JP3025076B2 - Acid gas measuring device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a technique for electrochemically detecting an acidic gas present in a gas.

[0002]

2. Description of the Related Art An electrochemical gas detector is provided in a container containing an electrolyte of a strong acid substance which generates a large amount of ions such as sulfuric acid.
A window is formed, the window is sealed with a gas-permeable membrane such as porous Teflon, and a working electrode made of platinum black is formed on the electrolyte side of the window. It is configured to always apply a potential lower than the potential at which water is electrolytically reduced, for example, about 0.5 to 1 volt (based on the hydrogen electrode). According to this configuration, gases such as carbon monoxide and hydrogen sulfide (H ₂ S) that are oxidized or reduced at a potential at which the electrolytic solution does not undergo electrolysis are
Although it can be detected, the potential required for oxidation and reduction, such as hydrogen fluoride (HF) or nitric acid (HNO ₃ ), is higher on the noble side than the potential at which water electrolysis starts. It is not possible to use an acidic gas that requires a potential, for example, a potential of about 1.2 volts, because the electrolysis current is superimposed on the current generated in proportion to the test gas concentration. Or the measurement error becomes extremely large.

In order to solve such a problem, when an acidic gas such as hydrogen fluoride (HF) and nitric acid (HNO ₃ ) is electrochemically detected, iodine ions (I
^-) and iodate (IO ₃ ^- using an aqueous solution containing a)
The hydrogen ions and the electrolytic solution caused by the acidic gas dissolved in the electrolytic solution through the gas permeable membrane are converted from the potential at which the electrolysis of water starts by the reaction of 6H ⁺ + 5I ⁻ + IO ₃ ⁻ → 3I ₂ + 3H ₂ O. Also, a gas detector is used which releases iodine (I ₂ ) which can be reduced at a base potential and detects a current value required for the reduction of iodine.

[0004]

Gas detectors utilizing such an iodine reaction are known as hydrogen fluoride (HF), nitric acid (H
Although it exhibits high sensitivity to strong acidic gases such as NO ₃ ), it reacts hydrogen ions generated by the test gas with the electrolyte to generate iodine, and the current required to reduce the iodine Because of the use of a two-step reaction process in which the measured value is used as a measured value, it is necessary to wait for sufficient diffusion of the generated hydrogen ions. Therefore, the response speed is low, and the hydrogen ions are neutralized in the process until iodine is generated. Therefore, there is a problem that the detection sensitivity to a weakly acidic gas such as carbon dioxide (CO ₂ ) is low. The present invention has been made in view of such a problem, and its purpose is to reduce or oxidize an acid gas to be detected at a working electrode. Your side,
Another object of the present invention is to provide a novel electrochemical gas detector capable of detecting an acidic gas requiring a base-shifted potential with a high response speed and a high sensitivity.

[0005]

According to the present invention, a window is provided in a container for accommodating an electrolytic solution having a hydrogen ion concentration of about -10 to -7 to about -7 mol / L. The liquid electrode is sealed with a gas permeable membrane, a metal electrode is provided on the electrolyte side of the gas permeable membrane to form a working electrode, and a counter electrode is immersed in the electrolyte to form the working electrode. The reduction current is detected while alternately applying a potential for oxidizing the metal and a potential for reducing the oxidized metal.

[0006]

When an oxidation potential is applied to a metal electrode constituting a working electrode, the working electrode is electrochemically oxidized to a metal oxide. Then, when a potential at which the metal oxide is reduced to the metal is applied, the metal gas returns to the metal when the concentration of the acidic gas that has permeated the diaphragm and entered the interface between the working electrode and the electrolyte has increased. PtO + 2H ⁺ + 2e ⁻ → Since the reaction of Pt + H ₂ O occurs and a reduction current flows in proportion to the concentration of the acidic gas, the concentration of the acidic gas can be known by measuring this current.

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The details of the present invention will be described below with reference to the illustrated embodiments. FIG. 1 shows an embodiment of the present invention. In the drawing, reference numeral 1 denotes a cell in which an opening is formed in an opposing wall surface to form a window 2, and the window 2 is formed by a diaphragm 4 described later. It is configured so as to house the electrolytic solution 6 in a sealed state. 4
Is a diaphragm provided in the window 2 in a liquid-tight manner. As shown in FIG. 2, a gas-permeable membrane 7 such as a porous fluororesin is used as a base, and a metal such as platinum is deposited on the side in contact with the electrolyte 6. The metal layer 9 is formed by a technique such as sputtering, baking, or the like. The metal layer 9 is made of platinum (Pt), gold (Au), iridium (Ir), rhodium (Rh), ruthenium (Ru),
Osmium (Os), palladium (Pd), titanium (T
One of those in which the hydrogen ion concentration-potential curve changes monotonously as in i) and the like is used.

The electrolytic solution 6 is composed of an aqueous solution of 0.1 mmol of sodium sulfate or the like, and has a hydrogen ion concentration of about -10 to -10 to about -7 mol / L, which is almost in a neutral region. ing.

Reference numeral 15 denotes a measuring circuit which applies an oxidation potential of 0.9 to 1.2 volts for a predetermined time, for example, 3 minutes, between the metal layer 9 serving as a working electrode and the reference electrode 11 as shown in FIG. Then, the potential is reduced to a potential of about 0.4 volts at which platinum oxide is reduced, and a reduction current flowing between the metal layer 9 serving as a working electrode and the counter electrode 5 while the reduction potential is applied is reduced. It is configured to measure. In addition, the code | symbol 12 in a figure shows a diaphragm mounting frame.

In this embodiment, an oxidation potential is applied to the metal layer 9 and the reference electrode 11 for a predetermined time so that the platinum metal layer 9 constituting the working electrode is oxidized in the first step T1.
In this embodiment, when the voltage is applied for 3 minutes, the metal layer 9
Is oxidized under the action of potential and platinum oxide (P
tO) is formed. In this step, electrolysis of the electrolytic solution occurs, but since there is no need to measure the current value at this time, there is no influence on the measurement of the gas concentration.

Next, in the second step T2, when the potential between the metal layer 9 serving as the working electrode and the reference electrode 11 is reduced to the reduction potential, the acidic gas that has passed through the diaphragm 4 is removed from the metal layer 9 and the electrolyte 6 by the acidic gas. Hydrogen ions are generated at the interface of the metal layer 9, and the hydrogen ions reduce the platinum oxide formed on the surface of the metal layer 9 by a reaction of PtO + 2H ⁺ + 2e ⁻ → Pt + H ₂ O. 5 will generate a current. Needless to say, in this reduction step, the potential applied to the metal layer 9 serving as the working electrode is 0.5 volt, which is lower than the potential 1.2 volt at which electrolytic oxidation of water starts, and the electrolytic reduction of water is performed. Is smaller than -1.2 volts, and the magnitude of the current is proportional to the concentration of hydrogen ions, and the hydrogen ions are proportional to the concentration of the acidic gas permeating the diaphragm 7, so that the reduction current By measuring, the concentration of the acid gas can be accurately known. Since this reaction occurs on the surface of the metal layer 9 constituting the working electrode, the hydrogen ions generated by the test gas do not diffuse into the electrolytic solution 6, and therefore the number of hydrogen ions by the test gas that has entered Is effectively reflected on the value of the measured current. Further, since the measurement is performed while consuming the hydrogen ions generated by the test gas, the test gas does not remain in the electrolytic solution, and exhibits extremely high response characteristics. Hereinafter, the concentration of the acid gas can be continuously measured by repeating the first step T1 and the second step T2 alternately.

FIG. 4 shows the oxidation potential V using the apparatus described above.
₂ at 1.2 volts for 3 minutes, and the reduction potential V ₁
Is set to 0.4 volts, and shows a current change when carbon dioxide is used as a sample. In the figure, I represents the case where carbon dioxide does not exist, and II represents the concentration of carbon dioxide of 1000.
ppm, III is 2000 ppm, I
V indicates 3000 ppm, and as is apparent from the figure, the magnitudes of the reduction currents I ₁ , I ₂ , I ₃ , and I ₄ at the reduction voltage of 0.4 V are proportional to the concentration of carbon dioxide. I have. According to this embodiment, the measurement operation is started after a certain amount of platinum oxide film is always formed on the metal layer 9 serving as the working electrode. Therefore, the gas concentration can be measured with platinum oxide having a constant activity, and the reliability is high. Measurement results can be obtained.

FIG. 5 shows another embodiment of the present invention. In the drawing, reference numeral 21 denotes a cell having a window 22 formed on a wall surface, and the window 22 is sealed with a diaphragm 24 described later. 0.
When the concentration of hydrogen ions such as 1 mmol sodium sulfate is 10
It is configured to contain an electrolyte which is in a substantially neutral region of about -10 to -7 mol / L. Numeral 24 denotes a liquid membrane provided in the window 22 in a liquid-tight manner. As shown in FIG.
A metal oxide such as platinum oxide on the side in contact with the electrolytic solution by vapor deposition, sputtering, or further applying a platinum oxide powder with a binder, followed by baking. The material layer 29 is formed. Reference numeral 35 denotes a measurement circuit, which includes a metal oxide layer 29 serving as a working electrode and a reference electrode 3
The reduction current generated between the metal oxide layer 29 and the counter electrode 30 is measured while applying a voltage capable of reducing the metal oxide in the presence of about 0.2 to 0.5 volts of hydrogen ions between the metal oxide layer 29 and the counter electrode 30. It is configured to be able to.

In this embodiment, a metal oxide serving as a working electrode between the metal oxide layer 29 and the reference electrode 31, in this embodiment, an acidic potential is applied to the diaphragm 24 while applying a potential for reducing platinum oxide. When the gas is supplied, the acidic gas permeates the gas permeable membrane 27 and generates hydrogen ions at the interface between the metal oxide layer 29 serving as a working electrode and the electrolyte 26. The hydrogen ions act on the metal oxide layer 29 constituting the working electrode, and reduce the metal oxide layer 29 to metal by the reaction of PtO + 2H ⁺ + 2e ⁻ → Pt + H ₂ O. During this time, a current proportional to the concentration of the acid gas is generated. Since this reaction occurs on the surface of the metal oxide layer 29 constituting the working electrode, hydrogen ions generated by the test gas do not diffuse into the electrolytic solution 26, and therefore, hydrogen ions generated by the penetrated test gas do not. The number of ions will be effectively reflected in the value of the measured current.

Needless to say, the potential applied to the metal oxide layer 29 serving as the working electrode is 0.5 volt, which is more negative than the 1.2 volt potential at which electrolysis of water starts, and the generated hydrogen Since the number of ions is proportional to the concentration of the acidic gas, the concentration of the acidic gas is not affected by the electrolysis of water. Therefore, the concentration of the acidic gas can be known by measuring the magnitude of the reduction current. Further, since the measurement is performed while consuming the hydrogen ions generated by the test gas, the test gas does not remain in the electrolytic solution, and exhibits extremely high response characteristics. At this time, the metal oxide constituting the working electrode is reduced to metal, but since the metal oxide is present in a large amount, it is possible to sufficiently cope with long-term measurement, Further, at the time of rest, by applying an oxidation potential in the same manner as in the above-described embodiment, the reduced metal is re-oxidized, thereby enabling semi-permanent use. According to this embodiment, since it is not necessary to apply an oxidation potential, not only can the acid gas be continuously measured, but also the measurement circuit can be simplified.

In the above-described embodiment, the case where the working electrode is made of platinum oxide has been described. However, ruthenium (R) which changes almost linearly with the hydrogen ion concentration is described.
u), oxidation of metal such as gold (Au), osmium (Os), iridium (Ir), palladium (Pd), rhodium (Rh), titanium (Ti), etc., whose hydrogen ion concentration-potential curve changes monotonously It was confirmed that the same action was exerted even when using the product.

Further, in the above-described embodiment, the description has been given by taking an example of a three-pole type having a reference pole. Has a similar effect. Further, in the above-described embodiment, the metal layer and the metal oxide constituting the working electrode are formed integrally with the gas permeable membrane, but the window 40 of the container is sealed as shown in FIG. The same effect can be obtained even if the metal 42 and the metal oxide 43 which are working electrodes facing the gas permeable membrane 41 are arranged with a gap through which the electrolyte can penetrate. it is obvious. According to this embodiment, since the gas permeable membrane and the working electrode can be configured separately, the step of forming a metal layer or a metal oxide layer on the gas permeable membrane becomes unnecessary, and the manufacturing process is simplified. Can be achieved.

[0018]

As described above, in the present invention,
A window is provided in a container containing an electrolyte having a hydrogen ion concentration of about -10 to -7 to the seventh power mol / liter, and a window is provided and sealed with a gas permeable membrane, and a metal electrode is arranged on the electrolyte side of the gas permeable membrane. As the working electrode is formed, the counter electrode is immersed in the electrolytic solution, and the reduction current is detected while applying a potential for reducing the metal oxide of the working electrode. Can reduce metal oxides in the presence of hydrogen ions generated due to water, and apply an electric potential smaller than the absolute value of the electrolysis onset potential of water. And the direct reaction on the working electrode is used, so that there is no reduction in sensitivity due to the diffusion of the test gas into the electrolytic solution. Small degree Carbon dioxide not only can be detected with high sensitivity also can be simplified handling because it does not require a strong acid to the electrolytic solution.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of an apparatus showing one embodiment of the present invention.

FIG. 2 is a sectional view showing a structure of a diaphragm in the device.

FIG. 3 is a diagram showing changes in potential applied to a working electrode and a counter electrode of the above device.

FIG. 4 is a diagram showing a result of measuring an acid gas by the same apparatus.

FIG. 5 is a configuration diagram showing another embodiment of the present invention.

FIG. 6 is a cross-sectional view showing a structure of a diaphragm in the device.

FIG. 7 is a configuration diagram showing another embodiment of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Cell 2 Window 4 Diaphragm 5 Counter electrode 6 Electrolyte 7 Gas permeable membrane 9 Metal layer used as working electrode 11 Reference electrode

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) G01N 27/416 G01N 27/404

Claims (4)

(57) [Claims] 1. A container for containing an electrolyte having a hydrogen ion concentration of about -10 to -7 to about -7 mol / l is provided with a window and sealed with a gas-permeable membrane, and the electrolyte of the gas-permeable membrane is provided. A metal electrode is provided on the side to form a working electrode, and a counter electrode is immersed in an electrolytic solution, and a potential for oxidizing a metal constituting the working electrode and a potential for reducing the oxidized metal are alternately applied to the working electrode. Acid gas measuring device that detects reduction current while applying. 2. The metal layer constituting the working electrode is made of platinum (Pt), ruthenium (Ru), gold (Au), osmium (Os), iridium (Ir), palladium (P
d), one of rhodium (Rh) and titanium (Ti)
The acid gas measuring device according to claim 1, wherein: 3. A container containing an electrolytic solution having a hydrogen ion concentration of about -10 to -7 to about -7 mol / L is provided with a window, and is sealed with a gas-permeable membrane. A metal oxide electrode is provided on the side to form a working electrode, a counter electrode is immersed in the electrolytic solution, and a reduction current is detected while applying a potential to reduce the metal oxide constituting the working electrode to the working electrode. Acid gas measuring device. 4. The metal oxide layer constituting the working electrode,
Platinum (Pt), ruthenium (Ru), gold (Au), osmium (Os), iridium (Ir), palladium (P
4. The acidic gas measuring device according to claim 3, wherein the acidic gas measuring device is one of oxides of d), rhodium (Rh), and titanium (Ti).