JP2019066328A - Electrochemical oxygen sensor - Google Patents

Abstract

To provide an electrochemical oxygen sensor that can prolong the life as an oxygen sensor.SOLUTION: The electrochemical oxygen sensor includes a holder 9 and a positive electrode 45, a negative electrode 8, and an electrolyte 7 in the holder 9. The negative electrode 8 is dissolved by discharge, the electrolyte 7 includes a chelate agent, and the pH of the electrolyte 7 is in the range of pKa±1.5 when a chelate agent is at the temperature of 25°C.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an electrochemical oxygen sensor.

  An electrochemical oxygen sensor (hereinafter also referred to as an oxygen sensor) has the advantage of being inexpensive, easy, and capable of operating at normal temperature, so checking for oxygen deficiency inside a hold or manhole, or an anesthesia machine or artificial respiration Are used in a wide range of fields such as detection of oxygen concentration in medical devices such as

  As such an electrochemical-type oxygen sensor, Patent Document 1 discloses that “electrochemical-type oxygen sensor including a cathode, an anode, and an electrolytic solution, wherein the electrolytic solution contains a chelating agent. "Oxygen sensor." (Claim [1]) is disclosed. Further, as an effect of the invention, Patent Document 1 discloses that “According to the present invention, an electrochemical oxygen sensor having a high response speed can be provided” (paragraph [0015]).

  Further, in Patent Document 2, “an end is provided with an oxygen permeable membrane that isolates the inside and the outside, a cathode and an anode are provided inside, and the cathode is in contact with the oxygen permeable membrane, and between the cathode and the anode In the galvanic oxygen electrode interposing an internal liquid, a cathode protective body surrounding the side of the cathode is provided inside the electrode, and the tip position of the cathode protective body is substantially flush with the tip position of the cathode And the tip of this cathode protector is also in contact with the oxygen-permeable membrane. "(Claim 1) is described," the internal liquid 9 is a dibasic acid or As the dibasic acid, organic acids such as citric acid, lactic acid, malic acid and the like can be exemplified, and since these organic acids are inexpensive and have low volatility, acetic acid and the like can be used. Compared to the above, there is an advantage such as no odor, long time stability etc. Especially, citric acid is preferable because it has high solubility and excellent stability when stored in a reagent. "(Paragraph [0029]) and" The concentration of the dibasic acid is preferably about 10 mM to 1 M. If the concentration is too low, the acid is consumed in the reaction with aluminum, and long-term stability can not be ensured. There is also a description “unsuitable.” (Paragraph [0030]).

WO2009 / 069749 JP 2004-132915 A

When citric acid is used as the internal liquid as described in Patent Document 2, the output voltage of the oxygen sensor may be unstable.
The present invention has been made to solve the above-mentioned problems, and it is an object of the present invention to provide an electrochemical oxygen sensor capable of obtaining a stable output voltage.

  In order to solve the above problems, the present invention comprises a holder and a positive electrode, a negative electrode and an electrolytic solution contained in the holder, wherein the negative electrode dissolves by electric discharge, and the electrolytic solution contains at least a chelating agent. And the pH of the electrolytic solution is within the range of pKa ± 1.5 with respect to the common logarithm pKa (= −log (Ka)) of the reciprocal of the acid dissociation constant Ka at 25 ° C. of the chelating agent. Adopt an oxygen sensor.

  According to the present invention, an electrochemical oxygen sensor capable of obtaining a stable output voltage is provided.

It is a conceptual diagram which shows the cross-section of the galvanic cell-type oxygen sensor which is one preferable embodiment of this invention. It is a conceptual diagram which shows the cross-section of the galvanic cell-type dissolved oxygen sensor which is another preferable embodiment of this invention. It is a conceptual diagram for demonstrating that an output voltage is stabilized.

  The configuration and effects of the present invention will be described together with technical ideas. However, the effects include the estimation, and the correctness does not limit the present invention. Moreover, among the components in the following embodiments, components not described in the independent claim showing the highest level concept are described as optional components.

The inventors have found that the output voltage stability of an oxygen sensor containing a chelating agent in the electrolyte depends on the pH of the electrolyte. Then, when the relationship between the output voltage stability of the oxygen sensor and the pH of the electrolyte was examined in detail, the pH of the electrolyte was the common logarithm pKa of the reciprocal of the acid dissociation constant Ka at 25 ° C. of the chelating agent It has been found that an oxygen sensor with high output voltage stability can be obtained when it is close to (Ka), and the present invention has been completed.
The term "chelating agent" as used in the present invention refers to a molecule having a plurality of functional groups such as a carboxyl group and an amino group which form a coordinate bond with a metal ion, and forms a complex with the metal ion to form a metal ion. To inactivate the

  Hereinafter, as a preferred embodiment of the present invention (hereinafter referred to as "the embodiment"), a galvanic cell type oxygen sensor among the electrochemical type oxygen sensor will be specifically described.

FIG. 1 is a conceptual view showing the cross-sectional structure of the galvanic cell type oxygen sensor according to the present embodiment.
In FIG. 1, 1 is a first holder lid (inner lid), 2 is an O-ring, 3 is a protective film for preventing dust and dirt adhesion to a diaphragm or water film adhesion, 4A is a diaphragm, 4B is a diaphragm. Catalyst electrode 5 positive electrode current collector 6 positive electrode lead wire 7 electrolyte 8 negative electrode 9 holder 9 second holder lid (outer lid) 11 perforation for electrolyte supply 12 Is a hole for a lead wire, 13 is a positive electrode current collector holding portion, 14 is a correction resistance, and 15 is a temperature compensation thermistor. The positive electrode 45 is composed of the catalyst electrode 4 B and the positive electrode current collector 5. Further, the holder lid 101 is configured by the first holder lid 1 and the second holder lid 10.

The operation principle of the above-described galvanic cell type oxygen sensor is considered to be as follows. Although tin is used for the negative electrode 8 in the present embodiment, the negative electrode material is not limited to this.
Oxygen which has selectively permeated oxygen and passes through the diaphragm 4A which limits the permeation amount to the battery reaction is reduced at the catalyst electrode 4B (positive electrode reaction). On the other hand, in the negative electrode 8, the following electrochemical reaction (negative electrode reaction) occurs via the electrolytic solution 7.

Positive electrode reaction: O ₂ + 4H ⁺ + 4e ⁻ → 2H ₂ O
Negative electrode reaction: Sn + 2 H ₂ O → Sn O ₂ + 4 H ⁺ + 4 e ⁻

  The electrochemical reaction generates a current according to the oxygen concentration between the catalyst electrode 4B and the negative electrode 8. The current generated by the positive electrode reaction at the catalyst electrode 4B is collected by the positive electrode current collector 5 in pressure contact with the catalyst electrode 4B, led to the outside by the positive electrode lead wire 6, and passed through the correction resistor 14 and the temperature compensation thermistor 15. It flows to the negative electrode 8. This current is converted to a voltage signal, which is obtained as the oxygen sensor output. Thereafter, the obtained output voltage is converted to the oxygen concentration by a known method and detected as the oxygen concentration.

At this time, if the electrolytic solution contains a chelating agent, SnO ₂ formed on the surface of the negative electrode 8 is dissolved in the electrolytic solution by reacting with the chelating agent by the reaction represented by the following formula, whereby It is considered that repetition of exposure of new Sn and oxidation of the new Sn, that is, consumption of the negative electrode (Sn) occurs.

Y ^X- + SnO ₂ + 4H ⁺ → YSn ^4-x + 2H ₂ O: Y is a chelating agent

The holder 9 is configured to receive the positive electrode 45, the negative electrode 8 and the electrolytic solution 7 therein. The material of the holder 9 is not particularly limited as long as there is no corrosion or the like due to the electrolytic solution 7 described later, but an ABS resin is preferably used.
The positive electrode 45 is accommodated in the holder 9. The material of the positive electrode 45 is not particularly limited as long as an electric current is generated by electrochemical reduction of oxygen on the positive electrode, but gold (Au), silver (Ag), platinum (Pt), titanium (Ti), etc. An active catalyst is preferably used.

The electrolytic solution 7 is contained in the holder 9. The electrolytic solution 7 contains at least a chelating agent.
The pH of the electrolytic solution 7 is in the range of pKa ± 1.5 with respect to the common logarithm pKa (= −log (Ka)) of the reciprocal of the acid dissociation constant Ka at 25 ° C. of the chelating agent. The pH is preferably in the range of pKa ± 1.0, more preferably in the range of pKa ± 0.5.
In addition, when there exist multiple pKa of the chelating agent to be used (pKa1, pKa2, pKa3, etc.), those are all included. As an example, when the chelating agent is citric acid, the pKa at 25 ° C. is pKa1 = 3.09, pKa2 = 4.75, and pKa3 = 6.40, so the pH of the electrolyte is 1.59. It is any one of 4.59 or less, 3.25 or more and 6.25 or less, and 4.90 or more and 7.90 or less.
As described above, when the pH of the electrolytic solution 7 is within the range of pKa ± 1.5 at 25 ° C. of the chelating agent, the pH buffering ability of the chelating agent is increased, so that the fluctuation of the pH of the electrolytic solution is suppressed. It is considered that the output voltage stability of the oxygen sensor can be improved.

  The pH of the electrolytic solution 7 described above is measured using a glass electrode type hydrogen ion concentration meter D-14 manufactured by HORIBA.

The pKa is preferably pKa2 or pKa3. Among the pKa, pKa1 is the common logarithm (-log (Ka1)) of the reciprocal of the first dissociation constant Ka1 of the acid, and pKa2 is the common logarithm (-log (Ka2)) of the reciprocal of the second dissociation constant Ka2 of the acid And pKa3 each represent the common logarithm (-log (Ka3)) of the reciprocal of the third dissociation constant Ka3 of the acid. It is considered that the pH buffering action of the above-mentioned chelating agent is larger in the case where the pH of the electrolytic solution is near pKa2 or pKa3 than in the case where the pH is near pKa1, and the stabilization effect of the output voltage is larger.
In addition, when a strong acid (a saturated aqueous solution having a pH of less than 2.0: for example, citric acid) is used as the chelating agent alone, when a chelating agent salt (for example, tripotassium citrate) is mixed, Although the pH of the electrolyte increases and approaches pKa2 or pKa3, the mixing of the salt of the chelating agent can increase the molar concentration of the chelating agent in the electrolytic solution, so that the lifetime of the oxygen sensor can be improved. it is conceivable that.

  The electrolyte solution of the said pH range mixes a chelating agent and a chelating agent salt (When a chelating agent is citric acid, it mixes citric acid and tripotassium citrate), and adjusts these composition ratio. You can get it. In addition, it may be adjusted by adding sodium hydroxide (NaOH), potassium hydroxide (KOH) or the like to a mixed solution of citric acid and tripotassium citrate. In addition, it may be adjusted by adding an acid (for example, acetic acid, phosphoric acid, carbonic acid) to tripotassium citrate (pH 7.8 to 8.6).

As the chelating agent, one containing either a weak acid or a weak base and a salt thereof is preferable in that the effect of stabilizing the output voltage of the oxygen sensor by the buffer action is high.
More preferably, the chelating agent is a mixed solution of citric acid and citrate. This is because citric acid and citrate have high solubility (right bracket) in water (25 ° C.) (citric acid (73 g / 100 ml), trisodium citrate (71 g / 100 ml) and tripotassium citrate ( 167 g / 100 ml)), because the molar concentration can be increased by selecting these.
Furthermore, the citrate preferably contains at least tripotassium citrate. As described above, tripotassium citrate has extremely high solubility in water, which makes it easier to control the pH of the electrolyte, and also has the effect of improving the life of the oxygen sensor.

  On the other hand, acetic acid, boric acid, phosphoric acid, ammonia, carbonic acid and salts thereof which are not included in the “chelating agent” in the present invention have weak complexing power, and thus when the electrolytic solution 7 is constituted by only this Is not preferable because the

  Further, in the electrolytic solution 7 in the present invention, in addition to the above-mentioned chelating agent, components other than the above-mentioned chelating agent (acetic acid, boric acid, phosphoric acid, ammonia, carbonic acid and salts thereof, etc. having weak complexing ability) May be included.

The molar concentration of the chelating agent in the electrolytic solution 7 is preferably 1.2 mol / L or more, more preferably 1.4 mol / L or more, and still more preferably 2.0 mol / L or more. By setting the molar concentration to the above range, the life as the oxygen sensor can be improved. The upper limit of the molar concentration of the chelating agent in the electrolytic solution 7 varies depending on the type of chelating agent, and the maximum number of moles that the chelating agent can dissolve in 1 L (liter) of solution (water) at 25 ° C. is there. For example, when the chelating agent is a mixture of citric acid and tripotassium citrate, the upper limit is 3.0 mol / L, preferably 2.3 mol / L.
Here, “molar concentration (mol / L)” refers to the number of moles of solute dissolved in 1 L (liter) of a solution at 25 ° C.

The negative electrode 8 is accommodated in the holder 9.
The material of the negative electrode 8 is not particularly limited as long as it does not deviate from the scope of the present invention. The negative electrode 8 can use Cu, Fe, Ag, Ti, Al, Mg, Zn, Ni, Sn and their alloys (hereinafter referred to as negative electrode metal).
The material of the negative electrode 8 may contain metals other than the negative electrode metal and other impurities as long as the material of the negative electrode 8 does not deviate from the scope of the present invention. Examples of metals other than the negative electrode metal and other impurities include In, Au, Bi, Na, S, Se and Ca.

The negative electrode 8 is preferably a Sn alloy.
The Sn alloy can suppress the generation of hydrogen during the electrochemical reduction reaction of oxygen in the oxygen sensor.
Further, the Sn alloy is more preferably a Sn-Sb alloy.
Compared with other Sn alloys, Sn-Sb alloy can densify the metal structure of Sn and suppress the transformation (phase transition) to α-Sn (alias: tin vest), so low temperature (for example, 0 ° C. Since it is considered that the sensor drive in (1) can be performed, it is possible to suppress the decrease in the output stability of the sensor even at the low temperature.

The negative electrode 8 (preferably a Sn alloy, more preferably a Sn-Sb alloy) preferably contains substantially no lead.
The term "substantially free of lead" as used herein means that the content of Pb contained in the negative electrode 8 is less than 1000 ppm. By using such a negative electrode 8, it is also possible to comply with the directive [Restriction of the Use of Certain Hazardous Substances in Electrical and Electronic Equipment] concerning the regulation of use of specific harmful substances in EU (European Union) An electrochemical oxygen sensor can be obtained.

The content of Sb in the Sn-Sb alloy is not particularly limited as long as the gist of the present invention is not deviated.
In the present invention, the content of Sb in the Sn-Sb alloy is subjected to EDX analysis (beam diameter: 1 mm) at any position of the negative electrode 8 to be measured, and the mass of Sb relative to the whole metal element measured there %.

  More specifically, as shown in FIG. 1, the galvanic cell type oxygen sensor according to the present embodiment includes a holder 9, a positive electrode 45 housed in the holder 9, a negative electrode 8 and an electrolyte 7, and the positive electrode 45, a protective film 3 provided on the diaphragm 4A, a first holder lid (inner lid) 1 for fixing the protective film 3, the holder 9 and the inner lid 1 An O-ring 2 disposed between them, a second holder lid (outer lid) 10 for fixing the inner lid 1, a correction resistor 14 connected in series to the positive electrode 45 and the negative electrode 8, and a thermistor for temperature compensation And 15. Further, the holder lid 101 constituted by the outer lid 10 and the inner lid 1 is provided with a through hole 16 serving as an oxygen supply path (space) passing through the protective film 3.

  The holder 9 has an opening at the top, and a screw thread and a screw groove are provided on the outer periphery of the opening, and a screw for screwing the outer lid 10 is provided.

  The positive electrode 45 is composed of a catalyst electrode 4B that electrochemically reduces oxygen and a positive electrode current collector 5 disposed on the side of the electrolyte 7 thereof. As the catalyst electrode 4B, gold (Au), silver (Ag), platinum (Pt) or the like is usually used, and as the positive electrode current collector 5, titanium (Ti) is usually used. The materials are not limited to these.

Below the positive electrode current collector 5, a positive electrode current collector holding portion 13 for holding the same is provided.
The positive electrode current collector holding portion 13 is provided with a hole 11 for supplying the electrolytic solution to the positive electrode 45 and a hole 12 for passing the positive electrode lead wire 6.
Although the material of the positive electrode current collector holding portion 13 is not particularly limited, normally, an ABS resin is used.

  The diaphragm 4A in the present embodiment is disposed to control the infiltration of oxygen so that the amount of oxygen reaching the catalyst electrode 4B is not too large. The diaphragm 4A is preferably one that can selectively permeate oxygen and limit the amount of oxygen gas to be permeated. The material and thickness of the diaphragm 4A are not particularly limited, but usually, fluorine resins such as tetrafluoroethylene resin and tetrafluoroethylene-hexafluoropropylene copolymer, and polyolefins such as polyethylene are used. The diaphragm 4A includes a porous film, a non-porous film, and a hole in which a capillary tube called a capillary type is formed.

  The protective film 3 in the present embodiment is a porous resin film provided on the diaphragm 4A, and has a function of preventing adhesion of dust and dirt to the diaphragm 4A or adhesion of a water film. The material of the protective film 3 is not particularly limited, but a fluorine resin such as tetrafluoroethylene resin is usually used.

  The inner lid 1 in the present embodiment functions as a pressing end plate of the protective film 3 and the positive electrode 45, and is pressed against the holder 9 via the O-ring 2 by screwing the holder 9 and the outer lid 10. Thus, the protective film 3 and the positive electrode 45 can be fixed to the holder 9 while maintaining the air tightness and the liquid tightness. The material of the inner lid 1 is not particularly limited, but usually, ABS resin, polypropylene, polycarbonate, fluorocarbon resin or the like is used.

  The O-ring 2 in the present embodiment is disposed between the holder 9 and the inner lid 1, and is pressed and deformed by screwing the holder 9 and the outer lid 10 to maintain airtightness and liquid tightness. It can be done. The material of the O-ring 2 is not particularly limited, but usually, nitrile rubber, silicone rubber, ethylene propylene rubber, fluorocarbon resin or the like is used.

  The outer lid 10 in the present embodiment is configured to seal the opening of the holder 9 together with the O-ring 2 and the inner lid 1 and is screwed with a screw formed on the outer periphery of the opening of the holder 9 As such, the screw portion is formed on the inner peripheral portion. The material of the outer lid 1 is not particularly limited, but normally, ABS resin, polypropylene, polycarbonate, fluorocarbon resin or the like is used.

The present invention is not limited to the above-described embodiment, and various modifications can be made within the scope of the technical idea thereof.
For example, the symbols 1 to 15 in FIG. 1 are not limited thereto, and various design changes are possible as long as they have the function as an oxygen sensor and the above-described oxygen supply path.
Moreover, although this embodiment demonstrated using the galvanic cell type oxygen sensor, this invention is applicable also to a constant potential type oxygen sensor.
The constant potential type oxygen sensor is a sensor that applies a constant voltage between the positive electrode and the negative electrode, and the applied voltage is set according to the electrochemical characteristics of each electrode and the gas type to be detected. In the constant potential type oxygen sensor, when an appropriate constant voltage is applied between the positive electrode and the negative electrode, the current flowing between them has a proportional relationship with the oxygen gas concentration, so if the current is converted to a voltage, the galvanic cell type oxygen sensor Similarly, by measuring the voltage, the oxygen gas concentration of the unknown gas can be detected.

The present invention is also applicable to a galvanic cell type dissolved oxygen sensor.
FIG. 2 is a conceptual view showing a cross-sectional structure of a galvanic cell type dissolved oxygen sensor according to another preferred embodiment of the present invention.
The galvanic cell type dissolved oxygen sensor (FIG. 2) according to the present invention comprises a diaphragm 21, a positive electrode 22, an electrolytic solution 23, a negative electrode 24, lead wires 25, a correction resistor 26 and a temperature compensating thermistor 27. The diaphragm 21 is made of, for example, a tetrafluoroethylene-hexafluoropropylene copolymer film. The positive electrode 22 is, for example, an electrode in which gold is supported on a carbon fiber electrode, and the gold functions as an effective catalyst for the electrochemical reduction of oxygen. The electrolytic solution 23 is made of, for example, a mixed aqueous solution of acetic acid, potassium acetate and lead acetate. The negative electrode 24 is, for example, a lead electrode, and the lead wire 25 is, for example, made of titanium. The temperature compensation thermistor 27 is attached to the outer wall of the galvanic cell type dissolved oxygen sensor.
By applying the present invention described above to a galvanic cell type dissolved oxygen sensor, it is possible to obtain a galvanic cell type dissolved oxygen sensor that has a quick response speed and improves the life as an oxygen sensor.

Next, the present invention will be more specifically described based on examples, but the present invention is not limited thereto.
(Examples 1 to 4)
The galvanic cell type oxygen sensor shown in FIG. 1 was produced. In FIG. 1, the inner lid 1 is made of ABS resin, the protective film 3 is a porous tetrafluoroethylene resin sheet, the diaphragm 4A is a tetrafluoroethylene hexafluoropropylene copolymer film, and the catalyst electrode 4B is gold (Au The positive electrode current collector 5 is made of titanium, the positive electrode lead wire 6 is made of titanium, and the positive electrode current collector 5 and the positive electrode lead wire 6 are integrated by welding.

  Further, the negative electrode 8 is made of Sn-5.0Sb alloy (the left number is the mass% of Sb in the Sn alloy), the holder main body 9 is made of ABS resin, and the outer lid 10 is made of ABS resin. Each is threaded.

The inner lid 1, the O-ring 2, the tetrafluoroethylene resin sheet (protective film) 3, the diaphragm 4A of the tetrafluoroethylene hexafluoropropylene copolymer film, the catalyst electrode 4B, and the positive electrode current collector 5 The outer cover 10 and the outer cover 10 are screwed so that a good contact state is maintained. The inner lid 1 functions as a pressing end plate, and the O-ring 2 ensures air tightness and liquid tightness.
Reference numeral 11 is a hole for supplying an electrolyte to the positive electrode and the diaphragm, and 12 is a hole for passing a titanium lead portion of the positive electrode current collector.

  As the electrolyte solution 7, citric acid-tripotassium citrate aqueous solution of the molar concentration shown in Table 1 was used, respectively.

(Comparative example 1)
A galvanic cell type oxygen sensor according to a comparative example was produced in the same manner as in Example 1 except that a citric acid aqueous solution having a molar concentration shown in Table 1 was used as the electrolytic solution 7.

(Reference example)
As a chelating agent, EDTA (Etylene Diamine Tetraacetic Acid) used in the example of Patent Document 1 · tetrasodium aqueous solution (60 g / 100 ml solubility in water at 25 ° C.) is used, and the solubility in water is close to the saturation concentration At the concentration (1.2 mol / L), a galvanic cell type oxygen sensor having a one-year life shown in FIG. 1 similar to that of Example 1 was manufactured.

[Characteristic comparison]
With respect to the plurality of galvanic cell type oxygen sensors prepared above, “90% response speed” after 2 months after preparation was evaluated. For the 90% response speed, after the output of each sensor is stabilized in the atmosphere (oxygen concentration 21%), the concentration 100% oxygen is ventilated to the sensor whose output is stabilized, and the concentration 100% The time required for the output to change by 90% was measured, with the output (saturation output) that can be generated by ventilating oxygen at 100%. In this evaluation test, the output after 30 minutes of ventilation was taken as the saturated output. The results obtained by the above evaluation test are shown in Table 1. In Table 1, ○ means that the 90% response speed is less than 15 seconds, Δ means that the response speed is 15 seconds or more and less than 60 seconds, and × indicates that the response speed is 60 seconds or more. means.

  Further, the plurality of galvanic cell type oxygen sensors prepared above were left at normal temperature (25 ° C.) in the same environment for 360 days to evaluate whether or not the output voltage of the oxygen sensor was stable. As shown in FIG. 3, when the output voltage is stable, the horizontal axis represents the measurement time, and the vertical axis represents the output voltage, as shown in FIG. When we draw a straight line.

Furthermore, an accelerated life test was conducted by passing 100% oxygen gas at a temperature of 40 ° C. At 40 ° C, the electrochemical reaction progresses about twice as much at room temperature and about five times as much as in air with 100% oxygen gas ventilated, so it is possible to judge the life at about ten times speed when left at room temperature in air. . The accelerated life test was performed by setting the life of the reference example to 1.0 and the other times.
Table 1 shows the results.

  As can be seen from Table 1, in the results of this test, Comparative Example 1 in which the pH of the electrolytic solution is out of the range of pKa1 ± 1.5, the output becomes unstable from the initial stage, and the response speed and life of the oxygen sensor It could not be measured. On the other hand, in Example 4 in which the pH of the electrolytic solution was in the range of pKa1 ± 1.5, a stable output voltage was obtained, and the life of the oxygen sensor was also improved as compared with the reference example. Furthermore, the life of the oxygen sensor was improved more than that of Example 4 in Examples 2 and 3 in which the pH of the electrolytic solution is in the range of pKa2 ± 1.5 and in Example 1 in which the pH of the electrolyte is pKa3 ± 1.5.

1 1st holder lid (middle lid)
2 O-ring 3 protective film 4A diaphragm 4B catalyst electrode 5 positive electrode current collector 6 positive electrode lead wire 7 electrolyte 8 negative electrode 9 holder 10 second holder lid (outer lid)
11: Perforation for electrolyte supply 12: Perforation for lead wire 13: Positive electrode current collector holding portion 14: Correction resistance 15: Temperature compensation thermistor 16: Through hole 45: Positive electrode 101: Holder cover

Claims (3)

With the holder,
And a positive electrode, a negative electrode and an electrolyte contained in the holder,
The negative electrode dissolves with an electrode reaction,
The electrolyte contains at least a chelating agent,
An electrochemical oxygen sensor wherein the pH of the electrolytic solution is within the range of pKa ± 1.5 with respect to the common logarithm pKa (= −log (Ka)) of the reciprocal of the acid dissociation constant Ka at 25 ° C. of the chelating agent .   The electrochemical oxygen sensor according to claim 1, wherein the pKa is pKa2 or pKa3. The electrochemical oxygen sensor according to claim 1 or 2, wherein the chelating agent is a mixed solution of citric acid and citrate.

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