JPH01239446A - Gas sensor - Google Patents

Abstract

PURPOSE:To freely adjust sensitivity and to provide the higher stability and the stabler signal output by forming a gas permeable membrane of a porous hydrophobic membrane uniformly dispersed with a conductive material and using this gas permeable membrane as a detecting electrode. CONSTITUTION:The aperture of a gas sensor body 1 is closed by the flat plate- shaped porous hydrophobic membrane 2 dispersed with the conductive material. The gas sensor is formed by using this porous membrane 2 as the detecting electrode and the gas permeable membrane and the gas sensor is formed by providing a counter electrode 3 and an electrolyte 4 in the body 1. A metallic wire 7 is brought into contact with the plural points of the surface of the porous membrane 2 with which gas comes into contact and led out to eliminate the influence of internal resistance. The gas permeable membrane 2 is adequate for detecting the specific gas in a naturally diffused sample gas if said membrane is made into a flat plate structure. In addition, the flat plate area is increased and the sensitivity is enhanced. The higher sensitivity and stabler signal output of the sensor are thereby obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a gas sensor that detects a specific gas component in a sample by electrochemically reacting it on the surface of a sensing electrode in the presence of an electrolyte.

[Conventional technology]

When attempting to detect toxic gases in the ambient air or work environment, the most commonly used are so-called electrochemical gas sensors that utilize the electrochemical activity of toxic gases.

Generally, an electrochemical gas sensor has a sensor body a partitioned off from the outside world by a hydrophobic gas-permeable membrane, and a sensing electrode C1, a counter electrode d, and an electrolytic solution e housed inside the body a, as shown in Figure 11. It is.

Reducing gases are usually electrolytically oxidized by the reaction G-+G'+e on the sensing electrode side with the involvement of electrode materials and electrolyte, and in the case of oxidizing gases, electrode materials and electrolyte Normally, an electrolyte is involved and an electrolytic reduction reaction of G+e-+G' occurs on the sensing electrode side.

At this time, as a matter of course, the electrode material and electrolyte are involved at the counter electrode, and an electrochemical reaction opposite to that at the sensing electrode occurs, and as a result, a current due to the electrochemical reaction is observed as a signal.

Poisonous gases that can be measured in this way include, for example, CQ2. O,, SO2, NO3°F2. B
r2, etc., and reducing gases include C02H2S, N
Examples include H3, PH, AsHl, N2H4, etc.

Of course, in order to cause an electrochemical reaction of a specific gas, it is necessary to appropriately select the materials of the sensing electrode and counter electrode, and also the electrolyte. Regarding the potential difference between the two electrodes, which is the driving force for electrochemical reactions, there are two methods: a galvanic method that uses the difference in standard potential based on the difference in the materials between the two electrodes, and a polarographic method that applies an externally applied voltage. , there is no essential electrochemical difference.

Such electrochemical gas sensors are used not only to detect toxic gases, but also to detect other electrochemically active gases such as oxygen gas.

[Problems to be Solved by the Invention] In order to construct such an electrochemical gas sensor, an electrolyte, a sensing electrode, and a counter electrode are essential for boat fishing, as shown in Figure 11. Furthermore, these are separated from the outside world,
A gas permeable membrane is also essential to allow gas components to permeate into the electrolyte.

The electrolyte, sensing electrode, and counter electrode materials are appropriately selected to cause an electrochemical reaction in the target gas, and the gas-permeable membrane is a material that traps the electrolyte inside while allowing outside gas to enter. For this purpose, a hydrophobic porous membrane is usually selected, and the material is preferably polytetrafluoroethylene or the like.

Gas sensors configured in this manner are widely used in practice because they are manufactured at relatively low cost, but they have the following practical problems.

First, the adhesion between the sensing electrode and the gas permeable membrane is not always constant and changes over time, which subtly affects the stability of the electrochemical reaction and tends to make the signal unstable. This is due to the fact that the three-dimensional positional relationship between the sensing electrode and the gas permeable membrane is microscopically inconsistent and unstable. In particular, a gas permeable membrane is a thin organic membrane on the order of μ and is therefore susceptible to changes over time.

Second, the detection electrode area is an important element that directly affects the electrolytic current value and sensitivity, but since the surface has a planar structure, it is practically limited to about 4 to 15 nnφ, and as a result, the sensitivity decreases. Limits arise.

Conventionally, the first method to solve these problems was
As shown in Figure 2, a gas permeable membrane is used to partition the inside of the main body a from the outside world, and a counter electrode d and an electrolyte e are placed inside the main body a.
A method is known in which an ultra-thin sensing film C is formed in a flat shape by vapor deposition on the inside of a gas-permeable membrane, but in this case also ■ If the electrode thin film becomes too thick, it will become a continuous film and the gas It becomes impermeable, making it impossible for gas electrochemical reactions to occur.■If the electrode thin film is made thin enough for gas to pass through, the surface area of the electrode (the area in contact with gas) cannot be expanded to the extent expected. . Furthermore, in this case, there is a problem that it becomes difficult to read out the electrode to the outside, and the control of the film thickness becomes extremely delicate.

The present invention was made in view of the above circumstances, and an object of the present invention is to provide a gas sensor with high sensitivity and excellent output stability.

[Means and effects for solving the problem] The present inventor has developed a hydrophobic membrane that can be used as a gas permeable membrane for gas sensors in order to solve the problems of conventional electrochemical gas sensors that have a wide range of applications. As a result of studying the porous membrane of
The present inventors have discovered that this porous membrane itself can be configured as a sensing electrode, thereby achieving higher sensitivity and output stabilization of the sensor, and have accomplished the present invention.

Therefore, the present invention includes a sensing electrode, a counter electrode, a gas permeable membrane,
It has a sensing electrode, a counter electrode, and an electrolyte in contact with the gas permeable membrane, and when the target gas component in the sample that has passed through the gas permeable membrane comes into contact with the sensing electrode, the target gas component is detected by an electrochemical reaction. In this gas sensor, the gas permeable membrane is formed of a hydrophobic porous membrane in which a conductive substance is uniformly dispersed, and the gas permeable membrane is configured as a sensing electrode.

The present invention will be explained in more detail below.

In the gas sensor of the present invention, as described above, the gas permeable membrane is formed of a hydrophobic porous membrane in which a conductive substance is uniformly dispersed, and this gas permeable membrane is configured as a sensing electrode.

In this case, there are no restrictions on the type of conductive substance used to make the porous membrane conductive, and metals can also be used.
A particularly suitable material is carbon. This is because carbon is not only electrochemically inert and a general-purpose electrode material, but when dispersed, carbon particles have a certain conductivity and act as a current collector even if they are not continuously connected. .

Furthermore, the type of hydrophobic porous membrane in which the conductive substance is dispersed is not limited, but it is preferable to use one made of polytetrafluoroethylene.

Furthermore, the average pore diameter of the hydrophobic porous membrane is about 0.05 to 1-1, and the amount of conductive material dispersed in the porous membrane has a volume resistivity of 0.
It is preferable to set the amount to about 5 to several Ω/an.

The gas sensor of the present invention uses a conductive hydrophobic porous membrane as a gas permeable membrane and a sensing electrode, and its structure is not particularly limited, but for example, as shown in FIG. The opening of the main body 1 is closed with a flat hydrophobic porous membrane 2 in which a conductive substance is dispersed.
is used as a sensing electrode and a gas permeable membrane, and a counter electrode 3 and an electrolyte 4 are arranged inside the sensor body 1, or a hydrophobic porous membrane in which a conductive substance is dispersed inside the sensor body 1 as shown in FIG. A tube 5 made of A configuration in which the hollow portion of No. 5 is used as a passage for the sample is preferably adopted. In this case, if a conductive gas permeable membrane is used in a flat plate structure as shown in FIG. 1, it is suitable for detecting a specific gas in naturally diffusing sample gas.

Sensitivity can be improved by increasing the area of this plate. Further, by molding the conductive gas permeable membrane into a tube shape and configuring it as shown in FIG. 2, it can be used as a sensor for sample gas. The specific gas is detected by sucking the sample gas into the conductive gas-permeable membrane tube using a suction pump or the like.

In this case, expanding the surface area of the membrane and increasing the sensitivity can be easily achieved by arbitrarily selecting the length of the tube.

Note that the hydrophobic porous membrane in which a conductive substance is dispersed is generally 1
■It may have a resistance of several Ω per electrode, and in order to increase the surface area of the sensing electrode, it is made into a large diameter flat plate film as shown in Figure 1, or formed into a tube shape as shown in Figure 2, and from these metal wires are formed. When leading out the conductive porous membrane, the internal resistance of the conductive porous membrane itself is several ohms, which poses a problem in extracting the electrode generated by the electrochemical reaction as a current. Therefore, as shown in Figures 1 and 2, metal wires 7 are brought into contact with multiple points on the surface of the conductive porous membrane that is in contact with the gas to provide lead-outs so as not to be affected by the internal resistance. It is desirable to do so.

In addition, it is preferable to apply hydrophilic treatment to a certain thickness on the surface of the conductive hydrophobic porous membrane that comes into contact with the electrolyte, thereby ensuring that the electrochemical reaction involving the electrolyte takes place. be able to. In this case, the electrode reaction takes place in this hydrophilically treated area, but since the portion of a certain thickness on the opposite side is hydrophobic, the electrolyte is retained and does not leak out.

Furthermore, the carbon material itself that makes the gas permeable membrane conductive generally serves as an excellent sensing electrode, but if necessary, other materials can be laminated as a thin film on the electrolyte side of the membrane to make it even more effective. be able to. For example, gold, platinum, silver, palladium, etc., which have catalytic activity, are effective.

Additionally, gas sensors equipped with gas permeable membranes are generally used not only to detect specific gases in gases, but also to detect gases dissolved in liquids. For example, a diaphragm-type residual chlorine electrode and a diaphragm-type dissolved oxygen electrode.

Dissolved ozone electrodes are widely used in practical applications.

The present invention relates to detection based on electrochemical reactions of gases, and can of course also be applied to gas detection in liquids. In the present invention, the gas-permeable membrane itself that separates the sensor components from the outside world serves as a current collector, so if the components are immersed in the liquid, the current generated during detection will also flow to the outside. Contamination can occur, such as leakage or conversely the pickup of external currents in the liquid sample.

Therefore, when detecting gas in a liquid, it is necessary to electrically insulate the side of the conductive gas permeable membrane 2 that contacts the liquid sample from the outside world as shown in Figure 3-4. A hydrophobic gas permeable membrane 8 is arranged to allow the gases to pass through the detection system. In this case, a lead-out metal conducting wire 7 is placed between the two gas permeable membranes 2 and 8. The sensor of the present invention also includes a sensor in which a hydrophobic gas permeable membrane is disposed on the outside for such liquid measurement.

When a gas sensor using a conductive gas permeable membrane as described above is used for a long period of time, it is inevitable that the electrolyte will decrease due to electrochemical reactions and evaporation. The configuration of a gas sensor equipped with an electrolyte replenisher can be easily created by using a tubular gas permeable membrane, for example, as shown in FIG.

That is, in FIG. 5, 1 is the sensor main body, 5 is a tube-shaped gas permeable membrane, 3 is a counter electrode, 4 is an electrolytic solution, 6.6 is a joint, 7 is a metal wire, and 8 is provided above the main body 1. An electrolytic solution reservoir inside communicates with the inside of the main body 1 via a communication pipe 9, 1o is an opening/closing cock provided in the communication pipe 9, and 11 is a replenishment port for the reservoir 8. In this device, the electrolytic solution in the main body 1 By opening the opening/closing cock 10 when the electrolytic solution 4' in the reservoir 8 decreases, the electrolytic solution 4' in the reservoir 8 can be replenished into the main body 1.

〔Effect of the invention〕

In this way, a gas sensor using a conductive gas permeable membrane can not only achieve high sensitivity and stabilize twice the output, but also can easily adjust the sensitivity freely, making it possible to easily adjust the sensitivity of a device with a built-in gas sensor. It is also easy to assemble, and has great practical value.

This gas sensor can be applied to all gases that directly or indirectly exhibit electrochemical reactions, and has a wide range of uses.

EXAMPLES Next, the present invention will be specifically explained with reference to Examples, but the present invention is not limited to the following Examples.

〔Example〕

Gas sensors were prepared and measured in the following items 1 to 2.

(2) Chlorine gas sensor It is known that a galvanic CQ2 or HOCQ sensor can be created by selecting carbon for the detection electrode, silver for the counter electrode, and potassium chloride for the electrolyte.

This reaction is carried out at the detection pole as follows: HOCL)e-+1/2H, +○cp-
H"+ e-→1/2 H2 At the opposite electrode, the reaction proceeds as follows: Ag-e-→Ag+ Ag"+CQ-→AgCfl Ag"+OCQ, -→AgC11/202.

A gas sensor as shown in FIG. 2 was constructed using a conductive tubular hydrophobic gas permeable membrane (manufactured by Coredex Co., Ltd.) in which carbon was dispersed, having an inner diameter of 2φ, a membrane thickness of 1 t, and a maximum pore diameter of 0.36 μm or less as a detection electrode. Note that silver was used as the counter electrode, and potassium chloride solution was used as the electrolyte. Note that the length of the detection pole can be arbitrarily selected within a range of about 30 to 150 m. When the sample gas is sucked with a small diameter pump, the T of the detection tube is less than lppm.
(Responses to CQ gas with high sensitivity. This situation is shown in Figure 6.

(2) Hydrogen sulfide gas sensor A sensor similar to (2) was selected for the detection electrode, silver for the counter electrode, and potassium chloride solution for the electrolyte to construct a sensor as shown in Figure 2. When a voltage of, for example, about 0.7 V is applied to the sensing electrode side, reducing gas such as H2S is oxidized on the sensing electrode side.

When used by applying a voltage between the two electrodes as shown in FIG. 7, it can be seen that it responds with high sensitivity to gas at the 10 pbb level as shown in FIG. 8.

(2) Oxygen gas sensor A galvanic diaphragm type 02 sensor as shown in Fig. 2 was constructed by selecting the same electrode as (2) for the detection electrode, lead for the counter electrode, and caustic alkali for the electrolyte.

Conventionally, the diaphragm type 02 sensor, which uses a gold or platinum flat plate electrode with a diameter of about 8 mm as the detection electrode, can only obtain a current of the order of a few tens of cubic meters even when a porous membrane is used as the gas permeable membrane to increase oxygen permeability. I can't.

In this system, by using 10 porous membrane tubes with a diameter of 2 and a diameter of 1, an output of 2 mA or more is produced with respect to atmospheric oxygen, as shown in Fig. 9.

However, because the electrode and gas permeable membrane are integrated, even though the sample gas is pulled by the pump,
Extremely stable, unaffected by dynamic pressure.

■Residual chlorine sensor in liquid As a gas sensor in liquid, a residual chlorine sensor having the configuration shown in Fig. 3 was manufactured. In this case, the conductive gas permeable membrane 2 has an outer diameter of 10 φm.

A carbon-dispersed conductive hydrophobic membrane with a thickness of 0.4 tmm, a silver electrode as the counter electrode 3, a potassium chloride aqueous solution as the internal liquid 4, and a hydrophobic gas permeable membrane 8 for insulating the sensor and the sample liquid with a micropore diameter of 0 A volite 1-lafluoroethylene membrane of .1- was used.

FIG. 10 shows the state of the signal when a sample with residual chlorine of 5 ρρm was adjusted to pH 4. Compared to a conventional residual chlorine sensor made of gold with a detection pole diameter of 4φ, which outputs a signal of approximately 0.5 pA for the same sample, several times higher sensitivity has been achieved. This can be confirmed.

From the results of (1) to (2) above, it is recognized that high sensitivity and output stabilization have been clearly achieved.

[Brief explanation of the drawing]

Figures 1 to 5 are cross-sectional views showing one embodiment of the gas sensor of the present invention, Figure 6 is a graph showing the output of the chlorine gas sensor of the present invention, and Figure 7 is the voltage applied to the sensor of Figure 2. 8 is a graph showing the output of the hydrogen sulfide gas sensor of the present invention, FIG. 9 is a graph showing the output of the oxygen gas sensor of the present invention, and FIG. 10 is a graph showing the output of the oxygen gas sensor of the present invention. Graphs showing the output of the residual chlorine sensor, FIGS. 11 and 12, are schematic diagrams each showing an example of a conventional gas sensor. Applicant: Electrochemical Meter Co., Ltd. Agent: Patent Attorney: Takashi Kojima Figure 1 Figure 2 Figure 3 Figure 4 Figure 6 Figure 7 Figure 8 b el)C

Claims (1)

[Claims] 1. A sensing electrode, a counter electrode, a gas permeable membrane, and these sensing electrodes,
It has an electrolyte in contact with a counter electrode and a gas permeable membrane, respectively,
In a gas sensor that detects a target gas component in a sample that has passed through the gas permeable membrane through an electrochemical reaction when it comes into contact with the detection electrode, the gas permeable membrane is coated with a conductive substance uniformly. A gas sensor characterized in that it is formed of a dispersed hydrophobic porous membrane and that this gas permeable membrane is configured as a detection electrode.