (en) * | 1984-12-21 | 1986-02-04 | Honeywell Inc. | Electrode system for an electro-chemical sensor for measuring vapor concentrations |
| US4842697A (en) * | 1985-04-26 | 1989-06-27 | Hnu Systems, Inc. | Method of determining ammonia in a gaseous or liquid sample |
| DE3841622A1 (en) * | 1988-12-10 | 1990-06-13 | Draegerwerk Ag | ELECTROCHEMICAL MEASURING CELL FOR THE AMPEROMETRIC DETERMINATION OF AMMONIA AND ITS DERIVATIVES |
| DE3914284A1 (en) * | 1989-04-29 | 1990-10-31 | Draegerwerk Ag | ELECTROCHEMICAL MEASURING CELL FOR DETERMINING AMMONIA OR HYDRAZINE IN A MEASURING SAMPLE |
-
1991
- 1991-01-25 GB GB919101643A patent/GB9101643D0/en active Pending
-
1992
- 1992-01-16 EP EP92300364A patent/EP0496527B1/en not_active Expired - Lifetime
- 1992-01-16 DE DE69201653T patent/DE69201653T2/en not_active Expired - Fee Related
- 1992-01-23 CA CA002059949A patent/CA2059949A1/en not_active Abandoned
- 1992-01-23 US US07/824,510 patent/US5234567A/en not_active Expired - Fee Related
- 1992-01-24 JP JP4032959A patent/JPH05196597A/en not_active Withdrawn
Also Published As
| Publication number | Publication date |
|---|---|
| DE69201653T2 (en) | 1995-07-13 |
| JPH05196597A (en) | 1993-08-06 |
| EP0496527A1 (en) | 1992-07-29 |
| DE69201653D1 (en) | 1995-04-20 |
| US5234567A (en) | 1993-08-10 |
| GB9101643D0 (en) | 1991-03-06 |
| EP0496527B1 (en) | 1995-03-15 |
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