JPH0327862B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a gas sensor for sulfur dioxide, and particularly to a gas sensor for sulfur dioxide that is simple, suitable for continuous measurement, and has high utility value in industrial applications.

[Prior art] Measuring the concentration of sulfur dioxide (hereinafter referred to as SO ₂ ) gas is an important technology for preventing air pollution caused by SO ₂ gas generated from non-ferrous metal smelting in the dry processing of sulfide ores, automobile exhaust, etc. .

Conventional methods for measuring the concentration of SO ₂ gas include collecting SO ₂ in the sample gas in an absorption liquid and performing chemical analysis (hydrogen peroxide method, Torine method, etc.), and quantitative determination using spectrophotometry. methods (pararosanine method, iodine starch method), methods to measure the electrical conductivity of the absorption liquid, and methods to introduce the test gas itself into an infrared spectrometer.
There are methods for quantifying SO ₂ .

However, these methods are difficult to operate, difficult to perform continuous measurements, complex equipment, responsiveness, etc.
When these measurement methods are adopted, this is one of the factors that reduces the overall efficiency. In addition, in recent years, with the automation of various tasks using microcomputers, the measurement methods and measuring devices that have the above-mentioned drawbacks are difficult to apply as sensors for microcomputers, and SO ₂ gas concentration measurement has become necessary for work. It has also become an obstacle to automation in industry.

Conventionally, one method to solve this problem is to use solid electrolyte pellets such as Na ₂ SO ₄ or K ₂ SO ₄ , contact one side with SO ₂ gas at a standard concentration as a reference electrode, and The SO ₂ gas to be measured is brought into contact with the surface of the SO 2 gas as a measurement electrode, and _the electromotive force generated on both sides due to the concentration difference is measured.
Methods have been proposed for determining the concentration of gas.

However, this method requires constantly supplying standard SO ₂ gas to the reference electrode with its concentration precisely adjusted, and requires large-scale equipment to measure the SO ₂ gas concentration. However, the standard SO ₂ gas concentration had to be checked periodically using other measurement methods.

[Objective of the Invention] The present invention provides an SO ₂ gas sensor that is easy to operate, allows continuous measurement, is compact, has good accuracy and responsiveness, and is suitable as a sensor for various automatic controls when measuring SO ₂ gas concentration. The purpose is to

[Structure of the Invention] The gist of the present invention is to provide a solid for sulfur dioxide that measures the sulfur dioxide gas concentration of a measurement electrode by measuring the electromotive force between a reference electrode and a measurement electrode provided on a solid electrolyte. In the electrolyte gas sensor, the solid electrolyte gas sensor for sulfur dioxide is characterized in that the solid electrolyte contains a sodium component and the reference electrode is sodium or a sodium alloy.

Here, it is desirable to use a sodium alloy instead of using sodium alone. That is,
In the case of pure sodium, it must be sealed because it easily evaporates at high temperatures. On the other hand, if an intermediate compound is created from sodium and another metal with strong chemical bonding strength, the activity of sodium can be reduced and evaporation can be prevented. In addition, if the intermediate compound and the metal coexist, the activity of sodium will be less likely to change compared to the case of a single phase of the intermediate compound, and it can be used as a reference electrode that shows a constant sodium activity. This is more preferable than the case of a phase.

As the alloy, NaAu ₂ is preferable in terms of physical properties, and a two-phase alloy of NaAu ₂ and Au is particularly preferable in terms of stability of sodium activity and composition at high temperatures.

As a solid electrolyte, Na ₂ O: Al ₂ O ₃ is 1:9 ~
β-Alumina, Na ₂ O with a composition of 1:11:
A solid electrolyte containing sodium such as β''-alumina and Na ₂ SO ₄ having a composition of Al ₂ O 3 of ₁ :5 to 1:7 is used.

Among the solid electrolytes mentioned above, it is preferable to use β-alumina and β″-alumina because of their high mechanical strength, good electrical conductivity, and good formability. It is the most preferred solid electrolyte because it has particularly excellent ⁺ conductivity.

For the contact portion between the solid electrolyte and the sodium or sodium alloy, it is sufficient to simply bring the two into contact; however, the sodium or sodium alloy may be heated and melted to bring it into close contact with the solid electrolyte;
Sodium or a sodium alloy may be powdered and brought into contact with the solid electrolyte to increase the contact area.

An example of the structure of the above sensor is shown in FIG. Here, 1 is a solid electrolyte, and 2 is sodium or a sodium alloy. This solid electrolyte 1 and sodium or sodium alloy 2 are in contact with each other on one side to form an interface 3.

The other side 1a of the solid electrolyte 1 that has been in contact with the Pt electrode is placed in an oxygen-containing atmosphere.
An electromotive force is generated in the solid electrolyte by exposure to SO ₂ gas, and if the electromotive force is measured by providing a voltmeter 4 between the surface 1a of the solid electrolyte 1 and the interface 3 or the sodium or sodium alloy 2, From this value, the concentration of SO ₂ gas in contact with surface 1a can be determined.

On the other hand, when used at a high temperature where sodium or a sodium alloy evaporates, a structure as shown in FIG. 2 may be used to prevent the sodium or sodium alloy from disappearing due to evaporation. here 5
is a solid electrolyte, especially a solid electrolyte with high mechanical strength such as β-alumina or β''-alumina, and 6 is sodium or a sodium alloy.This solid electrolyte 5
contains sodium or sodium alloy 6 as a container, and is sealed with an electrically insulating lid 7. This prevents the sodium or sodium alloy 6 from evaporating. An electrode 8 that is electrically connected to the sodium or sodium alloy 6 is provided through the lid 7 .

When the outer surface of the solid electrolyte 5 of a sensor of this type is exposed to SO ₂ gas in an atmosphere containing oxygen, an electromotive force is generated in the solid electrolyte 5, and conductivity between the outer surface and sodium or sodium alloy 6 is generated. By providing a voltmeter 4 between the electrode 8 and measuring the electromotive force, the concentration of SO ₂ gas can be determined. As an example of manufacturing the above-mentioned sensor, the electrode 8 is first made into a hollow tube shape, and sodium or sodium alloy 6 is injected from the electrode 8 in a molten state or powder state into a storage container consisting of a solid electrolyte 5 and a lid 7, and then the electrode 8 It may also be formed by sealing the hollow part of by brazing or the like.

The SO ₂ gas concentration measurement using the electromotive force of this sensor is based on the principle described below.

The electromotive force of the sensor of the present invention is that SO ₂ gas is SO ₂ +
This is because the reaction of 1/2O ₂ =SO ₃ occurs, forming a battery with the following configuration. Pt, SO ₂ +
O ₂ +SO ₃ /Solid electrolyte/Sodium or sodium alloy, Pt In this case, since the diffusion species are Na ions, the generated electromotive force E is given by the following equation (1).

E=RT/Flna ₂ /a ₁ ...(1) Here, a ₁ is near the left pole surface of the solid electrolyte.
The activity of Na, _a2 , is near the right extreme surface of the solid electrolyte.
Represents the activity of Na.

The above a ₂ is in sodium or sodium alloy.
Directly determined by Na activity.

In sodium or a sodium alloy, if the temperature is constant, the activity of sodium is constant, so the activity a ₂ of Na at the right pole is constant. Therefore,
The activity a ₁ of Na on the left pole surface is uniquely determined according to the electromotive force E. Moreover, the reaction at the left pole is 2Na + SO ₃ +
Since the concentration of SO ₃ from 1/2O ₂ = Na ₂ SO ₄ is determined by the above a ₁ , the reaction formula is SO ₂ + 1/2O ₂ = SO ₃ based on the electromotive force E and the oxygen concentration in the air. The concentration of SO ₂ is determined by the equilibrium constant K=Pso ₃ /Pso ₂・(Po ₂ ) ^1/2 . That is, the concentration of O ₂ is the concentration of O ₂ in the air,
Since it is considered to be approximately constant, the concentration of SO ₂ can be uniquely determined by the value of the electromotive force E.

[Effects of the Invention] As detailed above, the solid electrolyte gas sensor for sulfur dioxide of the present invention has the following features: by measuring the electromotive force between the reference electrode and the measurement electrode provided on the solid electrolyte, the sulfur dioxide gas at the measurement electrode is reduced. In a solid electrolyte gas sensor for sulfur dioxide that measures concentration, the solid electrolyte contains a sodium component and the reference electrode is sodium or sodium alloy, making it compact and easy to operate since SO ₂ gas is not used for the reference electrode. Moreover, the sensor can be continuously measured, has good accuracy and responsiveness, and is suitable as a sensor for various automatic controls.

[Example] Next, an example of manufacturing a sodium alloy Au ₂ Na of gold and sodium used in an example will be shown.

When producing Au ₂ Na, granular Au (Mitsubishi Metals
Co., Ltd.) and 98% rod-shaped Na (Wako Pure Chemical Industries, Ltd.) were used.

25 g of Au and 1.25 g of Na were placed in a covered carbon crucible with an inner diameter of 13 mm and a depth of 45 mm so that Au was 70 atomic% and Na was 30 atomic%.The crucible was placed in a silica tube and maintained at 500°C for 48 hours in an argon atmosphere. .

When I took it out after cooling it in the furnace, I found that it had turned a shiny ocher color overall. This is powdered in an agate mortar and examined using an X-ray diffractometer.
The presence of Au ₂ Na was confirmed.

As can be seen from the phase diagram of the Au-Na system shown in Figure 3, the coexistence region of Au and Au ₂ Na is wide;
The coexistence region S can be obtained even if Au ₂ Na contains only a very small amount of Au. It also has a relatively high melting point of 876°C or higher.

Next, specific examples will be shown.

Produced in the above sodium alloy production example,
A sensor as shown in FIG. 4 was fabricated using an alloy containing a trace amount of Au phase in the Au ₂ Na phase and β''-alumina as the solid electrolyte.

Here, 11 has an outer diameter of 15 mm, an inner diameter of 13 mm, and a length of 122 mm.
This β''-alumina tube is coated with a thin layer of platinum paste up to 7 mm from the bottom of its external bottom and outer surface.This β''-alumina tube 11
A high-purity alumina tube 13 was adhered to the open end of the tube using alumina cement 12, and the tube was dried at 1000° C. for about 2 hours.

This β″-alumina tube 11 contains Au ₂ Na.
A sodium alloy 14 consisting of a small amount of gold and a small amount of Au was placed in the container, and an alumina lid 15 was placed on top. In the center of this lid 15 is a platinum lead wire 16 on the reference electrode side.
A hole 15a is bored through the hole 15a.

The lid 15 is pressed down by the biasing force of a spring 18 via an alumina tube 17 which also serves as an argon gas introduction tube, and brings the sodium alloy 14 into close contact with the β''-alumina tube 11.
Further, the alumina tube 17 has an argon gas escape hole 17a near the lid 15.

The platinum lead wire 19 on the sample pole side is pressed against the outer bottom surface of the tube 11 by another spirally wound platinum wire 20, and this platinum wire 20 is further pressed against the tube 11 by an alumina tube 22 biased by a spring 21.
being pushed in the direction.

Further, the entire structure described above is housed in an alumina tube 23, and at its upper end, a glass cap is provided with an outlet 24a for the SO ₂ gas to be measured and an insertion port 24b for the platinum lead wire 19, and one end of the spring 18 is engaged. The stopped alumina ring 25, the argon gas inlet 26a and its outlet 26b, and the platinum lead wire 1
Glass cap 26 with 6 insertion openings 26c
is unit tube (joint material manufactured by Nippon Rikagakiki Co., Ltd.)
are sequentially connected by.

On the other hand, a rubber cap 28 is fitted into the lower end of the alumina tube 23 to lock one end of the spring 21 and allow an alumina tube 29 to pass through its center hole. A glass cap 31 having an insertion port 31a for a thermocouple 30 and an inlet 31b for SO ₂ gas to be measured is connected to the unit tube 27 in the alumina tube 29.
connected by. The thermocouple 30 has its tip disposed approximately at the outer bottom surface of the β''-alumina tube 11, and is configured to accurately capture the temperature of the alumina tube 11 as a solid electrolyte.
In the structure as described above, in order to maintain the 11 parts of the β''-alumina tube at a constant temperature,
The nichrome wire is inserted into the center hole of the resistance furnace 32.

The resistance furnace 32 is maintained within approximately ±1° C. of a predetermined temperature based on the temperature detected by the thermocouple 30.

[Measurement experiment] Next, using the sensor configured as above,
An experiment was conducted to measure the concentration of SO ₂ gas.

The inside of the system shown in Figure 4 is vacuumed to 1 x 10 ^-3 Torr using a rotary pump. In this state, open the furnace.
Heat to 100℃ to evaporate water etc. After this, the rotary pump is stopped and argon gas is gradually introduced. When the argon gas is completely replaced, the circuit on the sample electrode side is closed to create an argon gas atmosphere of 1 atm. On the reference electrode side, argon gas is passed and bubbled by a bubbler to increase the pressure. The furnace is heated in this state, and after the temperature has stabilized at the specified temperature, SO ₂
Gas flows in.

The electromotive force was measured by changing the temperature from 598°C to 835°C and the SO ₂ Ag concentration from 1.6ppm to 9020p.pm.

The gas used was an SO ₂ gas cylinder diluted with air. Concentrations are 542p.pm・, 1054p.pm・, 4940p.p.
m., 9020p.pm. In the low-concentration experiment, an SO ₂ gas cylinder diluted with air and a permeation tube (Gastetsu Co., Ltd.) were used, and an air cylinder (Ao Sangyo Co., Ltd.) was used as the diluent gas. SO ₂
Gas cylinder has SO ₂ concentration of 9020p.pm and 8940p.pm
For 8940p.pm cylinders, use a gas divider (SGD75- manufactured by Standard Technology Co., Ltd.).
C) to increase the concentration to 1.60ppm, 11.0ppm,
It was changed to 4 levels: 7.75ppm and 50.0ppm.

A permeation tube is a fluororesin tube of a certain quality that is filled with liquefied gas.
By keeping this in a constant temperature bath, the amount of liquefied gas permeating and diffusing inside the tube becomes constant.
If a constant flow of diluent gas is sent there, a gas of any concentration can be obtained continuously.

To raise the temperature of the furnace, an argon atmosphere was created inside the circuit. In addition, argon gas was kept flowing on the reference electrode side during the experiment. Argon gas is obtained by dehydrating gas obtained from a cylinder through silica gel, activated alumina, and a 3 ^1/2 effective pore size molecular sieve.
Furthermore, it was deoxidized through a titanium furnace and used.

[Experimental Results] Figures 5 to 10 show the results obtained in experiments using a sodium alloy reference electrode.

Figure 5 shows temperature on the horizontal axis and electromotive force EMF on the vertical axis.
This is a diagram plotting the results for each SO ₂ concentration in the gas used. SO ₂ concentration from 1.60ppm to 9020p.p.
Experiments were conducted using eight types of gases up to m.
The gas flow rate was 100 ml/min in both cases.

Since the amount of SO ₃ produced is a function of temperature, the plot at each SO ₂ concentration does not show a linear relationship. However, at the same temperature, the higher the SO ₂ concentration, the higher the electromotive force, indicating that the concentration can be determined from temperature and electromotive force.

The stability of the equilibrium electromotive force is best around 930K,
Within ±1 mV, the worst value was ±4 mV at 1108K. The same thing happened when the concentration of SO ₂ gas was changed.

Also, to check the reproducibility, the SO ₂ concentration was 542 p.pm.
3 times, 50.0ppm, 11.0ppm,
For each 1.60 ppm gas, the electromotive force was measured twice. The results are plotted in Figure 6.
The Au and Au ₂ Na alloys used for each measurement were 1
The first measurement and the second measurement were created at the same time, and the third measurement was created separately. Very good reproducibility was obtained above 930K.

The responsiveness of the electromotive force to changes in SO ₂ concentration and temperature was very good. Figure 7,
The results are plotted in Figure 8. FIG. 7 is a diagram showing the responsiveness of electromotive force to changes in SO ₂ concentration, and shows the results at 930K. The horizontal axis is taken in units of time, and the interval between each measurement point is 5 minutes. The time required for the electromotive force to reach equilibrium after changing the SO ₂ concentration is considered to be the time required for the gas in the reaction tube to be completely replaced.
When the concentration is changed, it is estimated that the electromotive force responds instantaneously to the change in concentration.

Moreover, FIG. 8 is a diagram showing the responsiveness of electromotive force to changes in temperature, and shows the results when the SO ₂ concentration is 11.0 ppm. The interval between each measurement point is 10 minutes. It can be seen that the responsiveness of the electromotive force to temperature changes is also very good.

Each of these responsivity is compared to conventional Na ₂ SO ₄ or K ₂ SO ₄
The performance was equivalent to or better than that of a measuring device using solid electrolyte pellets and a standard gas as a reference electrode.

Figure 9 shows the
The logarithm of Na activity a _Na(Na2SO4) in Na ₂ SO ₄ is plotted on the horizontal axis, and the obtained electromotive force values are plotted at each temperature to show the results.

On the gas electrode side, Na ₂ SO ₄ is generated by the reaction of the following equation (2).
is generated.

2Na+SO ₃ +1/2O ₂ =Na ₂ SO ₄ ...(2) The equilibrium constant of (2) is given by equation (3).

K(2)=a _Na2SO4 / (a _Na ) ²・Pso ₃・(Po ₂ ) ^1/2 ……(3) Therefore, from the free energy of formation in equation (2)
a _Na(Na2SO4) is determined.

Further, the electromotive force of the solid electrolyte is expressed by the following equation (4).

E=RT/Flna _Na (alloy)/a _Na(Na2SO4) ...(4) Considering that the temperature is constant, Au, Au ₂ Na in
Since the activity of Na is constant, (4) can be expressed as the following equation (5).

E=-C ₁ lna _Na(Na2SO4) +C ₂ ...(5) However, C ₁ = RT/F (constant) C ₂ = RT/Flna _{Na (} alloy ₎ From formula (5), lna _Na(Na2SO4) and If you plot the relationship between electromotive force, you should get a linear relationship.

The plot in FIG. 9 shows a good straight line, indicating that the obtained electromotive force value is a reasonable value.

Based on the electromotive force values obtained in this experiment, the activities of Na in Au and Au ₂ Na alloys were calculated at each temperature. FIG. 10 shows a plot of the logarithm value against the reciprocal of temperature.

The activity of Na in the alloy is expressed by formula (4), the measured value of the electromotive force, and
Substituting the calculated value of a _Na(Na2SO4) shown in Figure 9,
Obtain the value of a _{Na (} alloy ₎ calculated from the different concentrations at each temperature, average them, and calculate the value at that temperature.
a _{Na (} alloy ₎ .

Using the least squares method from each plot, we obtained the following straight line.

log a _{Na (} alloy ₎ = -4820/T+1.93 (±0.179) ...(6) By using the above data and formulas, for example, temperature and electromotive force can be set as parameters in the read-only memory of a microcomputer. By creating a map and referring to the map based on the actually measured temperature and electromotive force, it becomes possible to instantly and continuously detect the concentration of SO ₂ gas at that time.

[Brief explanation of drawings]

FIG. 1 is a vertical cross-sectional view of an example of the sensor structure of the present invention, FIG. 2 is a vertical cross-sectional view of another example of the sensor structure of the present invention, FIG. 3 is a state diagram of the Au-Na system, and FIG. Vertical sectional view showing a specific configuration example of the sensor of the present invention, No. 5
The figure is a graph showing the electromotive force for each SO ₂ gas concentration and ambient temperature using the sensor, and Figure 6 shows the electromotive force for each SO ₂ gas concentration and ambient temperature repeated two or three times to check reproducibility. Graphs showing the results, Figure 7 is a graph showing the responsiveness of electromotive force to changes in SO ₂ gas concentration, Figure 8 is a graph showing responsiveness of electromotive force to changes in temperature, and Figure 9 is a graph showing the response of the electromotive force to changes in SO 2 gas concentration. Na in Na ₂ SO ₄ generated at the measuring electrode)
Figure 10 is a graph showing the relationship between the activity of Na in the alloy and the temperature, which was determined through experiments. 1,5...solid electrolyte containing sodium component,
2,6...sodium or sodium alloy, 11
...β''-alumina (solid electrolyte), 14...Au ₂ Na (sodium alloy) containing a trace amount of Au.

Claims (1)

[Claims] 1. A solid electrolyte gas sensor for sulfur dioxide that measures the sulfur dioxide gas concentration at a measurement electrode by measuring the electromotive force between a reference electrode and a measurement electrode provided on a solid electrolyte, wherein the solid electrolyte is A solid electrolyte gas sensor for sulfur dioxide containing a sodium component and characterized in that a reference electrode is sodium or a sodium alloy. 2. The solid electrolyte gas sensor for sulfur dioxide according to claim 1, wherein the sodium alloy is an alloy of sodium and gold. 3. The solid electrolyte gas sensor for sulfur dioxide according to claim 1 or 2, wherein the solid electrolyte containing sodium is β-alumina or β″-alumina.