JPH0534314A - Method and device for measuring oxidizing substance concentration - Google Patents

Abstract

PURPOSE:To achieve low-concentration measurement of an oxidizing substance and a long-term concentration monitoring by using an electrolytic cell with three or more electrolytic electrodes including at least an operation electrode, an opposite electrode, and a reproduction electrode. CONSTITUTION:A container 9 where an electrolyte (potassium iodide aqueous solution) 7 is poured into a chamber which is partitioned into three portions by an ion-exchange film 3 is installed within a specimen gas 1 containing oxidizing gas so that permeability septum 2 contacts. An operation electrode 4 and an opposite electrode 5 are connected so that the operation electrode 4 becomes a cathode and the opposite electrode 5 becomes an anode. The opposite electrode 5 and a reproduction electrode 6 are connected so that opposite electrode 5 becomes the cathode and the reproduction electrode 6 becomes an anode by a lead wire 10. Also, an instrument 8 is provided between these, with which current which flows between the operation pole 4 and the opposite electrode 5 or between the opposite electrode 5 and the reproduction electrode 6 is measured. The current value is converted to ozone concentration, and then the concentration is displayed. By adding the reproduction electrode 6, halogen molecule which is conventionally accumulated by the operation electrode 4 and the opposite electrode 5 is accumulated within the electrolyte in the space where the reproduction electrode 6 is provided, thus preventing base current from being increased even in a continuous long-term measurement.

Description

[Brief description of drawings]

FIG. 1 is a diagram showing a measurement mechanism of the method of the present invention.

FIG. 2 is a schematic cross-sectional view of a measuring device showing an example of implementation of the present invention.

FIG. 3 is a graph showing a long-term change in measured current value with a conventional measuring device and a measuring device according to an embodiment of the present invention.

[Explanation of symbols]

1: sample gas, 2: breathable diaphragm, 3: ion exchange membrane,
4: Working electrode, 5: Counter electrode, 6: Regeneration electrode, 7: Electrolyte, 8:
Instrument, 9: container, 10: lead wire.

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Akira Umezawa             5-4-24 Ishigami, Niiza City, Saitama Prefecture (72) Inventor Shuichi Suzuki             1-40-6 Sugamo, Toshima-ku, Tokyo (72) Inventor Shunichi Uchiyama             1-265, Sukune, Fukaya City, Saitama Prefecture

Claims (7)

[Claims] 1. An electrolytic cell having at least three or more electrolytic electrodes of a working electrode, a counter electrode and a regenerating electrode, an ion exchange membrane partitioning each electrode, and an electrolytic cell containing an electrolyte solution containing a halogen ion, and added from the outside. A method for measuring the concentration of an oxidizing substance, characterized in that the concentration of the oxidizing substance added from the outside is measured by measuring the current flowing when electrolyzing the oxidizing substance or the total Coulomb number at that time. 2. The method for measuring the concentration of an oxidizing substance according to claim 1, wherein the oxidizing substance is dissolved or dispersed in a liquid, and the solution is injected into the working electrode to perform electrolysis. 3. The method for measuring the concentration of an oxidant substance according to claim 1, wherein the oxidant substance is a gas, and the gas is introduced into the working electrode through a gas permeable diaphragm to perform electrolysis. 4. An electrolytic cell having at least three or more electrolytic electrodes of a working electrode, a counter electrode and a regenerating electrode, an ion exchange membrane partitioning the electrodes, and an electrolytic cell containing an electrolyte solution containing a halogen ion. A device for measuring the concentration of an oxidant, which has a measuring device for measuring a current flowing when electrolyzing an oxidant added from the outside or a total Coulomb number at that time. 5. The oxidant concentration measuring device according to claim 4, wherein each electrode is formed of platinum, gold, silver or carbon. 6. The apparatus for measuring an oxidizing substance concentration according to claim 4, wherein an electrolytic electrode in which the surface area of each electrode is 10 times or more the apparent area is used. 7. The oxidant concentration measuring device according to claim 4, wherein each electrode is formed of carbon felt, carbon paper, or porous glassy carbon. Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for measuring the concentration of an oxidant present in a solution or in a gas, and a measuring device therefor. In particular, the present invention relates to a method for spot-concentrating a concentration in a trace amount of a sample simply, quickly and with high accuracy, a method for long-term monitoring, and a measuring device for these methods. In the present invention, the oxidizing substance means a substance capable of reacting with a solution of an alkali metal halide such as neutral potassium iodide to oxidize a halogen ion to release a halogen molecule, for example, a sulfur oxide. (SO _x ), nitrogen oxide (NO _x ), ozone (O ₃ ), and other substances having a strong oxidizing ability. 2. Description of the Related Art Conventionally, as a method for measuring the concentration of an oxidant substance in real time continuously or spot-wise in real time in a simple and highly accurate manner, for example, two electrolytic electrodes, a working electrode and a counter electrode, and an interelectrode electrode are used. There is a measurement method using an electrolysis method with an ion exchange membrane for separating water and an electrolytic solution containing a halogen ion, and in the case of an electrolysis cell with an electrolysis method using these two electrolytic electrodes, a working electrode is used. When reducing the halogen molecules generated by the reaction of the analyte with the halogen ions on the side, the halogen ions are oxidized on the counter electrode side to generate the halogen molecules. The halogen molecules generated on the opposite side are
When the electrolyte gradually accumulates in the electrolyte on the counter electrode side and its concentration rises, the rate of inhibition of halogen molecule migration by the ion exchange membrane decreases, and a slight amount of electrolytic reaction always occurs. There was a problem that the current gradually increased, and it was difficult to measure particularly low concentrations. Therefore, the electrolytic cell using these two electrolytic electrodes has a limit in use for the purpose of long-term monitoring of the concentration of the oxidizing substance in the field. SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide a method for measuring the concentration of an oxidizing substance in a solution or gas and a measuring apparatus therefor, which solves the above problems. [0006] That is, the present invention uses an electrolytic cell having at least three electrolytic electrodes of a working electrode, a counter electrode, and a regenerating electrode, and a sample is present and dissolved in a solution. If it is not a gas, inject the solution directly into the working electrode, and if the analyte is a gas dissolved in the solution, inject the solution directly into the working electrode or act through a breathable septum. When the sample is a gas that is present in the gas by introducing it into the electrode, by introducing it into the working electrode through a gas permeable membrane, the sample reacts with the halogen ion in the working electrode to form halogen molecules. It is a measuring method in which the concentration of an oxidizing substance in the sample is measured by the current or the total Coulomb number measured when the halogen molecules are produced and reduced. The present invention also includes at least a working electrode, a counter electrode,
A measuring device equipped with an electrolytic cell composed of three or more electrolytic electrodes of a regenerating electrode, an electrolytic solution containing halogen ions, an ion exchange membrane for separating each electrode, and a container for fixing these. And a measuring device equipped with a sensor in which a breathable diaphragm is added to the electrolytic cell. The electrolytic electrode used in the present invention is used by applying different applied voltages between the electrolytic electrodes, such as a working electrode and a counter electrode and a counter electrode and a reproducing electrode. Considering first the electrolytic reaction between the working electrode and the counter electrode, the working electrode converts the halogen molecule generated by the reaction between the oxidizing substance in the sample and the halogen ion contained in the electrolytic solution into the original halogen ion. And acts as an electrode that takes in electrons from the external circuit, and the counter electrode acts as an electrode that supplies electrons to the external circuit while oxidizing halogen ions contained in the electrolytic solution to generate halogen molecules. Next, considering the electrolytic reaction between the counter electrode and the regenerating electrode, at the counter electrode, the halogen molecules produced by the electrolytic reaction between the working electrode and the counter electrode are reduced to the original halogen ions, and electrons are emitted from the external circuit. It acts as an electrode for taking in, and at the regeneration electrode, it acts as an electrode that oxidizes halogen ions contained in the electrolytic solution to generate halogen molecules and also supplies electrons to an external circuit. Here, considering the working electrode, the counter electrode, and the regenerating electrode as a whole, the halogen molecules produced by the reaction between the oxidizing substance added from the outside and the halogen ion in the electrolytic solution are originally generated in the working electrode. When reducing to halogen ions,
When the electrolytic electrode had two electrodes, the working electrode and the counter electrode, halogen molecules were generated by oxidation of halogen ions at the counter electrode adjacent to the working electrode. Halogen molecules, which are supposed to be accumulated in the matching counter electrode, are accumulated in the electrolyte solution of the regeneration electrode and hardly accumulated in the electrolyte solution of the counter electrode due to the electrolytic reaction between the counter electrode and the regeneration electrode. Therefore, the halogen molecules in the electrolytic solution of the counter electrode are limited to the amount that has moved from the regeneration electrode through the ion exchange membrane, so that the concentration should be kept extremely low compared to the case where the regeneration electrode was not present. Is possible. Therefore,
Since the migration of halogen molecules from the counter electrode to the working electrode through the ion exchange membrane is practically negligible, current does not flow between the working electrode and the counter electrode unless an oxidizing substance is externally added to the working electrode. No longer flowing, no matter how long a continuous measurement is made and halogen molecules accumulate in the regenerating electrode, the base current of the sensor does not rise, which allows the sensor to have two electrolytic electrodes. You can prevent problems. Also, as these electrolytic electrodes, those having a large electrode area are used because it is necessary to rapidly react all the substances to be reacted at the electrodes, and the surface area thereof is usually 10 times or more of the apparent area, It is preferably 100 times or more. Examples of the electrode material that satisfies this condition include carbon felt, carbon paper, platinum black, and the like. Between the electrolytic electrodes, -50 mV to -1,
Although a DC voltage of about 000 mV is applied to each electrode reaction, it is not always necessary to apply the same applied voltage between the working electrode and the counter electrode and between the counter electrode and the reproducing electrode. Further, in determining the magnitude of the applied voltage, if an applied voltage more than necessary is applied, a reaction other than the intended electrolytic reaction occurs,
Not only will this cause a large error in the measured value, but reaction products other than the target reaction may interfere with the target electrolytic reaction, making it impossible to make measurements, so care must be taken in determining the applied voltage. . As the solute in the electrolytic solution,
For example, potassium iodide, sodium iodide, potassium bromide, alkali halides such as sodium bromide, preferably potassium iodide can be mentioned, and as the solvent, water, alcohol solvent, ester solvent, There is no particular limitation as long as it is a substance capable of dissolving the substance to be measured in the sample and the above-mentioned solute, such as an ether solvent, but if the solvent is highly volatile, the solvent volatilizes to cause electrolysis during measurement. Since the solute concentration of the liquid may change or the solute may be deposited to cause inconvenience to the measurement, a solvent having low volatility is preferable, and water or ethylene glycol can be exemplified. The solute concentration in this electrolyte is usually 0.1.
It is N or more, preferably around 1N. If it is lower than 0.1 N, it may not be possible to perform the measurement due to the limited current value. On the contrary, if the concentration is close to the saturation concentration, solute precipitation may occur, which is not preferable. Further, in the present invention, in order to prevent contact between adjacent electrodes and to prevent migration of halogen molecules to the adjacent electrodes while ensuring electrical conduction due to movement of ions, the distance between the electrodes is increased. It is necessary to provide a separating membrane, but as this diaphragm, it is preferable to use an ion exchange membrane because it prevents the permeation of low-molecular substances other than ions.
For example, Nafion (manufactured by DuPont, trade name), Flemion (manufactured by Asahi Glass Co., Ltd.), and the like can be mentioned. By using these ion exchange membranes, stable continuous measurement can be performed for a long term. When a breathable diaphragm is used in the present invention, for example, porous or microporous tetrafluoroethylene resin (PTFE), tetrafluoroethylene-hexafluoropropylene copolymer resin (FEP), Fluorine-based films such as fluorinated ethylene-perfluoroalkyl vinyl ether copolymer resin (PFA) and tetrafluoroethylene-ethylene copolymer resin (ETFE) can be mentioned. Regarding this breathable membrane, the velocity of the gas that permeates the breathable membrane is proportional to the gas concentration and the porosity of the membrane, inversely proportional to the film thickness, and further varies depending on the type of gas and the material of the film. So
Preferably, depending on the type and concentration of the gas to be measured,
It is good to properly select the material of the film, the film thickness, and the porosity of the film,
For example, when the gas having low permeability or the gas concentration is extremely low, a porous PTFE film having a film thickness of about 100 μm is preferable.
On the contrary, when the gas having high permeability or the gas concentration is extremely high, a microporous PTFE film or FEP film having a film thickness of about 25 μm is preferable. Further, as a measuring device for carrying out the method of the present invention, preferably, as its sensor part, an electrolyte solution containing the above-mentioned halogen ion, a container for containing the electrolyte solution, and at least three insides of the container. It is preferable to use an electrolytic cell composed of an ion exchange membrane that separates into the above chambers and a working electrode, a counter electrode, and a regenerating electrode that are located in the respective separated chambers. When used, the breathable membrane must be installed on the side wall of the container on the working electrode side, and the halogen molecules generated by the reaction between the gas that has permeated the breathable membrane and the halogen ions in the electrolyte are electrolyzed. In order to prevent diffusion into the liquid, it is desirable to place the working electrode in the immediate vicinity of the breathable diaphragm. The present invention will be specifically described below based on examples. FIG. 1 is a diagram showing a measuring mechanism of a measuring device according to the present invention when three electrodes, a working electrode 4, a counter electrode 5, and a regenerating electrode 6, are used and a breathable diaphragm 2 is used. When the sample gas 1 containing an oxidizing gas comes into contact with the gas permeable diaphragm 2, the oxidizing gas in the sample gas 1 passes through the gas permeable diaphragm 2 and the electrolytic solution of the working electrode 4 inside the sensor. It dissolves in 7 and reacts with a halogen ion contained in the electrolytic solution 7, for example, an iodine ion, and itself becomes its reduced body, and the halogen ion is oxidized into a halogen molecule. Since a predetermined voltage is applied between the working electrode 4 and the counter electrode 5, the halogen molecule thus generated is quickly reduced to the original halogen ion at the working electrode 4 serving as the cathode, and at the same time, the anode In the counter electrode 5 which becomes the above, the halogen ion in the electrolytic solution 7 is oxidized and halogen molecules are generated. At this time, a current corresponding to the amount of the electrode reaction flows between the working electrode 4 and the counter electrode 5. On the other hand, paying attention to between the counter electrode 5 and the reproducing electrode 6, a voltage is also applied to the counter electrode 5, so that in the counter electrode 5 which is a cathode between these two electrodes, the working electrode 4 and the counter electrode 5 described above are used.
The halogen molecules generated by the electrode reaction of are reduced,
It quickly returns to the original halogen ion, and the halogen ion in the electrolytic solution 7 is oxidized in the regeneration electrode 6 serving as the anode to generate a halogen molecule. At this time, the halogen molecule is generated between the counter electrode 5 and the regeneration electrode 6. An electric current flows according to the amount of electrode reaction. When the above electrode reactions occur, the current values flowing between the working electrode 4 and the counter electrode 5 and between the counter electrode 5 and the regeneration electrode 6 are equal and proportional to the concentration of the oxidizing gas contained in the sample gas 1. Because of the relationship, the concentration of the oxidizing gas dissolved in the electrolyte solution 7 from the sample gas 1 is the current value flowing between the working electrode 4 and the counter electrode 5, or the concentration between the counter electrode 5 and the regenerating electrode 6. The measured current value is measured by a meter 8 such as an ammeter, and the known concentration or the concentration measured by the conventional method using the same gas in advance and the current value measured by the method of the present invention are correlated to each other. By converting the concentration, it is possible to measure easily, quickly and with high accuracy. FIG. 2 is a schematic sectional view showing a measuring device for measuring the ozone concentration in the sample gas 1 as an example of the measuring device for carrying out the method of the present invention. The container 9 for containing the electrolytic solution 7 is made of a material resistant to ozone, for example, glass, polypropylene, polyethylene, fluororesin, or the like, and the electrolytic electrode (working electrode 4) installed in the container 9 is used. The counter electrode 5 and the reproducing electrode 6) are each formed of carbon felt, and preferably have a size that occupies most of the inside of the container 9. Further, in the opening formed on one side surface of the container 9 on the side of the working electrode 4, a microporous PTF as the breathable diaphragm 2 is provided.
An E membrane is attached, and the inside of the container 9 is separated by an ion exchange membrane 3 (Nafion membrane) into three chambers, a working electrode chamber, a counter electrode chamber, and a regeneration electrode chamber. As the electrolytic solution 7, a 1N potassium iodide aqueous solution (pH 9) is put in all three electrodes. The container 9 is installed so that the breathable diaphragm 2 comes into contact with the sample gas 1 containing the oxidizing gas. In that case, if there is a flow of gas, it is installed so that the flow is not disturbed as much as possible. Next, between the working electrode 4 and the counter electrode 5, the working electrode 4 becomes the cathode and the counter electrode 5 becomes the anode, and between the counter electrode 5 and the reproducing electrode 6, the counter electrode 5 becomes the cathode. The working electrode 4 and the counter electrode 5 and the counter electrode 5 and the reproducing electrode 6 are connected by a lead wire 10 so that the reproducing electrode 6 serves as an anode, and the working electrode 4 and the counter electrode 5 and the counter electrode 5 are connected. An instrument 8 for measuring a voltage and a current is provided between the regeneration electrode 6, the current flowing between the working electrode 4 and the counter electrode 5 or between the counter electrode 5 and the regeneration electrode 6 is measured, and the measured current value is ozone. It was converted into the concentration and displayed. FIG. 3 shows between the working electrode 4 and the counter electrode 5 and the counter electrode 5.
The above-mentioned ozone concentration measuring device in which a voltage of -200 mV is applied between the regenerating electrode 6 and the regenerating electrode 6 and the ozone concentration measuring device using a two-electrode type conventional electrolytic cell of the working electrode 4 and the counter electrode 5 are used for comparison. FIG. 3 is a diagram showing changes in measured current values of both measuring devices when a gas containing a constant concentration of ozone is used as the sample 1 and the current values measured by both measuring devices are continuously monitored. From this figure, in the case of the conventional measuring device using the two-electrode type electrolysis cell, the measured current value gradually increases during the long-term measurement. In the case of a measuring device using a three-electrode type electrolysis cell, which is an example of the device, it can be seen that the measured current value hardly changes. After one month, a gas containing no ozone was used as the sample 1, and the base current value of each ozone concentration measuring device was confirmed. The base current value of the conventional measuring device was found to be lower than the initial current value. Although the base current value of the measuring device according to the present invention has returned to almost the original current value, the measuring device using the three-electrode type electrolytic cell according to the present invention is Sample 1 containing
It can be seen that even when the measurement is continuously performed for a long time, the base current value does not change and the measurement can be performed accurately. Further, using the measuring apparatus of the present invention, the sample 1
The following shows the results of measurement by changing the ozone concentration in the sample and using the same sample 1 as a comparative example by the ultraviolet absorption method shown in JIS-B-7957. O ₃ concentration measurement value (ppb) Measurement No. Example Comparative Example 1 4.2 Not measurable 2 67.5 66.4 3 513.7 515.0 From these results, the method of the present invention is highly accurate and has extremely low concentration of ozone even when compared with the ultraviolet absorption method. It was found that it is possible to measure up to the concentration.