JP6466022B1 - Constant potential electrolytic gas sensor - Google Patents

Abstract

An object of the present invention is to provide a constant potential electrolysis gas sensor in which a change with time in sensitivity to hydrogen fluoride gas is suppressed.
A constant potential electrolysis gas sensor of the present invention is a constant potential electrolysis gas sensor 1 for detecting hydrogen fluoride gas, and as an electrode 2 for detecting hydrogen fluoride gas, a reaction electrode 21 and A lithium halide as an electrolytic solution 3 that includes a counter electrode 22 containing silver with respect to the reaction electrode 21 and a reference electrode 23 that serves as a reference for the potential of the reaction electrode 21, and contacts the reaction electrode 21, the counter electrode 22, and the reference electrode 23 In addition to the silver ions that spontaneously dissolve in the electrolyte solution 3 from the electrode 2, silver ions are added to the electrolyte solution 3.
[Selection] Figure 1

Description

  The present invention relates to a constant potential electrolytic gas sensor.

  As a sensor for detecting the detection target gas, for example, a constant potential electrolytic gas sensor as disclosed in Patent Document 1 is used. In general, a constant potential electrolytic gas sensor is in contact with a reaction electrode for electrochemically reacting a gas to be detected, a counter electrode for the reaction electrode, a reference electrode serving as a reference for the potential of the reaction electrode, and a reaction electrode, a counter electrode, and a reference electrode. An electrolyte solution. The constant potential electrolytic gas sensor controls the potential of the reaction electrode with respect to the reference electrode, and detects the electrolytic current generated between the reaction electrode and the counter electrode due to the electrochemical reaction of the detection target gas. Can be detected.

  The constant potential electrolytic gas sensor can detect various types of detection target gases by arbitrarily selecting the type of electrode and electrolyte, the potential of the reaction electrode with respect to the reference electrode, and the like. For example, a constant potential electrolytic gas sensor can detect hydrogen fluoride gas by using silver as a counter electrode material and a lithium halide solution such as lithium bromide as an electrolytic solution.

JP 2014-98679 A

  However, the constant potential electrolytic gas sensor can detect hydrogen fluoride gas with high sensitivity by using silver as a counter electrode material and a lithium halide solution as an electrolyte solution, while being sensitive to hydrogen fluoride gas. There is a problem that changes with time.

  The present invention has been made in view of the above problems, and an object of the present invention is to provide a constant potential electrolytic gas sensor in which a change with time in sensitivity to hydrogen fluoride gas is suppressed.

  The constant potential electrolysis gas sensor of the present invention is a constant potential electrolysis gas sensor for detecting hydrogen fluoride gas. As an electrode for detecting hydrogen fluoride gas, a reaction electrode and a silver for the reaction electrode And a reference electrode serving as a reference for the potential of the reaction electrode, and an electrolyte solution containing lithium halide as an electrolyte solution in contact with the reaction electrode, the counter electrode, and the reference electrode, and from the electrode In addition to silver ions that naturally dissolve in the electrolytic solution, silver ions are added to the electrolytic solution.

  Moreover, it is preferable that silver halide is added to the electrolytic solution.

  Moreover, it is preferable that the said silver halide is 1 or more types selected from the group containing silver chloride, silver bromide, and silver iodide.

  Moreover, it is preferable that 0.025 mol / L or more of the silver halide is added to the electrolytic solution.

  According to the present invention, it is possible to provide a constant potential electrolytic gas sensor in which a change with time in sensitivity to hydrogen fluoride gas is suppressed.

1 is a cross-sectional view of a constant potential electrolytic gas sensor according to an embodiment of the present invention. It shows the results of examining the difference in the response characteristics with respect to hydrogen fluoride gas over time depending on whether or not silver bromide is added to the electrolytic solution using a constant potential electrolytic gas sensor. (A) shows the electrolytic solution The result about Example 1 which added the silver bromide in is shown, (b) shows the result about the comparative example 1 which has not added silver bromide in electrolyte solution. It is the scanning electron micrograph (1,000 times) obtained from the counter electrode surface after one month after manufacture, (a) is the counter electrode surface used in Example 1 which added silver bromide in electrolyte solution (B) is a photograph of the counter electrode surface used in Comparative Example 1 in which no silver bromide was added to the electrolyte. It shows the results of investigating the difference in response characteristics to hydrogen fluoride gas with and without the addition of silver bromide in the electrolyte using multiple controlled potential electrolytic gas sensors manufactured in different production lots. a) shows the results for Example 2 in which silver bromide was added to the electrolytic solution, and (b) shows the results for Comparative Example 2 in which silver bromide was not added to the electrolytic solution. With respect to hydrogen fluoride gas when the installation environment of a constant potential electrolytic gas sensor is changed from a normal humidity environment (relative humidity 50% RH) to a low humidity environment (relative humidity 30% RH) using a constant potential electrolytic gas sensor. The result of having investigated the change of a response characteristic is shown, (a) shows the result about Example 3 which added silver bromide in electrolyte solution, and (b) shows silver bromide in electrolyte solution. The result about the comparative example 3 which is not added is shown. Hydrogen fluoride gas when the installation environment of a constant potential electrolytic gas sensor is changed from a normal humidity environment (relative humidity 50% RH) to a high humidity environment (relative humidity 80% RH) using a constant potential electrolytic gas sensor The result of having investigated the change of the response characteristic with respect to is shown, (a) shows the result about Example 3 which added silver bromide in electrolyte solution, (b) added silver bromide in electrolyte solution The result about the comparative example 3 which is not performed is shown. The result of having investigated the change of the response characteristic of the hydrogen fluoride gas with respect to the addition amount of silver bromide in electrolyte solution using the constant potential electrolytic gas sensor after one month progress after manufacture is shown.

  Hereinafter, a constant potential electrolytic gas sensor according to an embodiment of the present invention will be described with reference to the accompanying drawings. However, the embodiment shown below is an example, and the constant potential electrolytic gas sensor of the present invention is not limited to the following example.

  The constant potential electrolytic gas sensor 1 of the present embodiment is used for detecting hydrogen fluoride gas. As shown in FIG. 1, the constant potential electrolytic gas sensor 1 includes an electrode 2 for detecting hydrogen fluoride gas and an electrolytic solution 3 in contact with the electrode 2. The constant potential electrolytic gas sensor 1 detects hydrogen fluoride gas by detecting an electrolytic current generated in the electrode 2 by an electrochemical reaction involving the hydrogen fluoride gas.

  In the present embodiment, the constant potential electrolytic gas sensor 1 includes an electrolytic cell 4 that accommodates an electrode 2 and an electrolytic solution 3 as shown in FIG. The electrolytic cell 4 includes an electrolytic solution storage unit 41 that stores the electrolytic solution 3, an electrolytic solution supply hole 42 for supplying the electrolytic solution 3 to the electrolytic solution storage unit 41, and a measurement target gas including hydrogen fluoride gas is electrolyzed. Gas inflow holes 43 flowing from the outside of the tank 4 into the electrolytic solution storage part 41 and gases in the electrolytic solution 3 such as gas generated by electrochemical reaction and unreacted measurement target gas are electrolyzed from the electrolytic solution storage part 41. A gas outflow hole 44 that flows out of the tank 4 is provided.

  However, the electrolytic cell is not limited to the illustrated configuration as long as it can accommodate the electrode and the electrolytic solution and can be configured to allow gas to flow in and out. The material constituting the electrolytic cell is also not particularly limited, and known resin materials such as polycarbonate, vinyl chloride, polyethylene, polypropylene, and polytetrafluoroethylene can be employed.

  As shown in FIG. 1, the constant potential electrolytic gas sensor 1 includes a reaction electrode 21, a counter electrode 22 containing silver with respect to the reaction electrode 21, and a reaction electrode 21 as an electrode 2 for detecting hydrogen fluoride gas. A reference electrode 23 serving as a potential reference is provided. The reaction electrode 21, the counter electrode 22 and the reference electrode 23 are arranged in the electrolytic cell 4 so as to be in contact with the electrolytic solution 3. The reaction electrode 21, the counter electrode 22, and the reference electrode 23 are connected to a control means such as a known potentiostat (not shown) via a lead wire, and a predetermined voltage is applied to generate an electrolytic current generated as a result of an electrochemical reaction. Is measured.

  The reaction electrode 21 is an electrode that generates an electrochemical reaction involving hydrogen fluoride when a constant voltage is applied based on the potential of the reference electrode 23. In the present embodiment, the reaction electrode 21 is formed by applying and baking a paste made of an electrode material (for example, carbon) on the gas inflow side gas permeable film 5 as shown in FIG. . The reaction electrode 21 and the gas inflow side gas permeable membrane 5 are disposed so as to close the gas inflow hole 43 in a liquid-tight manner. More specifically, in the stacked reaction electrode 21 and gas inflow side gas permeable membrane 5, the reaction electrode 21 faces the electrolyte accommodating portion 41 and the gas inflow side gas permeable membrane 5 faces the gas inflow hole 43. Thus, the gas inflow side cover member 7 is fixed between the electrolyte solution containing portion 41 and the gas inflow hole 43 through a known sealing means such as an O-ring 6. The voltage applied to the reaction electrode 21 can be appropriately set so as to cause an electrochemical reaction involving hydrogen fluoride in the reaction electrode 21.

  The gas inflow side gas permeable membrane 5 suppresses the electrolytic solution 3 in the electrolytic solution storage part 41 from flowing out of the electrolytic cell 4 through the gas inflow hole 43, while measuring the hydrogen fluoride gas. Gas is allowed to flow into the electrolyte container 41 from the outside of the electrolytic cell 4. The gas inflow side gas permeable membrane 5 is not particularly limited as long as it is configured to suppress the outflow of the electrolyte solution 3 and to allow the inflow of the measurement target gas. For example, polytetrafluoroethylene A film formed porous using a fluororesin material such as perfluoroalkoxyalkane or perfluoroethylenepropene copolymer can be used.

  The counter electrode 22 is an electrode containing silver and causing another electrochemical reaction corresponding to the electrochemical reaction involving hydrogen fluoride at the reaction electrode 21. In this embodiment, the counter electrode 22 is configured as a silver wire fixed on the gas outflow side gas permeable membrane 8 as shown in FIG. The counter electrode 22 and the gas outflow side gas permeable membrane 8 are disposed so as to face the reaction electrode 21 through the electrolytic solution 3 so as to close the gas outflow hole 44 in a liquid-tight manner.

  The reference electrode 23 is an electrode serving as a reference for the potential of the reaction electrode 21. In this embodiment, the reference electrode 23 is configured as a silver wire fixed on the gas outflow side gas permeable membrane 8 together with the counter electrode 22 as shown in FIG. The reference electrode 23 is disposed so as to face the reaction electrode 21 through the electrolytic solution 3 so as to liquid-tightly close the gas outflow hole 44 together with the gas outflow side gas permeable membrane 8.

  The gas outflow side gas permeable membrane 8 prevents the electrolyte 3 in the electrolyte container 41 from flowing out of the electrolytic cell 4 through the gas outflow hole 44, while A gas in the electrolyte 3 such as an unreacted measurement target gas is allowed to flow out of the electrolytic cell 4 from the electrolyte container 41. As shown in FIG. 1, the gas outflow side gas permeable membrane 8 is disposed so that the surface opposite to the surface on which the counter electrode 22 and the reference electrode 23 are fixed faces the gas outflow hole 44. It is fixed between the electrolyte solution storage part 41 and the gas outflow hole 44 by the gas outflow side lid member 10 through a known sealing means such as 9. The gas outflow side gas permeable membrane 8 is not particularly limited as long as it is configured to suppress the outflow of the electrolytic solution 3 and allow the outflow of the gas in the electrolytic solution 3. Similarly to the inflow side gas permeable membrane 5, a film formed porous using a fluororesin material such as polytetrafluoroethylene, perfluoroalkoxyalkane, and perfluoroethylene propene copolymer can be used.

  In the constant potential electrolytic gas sensor 1, a constant voltage is applied to the reaction electrode 21 based on the potential of the reference electrode 23, and a constant potential difference is added to the reference electrode 23. The reaction electrode 21 to which a certain potential difference is added between the reference electrode 23 and the reaction electrode 21 causes an electrochemical reaction involving hydrogen fluoride flowing into the electrolytic solution 3 in contact with the reaction electrode 21. When an electrochemical reaction involving hydrogen fluoride occurs, another electrochemical reaction also occurs on the counter electrode 22 side corresponding to the electrochemical reaction. As a result of the electrochemical reaction occurring at the reaction electrode 21 and the counter electrode 22, an electrolysis voltage is generated between the reaction electrode 21 and the counter electrode 22, and by detecting the electrolysis current, hydrogen fluoride gas can be detected. The concentration of hydrogen fluoride gas can be determined according to the magnitude of the current.

  The constant potential electrolytic gas sensor 1 includes an electrolytic solution 3 containing lithium halide as the electrolytic solution 3 in contact with the reaction electrode 21, the counter electrode 22, and the reference electrode 23. Since the electrolytic solution 3 in contact with the reaction electrode 21, the counter electrode 22, and the reference electrode 23 contains lithium halide, (1) fluorination flowing into the electrolytic solution 3 in the reaction electrode 21 to which a certain voltage is applied. A hydrogen gas dissociation reaction, (2) a halogen gas generation reaction, and (3) a halogen gas reduction reaction occur, and (4) halogen ions at the counter electrode 22 corresponding to the above-described electrochemical reaction at the reaction electrode 21 Oxidation reaction occurs. In addition, in this embodiment, in order to accelerate | stimulate the reaction of above-mentioned (2) in the reaction electrode 21, the electrolyte solution 3 adds a halogenate ion. The reaction formulas (1) to (4) above when a halogen acid ion is added to the electrolytic solution 3 containing lithium halide are shown below.

(Reaction electrode)
(1) HF → H ⁺ + F ⁻
(2) 6H ⁺ + 5X ⁻ + XO ₃ ⁻ → 3X ₂ + 3H ₂ O
(3) X ₂ + 2e ⁻ → 2X ⁻
(Counter electrode)
(4) 2Ag + 2X ⁻ → 2AgX + 2e ⁻
However, X shows a halogen element.

  The concentration of lithium halide contained in the electrolytic solution 3 is not particularly limited, and is appropriately set so that an electrochemical reaction including a dissociation reaction of hydrogen fluoride gas occurs at the reaction electrode 21 and the counter electrode 22. Can do. In addition, any halogen element is applicable as the halogen element contained in the lithium halide, but one kind selected from the group containing chlorine (Cl), bromine (Br), and iodine (I) Preferably, it is bromine (Br). The addition of the halogenate ion to the electrolytic solution 3 is not particularly limited, but can be performed by adding, for example, potassium halide. As the halogen element contained in the potassium halide, any halogen element is applicable, but one or more selected from the group containing chlorine (Cl), bromine (Br) and iodine (I) Preferably, it is bromine (Br).

  In the constant potential electrolytic gas sensor 1, silver ions are added to the electrolytic solution 3 separately from the silver ions that are naturally dissolved from the electrode 2 into the electrolytic solution 3. The constant potential electrolytic gas sensor 1 can suppress a change with time in sensitivity to hydrogen fluoride gas by adding silver ions to the electrolytic solution 3. Thereby, the life of the constant potential electrolytic gas sensor 1 can be extended. As described in detail below, the addition of silver ions to the electrolytic solution 3 suppresses the deposition of lithium carbonate or lithium oxide that causes a change in sensitivity to hydrogen fluoride gas over time on the counter electrode 22. This is thought to be possible.

  Here, when a constant voltage is applied to the reaction electrode 21 with reference to the reference electrode 23, the silver contained in the counter electrode 22 and the halogen ions contained in the electrolytic solution 3 on the counter electrode 22 as described above. React to produce silver halide. This silver halide is produced by an electrochemical reaction (oxidation reaction) of halogen ions that occurs in response to the electrochemical reaction at the reaction electrode 21 and is very small. On the other hand, on the counter electrode 22, apart from the silver halide generated by the electrochemical reaction, the silver halide eluted from the counter electrode 22 and the halogen ion in the electrolytic solution 3 are also chemically reacted, so that the silver halide is also formed. Formation and precipitation. The silver halide produced by this chemical reaction is more than the silver halide produced by the electrochemical reaction. When the silver halide produced | generated by a chemical reaction precipitates on the counter electrode 22, lithium carbonate and lithium oxide will precipitate on the counter electrode 22 (refer FIG.3 (b)). This is thought to be because the silver halide deposited on the surface by the chemical reaction serves as a catalyst to promote the precipitation of lithium carbonate and lithium oxide. The lithium carbonate or lithium oxide deposited on the counter electrode 22 not only inhibits the electrochemical reaction on the counter electrode 22 but also elutes in the electrolyte 3 and reacts with hydrogen fluoride to be detected. This hinders detection of hydrogen gas. Thus, on the counter electrode 22, apart from the electrochemical reaction, the chemical reaction between silver ions eluted from the counter electrode 22 and the halogen ions in the electrolytic solution 3 proceeds with time, and accordingly lithium carbonate and lithium oxide are produced. Precipitate on the surface. As a result, it is considered that the sensitivity to hydrogen fluoride gas changes with time.

  On the other hand, by adding silver ions to the electrolytic solution 3 separately from the silver ions that naturally dissolve in the electrolytic solution 3 from the counter electrode 22, precipitation of lithium carbonate and lithium oxide is suppressed (FIG. 3A). reference). This is because by adding silver ions to the electrolytic solution 3, the silver contained in the counter electrode 22 is suppressed from eluting into the electrolytic solution 3, and the silver ions eluted from the counter electrode 22 and the halogen in the electrolytic solution 3 are reduced. This is probably because the chemical reaction with ions is suppressed, and the formation and precipitation of silver halide due to the chemical reaction is suppressed. As a result, it is considered that lithium carbonate or lithium oxide that causes a change in sensitivity with respect to hydrogen fluoride gas is prevented from being deposited on the counter electrode 22, and that a change in sensitivity with respect to hydrogen fluoride gas can be suppressed over time. .

  The addition of silver ions into the electrolytic solution 3 is not particularly limited, but can be performed, for example, by adding silver halide. By adding silver halide to the electrolytic solution 3, the silver halide can be dissociated into silver ions and halogen ions in the electrolytic solution 3, and silver ions can be contained in the electrolytic solution 3.

  The silver halide is not particularly limited, but is preferably at least one selected from the group comprising silver chloride (AgCl), silver bromide (AgBr) and silver iodide (AgI). More preferred is silver halide (AgBr). Further, the amount of silver halide to be added is not particularly limited, and can be appropriately set so as to add silver ions necessary for suppressing the change in sensitivity to hydrogen fluoride gas with time. . For example, silver halide can be added to the electrolytic solution 3 in an amount of 0.025 mol / L or more. By adding 0.025 mol / L or more of silver halide to the electrolytic solution 3, it is possible to more stably suppress a change with time in sensitivity to hydrogen fluoride gas.

  Further, in the constant potential electrolytic gas sensor 1 of the present embodiment, by adding silver ions to the electrolytic solution 3, it is possible to reduce a solid difference between production lots having sensitivity to hydrogen fluoride gas. As described in detail below, the addition of silver ions to the electrolyte 3 suppresses the deposition of lithium carbonate or lithium oxide that causes a change with time in the sensitivity to hydrogen fluoride gas on the counter electrode 22. This is thought to be possible.

  As described above, it is considered that lithium carbonate and lithium oxide are precipitated on the counter electrode 22 by the catalytic action of silver halide as silver halide is generated by a chemical reaction on the counter electrode 22. The electrochemical reaction on the counter electrode 22 is controlled by the potential with respect to the reference electrode 23, but the chemical reaction on the counter electrode 22 includes the properties of the surface of the counter electrode 22, the purity (impurity concentration) of the counter electrode 22, and the purity of the electrolyte ( It is influenced by the type and concentration of impurities) and the concentration of electrolyte. And the properties of the surface of the counter electrode 22 vary between production lots. That is, the production of silver halide by chemical reaction and the accompanying precipitation of lithium carbonate and lithium oxide are affected by the properties of the surface of the counter electrode 22 and the like, and are affected by fluctuations between production lots of the counter electrode 22. Therefore, the precipitation of lithium carbonate and lithium oxide fluctuates due to the variation between the production lots of the counter electrode 22, and the change with time in sensitivity to hydrogen fluoride gas fluctuates. However, even if the surface property of the counter electrode 22 varies between production lots, the addition of silver ions to the electrolytic solution 3 can suppress the precipitation of lithium carbonate and lithium oxide itself. It is considered that the fluctuation of precipitation of lithium carbonate and lithium oxide due to the fluctuation between the production lots can be suppressed, and the solid difference between production lots having sensitivity to hydrogen fluoride gas can be reduced.

  In the constant potential electrolytic gas sensor 1 of the present embodiment, by adding silver ions to the electrolytic solution 3, fluctuations in sensitivity to hydrogen fluoride gas due to humidity change in the installation environment of the constant potential electrolytic gas sensor 1 are suppressed. be able to. In general, when a constant potential electrolytic gas sensor is moved from a normal humidity environment to a different humidity environment, the concentration of the electrolytic solution changes. At this time, when a counter electrode containing silver is used as in the present embodiment, the solubility of silver ions in the electrolytic solution also changes, so that the elution amount of silver ions from the counter electrode also changes. As a result, the amount of chemical reaction that occurs with the halogen ions in the electrolyte also varies, resulting in a variation in sensitivity to hydrogen fluoride gas. On the other hand, in the constant potential electrolytic gas sensor 1 of the present embodiment, since silver ions are added to the electrolytic solution 3, elution of silver ions from the counter electrode 22 is suppressed. Therefore, even if the humidity in the installation environment of the constant potential electrolytic gas sensor 1 changes, the elution of silver ions from the counter electrode 22 itself is suppressed, so that fluctuations in the elution of silver ions are suppressed and Sensitivity fluctuations are suppressed.

  Below, the outstanding effect of the constant potential electrolytic gas sensor of this embodiment is demonstrated based on an Example. However, the constant potential electrolytic gas sensor of the present invention is not limited to the following examples.

(Constant potential electrolysis gas sensor)
The constant potential electrolytic gas sensor shown in FIG. 1 was produced by a known method. The basic configuration of the potentiostatic gas sensor excluding the electrolyte was as follows.
Reaction electrode: Carbon counter electrode: Silver wire Reference electrode: Silver wire permeable membrane: Porous membrane made of polytetrafluoroethylene

(Electrolyte)
As an electrolytic solution of an example used in a constant potential electrolytic gas sensor, an electrolytic solution (8 mol / L lithium bromide (LiBr) +0.4 mol / L potassium bromate (KBrO ₃ )) and silver bromide (silver bromide ( What added (AgBr) was used. As an electrolytic solution of a comparative example, an electrolytic solution (8 mol / L lithium bromide (LiBr) +0.4 mol / L potassium bromate (KBrO ₃ )) to which silver bromide (AgBr) was not added was used.

(Response characteristic measurement)
A constant potential electrolytic gas sensor is connected to a known potentiostat, a constant voltage is applied to the reaction electrode 21 for a certain time (300 seconds) with reference to the reference electrode 23, and an electrolytic current generated between the reaction electrode 21 and the counter electrode 22 Was measured.

(Measurement target gas)
The measurement target gas used was a mixture of 3.2 ppm of hydrogen fluoride gas in the atmosphere. The measurement target gas was supplied to the constant potential electrolytic gas sensor 1 for 60 to 240 seconds after applying a constant voltage to the reaction electrode 21.

(Aging conditions)
The constant potential electrolytic gas sensor in which the electrolyte solutions of the example and the comparative example are respectively stored is left for a predetermined period in an environment where the temperature is 20 ± 3 ° C. and the humidity is 44 to 73% RH while the energized state is maintained. did.

Example 1
Using a constant potential electrolytic gas sensor, changes in response characteristics to hydrogen fluoride gas were investigated over a period after the manufacture of the constant potential electrolytic gas sensor (after one month has elapsed since the manufacture). As the electrolytic solution, the electrolytic solution of the above-described example (Example 1) and the electrolytic solution of the comparative example (Comparative Example 1) were used, and the amount of silver bromide added in the electrolytic solution of the Example was 0.05 mol / L. As the constant potential electrolytic gas sensor, in Example 1 and Comparative Example 1, four different constant potential electrolytic gas sensors manufactured in the same production lot were used. The results obtained for Example 1 are shown in FIG. 2 (a), and the results obtained for Comparative Example 1 are shown in FIG. 2 (b). In the graph in the figure, the horizontal axis is the number of days elapsed after the production, and the vertical axis is the ratio of the current value obtained by the potentiostatic gas sensor after the lapse of one month after production to the absolute value. Referring to FIG. 2, in Comparative Example 1 using an electrolytic solution to which no silver bromide was added (FIG. 2 (b)), the same production lot as the period from one month passed after the production passed. In any of the constant-potential electrolytic sensors manufactured in 1), the amount of change is greatly changed, whereas in Example 1 using the electrolytic solution to which silver bromide was added (FIG. 2 (a)), Over the period, the constant potential electrolysis sensor manufactured in the same manufacturing lot shows a substantially constant change amount. From this, it can be seen that the addition of silver ions by silver bromide to the electrolytic solution containing lithium bromide can suppress the change with time in sensitivity to hydrogen fluoride gas.

  FIG. 3 shows a scanning electron micrograph (1,000 times) obtained from the surface of the counter electrode after one month has elapsed since the manufacture of the potentiostatic gas sensor. When FIG. 3 is seen, many columnar grains are observed on the counter electrode surface of Comparative Example 1 using an electrolyte solution to which no silver bromide is added (FIG. 3B). Such columnar particles are not observed on the counter electrode surface of Example 1 using the added electrolytic solution (FIG. 3A). This columnar particle is confirmed to be lithium carbonate from other analysis results. This shows that precipitation of lithium carbonate on the counter electrode can be suppressed by adding silver ions by silver bromide to the electrolytic solution containing lithium bromide.

  From the above results, it can be seen that the addition of silver ions by silver bromide to the electrolytic solution containing lithium bromide can suppress the change in sensitivity of the constant potential electrolytic gas sensor to hydrogen fluoride gas over time. And suppression of the time-dependent change of the sensitivity with respect to hydrogen fluoride gas is considered to be because precipitation of lithium carbonate on a counter electrode is suppressed.

(Example 2)
Using multiple controlled-potential electrolysis gas sensors with different production lots, the differences in response characteristics to hydrogen fluoride gas between production lots and between sensors within the same production lot were investigated. As the electrolytic solution, the electrolytic solution of the above-described example (Example 2) and the electrolytic solution of the comparative example (Comparative Example 2) were used, and the amount of silver bromide added in the electrolytic solution of the Example was 0.05 mol / L. As the constant potential electrolytic gas sensor, in each of Example 2 and Comparative Example 2, four different constant potential electrolytic gas sensors respectively produced in four different production lots were used. The results obtained for Example 2 are shown in FIG. 4 (a), and the results obtained for Comparative Example 2 are shown in FIG. 4 (b). When FIG. 4 is seen, in the comparative example 2 using the electrolyte solution which does not add silver bromide (FIG.4 (b)), according to the difference in a manufacturing lot, and between the sensors manufactured by the same manufacturing lot However, the current values are greatly different and the response characteristics are greatly different. On the other hand, in Example 2 using the electrolytic solution to which silver bromide was added (FIG. 4A), the current value was different even between the sensors manufactured in the same manufacturing lot even if the manufacturing lots were different. The response characteristics are almost the same. From this, it can be seen that the addition of silver ions by silver bromide to the electrolyte containing lithium bromide can reduce the production lot difference in sensitivity to hydrogen fluoride gas and the solid difference in the same production lot. . As described above, the elution of silver ions from the counter electrode is greatly affected by the properties of the counter electrode, so the properties of the counter electrode between different production lots and between different sensors in the same production lot. This is thought to be due to the fact that the addition of silver ions to the electrolyte suppresses the elution of silver ions from the counter electrode and suppresses fluctuations in the elution of silver ions. .

(Example 3)
Hydrogen fluoride when the installation environment of the potentiostatic gas sensor changes from normal humidity environment (relative humidity 50% RH) to low humidity environment (relative humidity 30% RH) or high humidity environment (relative humidity 80% RH) The change of response characteristics to gas was investigated. As the electrolytic solution, the electrolytic solution of the above-described example (Example 3) and the electrolytic solution of the comparative example (Comparative Example 3) were used. The amount of silver bromide added in the electrolytic solution of the Example was 0.05 mol / L. The results obtained for the change to the low humidity environment are shown in FIG. 5, and the results obtained for the change to the high humidity environment are shown in FIG.

  When FIG. 5 is seen, the change rate of the electric current value when the installation environment is changed from the normal humidity environment to the low humidity environment is a comparative example 3 using an electrolytic solution to which silver bromide is not added (FIG. 5B). ) Is smaller in Example 3 (FIG. 5A) using an electrolytic solution to which silver bromide is added. From this result, it can be seen that by adding silver ions to the electrolytic solution, even if the installation environment is changed to a low-humidity environment, the rate of change of the current value can be suppressed to a small value. In the first place, when the installation environment changes to a low-humidity environment, the concentration of the electrolytic solution changes. The amount of increase in the absolute value of the current value due to the change in concentration is that of Example 3 using the electrolytic solution to which silver bromide is added (see FIG. 5 (a)) and Comparative Example 3 (FIG. 5 (b)) using an electrolytic solution to which no silver bromide is added are substantially the same. On the other hand, as can be seen from the results of Example 2 and the results of Example 4 described later, the absolute value of the current value increases by adding silver ions to the electrolytic solution. In other words, when Example 3 that originally had an absolute value of a high current value was compared with Comparative Example 3 that originally had an absolute value of a low current value, the installation environment changed to a low-humidity environment, and both were almost the same amount. When the absolute value of the current value increases, the increase rate of the absolute value of the current value of Example 3 that originally has a high absolute value of the current value becomes smaller. This is considered to be a factor of the result shown in FIG.

  Next, referring to FIG. 6, the change rate of the absolute value of the current value when the installation environment is changed from a normal humidity environment to a high humidity environment is a comparative example using an electrolyte solution to which silver bromide is not added. 3 (FIG. 6 (b)) is smaller in Example 3 (FIG. 6 (a)) using the electrolytic solution added with silver bromide. From this result, it can be seen that the rate of change of the absolute value of the current value can be kept small by adding silver ions to the electrolytic solution, even if the installation environment is changed to a high humidity environment. This is thought to be related to a decrease in the solubility of silver bromide in the electrolyte due to the installation environment changing to a high humidity environment. By adding silver bromide to the electrolyte with reduced solubility, silver bromide is in a state of being much dissolved in the electrolyte, so the effect of adding silver bromide to the electrolyte is greater. Therefore, it is considered that the decrease in the absolute value of the current value due to the installation environment changing to a high humidity environment is suppressed.

Example 4
Response characteristics to hydrogen fluoride gas when the amount of silver bromide added in the electrolyte (8 mol / L lithium bromide (LiBr) +0.4 mol / L potassium bromate (KBrO ₃ )) was changed We examined changes. The addition amount of silver bromide was 0, 0.01, 0.025, 0.05, 0.1, and 0.2 mol / L. As a response characteristic to the hydrogen fluoride gas, an electrolytic current generated between the reaction electrode and the counter electrode was measured using a constant potential electrolytic gas sensor after one month had elapsed after production. The obtained results are shown in FIG. Referring to FIG. 7, when the addition amount of silver bromide is in the range of 0 to 0.025 mol / L, the absolute value of the current value increases as the addition amount of silver bromide increases, and the addition amount of silver bromide Is 0.025 mol / L or more, the current value is substantially constant with respect to an increase in the amount of silver bromide added. This result shows that when the addition amount of silver bromide is 0 mol / L, the absolute value of the current value is small due to the influence of the change in sensitivity with respect to hydrogen fluoride gas, and as the addition amount of silver bromide increases, When the influence of the sensitivity change with respect to hydrogen fluoride gas is reduced and the absolute value of the current value is increased, and the addition amount of silver bromide is 0.025 mol / L or more, the sensitivity change with time with respect to hydrogen fluoride gas is reduced. This shows that the influence has almost disappeared and the absolute value of the current value is almost constant at the highest value (normal value). From the above results, by adding silver ions by silver bromide to the electrolytic solution containing lithium bromide, it is possible to suppress the change over time in sensitivity to hydrogen fluoride gas, and the addition amount is 0.025 mol / L or more. It can be seen that the change with time in sensitivity to hydrogen fluoride gas can be suppressed more stably.

DESCRIPTION OF SYMBOLS 1 Constant potential electrolytic gas sensor 2 Electrode 21 Reaction electrode 22 Counter electrode 23 Reference electrode 3 Electrolytic solution 4 Electrolytic tank 41 Electrolyte accommodating part 42 Electrolyte supply hole 43 Gas inflow hole 44 Gas outflow hole 5 Gas inflow side gas permeable membrane 6 O ring 7 Gas inflow side lid member 8 Gas outflow side gas permeable membrane 9 O-ring 10 Gas outflow side lid member

Claims (4)

A constant potential electrolytic gas sensor for detecting hydrogen fluoride gas,
As an electrode for detecting hydrogen fluoride gas, a reaction electrode, a counter electrode containing silver with respect to the reaction electrode, and a reference electrode serving as a reference for the potential of the reaction electrode,
As an electrolytic solution in contact with the reaction electrode, the counter electrode, and the reference electrode, an electrolytic solution containing lithium halide is provided,
A constant potential electrolytic gas sensor, wherein silver ions are added to the electrolytic solution separately from silver ions that are naturally dissolved in the electrolytic solution from the electrode. The constant potential electrolytic gas sensor according to claim 1, wherein silver halide is added to the electrolytic solution. The constant potential electrolytic gas sensor according to claim 2, wherein the silver halide is at least one selected from the group comprising silver chloride, silver bromide and silver iodide. The constant potential electrolytic gas sensor according to claim 2 or 3, wherein 0.025 mol / L or more of the silver halide is added to the electrolytic solution.