JP2005241622A - Electrochemical sensor and electric conductivity cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electrochemical sensor and an electric conductivity cell with physical strength, in which a plating applied to an electrode surface is hardly peeled. <P>SOLUTION: The electrochemical sensor 1A for measuring the current carried to an electrode 4 according to the reaction of a component of measuring object on the surface of the electrode 4 comprises a platinum plating layer P gray in color on the surface of the electrode 4. This invention is suitably applicable to a diaphragm type sensor such as a dissolved hydrogen sensor. The electric conductivity cell 1C comprises the platinum plating layer P gray in color on the surface of the electrode 4. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an electrochemical sensor and an electrical conductivity cell, such as a dissolved hydrogen sensor, which are conventionally provided with a platinum black electrode.

  For example, in a nuclear power plant or the like, it is important to measure dissolved hydrogen (reduced substance) in a sample (test solution), and monitoring is performed by a meter. As a sensor of this dissolved hydrogen meter, a polarographic sensor using a hydrogen permeable diaphragm as a gas permeable membrane to be measured, that is, a diaphragm type polarographic sensor is mainly used.

  Taking a diaphragm type polarographic sensor (hereinafter referred to as “dissolved hydrogen sensor”) used as a sensor of the dissolved hydrogen meter as an example, a conventional diaphragm type polarographic sensor will be described. As shown in FIG. Is generally a working electrode 4 formed of platinum or a platinum group catalyst, a counter electrode 5 usually formed of silver (Ag) or silver-silver chloride (Ag / AgCl), and an electrolytic solution in contact with these electrodes 7 and a diaphragm 3 that selectively permeates hydrogen (bipolar type). In addition, there are also those equipped with a reference electrode (three-pole type).

  For example, with reference to FIG. 1, the bipolar type will be further described. Hydrogen that has passed through the diaphragm 3 is dissolved in the electrolytic solution 7 and is oxidized on the working electrode 4. An electrochemical reaction takes place between the working electrode 4 and the counter electrode 5 through the electrolytic solution 7 and the current flowing between the working electrode 4 and the counter electrode 5 is proportional to the hydrogen gas concentration. The hydrogen gas concentration can be known more.

  As described above, the dissolved hydrogen sensor 1A measures the amount of current flowing between the working electrode 4 and the counter electrode 5 due to the reaction at the working electrode 4, but the sensitivity gradually deteriorates immediately after the start of measurement. Sensitivity calibration had to be performed and continuous measurement was difficult.

  In order to solve this problem, a method of maintaining the sensitivity by performing platinum black plating on the surface of the working electrode 4 (platinum black electrode) and increasing the surface activity of the working electrode 4 is employed. Platinum black is a metallic spongy (powdered) metal platinum, and the electrode to which it is attached has a significantly larger actual surface area than the apparent surface area, and has a large microscopic unevenness. In the dissolved hydrogen sensor 1A provided with the platinum black electrode, the sensitivity can be recovered by removing the diaphragm when the sensitivity is deteriorated and replating the surface of the working electrode with platinum black.

  However, as described above, it is possible to reduce the sensitivity deterioration by performing platinum black plating on the surface of the working electrode 4 to increase the surface activity of the working electrode. Metal particles adhering to (depositing on), fragile and easily removed from the surface of the working electrode 4. In particular, in a diaphragm type sensor such as the dissolved hydrogen sensor 1A, platinum black plating can be easily removed during work such as replacement of the diaphragm 3, which is very difficult to handle. And when platinum black plating is taken, a sensitivity fall will be caused. Further, as described above, platinum black plating is very easy to remove, so it is impossible to physically remove dirt.

  By the way, in Patent Document 1, when high concentration hydrogen is measured or continuously measured, the sensitivity is not lowered or drifted over time by sputtering or vacuum deposition. In addition, it is disclosed that a platinum catalyst thin layer is formed on a diaphragm and the reaction parts are integrated to perform stable measurement. However, such prior art cannot be easily restored when sensitivity is deteriorated. Further, in order to form a catalyst electrode circuit, the electrode and the current collector are brought into close contact with each other. However, the contact state must be maintained in an ideal state, which is structurally complicated.

  Therefore, while maintaining the activity of the extreme surface, it is difficult to remove the plating applied to the surface, it has physical strength, and can be easily restored when the sensitivity deteriorates, and it is also structurally simple There is a need for an electrochemical sensor.

  A similar problem also exists in an electric conductivity measurement cell (hereinafter, referred to as “electric conductivity cell” or simply “cell”) in which a platinum black electrode is conventionally used. In other words, cells with various shapes have been put into practical use according to the application, and as the material of the electrode, an inert metal such as platinum, titanium, stainless steel is used, and platinum black plating is used for platinum. Has also been used. A cell with platinum black plating on the surface of the electrode has a large surface area, so it is strong against the effects of polarization and can measure electrical conductivity in a wide concentration range. As mentioned above, it is a metal fine particle adhering in a powdery state, it is fragile, easy to remove, and very difficult to handle. Also, as above, it is impossible to physically remove the dirt. And when platinum black plating is taken, a cell constant will change. In other words, a cell to which platinum black plating has been applied has a drawback that the performance gradually changes due to the dropping of platinum black over a long period of use.

  On the other hand, in cells using platinum, titanium, stainless steel, etc., with a smooth surface and no plating, it is necessary to secure a surface area to prevent polarization, the cell shape must be increased, and measurement Even in the concentration range, when high electrical conductivity is measured, the sensitivity is lowered, and the cell with platinum black plating is inferior in performance. The four-terminal method is resistant to dirt, but is not always a method that can be adopted. In other words, the four-terminal method has a complicated structure and device circuit, and it is difficult to reduce the size of the cell shape. Therefore, sensitivity may be lowered in the measurement of high electrical conductivity.

Therefore, it is difficult to remove the plating applied to the pole surface, it has physical strength, and it has a wide linearity range (measurement concentration range), a change in cell constant is reduced, and an electric conductivity cell capable of stable measurement. It has been demanded.
JP-A-9-138215

  An object of the present invention is to provide an electrochemical sensor and an electrical conductivity cell that are difficult to remove the plating applied to the pole surface and have physical strength.

  Another object of the present invention is to provide an electrochemical sensor that maintains the activity of the electrode surface, suppresses a decrease in sensitivity, and can be easily restored when the sensitivity decreases.

  Another object of the present invention is to provide an electrical conductivity cell that has a wide linearity range (measurement concentration range), reduces cell constant changes, and enables stable measurement.

  The above object is achieved by the electrochemical sensor according to the present invention. In summary, the present invention relates to an electrochemical sensor for measuring a current flowing through the electrode in response to a reaction of a component to be measured on the surface of the electrode, wherein a platinum plating layer exhibiting gray is provided on the surface of the electrode. An electrochemical sensor.

  According to an embodiment of the present invention, the electrochemical sensor includes a chamber that is partitioned from the outside by a diaphragm that allows the measurement target component in the sample to pass through at one end of the sensor body, and the working electrode and the counter electrode are provided inside the chamber. And the working electrode and the counter electrode are reacted with each other through the surface of the working electrode when the working electrode and the counter electrode are in contact with the electrolyte contained in the chamber. The platinum type plating layer which exhibits the said gray on the surface of the said working electrode is provided. In one embodiment, the electrochemical sensor is a polarographic sensor or a galvanic cell sensor. In one embodiment, the electrochemical sensor is a dissolved hydrogen sensor.

  According to another aspect of the present invention, in an electrochemical sensor for measuring the electrical conductivity of a sample by applying a voltage between electrodes arranged to be in contact with the sample, the surface of the electrode exhibits a gray color. An electrical conductivity cell is provided that is provided with a platinum plating layer.

According to one embodiment of each of the present invention, the platinum plating layer is white with a direct current having a current density of 0.05 to 0.3 mA / cm ² · s per unit time in the direction in which the platinum plating is performed. It is provided by gold plating. According to another embodiment, the platinum plating layer is platinum-plated with a direct current having a current density per unit time of 0.05 to 0.3 mA / cm ² · s in the direction in which the platinum plating is performed. At the same time, it is provided by flowing a direct current of the opposite polarity for a predetermined period. In one embodiment, (a) a forward current in which platinum plating is performed is applied for a predetermined period, and then a current having the opposite polarity is applied for a predetermined period, or (b) the forward current is supplied for a predetermined period. The reverse polarity current is allowed to flow for a predetermined period, and the forward current is allowed to flow for a predetermined period, or (c) the forward current and the reverse polarity current are alternately supplied a plurality of times for a predetermined period. In another embodiment of the present invention, the platinum plating layer is white with an alternating current having a current density of 0.3 to 6 mA / cm ² · s per unit time in the direction in which the platinum plating is performed. It is provided by gold plating.

  According to the present invention, the electrochemical sensor or the electrical conductivity cell is difficult to be plated on the surface of the electrode and has physical strength. And according to this invention, while maintaining the activity of the pole surface, the electrochemical sensor can suppress a sensitivity fall, and can be easily returned when a sensitivity fall is carried out. Further, according to the present invention, the electric conductivity cell has a wide linearity range (measurement concentration range), and the cell constant change is reduced, so that stable measurement is possible.

  Hereinafter, the electrochemical sensor according to the present invention will be described in more detail with reference to the drawings.

Example 1
The diaphragm type sensor according to the present invention can be suitably applied to a dissolved hydrogen sensor which is a diaphragm type polarographic sensor having the basic configuration described above with reference to FIG.

  The dissolved hydrogen sensor 1 </ b> A includes a hollow cylindrical sensor body 2, a gas permeable diaphragm 3 fixed to the opening at the tip thereof, and a working electrode 4 disposed in the sensor body 2 in the vicinity of the diaphragm 3. And a counter electrode 5 attached to the inner periphery of the support tube 6 that supports the working electrode 4. A chamber 2 a that is partitioned from the outside by the diaphragm 3 is formed between the sensor body 2 and the support tube 6. The electrolytic solution 7 is accommodated in the chamber 2a.

  For example, the support tube 6 made of glass is coaxially disposed inside the sensor, and the working electrode 4 is attached to the tip thereof. A slight gap having a constant thickness is formed between the diaphragm 3 and the working electrode 4 to form a layer (electrolytic solution layer) of the electrolytic solution 7 having a constant thickness.

  In the present embodiment, the working electrode 4 is made of platinum, and a platinum plating layer P according to the present invention, which will be described in detail later, is provided on the liquid contact surface. The counter electrode 5 is formed of silver-silver chloride (Ag / AgCl). Moreover, as the electrolyte solution 7, 0.1 mol / L HCl + 0.1 mol / L KCl can be suitably used.

  Lead wires 4 a and 5 a are connected to the working electrode 4 and the counter electrode 5, respectively, and these lead wires 4 a and 5 a are led out through the support tube 6 and connected to a voltage applying means (DC power source) 8. . Then, a predetermined electrolytic voltage is continuously applied from the power source 8 between the working electrode 4 and the counter electrode 5 via the lead wires 4 a and 5 a, and the steady value of the electrolytic current is measured by the ammeter 9. The dissolved gas concentration in the sample solution is obtained. That is, the gas to be measured (here, hydrogen gas) that has permeated through the diaphragm 3 reacts on the surface of the working electrode 4, and the electrolytic current of the dissolved gas flowing through the working electrode 4 at that time is measured by the ammeter 9. The dissolved hydrogen concentration detected by the sensor 1A is measured. The measurement result is instructed by an indicator provided in the measuring apparatus main body (not shown) or printed out.

  As a result of intensive studies to solve the above-mentioned problems, the present inventors can change the state of the platinum plating layer P while maintaining the catalytic function of the platinum plating layer P provided on the working electrode 4 depending on the plating conditions. I found out.

  More specifically, FIG. 6 shows a schematic configuration of a plating apparatus for providing the working electrode 4 with the platinum plating layer P according to the present invention. The platinum plating method according to the present invention is basically the same as the conventional platinum black plating method, and the current conditions are different as plating conditions. That is, in the plating operation, first, the plating solution 30 is placed in a plating container (beaker or the like) 40, and the liquid contact surface (cathode) of at least the working electrode 4 of the dissolved hydrogen sensor 1A is immersed in the plating solution 30 to generate current. The negative electrode of the vessel 10 is connected via the lead wire 4a. On the other hand, the platinum electrode 20 (anode) is immersed in the plating solution 30, and the positive electrode of the current generator 10 is connected via the lead wire 21. Then, platinum is deposited on the working electrode 4 by flowing a current from the current generator 10 between the working electrode 4 and the platinum electrode 20 as described in detail later.

  Then, as shown in the conceptual diagram of FIG. 2, generally, the platinum plating exhibiting a gray color according to the present invention is performed by causing the current in the plating operation to flow for a long time with a smaller current than in the conventional platinum black plating method. It has been found that the layer P can be provided on the working electrode 4. Such a platinum plating layer P is difficult to remove and has physical strength, so that a decrease in sensitivity of the dissolved hydrogen sensor 1A is suppressed. However, as shown in the conceptual diagram of FIG. 2, when the current during plating is excessively small and long, the platinum plating layer P becomes white and cannot maintain the catalytic activity.

  3 and FIG. 4, the surface of the deposited platinum crystal becomes more rough and rough as a result of the bonding of the metal particles as the current density per unit time of the current in the plating operation becomes smaller. Changes to a smooth state.

  3 and 4 show the results of observing the surface of the platinum plating layer P with a scanning electron microscope (SEM) when the current condition is changed as the plating condition. That is, as shown in FIG. 3A, the conventional platinum black plating generates a substantially acicular finely-plated platinum crystal lump (the outer diameter is about 1 to 3 μm). Then, as the current density per unit time decreases, a larger platinum crystal lump (generally about 5 to 15 μm) grows as shown in FIGS. 3B and 3C. . When the current density per unit time is further reduced, as shown in FIG. 4 (a), the deposited platinum crystal lump is further grown large (outer diameter 5 to 20 μm), and the surface approaches a smooth state. .

  However, as shown in FIGS. 4 (b) and 4 (c), when the current density per unit time is further reduced, the precipitated platinum crystals do not become particles and are in a smoother, substantially uniform state. It becomes.

  At this time, the appearance of the platinum plating layer P on the surface of the working electrode 4 is black in the state shown in FIG. 3A, and is in the state shown in FIGS. 3B, 3C, and 4A. Shows gray, and in the state shown in FIGS. 4B and 4C, white.

  The inventors of the present invention have found that the surface of the platinum-plated electrode is an ideal electrode that is robust and is less susceptible to a decrease in sensitivity within a range in which the surface is gray. A platinum-plated electrode having a black surface, that is, a platinum black electrode has the above-described problems because of its weakness. On the other hand, when the platinum-plated electrode is white, it becomes equivalent to smooth platinum and the catalytic activity cannot be obtained, so the sensitivity is lowered. Here, with respect to the appearance of the platinum plating layer P, the term “gray” includes those that are gray in a general color concept. More specifically, according to the study of the present inventor, a platinum plating layer P exhibiting a color represented by N (achromatic color) 2.0 to N8.0 in the Munsell color system is suitable, more preferably A platinum plating layer P exhibiting a color represented by N3.0 to N5.0, more preferably N3.0.

  As described above, the platinum plating method according to the present invention is the same as the conventional platinum black plating method except for the following current conditions.

As a plating solution, lead acetate (usually 0) is used in an aqueous solution of hexachloroplatinic (IV) acid (H ₂ [PtCl ₆ ]), which is the same as conventional platinum black plating (usually 3 g dissolved in 100 ml of water). 0.02 to 0.03 g.) Can be used.

As a current condition, when a direct current is used, a current density per unit time of a current in a direction in which platinum plating is performed on a pole (hereinafter referred to as “forward current”) is 0.3 mA / cm ² · s or less. It is preferable to use a direct current. On the other hand, the current density per unit time of the forward current is preferably 0.05 mA / cm ² · s or more. Therefore, when DC is used as the plating current, the current density per unit time of the forward current is preferably 0.05 to 0.3 mA / cm ² · s.

  In addition, a forward current having a current density per unit time as described above can flow, and a current having a polarity opposite to the forward current (hereinafter referred to as “reverse current”) can flow for a predetermined period. Such a reverse current may flow for a predetermined period after flowing the forward current for a predetermined period, or may flow for a predetermined period alternately for a predetermined period, alternately with the forward current.

  Note that the current value when the reverse current is passed as described above may be about 10 times the value of the forward current, and the platinum plating layer P is not peeled off by such a current. By flowing a reverse current, hexachloroplatinic (IV) acid and chlorine adhering to and occluding from the pole can be removed (electrolytic cleaning), and plating can be performed efficiently.

Further, an alternating current may be used. When alternating current is used, the current density per unit time of the forward current can be 6 mA / cm ² · s or less. On the other hand, the current density per unit time of the forward current is preferably 0.3 mA / cm ² · s or more. Therefore, when alternating current is used as the plating current, the current density per unit time of the forward current is preferably 0.3 to 6 mA / cm ² · s.

  The AC waveform is not particularly limited, and a sine wave, a rectangular wave, or the like can be used as appropriate. Moreover, although an alternating frequency can also be selected suitably, typically, DC to a commercial frequency (50/60 Hz) can be used conveniently.

  In order to confirm the effect of the present invention, a dissolved hydrogen sensor was prepared as follows, and the ease of removing the platinum plating layer P, the mechanical strength, and the sensitivity stability were evaluated. The results are shown in Table 1.

  Except for the plating conditions (or the presence or absence of plating) on the surface of the working electrode, all other configurations of the dissolved hydrogen sensor, such as the area and material of the working electrode and the counter electrode, were the same. The same plating solution (3 g of hexachloroplatinic (IV) acid dissolved in 100 ml of water and 0.03 g of lead acetate) was also used.

(Measurement condition)
・ Working electrode: Pt
・ Counter electrode: Ag / AgCl
-Applied voltage during measurement: 500 mV (between working electrode and counter electrode)
Electrolyte: 0.1 mol / L HCl, 0.1 mol / L KCl

  In Table 1, specific examples 1 to 3 are dissolved hydrogen sensors according to the present invention. In Example 1, direct current was passed as the current, the polarity of the current was reversed during plating, and electrolytic cleaning was performed. In the specific example 2, alternating current was passed as the current. In Example 3, direct current was passed as the current, but the polarity of the current was not reversed during plating.

  On the other hand, Comparative Examples 1 to 4 are dissolved hydrogen sensors not according to the present invention, and in particular, Comparative Examples 1 and 2 are dissolved hydrogen sensors provided with platinum black electrodes by a conventional platinum black plating method. Comparative Example 3 is an example in which the current density per unit time is further reduced as compared with Specific Examples 1 to 3, and Comparative Example 4 is an example in which platinum plating is not performed.

  In the specific example 1, the forward current is passed, the reverse current is passed, and the forward current is passed again (that is, the forward current is (1) and the reverse current is (2) (1) + (2) + (1)). However, when a forward current and a reverse current are caused to flow using a direct current, the present invention is not limited to the aspect of the first specific example. For example, assuming that the forward current is (1) and the reverse current is (2), (1) + (2), (1) + (2) + (1) + (2), or (1) + (2) may be repeated.

  It was found that the appearance of the platinum plating layers P of Specific Examples 1 to 3 was gray and had the surface shown in FIG. 3 (c) microscopically. As a result of further study, in another example, the dissolved hydrogen sensor in which the outer appearance is gray and the platinum plating having the surface of FIG. 3B and FIG. It was confirmed that the same characteristics as 1 to 3 were exhibited.

  It was found that the appearance of the platinum plating layer P of Comparative Examples 1 and 2 was black and had the surface shown in FIG. 3A microscopically. Moreover, the external appearance of the platinum plating layer P of the comparative example 3 was white, and it turned out that it has the surface shown in FIG.4 (c) microscopically. In addition, the external appearance of the platinum plating layer P of the comparative example 4 is exhibiting silver (metal color).

  Moreover, the ease of removing the platinum plating layer P and the physical strength were confirmed by wiping with a tissue paper. As a result, in Examples 1 to 3, the platinum plating layer P was not removed, and in Comparative Examples 1 and 2, black crystals were removed on the tissue paper. That is, it was found that the platinum plating layers P of Specific Examples 1 to 3 were more difficult to remove than the conventional platinum black plating layers P of Comparative Examples 1 and 2 and had a higher physical strength. Thus, it was found that in the platinum plating layer P (specific examples 1 to 3) according to the present invention, it is possible to physically remove dirt by wiping the surface with a tissue paper or the like.

  In FIG. 5, the result of having investigated sensitivity stability about the specific examples 1-3 and the comparative examples 1-4 is shown. In FIG. 5, the horizontal axis indicates the number of elapsed days, and the vertical axis indicates the output relative value (%) for each elapsed day with respect to the output at the start of the test (the number of elapsed days is 0).

  From Table 1 and FIG. 5, it can be seen that the specific examples 1 to 3 according to the present invention have less sensitivity reduction than the comparative examples 1 to 4. Further, in Examples 1 and 2 in which a reverse current (including the case of using an alternating current) was passed during plating, the output reduction was further reduced compared to Example 3 in which this was not applied. Specific examples 1 and 2 showed almost the same sensitivity stability.

  As can be seen from the results of Comparative Example 3, if the current density per unit time during plating is too small, the surface becomes too smooth, and there is no effect of applying platinum plating, almost the same as when not applying platinum plating. A similar decrease in sensitivity occurred.

  The dissolved hydrogen sensor 1A including the platinum plating layer P according to the present invention can restore the sensitivity by re-plating the platinum plating layer P when the sensitivity is lowered. When the platinum plating layer P is regenerated, the platinum plating layer P is once removed from the working electrode 4 by polishing or the like, and then the platinum plating is performed as in the case of regenerating platinum black plating. Is preferably performed. The plating conditions for re-plating are the same as described above.

  Conventionally, in a diaphragm type sensor equipped with a platinum black electrode, platinum black plating is usually taken out when the diaphragm is replaced and / or when the electrolyte is replaced. On the other hand, since the platinum plating layer P according to the present invention is more robust than the conventional platinum black plating, it is necessary to regenerate the platinum plating layer P each time the diaphragm is replaced and / or the electrolyte is replaced. No, it may be performed at the time when the decrease in sensitivity becomes significant.

  As described above, according to the present embodiment, the platinum plating layer P that is gray is provided on the working electrode of the dissolved hydrogen sensor, so that the platinum plating layer P is difficult to remove and has physical strength. It can be. In addition, it is possible to maintain the activity of the extreme surface and suppress a decrease in sensitivity. Further, according to the present invention, the pole provided with the platinum plating layer P can be easily reproduced when the sensitivity is lowered.

  In the above embodiment, the diaphragm type polarographic sensor provided with the platinum plating layer P according to the present invention has been described as a dissolved hydrogen sensor, but the present invention is not limited to this. Further, the present invention is not limited to the diaphragm type sensor, and the electrode provided with the platinum plating layer P may be exposed to the sample.

  In the above embodiment, the electrochemical sensor is described as a diaphragm type polarographic sensor, but the present invention is not limited to this. If an example is given, the platinum plating layer P according to this invention can be provided in the surface of the platinum electrode 4 with which a galvanic cell type sensor such as the diaphragm type galvanic cell type sensor 1B as shown in FIG. 7 is provided.

  Unlike the polarographic sensor 1A shown in FIG. 1, the diaphragm type galvanic cell type sensor 1B shown in FIG. 7 does not have a power source and does not apply a voltage from the outside during use, and the working electrode 4 and the counter electrode 5 are electrolyzed. The current flowing between the electrodes is measured by immersing each in the liquid 7 to form a circuit. The method of providing the platinum plating layer P according to the present invention on the electrode of the galvanic cell type sensor is substantially the same as that applied to the electrode of the diaphragm type polarographic sensor, and the platinum plating layer P according to the present invention is applied. By providing, the same operation effect as the case of the above-mentioned diaphragm type polarographic sensor can be produced.

Example 2
Next, another embodiment of the present invention will be described. In this embodiment, a case where the platinum plating layer according to the present invention is applied to an electric conductivity cell will be described.

  FIG. 8 shows a schematic cross section of the main part of the cell 1C. The cell 1C shown in FIG. 8 has a substantially columnar shape, and a plurality of platinum electrodes 4 are exposed at the tip of the support tube 6 so as to be exposed from the inner wall of a substantially cylindrical space (sample introduction part) 6a with one end open. In this embodiment, there are two. There may be more poles. And the electrical conductivity of a sample is measured by flowing an alternating current between platinum electrodes 4 and 4 and measuring the impedance of the sample introduced into the space between both electrodes.

  The method of providing the platinum plating layer P according to the present invention on the electrode of the cell 11 is substantially the same as that applied to the electrode of the diaphragm type polarographic sensor described in the first embodiment.

  By following the present invention, the physical strength can be improved by changing the state of the platinum plating layer P while maintaining the same or wider linearity range (measurement concentration range) as that of conventional platinum black. That is, according to the present invention, it is difficult to remove and it is possible to form a pole having a wide measurement concentration.

  Here, as described above, as the current density per unit time of the current in the plating operation decreases, the surface of the deposited platinum crystal changes from a rough, coarse needle-like crystal to a state in which smooth metal particles are bonded. . At this time, the surface color changes from black to gray and white, as described above. Then, the present inventors have found that the cell 11 is an ideal cell 11 that is robust and has a wide measurement concentration range in the range where the surface of the platinum plating electrode is gray. A platinum-plated electrode having a black surface, that is, a platinum black electrode has the above-described problems because of its weakness. On the other hand, when the surface of the platinum-plated electrode is white, it becomes equivalent to smooth platinum, and the surface area expansion effect cannot be obtained, so the measurement concentration range becomes narrow.

  Also in this embodiment, the plating conditions for providing the platinum plating layer P on the pole can be the same as those described in the first embodiment.

  In order to confirm the effect of the present invention, the cell 1C was produced as follows, and the ease of removing the platinum plating layer P, the mechanical strength, and the measured concentration range were evaluated. The results are shown in Table 2.

  Except for the plating conditions (or presence / absence of plating) of the electrode surface, the electrode area, body material, measurement frequency, etc. were the same. The same plating solution (3 g of hexachloroplatinic (IV) acid dissolved in 100 ml of water and 0.03 g of lead acetate) was also used.

  A platinum plating layer P was provided on the Pt electrode as shown in the table below, and the electrical conductivity of the potassium chloride standard solution was measured under the condition of 25 ° C.

  In Table 2, specific examples 4 to 6 are cells according to the present invention. In these specific examples, direct current was used as the plating current.

  On the other hand, Comparative Examples 5 to 7 are cells not according to the present invention. In particular, Comparative Example 5 is a cell including a platinum black electrode by a conventional platinum black plating method. Comparative Example 6 is an example in which the current density per unit time is further reduced as compared with Specific Examples 4 to 6, and Comparative Example 7 is an example in which platinum plating is not performed.

  The external appearance of the platinum plating layers P of Specific Examples 4 to 6 was gray, and it was found that the surface shown in FIG. On the other hand, it was found that the appearance of the platinum plating layer P of Comparative Example 5 was black and had the surface shown in FIG. 3A microscopically. In addition, it was found that the appearance of the platinum plating layer P of Comparative Example 6 was white and had the surface shown in FIG. 4 (c) microscopically. In addition, the external appearance of the platinum plating layer P of the comparative example 4 is exhibiting silver (metal color).

  Moreover, the ease of removing the platinum plating layer P and the physical strength were confirmed by wiping with a tissue paper. As a result, in Examples 4 to 6, the platinum plating layer P was not removed, and in Comparative Example 5, black crystals were removed on the tissue paper.

  FIG. 9 shows potassium chloride standard solution A (11134 mS / m, 25 ° C.), B (1286 mS / m, 25 ° C.), C (140.9 mS / m, 25 ° C.), D (14.7 mS / M) according to JIS K 0101. m, 25 ° C.), the error (%) from the actual concentration (electrical conductivity) of each standard solution and the measured values of the test cells (specific examples 4 to 6, comparative examples 5, 6, and 7). ) Is calculated. FIGS. 9A, 9B, and 9C show the results obtained when the measurement frequencies are 3000 Hz, 900 Hz, and 60 Hz, respectively. In addition, about the specific examples 4-6, the substantially same result was obtained. In Comparative Example 6 and Comparative Example 7, almost the same results were obtained.

  9 (a) to 9 (b), at any measurement frequency, the cells of specific examples 4 to 6 on which the platinum plating layer P exhibiting gray was applied had a low concentration (low electrical conductivity) to a high concentration ( It can be seen that the error (%) is the smallest (high electrical conductivity). 9A to 9C, even in the case where the measurement frequency is decreased, the cells of specific examples 4 to 6 to which the gray platinum plating layer P is applied have a low concentration to a high concentration. The increase in error (%) is relatively small.

  As described above, by providing the platinum plating layer P according to the present invention, it is confirmed that there is less error (%) with respect to the actual concentration (electric conductivity) of the standard solution and the measurement concentration range is wider than before. It was done. It is surprising that the platinum plating layer P according to the present invention is not only difficult to remove and has increased physical strength, but also has a wider measurement concentration range than conventional platinum black plating. This is thought to be because the surface state of the platinum plating layer P according to the present invention is less affected by polarization than conventional platinum black.

  In addition, when using a cell, when measuring a low concentration sample, the measurement frequency is increased. However, by providing a platinum plating layer P according to the present invention, measurement is performed on a sample having the same concentration. The frequency can be lower. This is extremely advantageous in terms of simplification of the circuit configuration for the cell.

  On the other hand, as shown in FIGS. 9A to 9C, in the cell of Comparative Example 6 in which the platinum plating layer P exhibiting white is applied, the measured concentration range is the same as the cell of Comparative Example 7 having a platinum electrode that is not plated. It can be seen that is narrow. This is considered to be because the surface area expansion effect cannot be obtained because the platinum crystals are dense.

  In Table 2, the evaluation result of the measured concentration range is shown as being measurable when the error (%) is within 5%.

  In addition, although the thing which reversed the polarity of the electric current during plating and the thing which used alternating current as a plating current were examined, all are easy to take platinum plating layer P, mechanical strength, and the measurement concentration range are plating. Same or better than not reversing current polarity.

  As described above, according to the present embodiment, like the first embodiment, by providing the platinum plating layer P exhibiting gray on the platinum electrode 4 of the cell, it is difficult to remove the platinum plating layer P, and the physical strength is increased. Can be provided. Further, the linearity range (measurement concentration range) of the cell is widened, and the change of the cell constant is reduced, thereby enabling stable measurement.

It is a principal part schematic block diagram of the diaphragm type polarographic sensor (dissolved hydrogen sensor) which can apply this invention. It is a conceptual diagram for demonstrating the principle of this invention. It is explanatory drawing which shows the scanning electron microscope (SEM) image of the platinum plating layer surface which changes with plating conditions. It is explanatory drawing which shows the scanning electron microscope (SEM) image of the platinum plating layer surface which changes with plating conditions. It is a graph which shows the sensitivity stability of the dissolved hydrogen sensor according to this invention. It is a schematic diagram for demonstrating the platinum plating method. It is a principal part schematic block diagram of the diaphragm type | mold galvanic-cell-type sensor which can apply this invention. It is a principal part schematic block diagram of the electrical conductivity cell which can apply this invention. It is a graph for demonstrating the measurement concentration range of the electrical conductivity cell according to this invention.

Explanation of symbols

1A Dissolved hydrogen sensor (diaphragm type polarographic sensor)
1B Diaphragm type galvanic cell type sensor 1C Conductivity cell 2 Sensor body 3 Diaphragm 4 Working electrode 5 Counter electrode 6 Support tube 7 Electrolytic solution 10 Current generator P Platinum plating layer

Claims (13)

In an electrochemical sensor for measuring a current flowing through the electrode in response to a reaction of a component to be measured on the surface of the electrode,
An electrochemical sensor characterized in that a gray platinum plating layer is provided on the surface of the electrode.   One end of the sensor body is provided with a chamber that is partitioned from the outside by a diaphragm that allows the measurement target component in the sample to pass through. A working electrode and a counter electrode are disposed inside the chamber, and the working electrode and the counter electrode are accommodated in the chamber. An electrochemical sensor for measuring a current flowing between the working electrode and the counter electrode when a component to be measured that has passed through the diaphragm reacts on the surface of the working electrode while being in contact with the applied electrolyte. The electrochemical sensor according to claim 1, wherein a platinum plating layer exhibiting the gray color is provided on a surface of the working electrode.   The electrochemical sensor according to claim 1 or 2, which is a polarographic sensor or a galvanic cell sensor.   The electrochemical sensor according to claim 1, 2 or 3, wherein the electrochemical sensor is a dissolved hydrogen sensor. The platinum plating layer is provided by platinum plating with a direct current having a current density per unit time in a direction in which the platinum plating is performed of 0.05 to 0.3 mA / cm ² · s. The electrochemical sensor according to any one of claims 1 to 4. The platinum plating layer is platinum-plated with a direct current having a current density of 0.05 to 0.3 mA / cm ² · s per unit time in the direction in which the platinum plating is performed, and a direct current of the opposite polarity is applied. The electrochemical sensor according to claim 1, wherein the electrochemical sensor is provided by flowing for a predetermined period.   (A) After flowing a forward current in which platinum plating is performed for a predetermined period, flow a reverse polarity current for a predetermined period, or (b) after flowing the forward current for a predetermined period, A current is allowed to flow for a predetermined period, and further, the forward current is allowed to flow for a predetermined period, or (c) the forward current and the reverse polarity current are alternately supplied multiple times for a predetermined period. Item 6. The electrochemical sensor according to Item 6. The platinum plating layer is provided by performing platinum plating with an alternating current having a current density per unit time of 0.3 to 6 mA / cm ² · s in a direction in which the platinum plating is performed. The electrochemical sensor according to any one of items 1 to 4. In an electrical conductivity cell for measuring the electrical conductivity of a sample by applying a voltage between electrodes arranged to contact the sample,
An electric conductivity cell, wherein a surface of the electrode is provided with a gray platinum plating layer. The platinum plating layer is provided by platinum plating with a direct current having a current density per unit time in a direction in which the platinum plating is performed of 0.05 to 0.3 mA / cm ² · s. The electrical conductivity cell of claim 9. The platinum plating layer is platinum-plated with a direct current having a current density of 0.05 to 0.3 mA / cm ² · s per unit time in the direction in which the platinum plating is performed, and a direct current of the opposite polarity is applied. The electric conductivity cell according to claim 9, which is provided by flowing for a predetermined period.   (A) After flowing a forward current in which platinum plating is performed for a predetermined period, flow a reverse polarity current for a predetermined period, or (b) after flowing the forward current for a predetermined period, A current is allowed to flow for a predetermined period, and further, the forward current is allowed to flow for a predetermined period, or (c) the forward current and the reverse polarity current are alternately supplied multiple times for a predetermined period. Item 11. The electric conductivity cell according to Item 11. The platinum plating layer is provided by performing platinum plating with an alternating current having a current density per unit time of 0.3 to 6 mA / cm ² · s in a direction in which the platinum plating is performed. 9 electrical conductivity cells.