JP3279928B2 - Diaphragm type electrochemical gas detector - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of contacting a gas with an electrolytic solution through a gas-permeable diaphragm and measuring the concentration of the gas based on an electrical change between a working electrode and a counter electrode formed on the diaphragm. The present invention relates to a membrane-type electrochemical gas detector for detecting a gas.

[0002]

2. Description of the Related Art In a diaphragm type electrochemical gas detector, a metal film serving as a working electrode is formed on the liquid contact side of a diaphragm made of a porous material such as Teflon having gas permeability, and immersed in an electrolytic solution together with a counter electrode. The gas concentration is detected by an electrical change between the working electrode and the counter electrode caused by the gas permeating the diaphragm.

In such a detector, since gas is taken into the electrolytic solution in the container through the diaphragm, the moisture of the electrolytic solution evaporates from the atmosphere opening port of the container constituting the diaphragm or the cell, and in an extreme case, The water level of the electrolyte drops below the diaphragm and detection becomes impossible, and the entire working electrode is exposed to the atmosphere due to a change in the attitude of the detector, and the characteristics of the working electrode fluctuate extremely, There is a problem that it takes time until the next measurement.

In order to solve such a problem, a porous sheet such as a filter paper extending to the bottom of the cell is usually brought into contact with the surface of the working electrode to lower the level of the electrolytic solution or to reduce the attitude of the entire detector. Even in the case of a change, the working electrode is prevented from coming into direct contact with air by the electrolyte solution absorbed in the porous sheet, thereby preventing a situation in which detection is not possible.

[0005]

However, when a sudden change in air pressure is caused by transportation or the like, the diaphragm is plastically deformed so that the outside becomes convex, and a gap is formed between the diaphragm and the porous sheet. Sometimes. Once the gap is thus formed, the porous sheet absorbing the electrolyte acts as a partition, preventing the electrolyte from moving to the working electrode side, regardless of the position of the detector. A new problem arises in that the working electrode is exposed to the atmosphere. The present invention has been made in view of such a problem, and an object thereof is to reliably maintain a working electrode in a liquid contact state regardless of a water level of an electrolytic solution or a posture of a cell. It is to provide a diaphragm type electrochemical gas detector.

[0006]

According to the present invention, there is provided a cell having a window sealed with a gas-permeable diaphragm having a working electrode formed on a liquid contact side. An electrolyte prepared by mixing sulfuric acid and polyvinyl alcohol and a counter electrode are accommodated, and the working electrode is always kept in a liquid-contact state by the tackiness of polyvinyl alcohol.

[0007]

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram showing a first embodiment of the present invention. FIG. 1 shows an embodiment of the present invention by taking a potentiostatic electrolytic gas detector as an example. In the drawing, reference numeral 1 denotes a cell container, and an electrolytic solution 2 described later.
And a through hole 3 is provided in one surface thereof.
In the through hole 3, a diaphragm 4 is liquid-tightly stretched by a holding lid 6 having gas inlets 6a, 6a # via a packing 5 such as an O-ring on the outside so as to seal the through hole. .

The diaphragm 4 is provided on the electrolyte side with a metal suitable for oxidizing and reducing the test gas, for example, platinum oxide (PtO),
A mixture of a fine metal powder such as ruthenium oxide (RuO2) and a fluororesin powder is applied to a porous film 4a made of a fluororesin or the like having air permeability and water repellency, sintered, reactive sputtering or the like. Electrode 4b formed by film formation
Is built-in.

A counter electrode 7 and a reference electrode 8 are provided in the cell container 1 at a certain distance from the working electrode 4b.

The working electrode 4b, the counter electrode 7, and the reference electrode 8
Is connected to the measurement circuit 10 via a lead wire while being kept in a liquid-tight state with the cell container 1 and is connected to the measurement circuit 10 to be tested based on changes in current and voltage between the working electrode 4b and the counter electrode 7. It measures the concentration of gas.

On the other hand, the electrolytic solution 2 adheres to sulfuric acid, which is a main component for causing oxidation and reduction reactions with the working electrode 4b, to the test gas together with the working electrode 4b. In this example, 5 wt% of polyvinyl alcohol having a saponification degree of 99.5% and a polymerization degree of 1500 or more is added.

In this embodiment, when the test gas passes through the diaphragm 4 and dissolves in the electrolytic solution 2, the dissociation reaction of the test gas in the electrolytic solution 2 and the oxidation reaction and reduction reaction at the working electrode 4b progress. Then, electrons proportional to the concentration of the transmitted test gas are generated, and the electrons are detected by the measurement circuit 10 as an electrolytic current.

On the other hand, during use, the sulfuric acid constituting the electrolytic solution 2 takes in atmospheric moisture into the container 1 due to its water absorption. The taken-in water dissolves in the electrolytic solution containing polyvinyl alcohol due to the extremely high hydrophilicity of polyvinyl alcohol, and cannot remain as water itself that easily flows into the cell.

On the other hand, when the water content of the electrolyte gradually evaporates,
As the concentration of polyvinyl alcohol contained in the electrolyte increases, the adhesive strength increases, and the electrolyte 2 lowers the water level while forming a thin layer 2a on the surface of the working electrode 4b as shown in FIG. Will be. Naturally, the diaphragm type electrochemical gas detector does not lose its gas detection function if the working electrode 4b is wetted by the electrolytic solution 2 and the conductive relationship between the working electrode 4b and the counter electrode 7 or the reference electrode 8 is maintained. Therefore, a current or a voltage is generated according to the concentration of the gas flowing from the outside.

Further, even when the attitude of the entire detector is extremely changed and the electrolytic solution 2 is separated from the working electrode 4b, as described above, the electrolytic solution 2 is not affected by the high adhesive force of the polyvinyl alcohol. A thin layer is formed on the inner surface of the cell 4b and the cell container 1, and the working electrode 4b is maintained in a conductive relationship with the counter electrode 7 and the working electrode 8 in a state of being moistened with the electrolytic solution 2, so that it is returned to the original posture. In this state, a detection signal corresponding to the gas concentration is immediately output.

By the way, 5 wt% of polyvinyl alcohol
In this embodiment, the test gas can be measured normally even when the detector is turned over and left for 24 hours, and even when the concentration of polyvinyl alcohol is reduced to 3 wt%, the overturn time is reduced. The test gas could be measured for about 3 hours.

In the case where the packing 5 such as the O-ring in the sealing region is worn down for a long time and the sealing force is reduced, or a foreign substance is mixed in the sealing region during the manufacturing process,
Since the adhesive strength and the viscosity of the electrolyte solution 2 are increased by the polyvinyl alcohol, the electrolyte solution 2 cannot leak. That is, the diameter between the packing 5 and the diaphragm 4 in FIG.
The sealing force in a part of the region was forcibly reduced by interposing a platinum wire of m length, and the presence or absence of leakage was inspected by changing the concentration of polyvinyl alcohol in the electrolytic solution. Leakage occurred when the content was less than 3 wt%, but no leakage was observed when the content was more than 3 wt%.

Further, in order to investigate the effect of polyvinyl alcohol on the detection sensitivity, the detection signal was examined while maintaining the concentration of the test gas constant and changing the concentration of polyvinyl alcohol in the electrolyte. No practical difference was observed with a detector using an electrolyte prepared only with sulfuric acid.

By the way, there are various grades of polyvinyl alcohol depending on the degree of saponification, that is, the degree of hydrolysis of the raw material polyvinyl acetate and the degree of polymerization. The electrolyte solution was adjusted using the method described above, and the time from the fall to the time when the electrolyte became undetectable and the leakage were examined in the same manner as described above.
With about 500, almost no difference was observed. This is because the polyvinyl acetate itself, which increases or decreases according to the degree of saponification of polyvinyl alcohol, has the property of increasing the viscosity of the solution and the adhesive property when dissolved in the solution. This is presumed to be less affected by the degree.

Then, the electrolyte solution into the cell container is usually measured with a syringe and a certain amount is injected using an air opening port formed in the cell container through an injection needle, and then the usual production is performed. In the case of following the process as it is, it is 3 wt% or more from the viewpoint of preventing a situation in which detection is not possible due to a fall and preventing leakage in addition, and at most 7 wt% from the viewpoint of fluidity for injection. It is desirable to suppress.

In the above embodiment, an example is described in which a porous sheet for maintaining liquid contact is not used. However, even when a porous sheet is provided,
If the porous sheet is made of a flexible material, the movement of the electrolytic solution due to the deformation of the diaphragm and the stickiness of the porous sheet move following the diaphragm, so that the porous sheet moves between the working electrode and the porous sheet. No gaps will occur.

On the other hand, even if the viscosity of polyvinyl alcohol is difficult to pass through an injection needle, the concentration still shows fluidity, for example, 7 wt% to 30 wt%.
In this case, the injection may be performed from the through-hole 3 for extending the diaphragm of the cell container or an injection port provided separately.

In the case where the viscosity is high as described above, if it is left for a while after injection, it adapts to the working electrode and the counter electrode due to its fluidity, and thus also functions as a detector.

Therefore, in the case of a detector in which the electrolytic solution is contained in the cell container, 3 wt% of polyvinyl alcohol is used in order to prevent the detection itself due to falling down as much as possible and to prevent leakage. % Is a practical choice.

Further, when the concentration of polyvinyl alcohol is 3
If it exceeds 0 wt%, injection itself becomes impossible,
At such a high concentration, the fluidity is extremely reduced and self-holding occurs. For this reason, the electrolyte is formed into an agar-like sheet, and a diaphragm having the above-described configuration is attached to one surface,
Even when a sheet having a layer of a material serving as a counter electrode is adhered to the other surface so as to be in close contact, the sheet-like electrolyte functions in the same manner as the liquid electrolyte with respect to the working electrode and the counter electrode formed on the diaphragm. It was confirmed that the device operated as a detector.

In the above embodiment, the case where polyvinyl alcohol is used has been described. However, the same action can be obtained even if polyethylene glycol having the same properties as polyvinyl alcohol is mixed with the electrolytic solution.

Polyethylene glycol has an extremely wide range of molecular weight as compared with polyvinyl alcohol.
Investigation of the concentration suitable as a diaphragm type electrochemical gas detector using sulfuric acid as an electrolytic solution revealed that it is desirable to mix a solid having a molecular weight of 1000 or more at room temperature with a sulfuric acid solution. That is, if the molecular weight is lower than 1,000, the amount of the compound to be mixed with the electrolytic solution to obtain the above-mentioned desired viscosity and adhesive strength becomes too large, and the function as the electrolytic solution cannot be sufficiently exhibited.

On the other hand, a material which is solid at room temperature, for example, a material having a molecular weight of about 10,000 has a minimum of about 45 wt%, a material having a molecular weight of about 2 million has a minimum of about 4 wt%, and a material having a molecular weight of about 4,000,000. By mixing at least about 4% by weight, the same operation and effect as those of the above-mentioned electrolytic solution containing 5% by weight of polyvinyl alcohol could be obtained. From these, it is desirable that the molecular weight is 200
If more than 10,000 polyethylene glycols are used, the mixing amount can be suppressed to an extremely small amount of about 4 wt%, and the function as an electrolyte solution can be secured, and the cost can be reduced by reducing the amount of polyethylene glycol used. Can be.

Further, polyethylene glycol has an economical effect that a product having a higher purity than polyvinyl alcohol can be obtained relatively inexpensively.

[0030]

As described above, according to the present invention, sulfuric acid and polyvinyl alcohol are added to a cell having a window sealed by a gas-permeable diaphragm having a working electrode formed on the liquid contact side. Since the electrolyte solution prepared by mixing and the counter electrode are housed, even when the liquid level or posture changes due to the adhesive force of polyvinyl alcohol, the working electrode is maintained in a liquid contact state through the electrolyte layer. To avoid undetectable situations, and no extra electrolyte is required,
Can be downsized. In addition, by increasing the viscosity of the electrolytic solution with polyvinyl alcohol, it is possible to suppress leakage from the joint region between the cell container and another member, thereby improving reliability.

[Brief description of the drawings]

FIG. 1 is a sectional view of an apparatus showing an embodiment of the present invention.

FIG. 2 is a diagram showing a state in which the water level of an electrolytic solution is reduced in the above device.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Cell 2 Electrolyte 3 Window 4 Diaphragm 4a Porous membrane 4b Working electrode 7 Counter electrode 8 Reference electrode

──────────────────────────────────────────────────続 き Continuation of the front page (72) Akira Akamatsu Inventor: Riken Keiki Co., Ltd. 2-7-6 Shozuzawa, Itabashi-ku, Tokyo (56) References JP-A-63-187149 (JP, A) JP-A-Hei 9-243594 (JP, A) JP-A-8-75693 (JP, A) JP-A-7-174728 (JP, A) JP-A-6-66761 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) G01N 27/404

Claims (4)

(57) [Claims] An electrolyte prepared by mixing sulfuric acid and polyvinyl alcohol in a cell having a window sealed with a gas-permeable diaphragm having a working electrode formed on the liquid contact side, and a counter electrode. A diaphragm-type electrochemical gas detector containing a gas. 2. The method according to claim 1, wherein the polyvinyl alcohol is 3 wt.
2. The membrane-type electrochemical gas detector according to claim 1, wherein the gas is contained in an amount of 30% by weight to 30% by weight. 3. An electrolytic solution prepared by mixing sulfuric acid and solid polyethylene glycol at room temperature in a cell having a window sealed with a gas-permeable diaphragm having a working electrode formed on the liquid contact side. And a counter electrode and a diaphragm type electrochemical gas detector. 4. The diaphragm type electrochemical gas detector according to claim 3, wherein the polyethylene glycol has a molecular weight of 2,000,000 or more and is contained in an amount of 4% by weight or more.