JP2000039417A - Diaphragm-type electrochemical gas detector - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress the aging of detection sensitivity, viscosity, and the like by accommodating the mixture of each specific sulfuric acid and polyethylene glycol as an electrolyte in a cell with a window being sealed by a gas- permeability diaphragm. SOLUTION: In a cell container 1, a diaphragm 7 where a film 9 that becomes an operation electrode by reactive sputtering or the like is extended in a through hole 3, and a diaphragm 11 where a film 10 made of a substance suited for a counter electrode is formed is provided in a through hole 4. Polyethyleneglycol is mixed to the following sulfuric acid by 1.0-3.5 wt.% for preparation. In this case, by using polyethyleneglycol with at least 4,000,000 molecular weight that shows a viscosity being bonded to the films 9 and 10 and cannot flow down, an electrolyte 2 can have high detection sensitivity and small economical change. Also, by using a sulfuric acid that is at least 10 prescription, the decomposition of polyethyleneglycol due to sulfuric acid, namely the reduction in viscosity, can be prevented.

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 the detection from becoming undetectable.

[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. In order to solve such a problem, as shown in JP-A-9-318589, a cell having a window sealed with a gas permeable diaphragm having a working electrode formed on the liquid contact side is provided with sulfuric acid. And proposed a diaphragm type electrochemical gas detector containing 4 wt% or more of polyethylene glycol which is solid at room temperature, for example, having a molecular weight of 2,000,000 or more.

According to this, an electrolytic solution prepared by mixing sulfuric acid and polyethylene glycol is placed in a cell having a window sealed with a gas-permeable diaphragm having a working electrode formed on the liquid contact side. Although the working electrode can be always maintained in a liquid contact state by the adhesiveness of polyethylene glycol by containing the counter electrode, the viscosity gradually decreases over time, and the gas detection sensitivity increases with time. Inconvenience such as change was found. The present invention has been made in view of such a problem, and it is an object of the present invention to suppress the change over time in detection sensitivity and suppress the change over time in viscosity and gas detection sensitivity to some extent. It is to provide a diaphragm type electrochemical gas detector that can be used.

[0007]

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. A mixed solution of sulfuric acid of 10 N or less and polyethylene glycol having a molecular weight of 4,000,000 or more and 1.0 to 3.5 wt% was accommodated as an electrolytic solution.

[0008]

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 using a potentiostatic electrolytic gas detector as an example. In FIG.
And through holes 3 and 4 are provided in two opposing surfaces thereof. One of the through holes 3 is provided with a gas inlet 6 through a packing 5 such as an O-ring so as to seal it.
The diaphragm 7 is liquid-tightly stretched by the holding lid 6 having a, 6a #.

The diaphragm 7 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 8 of a fluororesin or the like having air permeability and water repellency, sintered, reactive sputtering or the like. Thus, a film 9 serving as a working electrode is formed.

The other through hole 4 is provided with a diaphragm 11 on which a film 10 of a material suitable for a counter electrode is formed, which is more liquid-tight than the holding lid 12.

The working and counter electrodes 9 and 10 are connected to a measuring circuit via lead wires.
The concentration of the test gas is measured by measuring the detector current flowing between them.

On the other hand, the electrolytic solution 2 adheres to the membranes 9 and 10 to sulfuric acid of 10 N or less, which is a main component for causing oxidation and reduction reactions, together with the membrane 9 serving as a working electrode for the test gas. In this embodiment, the concentration is such that a polyethylene glycol having a molecular weight of 4,000,000 or more is mixed with 1.0 to 3.5 wt% to give a degree of adhesion that does not flow down from the surface.

In this embodiment, when the test gas permeates 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 in the film 9 serving as a working electrode occur. As the reaction proceeds, electrons are generated in proportion to the concentration of the permeated test gas, and the detector current flows.

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

On the other hand, when the water content of the electrolyte gradually evaporates,
The concentration of polyethylene glycol contained in the electrolytic solution increases, the adhesive strength increases, and the water level decreases while forming a thin layer 2a on the surfaces of the films 9, 10. Of course, the diaphragm type electrochemical gas detector does not lose the gas detection function if the working electrode and the counter electrode membranes 9 and 10 are moistened by the electrolyte solution 2 and the conductive relationship is maintained. Detector current corresponding to the concentration of gas flowing from the detector is generated.

Further, even when the attitude of the entire detector is extremely changed and the electrolytic solution 2 is separated from the membranes 9 and 10, as described above, the electrolytic solution 2 is kept at a high pressure by the polyethylene glycol. , 10 and a thin layer is formed on the inner surface of the cell container 1 and these are moistened with the electrolytic solution 2 to maintain the conductive relationship. Therefore, when returned to the original posture, a detection signal corresponding to the gas concentration is generated. Output immediately.

Incidentally, polyethylene glycol was added to 3.5
In this embodiment containing wt%, the test gas can be measured normally even when the detector is turned over and left for 24 hours,
Even when the concentration of polyethylene glycol was reduced to 1.0 wt%, the test gas could be measured if the overturn time was 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 matter is mixed in the sealing region during the manufacturing process,
Since the electrolytic solution 2 has increased adhesion and viscosity by polyethylene glycol, it cannot leak out. That is, the diameter between the packing 5 and the diaphragm 7 in FIG.
Then, the sealing force in a part of the region was forcibly reduced by interposing a platinum wire having a length of m, and the presence or absence of leakage was examined by changing the concentration of polyethylene glycol in the electrolytic solution. Leakage occurred when the content was less than 1.0 wt%, but no leakage was observed when the content was 1.0 wt% or more.

In order to investigate the effect of polyethylene glycol on detection sensitivity, the detector current was investigated while the concentration of the test gas was kept constant and the concentration of polyethylene glycol in the electrolyte was changed. As shown in FIG. 2, the electrolytic solution containing polyethylene glycol having a molecular weight of 4,000,000 or more had the highest detection sensitivity and little change over time.

On the other hand, as sulfuric acid, 6N, 10N, 18N
The specified ones were prepared, and 3.5 wt% of polyethylene glycol having a molecular weight of 4,000,000 or more was prepared to prepare an electrolytic solution, and the change over time in the detection sensitivity was investigated.
As shown in the figure, the change in the detector current was extremely small when the value was 10 norm or less. Similarly, a time-dependent change in the viscosity was investigated, and it was found that the sulfuric acid having a viscosity of 10 N or less did not show a decrease in the viscosity, and that the decomposition of polyethylene glycol by the sulfuric acid could be prevented.

Further, the color change of the electrolytic solution was examined just in case. As shown in Table 1, the electrolytic solution having a sulfuric acid concentration of 6N did not change its color. There was almost no change in the time.

[0022]

[Table 1]

When an electrolytic solution obtained by dissolving polyethylene glycol in 10N sulfuric acid is accommodated in a light-shielding container and allowed to stand under natural light, the degree of decrease in viscosity and the progress of discoloration can be effectively prevented. There was found. From this, by configuring the cell with a light-shielding material,
It is possible to more reliably prevent the electrolytic solution from changing over time.

[0024]

As described above, according to the present invention, in a cell having a window sealed with a gas-permeable diaphragm having a working electrode formed on the liquid contact side, sulfuric acid having a molecular weight of 10 N or less and a molecular weight of less than 10 N are provided. Used 4 million or more polyethylene glycols.
Since a mixed solution of 0 to 3.5 wt% is contained as an electrolyte, the working electrode is maintained in a liquid-contact state via the electrolyte layer even when the liquid level or posture changes due to the adhesive force of polyethylene glycol. As a result, it is possible to prevent a change in the sensitivity of the sensor over time and a decrease in the viscosity of the electrolytic solution as much as possible, while avoiding a situation in which detection is not possible.

[Brief description of the drawings]

FIG. 1 is a view showing an apparatus according to an embodiment of the present invention in a state where the water level of an electrolytic solution is lowered.

FIG. 2 is a diagram showing the change over time in the detection sensitivity for each molecular weight of polyethylene glycol.

FIG. 3 is a graph showing the change over time in the detection sensitivity for each concentration of sulfuric acid.

FIG. 4 is a diagram showing a change over time of an electrolytic solution concentration for each concentration of sulfuric acid.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Cell container 2 Electrolyte 3 Window 9 Film used as working electrode 10 Film used as counter electrode

Claims (2)

[Claims] 1. A cell having a window sealed with a gas-permeable diaphragm having a working electrode formed on the liquid contact side is charged with 1.0 N or less sulfuric acid and polyethylene glycol having a molecular weight of 4,000,000 or more in a cell. A diaphragm type electrochemical gas detector containing a mixed solution of about 3.5 wt% as an electrolyte. 2. The diaphragm type electrochemical gas detector according to claim 1, wherein the cell is made of a light-shielding material.