JPH0442624B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to an apparatus for measuring the concentration of dissolved substances such as dissolved oxygen or dissolved hydrogen.

[Background of the invention]

A conventional device for measuring the concentration of dissolved substances, such as a dissolved oxygen meter, is described in Japanese Patent Application Laid-Open No. 57-203945. This is shown in Fig. 1. In this dissolved oxygen meter, a detector 16 is housed in a pressure-resistant container 17, and sample water introduced from a sample water inlet 19 is passed through the water passage hole 5 throughout the pressure-resistant container. The entire detector is immersed in high-pressure sample water 18 to maintain pressure balance between the internal electrolyte 7 and the sample water, improve pressure resistance, and immerse the detector and selective (oxygen) permeable membrane 3 (hereinafter referred to as The purpose is to improve heat resistance by manufacturing an oxygen permeable membrane (abbreviated as oxygen permeable membrane) using a heat-resistant resin and absorbing thermal expansion of the electrolytic solution with bellows 10. Furthermore, in order to prevent the oxygen permeable membrane from being damaged due to an increase in the internal pressure of the detector due to the expansion force of the bellows, the oxygen permeable membrane is inserted between a porous cathode 4 (hereinafter referred to as cathode) and a metal filter 1. It supports and reinforces the oxygen permeable membrane. 1st
In the device shown in the figure, dissolved O ₂ in sample water first passes through a metal filter and an oxygen permeable membrane and enters the detector. The cathode is made of Au, Pt, Ag, etc., and is connected to the anodes B and 9 made of silver via a constant potential power source B14 and an ammeter 12. KCl in the electrolyte
An alkaline aqueous solution is used. The cathode is maintained at a constant potential more negative than the cathode B by a constant voltage power supply B14 and a potentiometer 15, and O ₂ entering the detector is reduced to OH ^- on this cathode according to equation (1).

O ₂ +2H ₂ O+4e ^- →4OH ^- ...(1) On the other hand, the reaction of formula (2) progresses on the anode, and a current flows between the two electrodes Ag+Cl ^- →AgCl+e ^- ...(2). This current is detected by an ammeter 12, and the dissolved O ₂ concentration is determined from this current value.

In conventional dissolved oxygen meters, the lower surface of the cathode is exposed in the electrolyte and is susceptible to interference from dissolved O ₂ and ions in the electrolyte, so these should be removed to prevent them from diffusing into the cathode. It is equipped with an interfering component removal electrode (hereinafter abbreviated as removal electrode) 6 for the purpose of removing interference components. The removal electrode is Au, Ag, Pt
It has a structure in which a conductor such as the like is provided with a through hole, and the electrolyte can communicate only through the electrolytic hole, and is placed near the porous metal electrode. This removal electrode is connected to the anode A8 via a constant voltage power source A13, and the potential of the removal electrode with respect to the anode A is maintained at the same potential as the potential of the cathode with respect to the anode B using the constant potential power source A. Dissolved particles diffusing from the anode A side to the cathode surface
Interfering components such as O ₂ and ions are reduced while passing through the through-hole of the removal electrode, and can be prevented from reaching the cathode and generating an interfering current. At this time, a reaction based on formula (2) proceeds on the anode A.

With the determination of dissolved O ₂ concentration in the known dissolved oxygen meter, according to formula (2), on anode A and anode B,
AgCl is generated and precipitated. The solubility of AgCl increases as the concentration increases, and at reactor water temperature it dissolves about 100 times as much as at room temperature, producing Ag ⁺ ions. This Ag ⁺ ion is reduced to Ag Ag ⁺ +e ^- →Ag (3) at the cathode according to equation (3), and this reduction current is used when measuring O ₂ concentration.
It becomes a hindrance. Therefore, at high temperatures, this Ag ⁺
Removal of ions becomes an important issue. Ag
Ions are deposited as Ag in the removal electrode through hole according to equation (3) and removed, but at this time the deposited Ag
There was a problem in that when the through-holes were accumulated and measurement was carried out for a long period of time, the through-holes were blocked and the conduction between the cathode and the anode B was lost, making measurements impossible.

[Purpose of the invention]

An object of the present invention is to provide a dissolved substance concentration measuring device that can be used continuously for an extended period of time.

[Summary of the invention]

The present invention relates to a measuring electrode consisting of a cathode and an anode having a through hole, one of which serves as a working electrode, an electrolytic solution sealed between the electrodes, a permselective membrane provided near the outside of the working electrode, and a permselective membrane provided near the outside of the working electrode. In a dissolved substance concentration measuring device whose main component is an interfering component removal electrode consisting of a cathode and an anode having a through hole arranged between measurement electrodes and arranged to have the same polarity as the measurement electrode,
The through hole of the interfering component removing electrode located on the working electrode side of the measuring electrode has a shape that widens continuously or stepwise toward the counter electrode side.

The conventional removal electrode has a through hole with a small diameter so that interfering components can be removed at a high rate. The rate of removal of interfering components by the removal electrode (hereinafter abbreviated as removal rate) is determined only by the length of the through hole, that is, the ratio of the thickness of the removal electrode to the radius of the through hole, and does not depend on the temperature or the type of the interfering component.
FIG. 2 shows the relationship between the ratio of the through hole length to the through hole radius and the removal rate. In Figure 2, the removal rate Y is defined by equation (4), and the relationship between the removal rate and the ratio of the through hole length to the through hole radius is expressed by the equation
It is given by (5).

Y=100Ci−Co/Ci ……(4) Y=100・{1−e _xp (−2・404・L/R ₀ )} ……(5) Here, Y represents the removal rate, and Ci and Co represents the concentration of interfering components at the inlet and outlet of the removal electrode. Further, R ₀ represents the radius of the through hole, and L represents the length of the through hole. As obtained from FIG. 2 and equation (5), as the ratio of the through hole length to the through hole radius increases,
Removal rate increases. For example, dissolved O ₂ at high temperatures
In measuring the concentration, it is required to remove a high concentration of Ag ⁺ ions, so a removal electrode with a through-hole radius smaller than the through-hole length is used. Ag ⁺ ions diffusing from the anode side are reduced on the inner surface of this through-hole, precipitated as Ag, and captured, but for long-term measurements, it is necessary to take into account the decrease in the through-hole diameter due to the accumulation of precipitated Ag. .

As shown in FIG. 2, more than 99% of the Ag ⁺ ions are removed and precipitated in a region corresponding to a depth twice the radius of the through hole from the entrance of the through hole. precipitated
As Ag reduces the through-hole radius, this region moves closer to the through-hole entrance with time. As a result, as shown in Figure 3, Ag thickly accumulates at the entrance of the through hole, further accelerating the plugging of the through hole, and eventually the through hole is completely plugged at the entrance, reducing the dissolved O ₂ concentration. Quantification becomes impossible. In the present invention, analysis shows that the change in the entrance radius over time can be calculated using the following formula:
We found that it is expressed by (6).

Here, t is the elapsed time, r is the entrance radius of the through hole at time t, R ₀ is the initial radius of the through hole at time 0, M is the atomic weight of Ag, D and C are the diffusion coefficient and concentration of Ag ⁺ ions, and ρ is Ag represents the density of formula
The time T from (6) until the through hole is closed is expressed by equation (7).

T=ρR ₀ ² /0.9MDC...(7) In other words, T is proportional to the square of the through hole radius,
and decreases in inverse proportion to the first power of Ag ⁺ ion concentration. Therefore, a removal electrode having a through-hole with a small diameter has an extremely short usage time, and the usage time of a high-temperature, high-pressure type dissolved oxygen meter is mainly determined by this time T. As an example, calculated using Equation 6,
Figure 4 shows the change over time of the entrance radius of a through hole with an initial radius of 0.025 cm caused by Ag ⁺ ions saturated in a 1 mol/KCl aqueous solution at the reactor water temperature (285° C.). The through holes become clogged in about 30 days, which is shorter than the lifetime determined by the consumption of Cl ^- ions in the electrolyte, deterioration of the oxygen permeable membrane, etc.

The present invention solves this problem and provides a high-efficiency and long-life removal electrode. According to equation (7), the occlusion time T can be increased by increasing the radius or decreasing the Ag ⁺ ion concentration. Focusing on this, we provided a plurality of removal electrodes with small through-hole diameters close to the cathode, and installed a removal electrode with the aim of removing interfering components such as Ag ⁺ ions at a high rate. In addition, it has a large through-hole diameter close to the anode, and after removing interfering components such as Ag ⁺ ions at a low rate and reducing the concentration, the interfering components are removed to a high-rate removal electrode placed on the cathode side. The removal electrode is difficult to block because it has a large through-hole, and the same potential is applied to both electrodes at the same time. We have devised a high-temperature, high-pressure dissolved oxygen analyzer that can perform measurements by removing interfering components at a high rate while also preventing blockage.

The feature of the present invention is that the electrolyte is filled inside the container,
This container has an oxygen permeable membrane installed on the surface of the sample water in contact with the sample water, and a cathode is placed in the electrolytic solution near the oxygen permeable membrane. and an anode for the cathode connected via an ammeter, and a plurality of electrodes for removing interfering components made of a conductor having a structure with a through hole through which the electrolyte can pass. In a diaphragm-type dissolved oxygen meter, the anode for the removal electrode is installed closer to the anode for the cathode than the removal electrode, and is connected to the removal electrode via a constant potential power supply. A through hole with a small through hole radius compared to the length of the through hole is provided adjacent to the side, and a removal electrode having a high interference component removal rate obtained by equations (4) and (5) is arranged. A removal electrode having a through hole with a radius larger than the through hole provided in the removal electrode placed close to the cathode is placed close to the anode, and a voltage is simultaneously applied to these by a constant potential power source. The solution lies in measuring the dissolved O ₂ concentration while applying and removing interfering components.

[Embodiments of the invention]

Embodiments of the present invention will be described below with reference to FIGS. 5 to 10. FIG. 5 is a schematic diagram of the dissolved oxygen meter of this embodiment. The dissolved oxygen meter of the embodiment includes a detector main body 16, a pressure-resistant container 17 that houses the detector main body 16,
And voltmeter 15, ammeter 12, constant voltage power supply A1
3. Consists of an external electric circuit system such as a constant voltage power source B14, which is connected to the electrodes within the detector body. An oxygen permeable membrane 3 is installed on the surface of the detector body, and an electrolytic solution 7 is sealed inside. The working electrode 4 is composed of a cathode having a through hole. Further, the interfering component removing electrode (hereinafter abbreviated as a removal electrode) having a through hole is a rear-stage cathode 6a (hereinafter abbreviated as a rear-stage removal electrode) and a front-stage cathode 6b (hereinafter abbreviated as a front-stage removal electrode).
It is composed of. 9 is an anode in the measurement electrode, and 8 is an anode paired with the removal electrode. In addition, the detector is equipped with a device aimed at improving the durability of the detector for measuring high-temperature sample water. That is, the detector is equipped with a bellows 10 communicating with the electrolyte to absorb thermal expansion of the electrolyte. In addition, the oxygen permeable membrane is inserted and supported by a porous metal filter 1 and a porous cathode to improve the durability of the oxygen permeable membrane.
This prevents the oxygen permeable membrane from being damaged due to an increase in the internal pressure of the electrolyte caused by an increase in the tension of the bellows. Further, the sample water enters the pressure container from the sample water inlet 19 and flows out from the sample water outlet 11. The pressure container is filled with sample water and the entire detector is immersed in it, so the pressure balance is always maintained between the sample water and the electrolyte, preventing damage to the detector body and oxygen permeable membrane. There is no. With these devices,
The detector can perform measurements without being damaged even in high-temperature, high-pressure sample water. Heat-resistant resin such as tetrafluoroethylene resin or polyimide resin is used for the detector body. For the oxygen permeable membrane, a heat-resistant resin with a large oxygen permeability coefficient, such as tetrafluoroethylene resin, is used. Cathode, front removal electrode,
The second stage removal electrode is manufactured using a metal that is less corrosive even at high temperatures, such as gold, platinum, or silver.
For the anodes A and B, Ag/AgCl electrodes are used, which do not decompose even at high temperatures and are stable, reversible, and highly reliable. Along with this, the electrolyte is an alkaline solution containing Cl ^- ions, for example, KCl1mol/,
An aqueous solution containing 1 mol/KOH is used.

The cathode is connected to the anode B via a constant voltage power supply B and an ammeter.
In addition, the front-stage removal electrode and the rear-stage removal electrode are respectively connected to the anode A via a constant voltage power supply A, and these constant voltage power supplies provide a cathode potential for the anode B, and a front-stage removal electrode and a rear-stage removal electrode for the electrode A. The potentials of the electrodes are each kept at the same potential, preferably about -08V at reactor water temperature.

O ₂ in the sample water passes through the oxygen permeable membrane and enters the electrolyte in the cathode hole, and while it diffuses in the electrode hole, it is reduced on the inner surface of the electrode hole according to equation (1). At this time, anode B
Above, the reaction of equation (2) progresses, and a current flows between the two electrodes. This current is detected by an ammeter, and the dissolved O ₂ concentration is determined from this current. At this time, AgCl generated on the anode B according to equation (2) is dissolved at high temperature and produces Ag ⁺ ions. This Ag ⁺ ion is reduced at the cathode,
Since a disturbance current is generated, this is reduced and removed using a front-stage removal electrode and a rear-stage removal electrode.

Since the purpose of the pre-stage removal electrode is to remove Ag ⁺ at a high rate, the radius of the through hole is smaller than the length of the through hole, that is, the thickness of the removal electrode, and is preferably 1/5.
It is manufactured as follows. At this time, the removal electrode removes 99.999% or more of the interfering components within its through hole. By reducing the radius of the through-hole, the electrical resistance of the electrolyte inside the through-hole increases, so this is reduced by providing a large number of through-holes.

This front-stage removal electrode has a small through-hole diameter, so the formula
As expected from (6), equation (7), and FIG. 4, the through holes are likely to be blocked by precipitated Ag, and the usage time is short. In order to solve this problem, in this embodiment, a post-removal electrode is installed in the detector. The rear removal electrode is installed between the front removal electrode, anode A, and anode B, is held at the same potential as the front removal electrode, and is used simultaneously with the removal electrode. The latter removal electrode is
A function that removes interfering components such as Ag ⁺ ions that diffuse from the anode side electrolyte toward the cathode at a lower rate than the front-stage removal electrode, and after reducing the concentration, diffuses them to the high-rate front-stage removal electrode side. has. The removal rate of the latter-stage removal electrode is relatively small, and preferably about 95%. Therefore, the diameter of the through hole is relatively large, preferably about 1.25 times the length of the through hole. As shown in Equation (7), the closure time of the through hole increases in proportion to the square of the radius of the through hole, and increases in inverse proportion to the first power of the Ag ⁺ ion concentration. Therefore, by using the rear removal electrode and the front removal electrode at the same time, the usage time of the former is reduced due to the effect of increasing the through-hole radius, and the latter is due to the effect of decreasing the Ag ⁺ ion concentration. At the same time, it is possible to significantly extend the time period and remove interfering components such as Ag ⁺ ions with a high removal rate. Figure 6 shows a post-removal electrode with a through hole with a radius of 0.2 cm and a thickness of 0.25 cm, and a back-stage removal electrode with a through hole with a radius of 0.2 cm and a thickness of 0.25 cm.
The graph shows the change in the through-hole radius of each electrode over time when a pre-removal electrode with a through-hole diameter of 0.125 cm was used at the same time at the reactor water temperature. The use time of the post-stage removal electrode is 2000 days or more, and the use time of the pre-stage removal electrode is 600 days or more, which is more than 20 times the usage time of the known removal electrode shown in FIG.

In this example, in addition to the front stage removal electrode, only one rear stage removal electrode was used, but as shown in FIG. It is also possible to do so. In FIG. 7, the latter removal electrode A6c is
The rear removal electrode B6d has a through hole larger than the front removal electrode and is closer to the anode B8 than the front removal electrode. These are placed on the B side, held at the same potential by a constant voltage power supply 13, and used at the same time. When this electrode system is used, it is possible to further extend the usage time of each electrode. Although FIG. 7 uses two rear-stage electrodes, it is also possible to use a larger number of rear-stage electrodes. In addition, in the example shown in Figure 7, the independent front-stage removal electrode and rear-stage removal electrode are used while being held at the same potential at the same time, but as shown in Figure 8, they are brought into direct contact with each other and used as a single electrode. Alternatively, as shown in Figure 9, a through hole whose radius decreases stepwise from the anode side to the cathode side is provided in the same metal plate, and the same electrode can be used to connect a front-stage removal electrode and multiple rear-stage removal electrodes. It is also possible to use an electrode that has the same function as when using an electrode, and as the pole root, it is also possible to use an electrode that has a through hole whose radius changes continuously as shown in Figure 10. be.

In this example, as anode A and anode B,
We explained an example using Ag/AsCl electrodes, but
The method of removing interfering components such as Ag ⁺ ions using the front-stage removal electrode and the rear-stage removal electrode of the present invention can be performed using other electrodes that are stable, reversible, and reliable at high temperatures, such as Ag/AgBr, Ag/ Ag, Ag/
It is also effective when using electrodes such as Ag ₃ PO ₄ , Pb/PbSO ₄ , Ag/Ag ₂ SO ₄ , etc.

As described above, according to this embodiment, the continuous measurement time of a high-temperature, high-pressure dissolved oxygen meter can be extended from the conventional 20 to 30 days to more than 20 times, that is, more than 600 days. be.

Other examples of the removal electrode arranged on the working electrode side of the measurement electrode are shown in FIGS. 11-14. Further, although not shown, the cross-sectional shape of the through hole may be triangular, quadrangular, or any other arbitrary shape other than circular.

In this embodiment, the same explanation was given to the O ₂ concentration measuring device, but the present invention can also be applied to other dissolved substances such as H ₂ , H ₂ S,
It is also possible to apply it to a device for measuring the concentration of H ₂ O ₂ or metal ions. Furthermore, although the cathode was used as the working electrode in this embodiment, it is also possible to use the anode as the working electrode for dissolved substances other than O ₂ .

〔Effect of the invention〕

In the case of a high-temperature, high-pressure type dissolved oxygen meter using the device of one embodiment of the present invention, the continuous measurement time is 20 to 30 minutes compared to the conventional one.
It can be extended from 200 days to 600 days or more.
It will be possible to apply and put the high-temperature, high-pressure dissolved oxygen meter into practical use as a measurement device for actual plants, such as light water reactors and heavy water reactors, where it is essential to have the ability to measure continuously for about one year. In light water reactors and heavy water reactors, in addition to O ₂ , H ₂ O ₂ is present in the reactor water.
During the sample water cooling operation, H ₂ O ₂ thermally decomposes into O ₂ and the O ₂ concentration changes, so it is important for reactor water quality management to directly measure the O ₂ concentration without cooling the sample water. . According to the present invention, in an actual plant,
It has become possible to provide a practical detector that can achieve this purpose, and it is possible to manage the water quality of reactor water and prevent H ₂ in the reactor water.
This can provide an extremely effective means for controlling the injection of dissolved O ₂ in the reactor water to reduce the concentration of dissolved O 2 in the reactor water.

[Brief explanation of the drawing]

Figure 1 is a schematic longitudinal cross-sectional view showing the basic structure of a conventional high-temperature, high-pressure dissolved oxygen analyzer, and Figure 2 is a graph showing the ratio of the removal electrode through-hole length to the through-hole radius and the removal rate of interfering components in the through-hole. be. FIG. 3 is a cross-sectional view of the removal electrode showing the pattern of Ag electrodeposition in the removal electrode through-hole, and FIG. 4 is a graph showing the change over time in the radius of the through-hole entrance of the removal electrode. FIG. 5 is a schematic cross-sectional view showing the structure of a concentration measuring device according to an embodiment of the present invention, and FIG. 6 shows changes over time in the entrance radius of the front-stage removal electrode and the rear-stage removal electrode through-hole when using this embodiment. FIG. 7 to 14 are cross-sectional views showing the structure of the removal electrode used in the measuring device of the present invention. DESCRIPTION OF SYMBOLS 1...Metal filter, 2...Pedestal, 3...Selective (oxygen) permeable membrane, 4...Cathode, 5...Water hole, 6...Interfering component removal electrode (removal electrode), 6a...Post-stage electrode of the interfering component removal electrode (back-stage removal electrode), 6b...front-stage electrode of the interfering component removal electrode (front-stage removal electrode), 6
c...Later stage removal electrode A, 6d...Later stage removal electrode B, 7
... Electrolyte, 8... Anode A, 9... Anode B, 10... Bellows, 11... Sample water outlet, 12... Ammeter, 13...
Constant potential power source A, 14... Constant potential power source B, 15... Voltmeter, 16... Detector body, 17... Pressure resistant container, 18...
Sample water, 19... Sample water inlet, 20... Through hole, 21
...Precipitated silver.

Claims (1)

[Scope of Claims] 1. A measuring electrode consisting of a cathode and an anode having a through hole, one of which serves as a working electrode, an electrolytic solution sealed between the electrodes, and a selective permeation electrode provided near the outside of the working electrode. Concentration measurement of dissolved substances, the main component of which is an interfering component removal electrode consisting of a cathode and an anode, which is provided between a membrane and the measurement electrode, and has a through hole arranged so as to have the same polarity as the measurement electrode. In the apparatus, the through-hole of the interfering component removing electrode located on the working electrode side of the measuring electrode includes a shape that expands continuously or stepwise toward the counter electrode side. Concentration measuring device. 2. The dissolved substance concentration measuring device according to claim 1, wherein the interfering component removing electrode disposed on the working electrode side is an integrally molded product. 3. The dissolved substance concentration measuring device according to claim 1, wherein the interfering component removing electrode disposed on the working electrode side is constructed by a combination of a plurality of molded products.