Electrochemical measurement apparatus and electrochemical measurement method

Abstract

To provide an electrochemical measurement device that can accurately measure the concentration of binding chlorine by a voltammetry technique not using a reaction reagent.SOLUTION: An electrochemical measurement device for electrochemically measuring the concentration of the remaining chlorine in a sample solution includes: an electrode arranged in contact with the sample solution, the electrode detecting remaining chlorine; and a voltage controller for controlling a voltage to apply to the electrode. The voltage controller applies an oxidation voltage for oxidizing binding chlorine to the electrode, and applies a measurement voltage for measuring the binding chlorine.SELECTED DRAWING: Figure 4

Description

Technical Field

[0001] The present invention relates to an electrochemical measurement device and an electrochemical measurement method.

Background Art

[0002] For example, in the process of washing and sterilizing food, it is necessary to strictly control the chlorine concentration range in which sterilization is properly performed. Therefore, it is required to accurately measure the residual chlorine concentration in the cleaning liquid after washing the food.

[0003] Most of the residual chlorine contained in the cleaning liquid exists as free chlorine (HClO or ClO-), but a very small part of this free chlorine reacts with nitrogen-containing compounds such as ammonia and exists as bound chlorine such as chloramine. Therefore, in order to accurately measure the residual chlorine concentration in the sample solution, it is necessary to measure the total concentration of free chlorine and bound chlorine contained in the cleaning liquid.

[0004] However, since the concentration of bound chlorine is very small, for example, 50 ppm or less, compared to free chlorine, it is particularly difficult to accurately measure the concentration of bound chlorine in reagentless voltammetry that measures the chlorine concentration by measuring the current caused by the redox reaction at the working electrode without using a reaction reagent such as iodine solution. Therefore, in the measurement of residual chlorine by voltammetry without using a reaction reagent, as described in Patent Document 1, only free chlorine is currently measured as residual chlorine.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] The present invention has been made in view of the above-described problems, and an object thereof is to provide an electrochemical measurement apparatus capable of accurately measuring the concentration of bound chlorine in a voltammetry method without using a reaction reagent.

Means for Solving the Problems

[0007] As a result of intensive studies by the present inventors to accurately measure bound chlorine, when a measurement voltage for measuring bound chlorine is applied after applying an oxidation voltage for oxidizing bound chlorine, a significant change in the current value depending on the concentration of bound chlorine can be observed. It was completed only after finding out that it can be done.

[0008] That is, the electrochemical measurement apparatus according to the present invention electrochemically measures the residual chlorine concentration contained in a sample solution, and includes an electrode disposed so as to be in contact with the sample solution for detecting residual chlorine, and a voltage for applying to the electrode. A voltage control unit for controlling the voltage, wherein the voltage control unit applies a measurement voltage for measuring bound chlorine after applying an oxidation voltage for oxidizing bound chlorine to the electrode.

[0009] According to the electrochemical measurement apparatus configured as described above, since the change in the current value depending on the concentration of bound chlorine can be detected more remarkably than before, the concentration of bound chlorine in the sample solution can be measured more accurately than before.

[0010] As a specific embodiment of the present invention, those in which the oxidation voltage is +1.0 V or more and +2.0 V or less can be mentioned. Further, those in which the measurement voltage applied by the voltage control unit to the electrode is -0.7 V or more and -0.2 V or less can be mentioned.

[0011] If the electrode is a diamond electrode or a diamond-like carbon electrode, it is preferable because it has high durability against voltage and can suppress the adverse effect on the electrode by applying the oxidation voltage.

[0012] A measurement cell having an inlet for introducing the sample solution and an outlet for discharging the sample solution, accommodating the sample solution in an internal flow path formed between the inlet and the outlet, and having the electrodes disposed in the internal flow path so as to be in contact with the sample solution; A flow path for supplying the sample solution to the measurement cell, 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 oxidation voltage and the measurement voltage to the electrodes, it is considered that the change in the current value can be detected more significantly because the bound chlorine oxidized by the oxidation voltage can be detected in a state where it exists around the electrodes.

[0013] The present invention also includes an electrochemical measurement method for electrochemically measuring the residual chlorine concentration contained in a sample solution, characterized in that after applying an oxidation voltage for oxidizing bound chlorine to an electrode disposed in contact with the sample solution to detect residual chlorine, a measurement voltage for measuring the bound chlorine is applied.

Advantages of the Invention

[0014] According to the present invention, even when measuring a sample solution containing an interfering substance having the same charge as the measurement target substance, the concentration of the measurement target substance can be accurately measured.

Brief Description of the Drawings

[0015]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Mode for Carrying Out the Invention

[0016] Hereinafter, the electrochemical measurement device 100 according to an embodiment of the present invention will be described with reference to the drawings. <Configuration of the Electrochemical Measurement Device According to this Embodiment> The electrochemical measurement device 100 according to this embodiment is, for example, a flow injection type electrochemical measurement device 100 that performs three-electrode voltammetry measurement for analyzing a sample by applying a voltage to a sample solution that is an electrolyte solution. As shown in FIGS. 1 and 2, its basic configuration includes a flow tube 2 in which a sample supply channel 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 sample supply channel 1, a sensor unit 3 provided on the sample supply channel 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 components in the sample based on the voltage, current, etc. obtained by the measurement circuit 4.

[0017] Such an electrochemical measurement device 100 can be used for various applications. In this embodiment, as an example, an explanation will be given of a device for measuring the residual chlorine concentration contained in a cleaning liquid obtained by cleaning foods such as vegetables.

[0018] Residual chlorine refers to all the available chlorine contained in the aqueous solution. Available chlorine consists of free chlorine such as hypochlorous acid (HClO), hypochlorite ion (ClO - -), dissolved chlorine (Cl2), etc., and combined chlorine such as monochloroamine (NH2Cl), dichloroamine (NHCl2), trichloroamine (NCl3), etc.

[0019] The sample supply channel 1 includes a main channel 11 through which the cleaning liquid, which is the sample solution, flows, and a branch channel 12 that branches off from the main channel 11 and is connected to the sensor unit 3. The flow rate control unit includes, for example, a valve 14 or a pump provided on the sample supply channel 1, and a valve control unit 53 that controls these valve 14 or pump, etc. In this embodiment, a valve 14 for controlling the inflow of the sample, etc. from the main channel 11 to the branch channel 12 is provided on the sample supply channel 1. When this valve 14 is opened, the liquid such as the sample flows from the main channel 11 to the branch channel 12, and when this valve 14 is closed, the liquid feeding from the main channel 11 to the branch channel 12 stops.

[0020] The sensor unit 3 is connected downstream of the valve 14 in the branch channel 12, and includes a measurement cell 31 that houses the sample solution inside, a working electrode 32 attached to the measurement cell 31 so as to contact the sample housed inside the measurement cell 31, a reference electrode 33, and a counter electrode 34.

[0021] The working electrode 32 is provided with a sensor surface 321 for applying a voltage in contact with the sample solution to detect the measurement 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.

[0022] 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.

[0023] When setting the potential at the working electrode 32, the counter electrode 34 is configured to allow 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.

[0024] 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.

[0025] 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 such that 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.

[0026] 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, 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 can be easily discharged from the outlet 312 to the outside of the measurement cell 31.

[0027] 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.

[0028] 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-voltage curve based on the voltage signal and current signal output from the measurement circuit 4 and calculates the concentration of residual chlorine in the sample based on this current-voltage curve, and a valve control unit 53 that is a fluid control unit that controls the opening and closing of a valve 14 provided in the sample supply flow path 1.

[0029] 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 perform the functions of the voltage control unit 51, the calculation unit 52, and the valve control unit 53. Further, 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 the above-described values 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, etc. for further arithmetic processing.

[0030] <Electrochemical Measurement Method Using the Electrochemical Measurement Device According to the Present Embodiment> A method and procedure for measuring the residual chlorine concentration in a sample solution using the electrochemical measurement device 100 as described above will be described below. In this embodiment, the case where an aqueous NaClO solution is used as a cleaning liquid for cleaning vegetables will be described.

[0031] The state of the measurement target substance contained in the sample solution changes depending on the pH of the sample solution. However, in the case of this embodiment, since an aqueous NaClO solution is used, the pH is 8.0 or higher.

[0032] First, as a pretreatment step, a voltage of -2.5 to +2.0 V is swept across the working electrode. The specific procedure of this pretreatment step is, for example, as follows. The sample solution is contained in the measurement cell 31, and with the sensor surface 321 of the working electrode 32 in contact with the sample, a voltage of +2.0 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.

[0033] 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. 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. Up to this liquid replacement is the pretreatment step.

[0034] After the above-described pretreatment step, the measurement step of free chlorine (NaClO) is started. The measurement step is performed, for example, according to the following procedure. After the pretreatment step is completed and the valve 14 is closed again and the flow rate of the sample solution flowing into the measurement cell 31 becomes zero, a measurement voltage of +1.3 V is applied to the working electrode 32, the reference electrode 33, and the counter electrode 34 by the measurement circuit 4 that has received a command from the voltage control unit 51 for 2 seconds. 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, and analyzed, and the free chlorine concentration in the sample is calculated. The measurement step of free chlorine ends when the voltage applied to the working electrode 32, the reference electrode 33, and the counter electrode 34 is returned to 0 V.

[0035] When the measurement step of free chlorine ends, the above-described pretreatment step is performed again. When the pretreatment step is completed, next, the measurement step of combined chlorine is started. The measurement step is performed, for example, according to the following procedure.

[0036] After the above-described pretreatment process is completed and the valve 14 is closed again so that the flow rate of the sample solution flowing into the measurement cell 31 becomes zero, an oxidation voltage (+2V, with reference to the counter electrode 34) that oxidizes the bound chlorine to the working electrode 32 is applied for 5 seconds by the measurement circuit 4 that has received a command from the voltage control unit 51. Immediately thereafter, a measurement voltage of -0.5V (with reference to the counter electrode 34) is applied to the working electrode 32 for 2 seconds. 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, and analyzed to calculate the concentration of bound chlorine in the sample. This measurement process of bound chlorine ends when the voltage applied to the working electrode 32 returns to 0V. In the present embodiment, since the oxidation voltage and the measurement voltage described above are continuously applied while the valve 14 remains closed after the pretreatment process is completed, no liquid replacement occurs in the measurement cell from the start of applying the oxidation voltage until the end of applying the measurement voltage.

[0037] The calculation unit 52 calculates the concentration of bound chlorine from the value of the electrical signal detected for the sample solution, which is the measurement mode, using a correlation relational expression such as a calibration curve between the electrical signal (for example, current value) created using bound chlorine with a known concentration and the bound chlorine concentration.

[0038] In the measurement process of bound chlorine according to the present embodiment, the voltage control unit 51 applies, for example, an oxidation voltage for oxidizing bound chlorine to the working electrode 32 for about 5 seconds immediately before applying the measurement voltage to the working electrode 32. Since the measurement voltage of bound chlorine in the present embodiment is -0.5V as described above, in this case, the oxidation voltage is preferably set to a voltage sufficient to oxidize various forms of bound chlorine regardless of the conditions, for example, +2.0V or the like.

[0039] Here, "immediately after" means a predetermined time until, for example, the time required for the oxidized bound chlorine to uniformly diffuse in the sample solution has elapsed. The time required for the bound chlorine oxidized by the oxidation voltage to uniformly diffuse is considered to vary depending on the volume of the internal flow path 313 of the measurement cell 31, the flow rate of the sample solution flowing into the measurement cell 31, etc. In the case of this embodiment, it is preferably within 5 seconds, more preferably within 3 seconds, and particularly preferably within 1 second.

[0040] Finally, the calculation unit 52 calculates the residual chlorine concentration obtained by summing the calculated free chlorine concentration and bound chlorine concentration, and outputs it to the display unit.

[0041] <Effects of this Embodiment> According to the electrochemical measurement device 100 or the electrochemical measurement method according to this embodiment, by oxidizing before measuring bound chlorine, the amount of change in the current value due to bound chlorine can be made larger than before. As a result, the concentration of bound chlorine contained in the sample solution can be accurately measured at a low concentration.

[0042] 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, corrosion resistance, etc., so they are suitable.

[0043] For diamond electrodes such as the boron-doped diamond electrodes described above, since they have high durability against voltage, a higher voltage can be applied compared to electrodes made of other materials, and the frequency of electrode replacement can be suppressed.

[0044] Since a three-electrode voltammetry measurement using the working electrode 32, the reference electrode 33, and the counter electrode 34 is adopted, no special reagent is required, and the influence of the potential window can be suppressed to accurately and easily measure the residual chlorine concentration.

[0045] The electrochemical measurement device 100 includes a main flow path 11 and a branch flow path 12, and since the sensor unit 3 is provided on the branch flow path 12, the response time of the liquid feeding accompanying the opening and closing of the valve 14 can be shortened.

[0046] 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 into the measurement cell 31 from the branch flow path 12 after closing the valve 14 can be made shorter.

[0047] The present invention is not limited to the above-described embodiment. For example, the electrochemical measurement device or the electrochemical measurement method according to the present invention can be used not only for food samples but also for various sample solutions such as tap water, drinking water, river and lake water, industrial wastewater, industrial waste liquid, and experimental reagents.

[0048] The size of the entire electrochemical 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 such as a food production line in a food factory and measured on-site.

[0049] The materials of the pipes forming each flow path, the measurement cell, the electrodes, etc. are not particularly limited, but when the electrochemical measurement device is incorporated into a food manufacturing process or the like as described above, it is preferable to use materials that comply with the Food Sanitation Law. For example, it is conceivable to use materials that have obtained certification under the Food Sanitation Law such as polymethylpentene (PMP), polypropylene (PP), polyethylene (PE), and polyvinyl chloride (PVC) as the material of the measurement cell.

[0050] The oxidation voltage applied to the working electrode is not limited to those within the aforementioned range, and any voltage can be used as long as it can oxidize the bound chlorine detected by the reduction reaction before measurement. It may be appropriately changed according to the type of bound chlorine predicted to be contained in the sample solution. Also, since the oxidation conditions of substances can be considered to change depending on conditions such as temperature and pH, an appropriate oxidation voltage may be selected according to the conditions such as the temperature and pH of the sample solution containing bound chlorine.

[0051] As an example of the oxidation voltage when using a carbon electrode containing carbon such as a conductive diamond electrode, a carbon electrode, a glassy carbon electrode, or a diamond-like carbon electrode as the working electrode and the counter electrode, it is preferably +1.0 V or more and +2.0 V or less, more preferably +1.5 V or more and +2.0 V or less, and particularly preferably +1.8 V or more and +2.0 V or less. Although a larger value of the oxidation voltage is preferable, it is preferable that it is +2.0 V or less because electrolysis of water can be suppressed.

[0052] The applied voltage and measurement voltage during the pretreatment applied to the working electrode are not limited to those described above, and can be appropriately changed according to the type of the sample solution, the detection target, the measurement environment, etc. Also, the length of time for applying these voltages and the length of time for performing liquid replacement can be appropriately changed. In the above-described embodiment, liquid replacement was prevented from occurring during the application of the oxidation voltage, during the application of the measurement voltage, and between the oxidation voltage and the measurement voltage. However, liquid replacement may be allowed to occur during the application of the oxidation voltage or during the application of the measurement voltage. The time for applying the oxidation voltage is preferably 1 second or more, more preferably 3 seconds or more, and particularly preferably 5 seconds or more in order to sufficiently oxidize the bound chlorine. There is no particular upper limit for the time for applying the oxidation voltage, but it is preferably 10 seconds or less from the viewpoint of speeding up the measurement.

[0053] When using carbon electrodes containing carbon such as conductive diamond electrodes, carbon electrodes, glassy carbon electrodes, diamond-like carbon electrodes, etc. as the working electrode and the counter electrode, the measurement voltage is preferably -0.7 V or higher in order to suppress the change in the current value derived from the electrolyte as much as possible. More specifically, the measurement voltage is preferably -0.7 V or higher and -0.2 V or lower, more preferably -0.7 V or higher and -0.3 V or lower, and particularly preferably -0.5 V or higher and -0.3 V or lower.

[0054] In the above-described embodiments, the measurement sequence for continuously measuring free chlorine and combined chlorine has been described. However, when only combined chlorine is to be measured, only the pretreatment step and the combined chlorine measurement step may be performed.

[0055] The electrochemical measurement device is not limited to the three-electrode type as described above, and may be a two-electrode type, a four-electrode type, or a six-electrode type.

[0056] 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, a diamond-like carbon electrode, etc., a noble metal such as gold or platinum, or an electrode using an alloy containing these noble metals.

[0057] 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, SnO2, etc. can be used.

[0058] 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.

[0059] Instead of using the main flow path and the branch flow path, a measurement cell may be provided in the main flow path and a valve may be used to control the flow in the main flow path. Alternatively, the fluid control unit may control a pump provided in the main flow path or the branch flow path instead of a valve. When a pump is provided instead of a valve in this way, the fluid control unit may supply a fluid such as a sample solution to the measurement cell by operating the pump, and may stop the supply of the fluid to the measurement cell by stopping the pump. During liquid replacement, 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 solution that does not contain the substance to be measured, a cleaning solution for the measurement cell, and the like.

[0060] The valve control unit is not limited to opening the valve only when no voltage is applied to the working electrode. It may keep the valve open at all times, 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.

[0061] 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. 8 and 9 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. 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 in the order of the reference electrode, the working electrode, and the counter electrode from the side closer to the inlet. However, the arrangement order of these electrodes is not limited to this and can be changed as appropriate. Also, the arrangement location of each electrode in the internal flow path can be changed as appropriate.

[0062] In the above-described embodiments, the case where the electrochemical measurement device is of the flow injection type has been described. However, a batch-type electrochemical measurement device that immerses the sensor unit in a beaker or the like for measurement may also be used. 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.

Example

[0063] Examples of the present invention will be described below, but the present invention is not limited to these examples.

[0064] <Example 1> Using the electrochemical measurement device described in the above embodiment, an attempt was made to detect monochloramine using an aqueous solution of monochloramine, which is a type of combined chlorine, as a sample solution. In this Example 1, after performing the pretreatment step, the valve was closed to stop the flow of the sample solution in the measurement cell, and the measurement voltage was swept from the oxidation voltage of +2.0 V to the measurement voltage of -2.0 V, and then continuously swept from -2.0 V to +2.0 V. The sweep rate was set to 0.1 V / sec. The results are shown in FIG. 5. The aqueous monochloramine solution used in this experiment was prepared by mixing an equal volume of a solution obtained by diluting a 0.071 M aqueous ammonium chloride solution with a 10 -4 M aqueous sodium hydroxide solution and a solution obtained by diluting an aqueous sodium hypochlorite solution with a 10 -4 M aqueous sodium hydroxide solution so that the molar concentration ratio of "sodium hypochlorite: ammonium chloride" was 1:1.03.

[0065] <Comparative Example 1-1> Also, as Comparative Example 1, an experiment was conducted in the same procedure as in Example 1 except that the oxidation voltage was not applied immediately before applying the measurement voltage for the combined chlorine concentration of the same combined chlorine aqueous solution. The results are shown in FIG. 5. <Comparative Examples 1-2 and 1-3> Comparative Examples 1-2 and 1-3 were experiments using an aqueous NaClO solution as a sample. Comparative Example 1-2 was conducted using the same procedure as Example 1, and Comparative Example 1-3 was conducted using the same procedure as Comparative Example 1-1. The results are shown in Fig. 5.

[0066] <Considerations on Example 1 and Each Comparative Example> As shown in Fig. 5, in Example 1 using the electrochemical measurement method according to the present invention, a peak in the current value could be detected near -0.5. On the other hand, in Comparative Example 1-1, Comparative Example 1-2, and Comparative Example 1-3, almost no peak in the current value could be observed near -0.5 V. In addition, these experimental results did not change even when different devices with the same configuration were used. From these results, it was found that by applying an oxidation voltage before applying a measurement voltage for measuring combined chlorine using the electrochemical measurement device and the electrochemical measurement method according to the above embodiment, a change in the current value that seems to be derived from combined chlorine can be detected by the voltammetry method without using a reaction reagent.

[0067] <Example 2> Next, it was investigated whether the change in the current value observed in Fig. 5 depends on the concentration of combined chlorine. In this experiment, using the measurement method according to the above embodiment, the change in the current value was observed when the concentration of combined chlorine (monochloramine) was changed and when the measurement voltage was changed variously. An aqueous monochloramine solution adjusted by the same method as the sample used in Fig. 5 was diluted step by step and measured for 10 minutes each at each measurement voltage, and the current value after 10 minutes was observed. The results are shown in Fig. 6.

[0068] <Considerations on Example 2> From the results of Fig. 6, it was found that there is an obvious correlation between the change in the current value observed in Fig. 5 and the concentration of combined chlorine. Also, from the results of Fig. 6, it was found that the correlation between the combined chlorine concentration and the potential change is maintained even when the measured voltage is varied. The approximate formula obtained based on these correlations shows a very high correlation when approximated in the range of 50 ppm or less, and an even higher correlation when approximated at 25 ppm or less. From the above results, if the correlation formula between the combined chlorine concentration and the current value at a certain measured voltage is obtained, the combined chlorine concentration of a sample solution with an unknown combined chlorine concentration can be calculated.

[0069] <Example 3> In Example 3, the chlorine concentration of the cleaning liquid sampled from the cleaning tank that actually cleans vegetables (cabbage or onion) was measured while continuously supplying the cleaning liquid to the measurement cell of the measuring device according to the above embodiment. The experimental procedure is as follows. After starting the supply of tap water to the cleaning tank, the chlorine concentration in the cleaning liquid sampled from the cleaning tank was measured for 20 minutes. The supply of tap water continued at the same flow rate until the measurement was completed. Next, while supplying an aqueous sodium hypochlorite solution adjusted so that the total chlorine concentration is about 200 ppm to the cleaning tank at a constant flow rate, the chlorine concentration in the cleaning liquid was measured for 20 minutes. Subsequently, the supply of the aqueous sodium hypochlorite solution to the cleaning tank was stopped, 100 g of the above vegetables were put into the cleaning tank, and the chlorine concentration in the cleaning liquid in the state where the vegetables were immersed was measured for 10 minutes. The vegetables were cut into pieces, and 100 g each were placed in a draining net so that the cut vegetables would not diffuse in the cleaning tank. The vegetables were taken out, and the chlorine concentration in the cleaning liquid was continuously measured for 20 minutes. To confirm whether there is no change in sensitivity from the start of the measurement, the aqueous sodium hypochlorite solution was restarted, and the chlorine concentration in the cleaning liquid was measured. After the supply of the aqueous sodium hypochlorite solution was stopped, the chlorine concentration in the cleaning liquid was measured for an additional 10 minutes to complete the measurement. The results are shown in Fig. 7.

[0070] <Comparative Example 3-1> The residual chlorine concentration was measured for the same sample as in Example 3 in the same procedure as in Example 3, except that an oxidation voltage was not applied before applying the measurement voltage for combined chlorine. The results are shown in Fig. 7.

[0071] <Comparative Example 3-2> For the same sample as in Example 3, the residual chlorine concentration was measured by the spectrophotometric method (colorimetric method) using an iodine reagent as the reaction reagent. Since the iodine reagent reacts with not only free chlorine but also combined chlorine, the residual chlorine concentration including both free chlorine and combined chlorine can be measured by the colorimetric method. The results are shown in Fig. 7.

[0072] <Considerations on Example 3, Comparative Example 3-1, and Comparative Example 3-2> As can be seen from Fig. 8, it can be seen that an error occurs between the measurement results of Comparative Example 3-1 (when no oxidation voltage is applied) and the measurement results by Comparative Example 3-2 (colorimetric method). This error is a small one of about 20 ppm. On the other hand, in Example 3 using the electrochemical measurement apparatus and measurement method according to the present invention, it can be seen that the error from Comparative Example 3-2 is clearly smaller than that from Comparative Example 3-1. From this result, according to the electrochemical measurement apparatus and the electrochemical measurement method of Example 3, combined chlorine that was not detected in Comparative Example 3-1 where no oxidation voltage was applied was accurately detected, and as a result, it can be seen that the residual chlorine concentration closer to the result of the colorimetric method (Comparative Example 3-2) that measures the total amount of free chlorine and combined chlorine could be measured.

Explanation of Signs

[0073] 100 ··· Electrochemical measurement apparatus 31 ··· Measurement cell 311 ··· Inlet 312 ··· Outlet 313 ··· Internal flow path 32 ··· Working electrode 51 ··· Voltage control unit

Claims

1. An electrochemical measurement device for electrochemically measuring the concentration of bound chlorine among the residual chlorine concentrations contained in a sample solution, comprising: an electrode arranged to contact the sample solution for detecting residual chlorine; a voltage control unit for controlling the voltage applied to the electrode; The electrochemical measurement device is characterized in that the voltage control unit applies a measurement voltage for measuring bound chlorine after applying an oxidation voltage for oxidizing bound chlorine to the electrode.

2. The electrochemical measurement device according to claim 1, wherein the voltage control unit applies the measurement voltage immediately after applying the oxidation voltage to the electrode.

3. The oxidation voltage is +1.0 V or more and +2.0 V or less, The electrochemical measurement device according to claim 1 or 2, wherein the measurement voltage is -0.7 V or more and -0.2 V or less.

4. The electrochemical measurement device according to any one of claims 1 to 3, wherein the electrode is a diamond electrode or a diamond-like carbon electrode.

5. A measurement cell having an inlet for introducing the sample solution and an outlet for discharging the sample solution, accommodating the sample solution in an internal flow path formed between the inlet and the outlet, and accommodating the electrode in the internal flow path so as to contact the sample solution; a sample supply flow path for supplying the sample solution to the measurement cell; a flow rate control unit provided on the sample supply flow path for controlling the inflow rate of the sample solution into the measurement cell; The electrochemical measurement device according to any one of claims 1 to 4, wherein the voltage control unit applies the oxidation voltage and the measurement voltage to the electrode after the flow rate control unit reduces or sets to zero the inflow rate of the sample solution into the measurement cell.

6. The electrochemical measurement device according to any one of claims 1 to 5, characterized in that the electrode comprises a working electrode, a counter electrode, and a reference electrode.

7. A method for electrochemically measuring the concentration of bound chlorine among the residual chlorine concentrations contained in a sample solution, comprising: applying a measurement voltage for measuring bound chlorine after applying an oxidation voltage for oxidizing bound chlorine to an electrode arranged to contact the sample solution for detecting residual chlorine.

8. A program characterized by causing a computer to function as a voltage control unit that applies an oxidation voltage for oxidizing bound chlorine to an electrode that is arranged to touch a sample solution and detects residual chlorine contained in the sample solution, and then applies a measurement voltage for measuring the bound chlorine.

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