Nitrite ion measurement device and nitrite ion measurement method

Abstract

To provide a nitrite ion measurement device and a nitrite ion measurement method which can continuously and electrochemically measure the concentration of nitrite ions even when an interfering substance derived from an organism exists.SOLUTION: A nitrite ion measurement device for electrochemically measuring the concentration of nitrite ions includes: an electrode which is arranged contacting with a measured liquid and which has a sensor surface for detecting the nitrite ions in the measured liquid; and a voltage control part for controlling a voltage applied to the electrode. The voltage control part applies positive and negative voltages to the electrode so as to electropolish the electrode before applying a measurement voltage for detecting the nitrite ions to the electrode.SELECTED DRAWING: Figure 4

Description

Technical Field

[0001] The present invention relates to a nitrite ion measuring device and a nitrite ion measuring method.

Background Art

[0002] For example, in an environment where organisms exist in water, the concentration of nitrite ions is an important indicator that even affects the life and death of organisms. Therefore, it is desired to continuously measure the nitrite ion concentration with a configuration as simple as possible.

[0003] On the other hand, when electrochemically measuring the nitrite ion concentration, there is a risk that the measurement may be disturbed by substances derived from organisms present in the measurement target solution, resulting in a decrease in measurement accuracy.

[0004] As a method for suppressing a decrease in measurement accuracy due to substances that interfere with the measurement (hereinafter referred to as interfering substances), for example, as shown in Patent Document 1, a method of oxidizing interfering substances by applying a voltage to the measurement target solution and electrolyzing the measurement target solution is considered.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] However, it is still difficult to sufficiently remove interfering substances even by the method described in Patent Document 1, and at present, a measuring device capable of electrochemically continuously measuring the nitrite concentration has not been put into practical use.

[0007] The present invention has been made in view of such problems, and an object of the present invention is to provide a nitrite ion measuring device and a nitrite ion measuring method capable of electrochemically and continuously measuring the nitrite concentration even when there are interfering substances derived from organisms.

Means for Solving the Problems

[0008] That is, the nitrite ion measuring device according to the present invention is an electrochemical measuring device that electrochemically measures the nitrite ion concentration, and is arranged to be in contact with the measurement target liquid and has a sensor surface for detecting nitrite ions in the measurement target liquid. It comprises an electrode and a control unit for controlling the voltage applied to the electrode, and the control unit applies a positive voltage and a negative voltage to electrolytically polish the electrode before applying a measurement voltage for detecting nitrite ions to the electrode.

[0009] According to the nitrite ion measuring device configured as described above, since the electrode is electrolytically polished before applying the measurement voltage, even when continuously measuring nitrite ions, interfering substances adhering to the surface of the electrode can be removed, and furthermore, the surface of the electrode can be hydrogen-terminated, so the conductivity of the electrode surface can be increased and the accuracy of nitrite ion measurement can be improved.

[0010] As a specific embodiment of the present invention, an example can be given in which the control unit applies a pretreatment voltage different from the measurement voltage to the electrode after the electrolytic polishing and before applying the measurement voltage. The inventor has found that when not only nitrite ions but also nitrate ions coexist in the measurement target liquid, these nitrate ions affect the measurement result of the nitrite ion concentration. Therefore, as a result of further intensive studies to reduce the influence of this nitrate, the inventor has found that if a pretreatment voltage is applied before applying the measurement voltage, the influence of nitrate ions on the measurement result of the nitrite ion concentration can be suppressed to a small extent.

[0011] The pretreatment voltage is preferably in the range exceeding 0V and not exceeding +2.0V.

[0012] If the electrode is a diamond electrode or a diamond-like carbon electrode, it has high durability against voltage, so it is preferable because a higher voltage can be applied during electrolytic polishing for cleaning.

[0013] The measurement cell is provided with a flow path for supplying the sample solution and a flow rate control unit provided on the flow path for controlling the inflow rate of the sample solution into the measurement cell. If, after the flow rate control unit reduces the inflow rate of the sample solution into the measurement cell or sets it to zero, the control unit applies the pretreatment voltage and the measurement voltage to the electrode, the amount of interfering substances newly supplied into the measurement cell after the interfering substances are removed by the pretreatment voltage can be reduced, so the measurement accuracy of the nitrite ion concentration can be further improved. Further, if, after applying the pretreatment voltage and before applying the measurement voltage, the flow rate control unit allows the sample solution to flow around the electrode, the oxidized interfering substances can be removed from the electrode surface, so the measurement accuracy of the nitrite ion concentration can be further improved.

[0014] The present invention also includes a method for electrochemically measuring the nitrite ion concentration, which is an electrochemical measurement method for electrochemically measuring the nitrite ion concentration, and is characterized in that, before applying a measurement electrode for detecting nitrite ions, a positive voltage and a negative voltage are applied to electrolytically polish the electrode.

Advantages of the Invention

[0015] According to the present invention, even when interfering substances derived from organisms are present, the nitrite ion concentration can be electrochemically continuously measured.

Brief Description of the Drawings

[0016]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Mode for Carrying Out the Invention

[0017] Hereinafter, the nitrous acid concentration measuring device 100 according to an embodiment of the present invention will be described with reference to the drawings.

[0018] <Configuration of the nitrous acid concentration measuring device according to the present embodiment> The nitrous acid concentration measuring device 100 according to the present embodiment is, for example, a flow injection type electrochemical measuring device that performs three-electrode chronoamperometry measurement for analyzing the concentration of each component contained in a measurement target solution (hereinafter, also referred to as a sample solution), which is an electrolyte solution, by applying a voltage to the sample solution. As shown in FIGS. 1 and 2, its basic configuration includes a flow tube 2 in which a flow path 1 through which the sample solution flows is formed inside, a flow rate control unit that controls the flow rate of the sample solution flowing through the flow path 1, a sensor unit 3 provided on the flow path 1, a measurement circuit 4 for extracting a signal from the sensor unit 3, and an information processing device 5 that calculates the concentration of each component in the sample solution based on the voltage, current, etc. obtained by the measurement circuit 4.

[0019] Such a nitrous acid concentration measuring device 100 can be used for various applications. In this embodiment, as an example, the concentration of nitrite ions contained in a measurement target liquid in which organisms exist, such as treated water in factories and wastewater treatment plants, and environmental water such as rivers, marshes, lakes, seas, and fish farms, will be described.

[0020] The flow path 1 includes a main flow path 11 through which a sample solution flows, and a branch flow path 12 that branches from the main flow path 11 and is connected to the sensor unit 3. The flow rate control unit includes, for example, a valve 14 or a pump (not shown) provided on the flow path 1, and a valve (pump) control unit 53 that controls the operation of the valve 14 and the pump. In this embodiment, a valve 14 for controlling the inflow of a sample solution or the like from the main flow path 11 to the branch flow path 12 is provided on the flow path 1. When this valve 14 is opened, a liquid such as a sample solution flows from the main flow path 11 to the branch flow path 12, and when this valve 14 is closed, the liquid feeding from the main flow path 11 to the branch flow path 12 stops.

[0021] The sensor unit 3 is connected downstream of the valve 14 in the branch flow path 12, and includes a measurement cell 31 that houses a sample solution therein, a working electrode 32 attached to the measurement cell 31 so as to contact the sample solution housed inside the measurement cell 31, a reference electrode 33, and a counter electrode 34 (also referred to as a counter electrode or an opposite electrode).

[0022] The working electrode 32 is provided with a sensor surface 321 for applying a voltage in contact with the sample solution to detect a measurement target object. For example, the sensor surface 321 is a diamond electrode formed of boron-doped diamond having conductivity by adding boron at a high concentration.

[0023] The reference electrode 33 is an electrode that serves as a reference for the potential of the working electrode 32. In this embodiment, a silver / silver chloride electrode is used.

[0024] When setting the potential at the working electrode 32, the counter electrode 34 allows current to flow through the working electrode 32 without any hindrance. In the present embodiment, similar to the working electrode 32, a boron-doped diamond electrode is used.

[0025] The measurement cell 31 is, for example, in the shape of a block as shown in FIG. 3, and has an inlet 311 for introducing the sample solution and an outlet 312 for discharging the sample solution. The sample solution is accommodated in an internal flow path 313 formed between the inlet 311 and the outlet 312.

[0026] The aforementioned working electrode 32, reference electrode 33, and counter electrode 34 are arranged such that each is in contact with the sample solution accommodated in the internal flow path 313. In particular, the working electrode 32 is arranged in a posture in which its sensor surface 321 is inclined or perpendicular to the flow of the sample solution flowing through the internal flow path 313. Specifically, as shown in FIG. 3, the internal flow path 313 is shaped to guide the sample solution flowing toward the sensor surface 321 of the working electrode 32 to hit the sensor surface 321 at a perpendicular or nearly perpendicular angle. By shaping the internal flow path 313 in such a way, the flow of the sample solution hitting the sensor surface 321 of the working electrode 32 becomes a turbulent flow, and the bubbles generated on the sensor surface 321 can be efficiently flushed away. As a result, the measurement accuracy can be further improved.

[0027] Furthermore, in the measurement cell 31 according to the present embodiment, since the inlet 311 is arranged below the sensor surface 321 and the outlet 312 is arranged above the sensor surface 321, it is easy to discharge the bubbles mixed with the sample solution from the inlet 311 and the bubbles generated on the sensor surface 321 of the working electrode 32 to the outside of the measurement cell 31 from the outlet 312.

[0028] The measurement circuit 4 applies a voltage to the working electrode 32, reference electrode 33, and counter electrode 34, and detects the current value at the applied voltage. For example, it includes a potentiostat.

[0029] The information processing device 5 includes a voltage control unit 51 that controls the voltage applied to the measurement circuit 4, a calculation unit 52 that obtains a current-time (voltage) curve based on the voltage signal and current signal output from the measurement circuit 4, and calculates the concentration of nitrite ions in the sample solution based on this current-time (voltage) curve, and a valve control unit 53 that controls the opening and closing of the valve 14 provided in the flow path 1.

[0030] The specific configuration of the information processing device 5 includes a CPU, a memory, an A / D converter, a D / A converter, etc. The CPU and peripheral devices cooperate according to a program stored in a predetermined area of the memory to function as the voltage control unit 51, the calculation unit 52, and the valve control unit 53. Also, as shown in FIG. 2, the information processing device 5 may include a recording unit that records the potential output from the sensor unit 3, the potential difference obtained by the measurement circuit 4, the calculated value calculated by the calculation unit 52, etc., and a display unit that displays these values. The information processing device 5 may further include an output unit that outputs values as described above to, for example, a recording medium. The output value output from the output unit may be displayed on an external display unit, or may be taken into a general-purpose PC or the like for further arithmetic processing.

[0031] <Electrochemical Measurement Method Using the Electrochemical Measurement Device According to the Present Embodiment> A method and procedure for measuring the concentration of nitrite ions in a sample solution using the nitrite concentration measurement device 100 as described above will be described below.

[0032] (Electropolishing Step) For example, as shown in FIG. 4, the voltage control unit 51 performs electropolishing of the working electrode 32 by sweeping a positive voltage greater than the measurement voltage for detecting nitrite ions on the working electrode 32 and a negative voltage having an absolute value approximately the same as that of the positive voltage as an electropolishing step. The absolute values of these positive and negative voltages are preferably 2 times or more and 10 times or less, and more preferably 2 times or more and 5 times or less of the measurement voltage described later. The procedure of this electropolishing step is, for example, as follows. With the sample solution contained in the measurement cell 31 and the sensor surface 321 of the working electrode 32 in contact with the sample, a voltage of, for example, +2.5 V is applied to the working electrode 32 by the measurement circuit 4 that has received a command from the voltage control unit 51 for 2 seconds, and then the voltage applied to the working electrode 32 is returned to 0. Next, the valve 14 provided on the branch flow path 12 is opened for 1 second by a signal from the valve control unit 53, so that the sample solution flows into the measurement cell 31 through the branch flow path 12, and liquid replacement in the measurement cell 31 is performed. After the valve 14 is closed, a voltage of -2.5 V is applied to the working electrode 32 by the measurement circuit 4 that has received a command from the voltage control unit 51 for 1 second, and then a voltage of -3.5 V is applied for 1 second. Subsequently, the voltage applied to the working electrode 32 is returned to 0, and the electrolytic polishing process ends.

[0033] This electrolytic polishing is performed, for example, when the first measurement is made after turning on the power of the nitrous acid concentration measuring device 100, or when a measurement sequence is repeated a predetermined number of times or more, for example, four times. In other cases, it is set to be performed once each time a measurement sequence ends.

[0034] (Pretreatment process) After the electrolytic polishing process described above, the pretreatment process is started. The pretreatment process is performed, for example, according to the following procedure. The valve 14 provided on the branch flow path 12 is opened for 1 second by a signal from the valve control unit 53, so that the sample solution flows into the measurement cell 31 through the branch flow path 12, and liquid replacement is performed. After the valve 14 is closed again and the flow rate of the sample solution flowing into the measurement cell 31 becomes zero, a pretreatment voltage of +1.3 V is applied to the working electrode 32 by the measurement circuit 4 that has received a command from the voltage control unit 51 for 2 seconds, and the pretreatment process is performed. In this embodiment, the measurement circuit 4 applies a voltage to the working electrode 32 while performing liquid replacement so that the timing when the valve 14 is closed coincides with the timing when the pretreatment voltage of +1.3 V starts to be applied to the working electrode 32.

[0035] (Measurement process) The measurement process is performed, for example, according to the following procedure. A measurement voltage of +1.0 V is applied to the working electrode 32, the reference electrode 33, and the counter electrode 34 for 2 seconds by the measurement circuit 4 that has received a command from the voltage control unit 51. At this time, an electrochemical reaction corresponding to the applied voltage occurs on the surface of the working electrode 32, and the electrical signal generated at this time is detected by the measurement circuit 4, sent to the calculation unit 52, analyzed, and the nitrite ion concentration in the sample solution is calculated. The measurement process ends when the voltage applied to the working electrode 32, the reference electrode 33, and the counter electrode 34 is returned to 0 V. In the present embodiment, during the process of returning the voltage applied to the working electrode 32 to 0 after the above-described pretreatment process, and also during the period from when the voltage applied to the working electrode 32 is 0 until the measurement voltage is applied, liquid replacement is performed. This liquid replacement is performed by opening and closing the valve 14 provided on the branch flow path 12 by a signal from the valve control unit 53 as described above. When the application of the pretreatment voltage ends in the pretreatment process and the voltage starts to drop from +1.3 V, the valve 14 is opened, and when the voltage applied to the working electrode 32 reaches the measurement voltage of +1.0 V, the valve 14 is closed at the same time.

[0036] <Effects of the present embodiment> According to the nitrite concentration measuring apparatus 100 or the nitrite concentration measuring method according to the present embodiment, since electropolishing is performed before applying the measurement voltage, contaminants contained in the sample solution adhering to the surface of the working electrode 32 can be removed before measurement and the surface of the working electrode 32 can be hydrogen-terminated. Therefore, the concentration of nitrite ions can be accurately measured while avoiding the influence of the contaminants as much as possible.

[0037] In this embodiment, since a positive pretreatment voltage is applied before applying the measurement voltage to the working electrode 32, even when there are other reducible substances (e.g., nitric acid) in the sample solution in addition to nitrite ions, the nitrite ion concentration can be measured as accurately as possible. The reasons are considered as follows.

[0038] During electropolishing, reducible substances (e.g., nitrate ions) contained in the sample solution may be reduced by the negative voltage applied, resulting in interfering substances (e.g., ammonium ions) that may adsorb on the surface of the working electrode 32. If the reduced interfering substances are adsorbed on the surface of the working electrode 32 in this way, these interfering substances cover the surface of the working electrode 32, preventing nitrite ions from approaching the surface of the working electrode 32, and an error will occur in the detection value. Therefore, after electropolishing, a positive pretreatment voltage is applied to remove the reduced interfering substances (e.g., ions with positive charges such as ammonium ions), and by removing these interfering substances in advance, only the oxidation reaction derived from nitrite ions in the sample solution can be detected when the measurement voltage is applied.

[0039] The effect of reducing the interference of interfering substances by applying the pretreatment voltage described above has been confirmed by the following experiment. (Experimental method) As the sample solution, an aqueous potassium chloride solution (zero water), an aqueous sodium nitrate solution, and sodium nitrite were used. For the aqueous sodium nitrate solution and the aqueous sodium nitrite solution, sodium nitrate or sodium nitrite was dissolved in pure water, and after adjusting to a nitrate ion concentration of 100 mg-N / L and a nitrite ion concentration of 1 mg-N / L, potassium chloride was dissolved to adjust the electrical conductivity to about 0.2 mS / cm for use. Using this sample solution, the nitrite ion concentration was measured for both the case where the pretreatment voltage was not applied and the case where it was applied. The results are shown in FIG. 5.

[0040] (Results) In FIG. 5, the horizontal axis represents time and the vertical axis represents the detected value (current value). Also, the upper graph in FIG. 5 shows the results when the pretreatment voltage is 0 V (no pretreatment voltage), 1.1 V, 1.3 V, and 1.5 V superimposed, and the two lower graphs separately show the results for 1.3 V and 1.5 V in the upper graph for easy viewing.

[0041] From the results in FIG. 5, when no pretreatment voltage is applied, a large peak due to sodium nitrate appears around 5 to 10 minutes. However, when a pretreatment voltage is applied, regardless of whether 1.1 V, 1.3 V, or 1.5 V is adopted, the peak due to sodium nitrate becomes smaller compared to the case where no pretreatment voltage is applied, and the peak of nitrite ions due to sodium nitrite around 20 minutes also approaches the original peak magnitude. From this result, it was found that even when there are reducible interfering substances such as nitrate ions, by applying a pretreatment voltage before applying the measurement voltage, the influence can be suppressed as much as possible and nitrite ions can be accurately measured. It was found that the voltage of the pretreatment voltage may be 0 V or higher, but if it is 1.0 V or higher, the influence of the interfering substance can be suppressed more. On the other hand, if a voltage that is too high as the pretreatment voltage is applied for a long time, the surface of the working electrode may be oxygen-terminated and the measurement sensitivity may decrease. From such a viewpoint, it is preferable to apply a voltage of 3 V or less as the pretreatment voltage in the range of 2 seconds or less. Note that the pretreatment voltage may be a voltage different from the measurement voltage or the same voltage.

[0042] While the voltage control unit is electrochemically polishing the working electrode, while applying the pretreatment voltage to the working electrode, and while applying the measurement voltage to the working electrode, the flow of the sample solution around the working electrode is stopped so that no new sample solution is supplied to the working electrode. Therefore, it is possible to suppress the supply of new interfering substances to the surface of the working electrode and make the surface state of the working electrode more suitable for measurement. Also, since liquid replacement is performed after applying the pretreatment voltage and before applying the measurement voltage, substances derived from interfering substances oxidized by the application of the pretreatment voltage can be removed from the surface of the working electrode. As a result, the accuracy of nitrite ion concentration measurement when applying the measurement voltage can be further improved.

[0043] By performing liquid replacement between applying the pretreatment voltage to the working electrode and applying the first measurement voltage after that, interfering substances oxidized by applying the pretreatment voltage can be removed, so the accuracy of nitrite measurement can be further enhanced.

[0044] Also, as described above, since the shape of the measurement cell 31 and the arrangement of the working electrode 32 are such that bubbles are less likely to stay near the sensor surface 321 of the working electrode 32, the influence of bubbles generated when the voltage control unit 51 applies a removal voltage to the working electrode 32 can also be reduced.

[0045] Furthermore, since the working electrode 32 and the counter electrode 34 are boron-doped diamond electrodes, they have a wide potential window (wide oxidation potential and reduction potential), a low background current compared to other electrode materials, and further advantages such as excellent chemical resistance, durability, electrical conductivity, and corrosion resistance, so they are suitable. If it is a diamond electrode, since the durability against voltage is high, a higher voltage can be applied than electrodes of other materials, and the frequency of electrode replacement can be suppressed.

[0046] Since voltammetry measurement using a three-electrode system with the working electrode 32, the reference electrode 33, and the counter electrode 34 is adopted, no special reagent is required, and the nitrite concentration can be measured accurately and easily while suppressing the influence of the potential window.

[0047] Since the nitrite concentration measuring device 100 includes the main flow path 11 and the branch flow path 12, and the sensor unit 3 is provided on the branch flow path 12, the response time of liquid feeding accompanying the opening and closing of the valve 14 can be shortened.

[0048] Since the valve 14 is arranged upstream of the measurement cell 31, the time until the flow of the sample solution in the measurement cell 31 stops due to the sample flowing from the branch flow path 12 into the measurement cell 31 after closing the valve 14 can be made shorter.

[0049] <Other modified embodiments of the electrochemical measurement device according to the present invention> The present invention is not limited to the above-described embodiments. For example, the calculation unit is not limited to calculating the nitrite ion concentration, and may calculate the nitrous acid concentration.

[0050] The size of the entire nitrite ion measurement device according to the present invention is not particularly limited, but it is preferably of a size that can be carried so that it can be easily installed at the site where the measurement target liquid is present and the nitrite ion concentration can be measured on-site.

[0051] In the electrolytic polishing process described above, liquid replacement for causing a liquid flow around the working electrode may be performed before the positive voltage starts to be applied or after the negative voltage is applied, or liquid replacement may be performed after the positive voltage is applied and before the negative voltage is applied.

[0052] The voltage during electrolytic polishing applied to the working electrode, the voltage during pretreatment, the measurement voltage, etc. are not limited to those described above, and can be appropriately changed according to the type of the sample solution. For example, as shown in FIG. 6, the measurement process may be started without returning the voltage applied to the measurement electrode to 0 immediately after the above-described pretreatment process. Also, the time for applying these voltages and the length of the time for performing liquid replacement can be appropriately changed.

[0053] In the above-described embodiments, the case where nitrate ions coexist has been described. However, when there are no competing factors that affect the measurement of nitrite ions such as nitrate ions or when the concentration of the competing factors is known, the pretreatment voltage does not necessarily have to be applied.

[0054] The electrochemical measurement device is not limited to the three-electrode type as described above, and may also be a two-electrode type, a four-electrode type, or a six-electrode type.

[0055] The working electrode is not limited to a boron-doped diamond electrode, and may be a conductive diamond electrode doped with an element of Group 13 or Group 15 such as nitrogen or phosphorus. Further, it is not limited to a diamond electrode, and may be a carbon electrode containing carbon such as a carbon electrode, a glassy carbon electrode, or a diamond-like carbon electrode, an electrode using a noble metal such as gold or platinum, or an electrode using an alloy containing these noble metals. The reference electrode is not limited to the silver / silver chloride electrode described above, and for example, a standard hydrogen electrode, a mercury / mercury chloride electrode, a hydrogen palladium electrode, etc. can also be used. Furthermore, regarding the counter electrode as well, it is not limited to a diamond electrode, and for example, electrodes such as carbon, stainless steel, gold, silver, silver chloride, platinum, and SnO2 can be used.

[0056] The valve only needs to be able to control the flow of the sample solution to the measurement cell 31, and may be arranged downstream of the measurement cell 31, or may be provided on both the upstream side and the downstream side of the measurement cell.

[0057] Instead of using the main flow path and the branch flow path, a measurement cell may be provided in the main flow path and the valve may control the flow of the main flow path.

[0058] Also, the flow rate control unit only needs to be able to create or block the flow of the sample solution around each electrode. For example, the flow rate control unit is not limited to one equipped with a valve, and may be one that controls a pump provided in the main flow path or the branch flow path. When a pump is provided instead of a valve in this way, the flow rate control unit operates the pump to supply a fluid such as a sample solution to the measurement cell, and stops the pump to stop the supply of the fluid to the measurement cell. Also, the flow rate control unit may be, for example, a valve, a pump, a rotating member, etc. provided on the internal flow path inside the measurement cell other than those described above. When replacing the liquid, the liquid flowing through the main flow path, the branch flow path, and the measurement cell may be not only the sample solution described above, but also a calibration liquid that does not contain the substance to be measured, a cleaning liquid for the measurement cell, and the like.

[0059] The valve control unit is not limited to opening the valve only while no voltage is applied to the working electrode, and may always keep the valve open, or may control the flow rate of the sample solution flowing into the measurement cell when a voltage equal to or higher than a predetermined value is applied to the working electrode to be smaller than that when a voltage smaller than the predetermined value is applied to the working electrode.

[0060] The measurement cell is not limited to having the sensor surface above the inlet and the outlet above the sensor surface as described above. For example, various shapes as shown in FIGS. 7 and 8 can be adopted. Also, regarding the shape of the internal flow path formed in the measurement cell, for example, when measuring a sample solution that is less likely to generate bubbles, it does not have to be a shape in which the flow of the sample solution collides vertically or obliquely with the sensor surface of the working electrode as described above. Further, in the above embodiment, the arrangement order of the working electrode, the reference electrode, and the counter electrode in the internal flow path is described as being arranged in the order of the reference electrode, the working electrode, and the counter electrode from the side closer to the inlet, but the arrangement order of these electrodes is not limited to this and can be appropriately changed. Also, the arrangement location of each electrode in the internal flow path can be appropriately changed.

[0061] In the above-described embodiment, the case where the electrochemical measurement device is of the flow injection type has been described, but a batch-type electrochemical measurement device that immerses the sensor unit in a sample solution stored in a treatment tank, an aquarium, a beaker, etc. may also be used. In this case, by using a rotary electrode as shown in FIG. 9 that can create a flow of the sample solution around each electrode, the electrode itself may function as a flow rate control unit. In addition, various modifications and combinations of embodiments may be made as long as they do not depart from the spirit of the present invention.

Description of Symbols

[0062] 100 ··· Electrochemical measurement device 31 ··· Measurement cell 311 ··· Inlet 312 ··· Outlet 313 ··· Internal flow path 32 ··· Working electrode 33 ··· Reference electrode 34 ··· Counter electrode 321 ··· Sensor surface 51 ··· Voltage control unit

Claims

1. A measuring device for electrochemically measuring the concentration of nitrite ions, comprising: an electrode arranged to contact a liquid to be measured and having a sensor surface for detecting nitrite ions in the liquid to be measured; a voltage control unit for controlling the voltage applied to the electrode; The voltage control unit is characterized in that, before applying a measurement voltage for detecting nitrite ions to the electrode, a positive voltage and a negative voltage are applied to electrolytically polish the electrode. A nitrite ion measuring device.

2. The nitrite ion measuring device according to claim 1, wherein the voltage control unit applies a positive pretreatment voltage to the electrode, which is different from or the same as the measurement voltage, after the electrolytic polishing and before applying the measurement voltage.

3. The nitrite ion measuring device according to claim 1 or 2, wherein the pretreatment voltage ranges from more than 0 V to 2.0 V or less.

4. The nitrite ion measuring device according to any one of claims 1 to 3, wherein the voltage control unit applies the pretreatment voltage for a period exceeding 0 seconds and not exceeding 3 seconds.

5. The nitrite ion measuring device according to any one of claims 1 to 4, wherein the electrode is a diamond electrode or a diamond-like carbon electrode.

6. The device further comprises a measurement cell having an inlet for introducing the sample solution and an outlet for discharging the sample solution, and accommodating the sample solution in an internal flow path formed between the inlet and the outlet. The nitrite ion measuring device according to any one of claims 1 to 5, wherein the sensor surface is arranged in the internal flow path so as to contact the sample solution.

7. a flow path for supplying the sample solution to the measurement cell; a flow rate control unit provided on the flow path or the internal flow path for controlling the flow rate of the sample solution supplied to the measurement cell; The nitrite ion measuring device according to any one of claims 6 to 8, wherein after the flow rate control unit reduces the flow rate or sets it to zero, the voltage control unit applies the pretreatment voltage and the measurement voltage to the electrode.

8. The nitrite ion measuring device according to claim 7, wherein while the voltage control unit applies a voltage to the electrode for electrolytic polishing, while applying the pretreatment voltage and / or while applying the measurement voltage, the flow rate control unit sets the flow rate to zero.

9. The nitrite ion measuring device according to claim 7 or 8, wherein after applying the pretreatment voltage and before applying the measurement voltage, the flow control unit causes a sample solution to flow around the electrode.

10. The nitrite ion measuring device according to any one of claims 1 to 9, characterized in that the electrode includes a working electrode, a counter electrode, and a reference electrode.

11. A measuring method for electrochemically measuring the concentration of nitrite ions, A nitrite ion measuring method, characterized in that before applying a measurement electrode for detecting nitrite ions to an electrode for detecting nitrite ions, a positive voltage and a negative voltage are applied to electrolytically polish the electrode.

12. A program for a measuring device for electrochemically measuring the concentration of nitrite ions, comprising: an electrode having a sensor surface disposed so as to be in contact with a measurement target solution and detecting nitrite ions in the measurement target solution; and a voltage control unit for controlling a voltage applied to the electrode. A program for a nitrite ion measuring device, which causes a computer to function as a voltage control unit that applies a positive voltage and a negative voltage to electrolytically polish the electrode before applying a measurement electrode for detecting nitrite ions to the electrode.

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