JP2006084375A - Measuring method using crystalline carbon as electrode - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide various methods relevant to measurement using polarography with favorable sensitivity improving repeatability of measurement accuracy, and providing recycling of an electrode without requiring labor or work for polishing by properly processing a surface of the electrode. <P>SOLUTION: The electrode provided by an activation process of dipping an electrode material which is crystalline carbon in an electrolyte solution, and applying a first boundary voltage which is a voltage of a boundary wherein oxidation of a first negative ion occurs is used as a first working electrode 1. There are provided: a measurement process of carrying out the measurement using polarography; and a conditioning processing of removing substances attached to the working electrode 1 by applying a second boundary voltage which is a voltage of a boundary wherein oxidation of a second negative ion occurs, and the same or below the first boundary voltage to the first working electrode 1 after the measurement process. The negative ions derived from the electrolyte solution and oxidized by the voltage the same or below the first boundary voltage are the same that negative ions included in a supporting electrolyte solution used in the conditioning process. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a method for qualitatively and quantitatively measuring a substance by an electrochemical method using crystalline carbon as an electrode, and an electrode activation method, an electrode manufacturing method, an electrode conditioning method, and the like suitable for the measurement. It is.

  Conventionally, in a concentration measurement method such as a polarographic method, a mercury dropping electrode is used as shown in Patent Document 1, or a mercury salt solution is added using glassy carbon as an electrode. However, due to the correct understanding of the effects of heavy metals such as mercury on the natural environment, the adverse effects on the ecosystem are considered to be significant, and the abolition of mercury use is now desired.

  Therefore, measurement using an electrode made of glassy carbon or conductive diamond as a raw material is performed.

  However, in the case of glassy carbon or the like, the measurement substance or the like is adsorbed on the electrode surface and the sensitivity of the electrode is lowered. In order to recover this sensitivity, conditioning is performed by applying a high voltage opposite to that during measurement, and the adsorbed substances are redissolved in the solution, but the sensitivity of the electrode cannot be fully recovered. The electrode is reused by polishing the electrode and exposing a new surface.

However, if such a method is used, not only the labor and apparatus for performing polishing are required, but also the measurement accuracy depends on the degree of polishing. Mercury is indispensable to restore the accuracy to near the time of new electrode construction.
JP-A-61-210940

  Therefore, the present invention does not require the labor and equipment required for polishing by appropriately treating the surface of the electrode, enables the electrode to be reused without using mercury, has higher sensitivity, and allows the reproducibility of measurement accuracy. The main challenge is to provide methods for improved measurement.

  That is, the present invention immerses a working electrode, which is an electrode comprising crystalline carbon, and a counter electrode, which is a pair of electrodes, in a supporting electrolyte solution in which a predetermined substance and a supporting electrolyte are dissolved in a solvent. A measurement method using a polarographic method for quantifying and / or qualifying the predetermined substance by applying a voltage and measuring a flowing current amount and / or a change position thereof, and a crystalline carbon serving as an electrode The electrode raw material is immersed in an electrolyte solution, and obtained by an activation step in which a first boundary voltage, which is a voltage at which a predetermined first anion derived from the electrolyte solution oxidizes, is applied for a predetermined time. A measurement step in which measurement is performed using a polarographic method with an electrode as a working electrode, and a boundary where oxidation of a predetermined second anion derived from the supporting electrolyte or the solvent occurs after the measurement step. And applying a second boundary consumption side voltage, which is a voltage larger than the second boundary voltage, which is a voltage equal to or lower than the first boundary voltage, by a predetermined amount to the working electrode for a predetermined time, thereby causing a substance attached to the surface of the working electrode to A supporting electrolyte solution used in the conditioning step, wherein a part or all of anions derived from the electrolyte solution used in the activation step and oxidized at a voltage equal to or lower than the first boundary voltage are provided. It is characterized by being common to the anions contained in.

  With such a concentration measuring method, for example, crystalline carbon such as diamond or pyrolytic graphite is used as an electrode raw material, so that it is not necessary to use mercury. Further, it is considered that the surface of the electrode raw material is terminated by, for example, an anion in the electrolyte solution in the activation step, and the sensitivity of the electrode is improved.

  In addition, by performing the conditioning process after the measurement process, for example, many anions whose reactivity has been increased by the second boundary consumption side voltage are present in the vicinity of the electrode surface, react with the substance adsorbed on the electrode, and adsorb on the electrode surface It is thought that the same substances can be removed more reliably.

  If a common anion is used in the electrolyte solution in the activation step and the supporting electrolyte solution in the concentration measurement step, the electrode surface can be used in the conditioning step even when an anion in the aqueous solution reacts with the electrode surface. It is considered that the sensitivity is not easily lowered without significantly changing the composition.

  Furthermore, experiments have shown that such a method can be measured with high sensitivity and good reproducibility.

  The electrode conditioning method used in this conditioning step can be used not only for the measurement method, but also for example, it is considered that it is possible to remove substances adhering to the electrode surface with high accuracy, enabling measurement with good reproducibility. .

  Further, the supporting electrolyte solution dissolves chloride ions. By applying a voltage of 0.05 V or more and 0.1 V or less on the SCE basis to the working electrode, a substance attached to the surface of the working electrode is removed. If it is characterized by removal, for example, chlorine, which is a highly oxidizing element, can be used to remove substances that adhere to the electrode surface, so that the substances attached to the electrode surface can be removed more effectively. It will be possible.

  On the other hand, the electrode activation method used in the activation step not only allows the electrode activated by this method to be suitably used as a working electrode used in the measurement method, but also appropriately uses, for example, a solvent and an electrolyte. Therefore, it is possible to terminate the surface of the electrode with any composition using ions in the electrolyte solution. It is considered possible.

  In order to uniformly activate the surface, it is preferable to apply the first boundary voltage after mirroring the contact surface of the electrode material with the electrolyte solution.

  Furthermore, the electrode activation method includes an application step of applying the first boundary voltage to the electrode raw material, and a pause step of stopping application of the first boundary voltage to the electrode raw material, It is considered that the termination of the electrode surface can be controlled by providing a plurality of application steps and applying the first boundary voltage for the predetermined time in total.

  If the electrolyte solution is an aqueous solution in which chloride ions are dissolved, it is generally easy to use.

  If the first boundary voltage is an oxygen generation boundary voltage that is a voltage that oxidizes water contained in the electrolyte solution and generates oxygen gas, for example, the electrode surface may be oxygen-terminated. It is possible to improve the hydrophilicity of the electrode surface, and it is considered to be particularly suitable when an aqueous solution is used in the measurement process.

  The crystalline carbon used in the electrode material is pyrolytic graphite, and the surface of the electrode material is activated by applying a voltage of 1.6 V or less on the basis of SCE. For example, when the surface is terminated, oxygen atoms or the like do not enter and react with the inside of the electrode, so that the activation can be suitably performed.

  By using such a method, it is possible to perform highly sensitive measurement with good reproducibility without using mercury and without requiring labor and equipment required for polishing.

  Hereinafter, embodiments of the present invention will be described.

  In the present embodiment, an activation step of activating a working electrode 1 that is an electrode made of crystalline carbon as a raw material, and a concentration measurement in which the concentration of a predetermined substance is measured by a polarographic method using the activated working electrode 1. A process and a conditioning process for removing substances adsorbed on the working electrode 1 after concentration measurement are performed.

  Each step will be described in detail below.

  In the activation step, an electrolyte solution in which pyrolytic carbon, which is one of crystalline carbons, is used as an electrode raw material, and the electrode raw material is obtained by dissolving potassium chloride (hereinafter referred to as KCl) as an electrolyte in pure water. Then, a first boundary voltage that is a voltage at which a predetermined first anion contained in the electrolyte solution starts to transfer electrons is applied to the electrode material for a predetermined time. As shown in FIG. 1, a three-terminal device including a cell 4 filled with an electrolyte solution, a working electrode 1, a counter electrode 2, and a reference electrode 3 is used. The potentiostat 5 generates a direct current so that the current value can be monitored from the ammeter 6.

  Crystalline carbon includes diamond and graphite. Pyrolytic graphite is a kind of graphite, which is obtained by calcination and recrystallization of carbon produced by a pyrolysis method in a reducing atmosphere. In this embodiment, the wetted surface of pyrolytic graphite that is available as Pyroid (registered trademark) in the form of a cylinder with a diameter of 2 mm is mirror-polished by polishing until a metallic luster appears, and further recrystallized. Used and the surface is hydrogen terminated. Moreover, in this embodiment, an electrode raw material refers to the electrode which has not been activated yet.

  The solvent is pure water, and the electrolyte solution is a saturated solution of KCl at about 0.1 mol / liter. This is because it is adapted to the concentration measurement process described later.

  Further, as the first boundary voltage, in this embodiment, the voltage that is the upper limit of the potential window with respect to the solvent of the electrode material, that is, the boundary that generates oxygen gas by oxidizing water contained in the electrolyte solution is about 1.5V. An oxygen generation boundary voltage, which is a voltage, is applied. Unless otherwise indicated, the voltage is based on SCE.

The activation process includes an application process in which a first boundary voltage is applied to the electrode material, and a pause process in which application of the first boundary voltage to the electrode material is suspended. By alternately repeating the steps, the first boundary voltage is applied to the electrode material for a predetermined time to control the activation of the electrode material. Each application process is continuous for about 1 hour, each rest process is continuous for about 23 hours, and one turn per day. Further, in the pause process, the electrode raw material is used as an electrode, a concentration measuring step described later is performed, a voltage of −1 V or more and less than 1.5 V is swept, and the sensitivity test is performed by measuring the concentration of the sample. For example, using a solution in which an equal amount of a sample ionized to Cu ²⁺ is mixed with an electrolyte, a current of about 6 μA or more flows with respect to 10 ppb of copper as a criterion for completion of activation of the electrode raw material. According to the standard, it is necessary to provide a total of about 20 application steps until the activation is completed, but it is preferable to adjust the reference, the number of turns of the application process, and the application time of the first boundary voltage according to the application.

  In addition, activation of the surface of this electrode raw material is terminated by attaching an appropriate amount of, for example, chlorine or oxygen derived from anions in the electrolyte solution to the carbon atoms on the surface of the electrode raw material having a hydrogen termination, It can be estimated that the modification is performed. Therefore, it is preferable to determine the first boundary voltage so that such a state can be maintained. Furthermore, experiments have shown that it is efficient if surface modification is performed at a voltage indicating the vicinity of the end of the plateau region of the elution curve. This voltage is about 1.4 V to 1.6 V for graphite, for example, and there is a margin of about 0.2 V to 0.3 V in the band where the reforming can be effectively performed. Moreover, it is preferable not to exceed 1.6V especially when oxygen gas can be generated. In this case, it is also preferable to obtain a highly sensitive carbon electrode by making the composition of the electrolyte solution used for activation coincide with or similar to the composition of the supporting electrolyte solution used in the subsequent concentration measurement step.

  In the concentration measurement step, the working electrode 1 that is the electrode obtained in the activation step and the counter electrode 2 that is the Pt electrode that is a pair thereof, KCl that is the supporting electrolyte, and ions that are the measurement target are added to pure water. A concentration of a predetermined substance is measured using a polarographic method in which a voltage is applied by immersing in a supporting electrolyte solution that is dissolved, and a current amount flowing through the solution and / or a change position thereof is measured. As shown in FIG. 1, a three-terminal device including a cell 4 filled with a solution, a working electrode 1, a counter electrode 2, and a reference electrode 3 using Ag / AgCl is used.

  In the present embodiment, the solvent is pure water and the supporting electrolyte solution is a saturated solution of KCl. Here, in order to distinguish from the electrolyte and electrolyte solution used in the activation step, they are referred to as a supporting electrolyte and a supporting electrolyte solution, and do not mean a material difference. Further, the concentration measurement is performed using a stripping polarographic method which is one of polarographic methods, more specifically, an anode stripping method which is a stripping polarographic method for analyzing metal ions and the like.

  The stripping polarographic method is a method in which a voltage is applied to the working electrode 1 and the counter electrode 2 to concentrate ions in the solution on the working electrode 1 and measure the amount of current and / or its change position while eluting the ions. It is a method for performing quantitative and / or qualitative analysis of these substances with extremely high sensitivity. For the purpose of analysis of metal ions, etc., the metal ions etc. are reduced and concentrated on the working electrode 1 and then oxidized and eluted to measure the oxidation potential, the current flowing at the potential and the quantity of electricity, and the halogen ions, etc. For the purpose of analysis, a cathode stripping method and the like that are electrically reversed from the anode stripping method are included.

  The concentration measurement step using the stripping polarographic method includes a deposition step in which nitrogen gas or the like is dissolved in the supporting electrolyte solution to remove dissolved oxygen, and then the ions are concentrated on the working electrode 1 while stirring the solution. And a stripping step for eluting ions, which are performed in the order described above. FIG. 2 schematically shows the relationship between the time of each step and the voltage applied to the working electrode.

  In the deposition step, for example, a voltage of −1 V is applied to the working electrode 1 for 300 seconds to concentrate metal ions in the solution on the surface of the working electrode 1.

  In the rest process, the solution is kept stationary for about 30 seconds.

  In the stripping step, the elution curve is obtained by sweeping from -1 V to +0.05 V using a differential pulse polarographic equation in which pulses are superimposed on DC sweep at regular intervals and measuring the current.

  3, 4 and 6 are graphs showing elution curves obtained when measuring a solution in which 10 ppb of Cu + is dissolved 5 times, and FIG. 3 shows an electrode without any activation step. Fig. 6 shows the case where the electrode on the 9th day is used, and Fig. 6 shows the case where the electrode on the 30th day is used. In the purified pyroid without the activation step, almost no peak appears and the sensitivity is low, so that it is not suitable as an electrode. FIG. 5 and FIG. 7 show the sensitivity of the electrodes for the electrodes on the 9th and 30th days, with the average peak height being 100 in each measurement and the ratio of the peak height for each measurement. It is a graph which shows the change of. The electrode on the 9th day has the lowest sensitivity in the first measurement, so that it is not completed as an electrode, and it is considered that the sensitivity is increased by the conditioning process described later. On the other hand, the electrode on the 30th day has the highest sensitivity in the first measurement, and it is considered that the activation has been completed.

  In the conditioning process, after the concentration measurement process, the second boundary consumption is a voltage that is a predetermined amount larger than the second boundary voltage, which is a voltage at which the second anion derived from the supporting electrolyte solution is oxidized, on the SCE basis. By applying a side voltage to the working electrode 1 for a predetermined time, the substance adhering to the surface of the working electrode 1 is removed. The apparatus used in the concentration measurement step shown in FIG. 1 is used as it is.

  In the present embodiment, the electrolyte solution dissolves chloride ions. By applying a voltage of +0.05 V or more and 0.1 V or less on the basis of the SCE to the working electrode 1 for about 900 seconds, the working electrode 1 The material attached to the surface is removed. The predetermined second anion may be the same as the predetermined first anion.

  It has been found that by performing this conditioning process, the density value measured in the density measurement process has better reproducibility than when the conventional conditioning process for applying a higher voltage is performed. From this, it can be presumed that an appropriate amount of chlorine that has been radicalized, for example, increased in reactivity at the electrolyte consumption side boundary voltage reacts with the metal adhering to the working electrode 1 and is more reliably dissolved as metal ions. Therefore, it is preferable to determine the second boundary voltage so that such a state can be maintained. If the band is, for example, graphite, it is considered that there is a margin of about 0.2 V to 0.3 V. Further, for example, since the second boundary voltage is equal to or lower than the first boundary voltage, new anions that are oxidized at a voltage higher than the first boundary voltage do not react with the surface of the electrode, and are active in the conditioning process. Since the KCl saturated solution is used in the same way as the crystallization step, the reaction between the electrode surface and chlorine is in an equilibrium state, the composition of the atoms terminating the surface does not change significantly, and the sensitivity of the electrode does not change significantly. Can be estimated.

  By using such an activation step and a conditioning step, it is possible to use a highly sensitive electrode without using mercury, and to simplify the process for electrode reuse.

  The present invention is not limited to the above embodiment.

  For example, glassy graphite may be used instead of pyrolytic graphite. In this way, the cost of the electrode raw material can be reduced.

The electrolyte that is ionized to chloride ions is not limited to KCl, and may be NaCl. Anions are not limited to chloride ions but may be other halogen ions with strong oxidizing power, etc., or ions derived from multiple atoms such as proteins or oxygen acids that are highly polarized molecules Etc.

The counter electrode and the reference electrode are not limited to Pt or Ag / AgCl, and general electrodes may be used.

The concentration measurement step may be a cathode stripping method instead of the anode stripping method depending on the measurement target, or a polarographic method instead of the stripping polarographic method. Further, the measurement is not limited to the concentration measurement, and may be other measurement for quantitative or qualitative purposes.

Further, the supporting electrolyte solution and the electrolyte solution are not limited to aqueous solutions. Moreover, not only a liquid but gel, resin, etc. may be sufficient. In addition, when an electrolyte or supporting electrolyte is not required, it is not necessary to add them.

  A part or all of the above-described components may be appropriately combined.

Various other modifications are possible without departing from the spirit of the present invention.

The schematic diagram which shows the apparatus used at the activation process in this embodiment, a density | concentration measurement process, and a conditioning process. The density | concentration measurement process voltage transition diagram which shows the transition of the voltage applied at the density | concentration measurement process in the embodiment. The graph showing the elution curve obtained when using the density | concentration measuring method in the same embodiment, and using the electrode which does not perform an activation process as a carbon electrode, and measuring the solution which melt | dissolved 10 ppb Cu + 5 times. The graph showing the elution curve obtained when the solution which melt | dissolved 10 ppb Cu + was measured 5 times with the electrode of the activation process 9th day using the concentration measuring method in the embodiment. Using the concentration measurement method in the same embodiment, the measurement was performed 5 times with the electrode on the 9th day of the activation step, the average value of the peak height was set to 100, and the peak height for each measurement was displayed as a ratio. The graph which shows the change of the sensitivity of a carbon electrode. The graph showing the elution curve obtained when the solution which melt | dissolved 10 ppb Cu + was measured 5 times with the electrode of the activation process 30th day using the concentration measuring method in the embodiment. Using the concentration measurement method in the same embodiment, the measurement was performed 5 times with the electrode on the 30th day of the activation step, the average value of the peak height was set to 100, and the peak height for each measurement was displayed as a ratio. The graph which shows the change of the sensitivity of a carbon electrode.

Explanation of symbols

1 ... Working electrode 2 ... Counter electrode

Claims (9)

A voltage is applied by immersing a working electrode that is an electrode comprising crystalline carbon and a counter electrode that is a pair of electrodes in a supporting electrolyte solution in which a predetermined substance and a supporting electrolyte are dissolved in a solvent. A measurement method using a polarographic method for quantifying and / or qualifying the predetermined substance by measuring a flowing current amount and / or a change position thereof,
An activity of immersing an electrode raw material, which is crystalline carbon to be an electrode, in an electrolyte solution, and applying a first boundary voltage, which is a voltage at which a predetermined first anion derived from the electrolyte solution oxidizes, for a predetermined time A measurement step of performing a measurement using a polarographic method, with the electrode obtained by the conversion step as a working electrode;
After the measurement step, a voltage that becomes a boundary where oxidation of a predetermined second anion derived from the supporting electrolyte or the solvent occurs and is larger than a second boundary voltage that is a voltage equal to or lower than the first boundary voltage by a predetermined amount. And a conditioning process for removing substances adhering to the surface of the working electrode by applying the second boundary consumption side voltage to the working electrode for a predetermined time.
A part or all of the anion derived from the electrolyte solution used in the activation step and oxidized at a voltage equal to or lower than the first boundary voltage is common with the anion contained in the supporting electrolyte solution used in the conditioning step. A measuring method using a polarographic method, characterized by Sweeping the voltage by immersing the working electrode, which is an electrode comprising crystalline carbon, and the counter electrode, which is a pair of electrodes, in a supporting electrolyte solution in which a predetermined substance and the supporting electrolyte are dissolved in a solvent And a method of recovering the working electrode after performing a polarographic method for quantifying and / or qualifying the predetermined substance by measuring a flowing current amount and / or a change position thereof,
A second boundary consumption side voltage, which is a voltage larger by a predetermined amount than a second boundary voltage, which is a boundary voltage at which oxidation of a predetermined second anion derived from the supporting electrolyte or the solvent occurs, is performed for a predetermined time. The electrode conditioning method characterized by removing the substance adhering to the surface of the said working electrode by applying. The supporting electrolyte solution is a solution in which chloride ions are dissolved, and the substance adhering to the surface of the working electrode is removed by applying a voltage of 0.05 V or more and 0.1 V or less on the SCE standard to the working electrode. The electrode conditioning method according to claim 2. A method of activating the surface of an electrode raw material that is crystalline carbon that is a raw material of an electrode used for quantitative and / or qualitative analysis of a substance,
The electrode raw material is immersed in an aqueous electrolyte solution in which an electrolyte is dissolved, and a first boundary voltage, which is a boundary at which oxidation of a predetermined first anion derived from the electrolytic aqueous solution occurs, is applied to the electrode raw material for a predetermined time. Then, the electrode activation method characterized by activating the surface. The electrode activation method according to claim 4, wherein after the contact surface of the electrode material with the electrolyte aqueous solution is mirror-finished, the first boundary voltage is applied to activate the surface of the electrode material. An application step of applying the first boundary voltage to the electrode material, and a pause step of stopping application of the first boundary voltage to the electrode material, comprising a plurality of the application steps, 6. The electrode activation method according to claim 4, wherein the first boundary voltage is applied for the predetermined time in total. The electrode activation method according to claim 4, wherein the aqueous electrolyte solution is an aqueous solution in which chloride ions are dissolved. The electrode activation method according to claim 7, wherein the first boundary voltage is an oxygen generation boundary voltage that is a voltage that oxidizes water contained in the electrolyte aqueous solution and generates oxygen gas. An electrode manufacturing method comprising manufacturing an electrode using the electrode activation method according to claim 4.