JPH09318589A - Diaphragm type electrochemical gas detector - Google Patents

Abstract

PROBLEM TO BE SOLVED: To certainly hold an acting electrode to a liquid contact state regardless of the liquid level of an electrolyte or the posture of a cell by the stickiness of polyvinyl alcohol by preparing the electrolyte by mixing sulfuric acid and polyvinyl alcohol. SOLUTION: An electrolyte 2 is prepared, for example, by adding 3-5wt. of polyvinyl alcohol with sulfidity of 99.5% and a polymerization degree of 1,500 or more to sulfuric acid becoming a main component for generating redox reaction to gas to be inspected in concn. developing stickiness of a degree adhering to an acting electrode 4b and not flowing down from the surface thereof. By this constitution, even if the posture of the whole of a detector is extremely changed and the electrolyte 2 is separated from the acting electrode 4b, the electrolyte 2 forms a thin layer 2a on the surface of the acting electrode 4b and the inner surface of a cell container 1 by the high sticking force of polyvinyl alcohol and, since the acting electrode 4b holds the conductive relation with an opposed electrode 7 or an acting electrode 8 as a whole in the state wetted with the electrolyte 2, in the state restored to the original posture, the detection signal corresponding to the concn. of the gas to be inspected can be immediately outputted.

Description

Detailed Description of the Invention

[0001]

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

[0002]

2. Description of the Related Art A diaphragm-type electrochemical gas detector is formed by forming a metal film as a working electrode on the liquid contact side of a diaphragm made of a porous material such as Teflon having gas permeability and immersing it in an electrolyte together with a counter electrode. Then, the gas concentration is detected by an electrical change between the working electrode and the counter electrode due to the gas that has passed through the diaphragm.

In such a detector, since the 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, or the working electrode as a whole is exposed to the atmosphere due to changes 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 so that the liquid level of the electrolytic solution is lowered or the posture of the whole detector is changed. Even when it changes, the working electrode is prevented from coming into direct contact with the air due to the electrolyte solution absorbed in the porous sheet, thereby preventing undetectable situations.

[0005]

However, when the atmospheric pressure is suddenly changed due to transportation or the like, the diaphragm is plastically deformed so that the outside is convex, and a gap is formed between the diaphragm and the porous sheet. Sometimes. Once the gap is formed in this way, the porous sheet that absorbs the electrolyte acts as a partition wall, blocking the movement of the electrolyte to the working electrode side, regardless of the detector attitude. A new problem occurs 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 the working electrode in the liquid contact state regardless of the water level of the electrolytic solution and the attitude of the cell. An object of the present invention is to provide a diaphragm type electrochemical gas detector.

[0006]

In order to solve such a problem, in the present invention, a cell having a window sealed with a gas permeable diaphragm having a working electrode formed on the liquid contact side is provided. An electrolytic solution prepared by mixing sulfuric acid and polyvinyl alcohol and a counter electrode are accommodated, and the working electrode is always kept in contact with the liquid due to the adhesiveness 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 electrolysis gas detector as an example. In the figure, reference numeral 1 is a cell container, and an electrolyte solution 2 to be described later.
And a through hole 3 is provided on one surface thereof.
A diaphragm 4 is stretched liquid-tightly in the through hole 3 via a packing 5 such as an O-ring or the like so as to seal the through hole 3 with a pressing lid 6 having gas inlets 6a, 6a. .

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

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

These working electrode 4b, counter electrode 7 and reference electrode 8
Is connected to the measuring circuit 10 by being drawn out to the outside via a lead wire so as to maintain a liquid-tight state with the cell container 1, and is tested based on changes in current and voltage between the working electrode 4b and the counter electrode 7. It is designed to measure the concentration of gas.

On the other hand, the electrolytic solution 2 has a degree of adhesion such that sulfuric acid, which is a main component for causing oxidation and reduction reactions with the working electrode 4b with respect to the test gas, sticks to the working electrode 4b and does not flow down from the surface. In this example, 5 wt% of polyvinyl alcohol having a saponification degree of 99.5% and a polymerization degree of 1500 or more was added.

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

On the other hand, during use, the sulfuric acid forming the electrolytic solution 2 absorbs moisture in the atmosphere into the container 1 due to its water absorption. Due to the extremely high hydrophilicity of polyvinyl alcohol, this taken-in water is dissolved in the electrolytic solution containing polyvinyl alcohol and cannot remain as water itself which easily flows out into the cell.

On the other hand, when the water content of the electrolytic solution gradually evaporates,
As the concentration of polyvinyl alcohol contained in the electrolytic solution increases and the adhesive strength increases, the electrolytic solution 2 lowers the water level while forming a thin layer 2a on the surface of the working electrode 4b as shown in FIG. I will let you. Of course, the diaphragm-type electrochemical gas detector does not lose its gas detection function as long as the working electrode 4b is moistened with the electrolytic solution 2 and the conductive relationship with the counter electrode 7 and the reference electrode 8 is maintained. Therefore, a current or voltage is generated according to the concentration of the gas flowing from the outside.

Even when the attitude of the whole detector is extremely changed and the electrolytic solution 2 is separated from the working electrode 4b, the electrolytic solution 2 is caused by the high adhesive force of polyvinyl alcohol as described above. 4b and the inner surface of the cell container 1, a conductive layer is maintained with the counter electrode 7 and the working electrode 8 while the working electrode 4b is wetted with the electrolytic solution 2 as a whole. In this state, the detection signal corresponding to the gas concentration is immediately output.

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

Even when the packing 5 such as an O-ring in the sealing region is deteriorated due to long-term use and the sealing force is reduced, or foreign matter is mixed in the sealing region during the manufacturing process,
Since the electrolytic solution 2 has increased adhesive force and viscosity due to polyvinyl alcohol, it cannot leak out. That is, the diameter between the packing 5 and the diaphragm 4 in FIG.
The platinum wire of m was forcibly inserted to partially reduce the sealing force, and the concentration of polyvinyl alcohol in the electrolytic solution was changed to inspect for the presence or absence of leakage. Leakage occurred with less than 3 wt%, but no leakage was observed with more than 3 wt%.

In order to investigate the influence of polyvinyl alcohol on the detection sensitivity, the detection signal was investigated while the concentration of the test gas was kept constant and the concentration of polyvinyl alcohol in the electrolytic solution was changed. Practically no difference was observed with the detector using the electrolytic solution prepared only with sulfuric acid.

Incidentally, 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. Therefore, polyvinyl alcohols having different degrees of saponification and degree of polymerization are selected. When the electrolyte solution was prepared and used, and the time from falling to undetectability and leakage were investigated in the same manner as described above, the saponification degree was 80 to 100 and the polymerization degree was 1000 to 1
With about 500, almost no difference was observed. This is because when polyvinyl acetate itself, which increases or decreases depending on the degree of saponification of polyvinyl alcohol, is also dissolved in the solution, it has the property of increasing the viscosity of the solution and the adhesive property. It is presumed that this is because it is not much affected by the degree.

Then, the electrolytic solution to be injected into the cell container is usually measured by a syringe and injected through the atmosphere opening port provided in the cell container through an injection needle, and then the usual manufacturing process is performed. When following the process as it is, it is 3 wt% or more from the viewpoint of preventing undetectable situations due to falling and also preventing leakage, and at the maximum 7 wt% from the viewpoint of fluidity for injection. It is desirable to suppress it.

In the above-mentioned embodiment, the example in which the porous sheet for maintaining the liquid contact is not used has been described, but even when the porous sheet is provided,
If the porous sheet is made of a flexible material, the electrolyte moves due to the deformation of the diaphragm, and the adhesiveness causes the porous sheet to move following the diaphragm, so that the gap between the working electrode and the porous sheet It does not cause a gap.

On the other hand, even if the viscosity of polyvinyl alcohol is difficult to pass through the injection needle, the concentration is such that it still has fluidity, for example, 7 wt% to 30 wt%.
If it is within the range, it may be injected from the through hole 3 for tensioning the diaphragm of the cell container or a separately provided injection port.

When the viscosity is high as described above, if it is left for a while after the injection, the fluidity allows it to adapt to the working electrode, the counter electrode, etc., so that it also functions as a detector.

Therefore, in the detector of the type in which the electrolytic solution is contained in the cell container, 3 wt% of polyvinyl alcohol is used in order to prevent undetectability due to a fall as much as possible and also to prevent leakage. It is a practical choice to contain it in a concentration of 30% by weight or more and 30% by weight or less.

Further, 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 the self-holding property is generated. Therefore, the electrolytic solution is molded as an agar-like sheet, and the diaphragm having the above-described structure is attached to one surface of the sheet,
Even if the sheet having the layer of the material serving as the counter electrode is adhered to the other surface so as to be in close contact, the sheet-shaped electrolytic solution functions similarly to the liquid electrolytic solution for the working electrode and the counter electrode formed in the diaphragm. It was confirmed that it works as a detector.

In the above embodiments, the case where polyvinyl alcohol is used has been described. However, the same effect can be obtained by mixing polyethylene glycol having the same properties as polyvinyl alcohol with the electrolytic solution.

Since polyethylene glycol has an extremely wide molecular weight range as compared with polyvinyl alcohol,
When the concentration suitable for a diaphragm type electrochemical gas detector using sulfuric acid as an electrolytic solution was investigated, it was found that it was desirable to mix a sulfuric acid solution having a molecular weight of 1,000 or more and existing as a solid at room temperature. That is, if the molecular weight is lower than 1000, the amount mixed with the electrolytic solution in order 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 solid substance at room temperature, for example, one having a molecular weight of about 10,000 has a minimum content of about 45 wt%, a substance having a molecular weight of about 2 million has a minimum content of about 4 wt%, and a molecular weight of about 4 million. By mixing at least about 4 wt%, it was possible to obtain the same function and effect as the above-mentioned electrolytic solution containing 5 wt% of polyvinyl alcohol. From these things, the molecular weight is preferably 200
If more than 10,000 polyethylene glycols are used, the mixing amount can be kept to a very small amount of about 4wt%, and the function as an electrolyte can be secured, and the amount of polyethylene glycol used should be reduced to reduce the cost. You can

Polyethylene glycol has an economical effect that it can be obtained at a relatively low cost as it has a higher purity than that of polyvinyl alcohol.

[0030]

As described above, in the present invention, sulfuric acid and polyvinyl alcohol are added to a cell having a window sealed with a gas permeable diaphragm having a working electrode formed on the liquid contact side. Since the electrolyte prepared by mixing and the counter electrode are contained, the working electrode is maintained in the liquid contact state through the electrolyte layer even when the liquid level or posture changes due to the adhesive force of polyvinyl alcohol. It is possible to avoid the situation where it becomes undetectable due to the fact that it does not require extra electrolyte,
Can be miniaturized. Further, by increasing the viscosity of the electrolytic solution with polyvinyl alcohol, it is possible to suppress leakage from the joining region between the cell container and the other member and improve the reliability.

[Brief description of drawings]

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

FIG. 2 is a diagram showing a state when the water level of an electrolytic solution is lowered in the same apparatus.

[Explanation of symbols]

 1 Cell 2 Electrolyte 3 Window 4 Septum 4a Porous Membrane 4b Working Electrode 7 Counter Electrode 8 Reference Electrode

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Akira Akamatsu 2-7-6 Shozuzawa, Itabashi-ku, Tokyo Riken Keiki Co., Ltd.

Claims (4)

[Claims] 1. An electrolytic solution 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 polyvinyl alcohol is 3 wt.
% To 30 wt%, The diaphragm type electrochemical gas detector according to claim 1. 3. An electrolytic solution prepared by mixing sulfuric acid and polyethylene glycol, which is solid at room temperature, into a cell having a window sealed with a gas-permeable diaphragm having a working electrode formed on the liquid contact side. And a diaphragm-type electrochemical gas detector containing a counter electrode. 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 4 wt% or more.