JP2014232052A - Electrochemical analysis method and apparatus for selenium - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electrochemical analysis method and an apparatus for selenium, capable of distinguishing and detecting Seand Se.SOLUTION: There is provided an analysis method of electrochemically analysing by voltammetry selenium ions each having different ionic valences contained in a sample solution using a carbon electrode as work electrode. The analysis method includes the steps of: adjusting pH of the sample solution to 9 or less; changing a potential of the work electrode to a negative potential direction to reduce and precipitate Seon the work electrode surface; and sweeping the potential of the work electrode to a positive potential direction to oxidize and elute selenium ions from the work electrode surface.

Description

The present invention relates to a method and an apparatus for electrochemical analysis of selenium capable of analyzing Se ⁴⁺ and Se ⁶⁺ separately.

  Selenium (Se) is an element that exists widely in nature, and plays an important role in the antioxidant system as a component of glutathione peroxidase in the human body. For this reason, selenium is an essential element for the human body as long as it is at a trace level.

  However, selenium is also toxic, and overdose over 400 mg / day causes symptoms such as nail deformity and hair loss, gastrointestinal disorders, diarrhea, fatigue, irritation, peripheral neuropathy, and skin diseases. Furthermore, ingestion of selenium in grams can cause severe gastrointestinal disorders, neurological disorders, myocardial infarction, acute dyspnea, renal failure and the like.

  For such selenium, for example, the water pollution standard is regulated to 0.1 mg / L or less in the Water Pollution Control Law, and the water quality standard is regulated to 0.1 mg / L or less in the Sewerage Law. In the United States, the selenium concentration in wastewater is regulated to 50 μg / L or less by the Environmental Protection Agency. For this reason, a method for analyzing selenium in waste water simply and accurately is required. Selenium is used in the fields of rectifiers, copiers, glass, chemicals, pigments, metallurgy, etc., and selenium drums, rectifiers, and the like use high-purity products obtained by further purifying metal selenium. In the manufacturing process and the purification process, a method for analyzing selenium simply and accurately is required.

Selenium can take various forms in water, and the main ones include H ₂ SeO ₃ (oxidation number + IV), H ₂ SeO ₄ (oxidation number + VI), and derivatives thereof. Although Se toxicity ⁴⁺ and Se ⁶⁺ are different from each other, conventionally, atomic absorption spectrometry used to detect selenium ions, atomic fluorescence spectroscopy, the method of gas chromatography, and ion valence different Se ⁴⁺ Se ⁶⁺ cannot be distinguished and analyzed.

  In addition, the present inventors have succeeded in analyzing selenium ions by anodic stripping voltammetry (Patent Document 1), but the method does not make a distinction based on ionic valence.

JP2013-24581A

Therefore, the present invention is intended to provide an electrochemical analysis method and apparatus for selenium capable of distinguishing and detecting Se ⁴⁺ and Se ⁶⁺ .

As a result of intensive studies by the present inventors, when analyzing the selenium ion by voltammetry, if the pH of the sample solution is set to 9 or more, only the current peak associated with the oxidation elution of Se ⁴⁺ is detected, and the oxidation elution of Se ⁶⁺ occurs. The accompanying current peak was not detected. However, when the pH of the sample solution was less than 9, the current peak associated with Se ⁴⁺ oxidation elution and the current peak associated with Se ⁶⁺ oxidation elution were detected at the same potential. The present invention has been completed based on this novel finding.

That is, the electrochemical analysis method of selenium according to the present invention is a method of electrochemically analyzing selenium ions having different ionic values contained in a sample solution by voltammetry using a carbon electrode as a working electrode, Adjusting the pH of the sample solution to 9 or more, changing the potential of the working electrode in a negative potential direction to reduce Se ⁴⁺ on the surface of the working electrode, and setting the potential of the working electrode to a positive potential It is characterized by comprising a step of sweeping in the direction and oxidizing and eluting selenium ions from the surface of the working electrode. In the present invention, a reaction of Se ⁰ → Se ⁴⁺ occurs in the step of oxidizing and eluting selenium ions. The electrochemical analysis method for selenium according to the present invention may be either cyclic voltammetry, stripping voltammetry (anodic stripping voltammetry), or any other method.

According to the present invention, Se ⁴⁺ can be selectively detected and its content can be measured by adjusting the pH of the sample solution to 9 or more.

  Here, the “content” includes not only the total amount contained in the sample solution but also the concentration that is the content per unit volume of the sample solution. Moreover, the relative value which can understand relatively whether the content is contained or not, regardless of the precision may be sufficient.

A step of adjusting the pH of the sample solution to less than 9, a step of reducing Se ⁴⁺ and Se ^{6+ on the} surface of the working electrode by changing the potential of the working electrode in a negative potential direction, and the action If the step of sweeping the potential of the electrode in the positive potential direction and oxidizing and eluting selenium ions from the surface of the working electrode is performed, Se ⁴⁺ and Se ⁶⁺ are detected together and the total content is measured. Can do. And the content of Se6 ⁺ can also be ^{calculated |} required by subtracting the content of Se4 ⁺ from the total content of ^{Se4 +} and ^{Se6 +} .

  The sample solution for adjusting the pH to 9 or more and the sample solution for adjusting the pH to less than 9 may be separated from the same sample solution, but may be the same.

As the order of analysis, first, the pH of the sample solution was adjusted to 9 or more to perform reduction precipitation and oxidation elution of Se ⁴⁺ , and then the pH of the sample solution was adjusted to less than 9 and Se ⁴⁺ and Se ⁶⁺ The reduction deposition and oxidation elution of the electrode require less deterioration of the working electrode and can maintain the analysis accuracy.

  Examples of the carbon electrode used as the working electrode include a glassy carbon electrode and a conductive diamond electrode. Among these, a conductive diamond electrode is preferably used. As the conductive diamond electrode, for example, one doped with boron, nitrogen, phosphorus, or the like is used, and a boron-doped diamond electrode doped with boron at a high concentration is particularly preferable. The boron-doped diamond electrode has a wide potential window (wide oxidation potential and reduction potential), low background current compared to other electrode materials, high sensitivity to redox species, compared to gold, platinum, etc. In addition, since physical adsorption hardly occurs on the electrode surface, it has an excellent property that peaks other than the generation of oxygen and hydrogen hardly occur. Further, the boron-doped diamond electrode is excellent in chemical durability, mechanical durability, electrical conductivity, corrosion resistance, and the like. Furthermore, the boron-doped diamond electrode also has the advantage that it is easy to perform chemical and physical cleaning due to its hardness, and it is easy to maintain the electrode surface in a clean state.

  It is preferable to deposit gold on the surface of the conductive diamond electrode before reducing or precipitating selenium ions or in parallel with reducing and precipitating selenium ions. Since gold functions catalytically as an active site for electrode reaction, the reduction and precipitation of selenium ions can be promoted by modifying the surface of the conductive diamond electrode with gold. As a result, even a very small amount of selenium ion can be analyzed with high sensitivity.

  The electrochemical analysis method for selenium according to the present invention can be carried out, for example, by an analyzer having the following configuration. That is, an apparatus for electrochemically analyzing selenium ions having different ionic values contained in a sample solution by voltammetry, the first pH adjusting unit for adjusting the pH of the sample solution to 9 or more, and the sample A second pH adjuster for adjusting the pH of the solution to less than 9; a working electrode comprising a carbon electrode; a cell for containing the sample solution; and the potential of the working electrode being varied in the negative potential direction, A potential fluctuation unit that supplies a potential for reducing and precipitating selenium ions on the surface of the working electrode, and then sweeps the potential of the working electrode in a positive potential direction to supply a potential for oxidizing and eluting selenium ions from the surface of the working electrode. And a current detector for detecting a current change accompanying a change in potential of the working electrode. Such an apparatus for electrochemical analysis of selenium is also one aspect of the present invention.

  The first pH adjusting unit and the second pH adjusting unit may be provided separately, but the first pH adjusting unit may also serve as the second pH adjusting unit. When the first pH adjustment unit also serves as the second pH adjustment unit, the cell may be used as the pH adjustment unit.

According to the present invention, Se ⁴⁺ and Se ⁶⁺ can be distinguished and detected.

The typical block diagram of the electrochemical analyzer which concerns on one Embodiment of this invention. The flowchart which shows the analysis method which concerns on the same embodiment. The typical block diagram of the electrochemical analyzer which concerns on other embodiment. The flowchart which shows the analysis method which concerns on the same embodiment. The flowchart which shows the other analysis method which concerns on the same embodiment. The voltammogram at the time of performing cyclic voltammetry using the sample solution (pH1) containing ^{Se4 +} (a) or Se6 ⁺ (b). The graph which shows the relationship between the electric potential at the time of performing cyclic voltammetry using the sample solution (pH1) containing ^{Se4 +} or ^{Se6 +,} and a peak electric current value. Voltammogram when cyclic voltammetry is performed using a sample solution (pH 1) containing Se ⁴⁺ alone, Se ⁶⁺ alone, or both Se ⁴⁺ and Se ⁶⁺ . The voltammogram at the time of performing cyclic voltammetry by changing pH (pH 1.00-5.99 (a), pH 6.30-11.47 (b)) using the sample solution containing Se4 ⁺ . PH using a sample solution containing ^{Se 6+ (pH1.00~5.99 (a),} pH6.30~11.43 (b)) voltammogram in the case of performing cyclic voltammetry with different. The graph which shows the relationship between pH and a peak electric current value when performing cyclic voltammetry by changing pH using the sample solution containing ^{Se4 +} or ^{Se6 +} . Voltammograms when cyclic voltammetry was performed using a sample solution containing Se ⁴⁺ alone, Se ⁶⁺ alone, or both Se ⁴⁺ and Se ⁶⁺ (pH 3.55 (a) or pH 10.22 (b)). .

<First Embodiment>
A first embodiment of the present invention will be described below with reference to the drawings.

The electrochemical analysis apparatus 100 according to the present embodiment uses, for example, selenium ions contained in the sample solution, specifically Se ^4+, using waste water or the like flowing into or out of the waste water treatment plant as a sample solution. And Se ⁶⁺ are analyzed by cyclic voltammetry.

  As shown in FIG. 1, an electrochemical analysis apparatus 100 according to the present embodiment includes a pretreatment unit 10 that performs pretreatment of a sample solution, and an analysis unit that performs detection, concentration measurement, and the like of selenium ions contained in the sample solution. 20.

  The pretreatment unit 10 includes a storage tank 11 that temporarily stores a sample solution such as drainage and a pH adjustment unit 12 that adjusts the pH of the sample solution flowing from the storage tank 11 from the upstream side.

  The storage tank 11 is for temporarily storing a sample solution such as waste water that has flowed out of the waste water treatment plant, and is provided with a drain 111 for discharging excess sample solution.

  The pH adjusting unit 12 is for adjusting the pH of the sample solution flowing from the storage tank 11, and the first pH adjusting unit 12a for adjusting the pH of the sample solution to 9 or more, and the pH of the sample solution being 9 And a second pH adjusting unit 12b for preparing less.

  The first pH adjusting unit 12a and the second pH adjusting unit 12b are provided with pH adjusting tanks 121a and 121b and pH adjusting agent tanks 122a and 122b containing pH adjusting agents, respectively. Examples of the pH adjuster accommodated in the pH adjuster tanks 122a and 122b include alkaline pH adjusters such as sodium hydroxide and potassium hydroxide, acidic pH adjusters such as hydrochloric acid and acetic acid, and Briton-Robinson buffer. Which pH adjuster is used may be appropriately selected depending on the pH of the sample solution to be adjusted.

  The pH adjusting tanks 121a and 121b are provided below the stirring bars SBa and SBb accommodated in the pH adjusting tanks 121a and 121b and the pH adjusting tanks 121a and 121b, respectively. Are provided with stirring mechanisms STa and STb comprising magnetic actuators MSa and MSb for rotation by rotation.

  Each of the pH adjustment tanks 121a and 121b is further provided with pH sensors 124a and 124b, and it is confirmed by the pH sensors 124a and 124b whether or not the sample solution has been adjusted to a desired pH. The pH sensors 124a and 124b are connected to an arithmetic processing unit 27 described later.

  The pH adjusting agent tanks 122a and 122b have pH adjusting agent supply pipes 123a and 123b provided with pumps for supplying the pH adjusting agent from the pH adjusting agent tanks 122a and 122b to the pH adjusting tanks 121a and 121b, respectively. Is equipped.

  The storage tank 11, the pH adjustment unit 12, and the analysis unit 20 are connected by sample solution supply pipes 13a and 13b equipped with a pump.

  In addition, the branch point to the first pH adjustment unit 12a or the second pH adjustment unit 12a on the sample solution supply pipe 13a and the junction point from the first pH adjustment unit 12a and the second pH adjustment unit 12a on the sample solution supply pipe 13b. Are provided with switching valves 131a and 131b, respectively, to which of the pH adjustment tanks 121a and 121b the sample solution is supplied from the storage tank 11, and from which of the pH adjustment tanks 121a and 121b to the analysis unit 20 Whether to supply the solution can be switched.

  The analysis unit 20 includes a boron-doped diamond electrode 22, which is a working electrode, a counter electrode 23 and a reference electrode 24, and an analysis cell 25 provided with these three electrodes. A potentiogalvanostat 26 provided with a processing unit 27 is connected to the boron-doped diamond electrode 22, the counter electrode 23, and the reference electrode 24.

  The boron-doped diamond electrode 22 has conductivity imparted by mixing boron into diamond as an insulator. In the present embodiment, the bottom of the analysis cell 25 is closed by the planar boron-doped diamond electrode 22.

As the counter electrode 23, for example, an electrode made of platinum, carbon, stainless steel, gold, diamond, SnO ₂ or the like can be used.

  As the reference electrode 24, for example, an Ag / AgCl electrode, a calomel electrode, a standard hydrogen electrode, a hydrogen palladium electrode, or the like can be used.

  The analysis cell 25 accommodates a sample solution therein, and is configured so that the accommodated sample solution can contact the boron-doped diamond electrode 22, the counter electrode 23, and the reference electrode 24.

  The analysis cell 25 is provided with a stirring mechanism ST and a supply / drainage mechanism 28. The stirring mechanism ST includes a stirrer SB accommodated in the analysis cell 25 and a magnetic actuator MS that is provided below the analysis cell 25 and rotates the stirrer SB by magnetic force.

  The supply / drainage mechanism 28 includes a first buffer tank T1a, a second buffer tank T1b, a gold compound tank T2, and a waste liquid tank T3.

  The first buffer tank T1a contains a buffer solution having a pH of 9 or more, and the second buffer tank T1b contains a buffer solution having a pH of less than 9. Examples of the buffer solution having a pH of 9 or more include a Briton-Robinson buffer solution and a borate buffer solution. Examples of the buffer solution having a pH of less than 9 include a Briton-Robinson buffer solution and an acetate buffer solution. Phosphate buffer solution, borate buffer solution and the like are used.

  The first buffer tank T1a and the second buffer tank T1b are provided with a buffer solution supply pipe 28a provided with a pump for supplying a buffer solution from each of the buffer solution tanks T1a, T1b to the analysis cell 25. . A switching valve 281a is provided at the junction point from the first buffer tank T1a and the second buffer tank T1b on the buffer solution supply pipe 28a, and the buffer solution is analyzed from any of the buffer tanks T1a, T1b. The supply to the cell 25 can be switched.

The gold compound tank T2 contains a gold compound for electrodepositing gold on the surface of the boron-doped diamond electrode 22 to modify the electrode surface. The gold compound is not particularly limited as long as it can supply gold in an oxidized state capable of reducing precipitation, such as gold ions and gold complex ions, to the sample solution, and examples thereof include AuCl ₃ . The gold compound tank T2 includes a gold compound supply pipe 28b provided with a pump for supplying the gold compound from the gold compound tank T2 to the analysis cell 25.

  The waste liquid tank T3 includes a discharge pipe 28c provided with a pump for discharging the liquid stored in the analysis cell 25 to the outside.

  The potentiogalvanostat 26 functions as a potential fluctuation unit and a current detection unit. The potentiogalvanostat 26 applies a voltage between the boron-doped diamond electrode 22 and the reference electrode 24, detects a current generated between the boron-doped diamond electrode 22 and the counter electrode 23, and calculates the detection signal. This is transmitted to the processing device 27.

  The arithmetic processing unit 27 is a general purpose or dedicated device including a CPU, memory, input / output channels, input means such as a keyboard, output means such as a display, A / D converter, D / A converter, etc. The CPU and its peripheral devices cooperate with each other according to a program stored in a predetermined area of the memory, so that a signal detected by the potentiogalvanostat 26 is analyzed, and detection of selenium ions and concentration measurement are performed. . Further, the signals detected by the pH sensors 124a and 124b are also analyzed by the arithmetic processing unit 27, and the pH of the sample solution is measured. The arithmetic processing unit 27 does not need to be physically integrated, and may be divided into a plurality of devices by wire or wireless.

  Next, a method for analyzing selenium ions by cyclic voltammetry using the electrochemical analyzer 100 according to the present embodiment will be described with reference to the flowchart shown in FIG.

1. pH adjustment step First, the sample solution is poured from the storage tank 11 into each of the pH adjustment tanks 121a and 121b. Next, when the pH of the sample solution is less than 9, an alkaline pH adjuster is supplied from the first pH adjuster tank 122a to the first pH adjuster tank 121a, and the pH of the sample solution stored in the first pH adjuster tank 121a is 9 or more. Adjust to. When the pH of the sample solution is 9 or more, pH adjustment in the first pH adjustment tank 121a is unnecessary, but an acidic pH adjuster is supplied from the second pH adjuster tank 122b to the second pH adjuster tank 121b, The pH of the sample solution stored in the second pH adjustment tank 121b is adjusted to less than 9. In addition, when pH adjustment of the sample solution accommodated in the 2nd pH adjustment tank 121b is required, what is necessary is just to perform at any time before the 2nd reduction | restoration precipitation process explained in full detail below.

2. First Washing Step In parallel, the first buffer solution (pH ≧ 9) is supplied from the first buffer solution tank T1a to the analysis cell 25, and the inside of the analysis cell 25 is washed while stirring the buffer solution. The buffer solution after washing is discharged to the waste liquid tank T3.

3. Electrode surface modification process (gold electrodeposition)
Next, the first buffer solution (pH ≧ 9) is again supplied from the first buffer solution tank T1a to the analysis cell 25, and the AuCl concentration is adjusted to about 75 to 250 ppb from the gold compound tank T2. ₃ or the like is supplied to the analysis cell 25. Then, while stirring the buffer solution to which the gold compound is added, the potential of the boron-doped diamond electrode 22 is changed in the negative potential direction, and gold is electrodeposited on the surface of the boron-doped diamond electrode 22 so that the electrode surface becomes gold. Qualify with

4). First reduction precipitation step (reduction precipitation of Se ⁴⁺ )
A sample solution having a pH of 9 or more is supplied from the first pH adjustment tank 121a to the analysis cell 25, and the potential of the boron-doped diamond electrode 22 is set to a negative potential direction using the potentiogalvanostat 26 while stirring the sample solution. by varying, by a lower reduction potential of Se ⁴⁺ in the pH potential to reductive deposition of Se ⁴⁺ on the surface of the boron-doped diamond electrode 22. Then, after the potential of the boron-doped diamond electrode 22 to a potential lower than the reduction potential of the Se ⁴⁺ it is sufficiently concentrated Se ⁴⁺ by maintaining the potential for some time.

5. First liquid replacement step If Se ⁴⁺ is reduced and deposited on the surface of the boron-doped diamond electrode 22, analysis is performed in a state where the potential of the boron-doped diamond electrode 22 is maintained at a potential lower than the reduction potential by a potentiogalvanostat 26. The sample solution in the analysis cell 25 is discharged to the waste liquid tank T3, and the first buffer solution (pH ≧ 9) is supplied from the first buffer solution tank T1a to the analysis cell 25, so that the sample solution in the analysis cell 25 is supplied. Is replaced with the first buffer (pH ≧ 9). Thus, in the liquid replacement step, Se ⁴⁺ reduced and deposited on the boron-doped diamond electrode 22 is eluted in the liquid replacement step by maintaining the potential of the boron-doped diamond electrode 22 at a potential lower than the reduction potential. Can be prevented. This also applies to the second liquid replacement step described in detail below.

6). First oxidation elution process (Se ⁴⁺ oxidation elution)
After the sample solution in the analysis cell 25 is replaced with the first buffer (pH ≧ 9), the potential of the boron-doped diamond electrode 22 is increased to a potential higher than the reduction potential of Se ⁴⁺ by the potentiogalvanostat 26 in the positive potential direction. To oxidize and elute Se ⁴⁺ into the buffer. Then, a current is generated with Se ⁴⁺ oxidation elution.

A current value or a charge value (electric signal) generated by such an electrochemical reaction is transmitted to a potentio galvanostat 26, and signals are controlled and detected at each electrode. The signal detected by the potentiogalvanostat 26 is transmitted to the arithmetic processing unit 27, and a calibration curve of Se ⁴⁺ concentration and current value or charge value prepared in advance is compared with the obtained current value or charge value. Thus, the Se ⁴⁺ concentration in the sample solution is calculated. At this time, a differential current value or a differential charge amount obtained by subtracting a base current value or a base charge amount measured using only the first buffer solution (pH ≧ 9) in advance from a current value or charge amount that is an actual measurement value is used. by calculating the Se ⁴⁺ concentration Te, it is possible to calculate the Se ⁴⁺ concentration with higher accuracy. This also applies to the calculation of the total concentration of Se ⁴⁺ and Se ⁶⁺ described below.

7). Second Washing Step The first buffer solution (pH ≧ 9) after completion of the oxidation elution of Se ⁴⁺ is discharged to the waste liquid tank T3. Subsequently, the second buffer solution (pH <9) is supplied from the second buffer solution tank T1b to the analysis cell 25, and the inside of the analysis cell 25 is washed while stirring the buffer solution. The buffer solution after washing is discharged to the waste liquid tank T3.

8). Second reduction precipitation step (reduction precipitation of Se ⁴⁺ and Se ⁶⁺ )
A sample solution having a pH of less than 9 is supplied from the second pH adjustment tank 121b to the analysis cell 25, and the potential of the boron-doped diamond electrode 22 is set to a negative potential direction using the potentiogalvanostat 26 while stirring the sample solution. by varying the, the ^{Se 4+} and ^{Se 6+} is reduced and deposited on the surface of the boron-doped diamond electrode 22 by the lower than the reduction potential of the ^{Se 4+} and ^{Se 6+} in the pH potential. Then, after the potential of the boron-doped diamond electrode 22 to a potential lower than the reduction potential of the ^{Se 4+} and ^{Se 6+} is sufficiently concentrated the ^{Se 4+} and ^{Se 6+} by maintaining for some time the electric potential.

9. Second liquid replacement step After Se ⁴⁺ and Se ⁶⁺ are reduced and deposited on the surface of the boron-doped diamond electrode 22, the potential of the boron-doped diamond electrode 22 is maintained at a potential lower than the reduction potential by the potentiogalvanostat 26. The sample solution in the analysis cell 25 is discharged to the waste liquid tank T3, and the second buffer solution (pH <9) is supplied from the second buffer solution tank T1b to the analysis cell 25. Replace sample solution with second buffer (pH <9).

10. Second oxidation elution step (oxidation elution of Se ⁴⁺ and Se ⁶⁺ )
After replacing the sample solution in the analysis cell 25 with the second buffer solution (pH <9), the potential of the boron-doped diamond electrode 22 is positively increased to a potential higher than the reduction potential of Se ⁴⁺ and Se ⁶⁺ by a potentiogalvanostat 26. By sweeping in the potential direction, Se ⁴⁺ and Se ⁶⁺ are oxidized and eluted in the second buffer solution (pH <9). Then, an electric current is generated with the oxidation elution of Se ⁴⁺ and Se ⁶⁺ . In addition, the current peak accompanying the oxidation elution of Se ⁴⁺ and Se ⁶⁺ appears as a single thing.

A current value or a charge value (electric signal) generated by such an electrochemical reaction is transmitted to a potentio galvanostat 26, and signals are controlled and detected at each electrode. A signal detected by the potentiogalvanostat 26 is transmitted to the arithmetic processing unit 27, and a calibration curve of the total concentration of Se ⁴⁺ and Se ⁶⁺ and the current value or charge value prepared in advance and the obtained current value or charge value are obtained. And the total concentration of Se ⁴⁺ and Se ^{6+ in} the sample solution is calculated.

Further, the Se ⁶⁺ concentration is calculated by subtracting the previously obtained Se ⁴⁺ concentration from the calculated total concentration of Se ⁴⁺ and Se ⁶⁺ .

11. Post-electrolysis step After the potential sweeping is completed, the potential of the boron-doped diamond electrode 22 is kept at about +1.0 V for a while, thereby completely oxidizing and eluting gold and residual selenium deposited on the electrode surface. The boron-doped diamond electrode 22 can be returned to the state before use and regenerated. By regenerating the boron-doped diamond electrode 22 in this way, the same electrode can be used repeatedly. The boron-doped diamond electrode 22 can be regenerated not only by holding at a constant potential but also by repeatedly performing sweeping at a wide potential. When post-electrolysis is completed, the sample solution in the analysis cell 25 is discharged to the waste liquid tank T3.

According to the present embodiment configured as described above, Se ⁴⁺ can be selectively detected by setting the pH of the sample solution to 9 or more, and therefore the concentration of Se ⁴⁺ alone can be measured. Furthermore, since Se ⁴⁺ and Se ⁶⁺ can be detected together by setting the pH of the sample solution to less than 9, by subtracting the Se ⁴⁺ concentration from the total concentration of Se ⁴⁺ and Se ⁶⁺ , The concentration of Se ⁶⁺ can also be determined.

Also, gold deposited on the surface of the boron-doped diamond electrode 22 is more stable under alkaline conditions than under acidic conditions. For this reason, if the analysis of Se ⁴⁺ performed with the sample solution having a pH of 9 or more is performed prior to the batch analysis of Se ⁴⁺ and Se ⁶⁺ performed with the sample solution having a pH of less than 9, the surface of the boron-doped diamond electrode 22 is obtained. Since the gold deposited on the layer is difficult to peel off, a highly accurate analysis can be performed.

Second Embodiment
A second embodiment of the present invention will be described below with reference to the drawings. In the following, description will be made centering on differences from the first embodiment.

  As shown in FIG. 3, the electrochemical analyzer 100 according to the present embodiment does not have the pH adjusting unit 12 in the first embodiment, and the analysis cell 25 also serves as the pH adjusting tanks 121a and 121b. Therefore, the pH adjusting agent tanks 122a and 122b and the pH sensor 124 are provided in the analysis cell 25.

  Next, a method for analyzing selenium ions by cyclic voltammetry using the electrochemical analyzer 100 according to the present embodiment will be described with reference to the drawings.

<2-1st Embodiment>
First, the case where the sample solution whose pH is 9 or more and the sample solution whose pH is less than 9 are separately separated will be described with reference to the flowchart shown in FIG.

1. First washing step-2. Electrode surface modification process (gold electrodeposition)
The first cleaning step and the electrode surface modification step (gold electrodeposition) are performed in the same manner as in the first embodiment.

3. First pH adjustment step (pH ≧ 9)
First, a sample solution is poured from the storage tank 11 into the analysis cell 25. Next, an alkaline pH adjuster is supplied from the first pH adjuster tank 122a to the analysis cell 25, and the pH of the sample solution in the analysis cell 25 is adjusted to 9 or more. When the pH of the sample solution is already 9 or higher, the pH adjustment is not necessary.

4). First reduction deposition step (reduction deposition of Se ⁴⁺ ) to 7. Second Cleaning Step The first reduction deposition step, the first liquid replacement step, the first oxidation elution step, and the second cleaning step are performed in the same manner as in the first embodiment.

8). Second pH adjustment step (pH <9)
First, a new sample solution is poured from the storage tank 11 into the analysis cell 25. Next, an acidic pH adjuster is supplied from the second pH adjuster tank 122b to the analysis cell 25 to adjust the pH of the sample solution in the analysis cell 25 to less than 9. When the pH of the sample solution is already less than 9, the pH adjustment is not necessary.

9. Second reduction precipitation step (reduction precipitation of Se ⁴⁺ and Se ⁶⁺ ) to 12. Post-electrolysis step Each step of the second reduction deposition step, the second liquid replacement step, the second oxidation elution step, and the post-electrolysis step is performed in the same manner as in the first embodiment.

<Second Embodiment>
Next, the case where the sample solution whose pH is adjusted to 9 or more and the sample solution whose pH is adjusted to less than 9 are the same will be described with reference to the flowchart shown in FIG.

1. First cleaning step to 4. First reduction precipitation step (reduction precipitation of Se ⁴⁺ )
Each process of the 1st washing process, the electrode surface modification process, the 1st pH adjustment process, and the 1st reduction precipitation process is performed like a 2nd-1 embodiment.

5. First oxidation elution process (Se ⁴⁺ oxidation elution)
Without replacing the sample solution in the analysis cell 25, the potential of the boron-doped diamond electrode 22 is swept in the positive potential direction to a potential higher than the reduction potential of Se ⁴⁺ by the potentiogalvanostat 26, and Se in the sample solution is obtained. ⁴⁺ is oxidized and eluted, and the Se ⁴⁺ concentration in the sample solution is calculated based on the generated current.

6). Second pH adjustment step (pH <9)
Without replacing the sample solution in the analysis cell 25, an acidic pH adjuster is supplied from the second pH adjuster tank 122b to the analysis cell 25, and the pH of the sample solution in the analysis cell 25 is adjusted to less than 9. .

7). Second reduction precipitation step (reduction precipitation of Se ⁴⁺ and Se ⁶⁺ )
In the same manner as in the 2-1 embodiment, Se ⁴⁺ and Se ⁶⁺ are reduced and deposited on the surface of the boron-doped diamond electrode 22.

8). Second oxidation elution step (oxidation elution of Se ⁴⁺ and Se ⁶⁺ )
Without replacing the sample solution in the analysis cell 25, the potential of the boron-doped diamond electrode 22 is swept in the positive potential direction to a potential higher than the reduction potential of Se ⁴⁺ and Se ⁶⁺ by the potentiogalvanostat 26. ⁴⁺ and Se ⁶⁺ are oxidized and eluted in the sample solution, and the total concentration of Se ⁴⁺ and Se ⁶⁺ in the sample solution is calculated based on the generated current. Further, the Se ⁶⁺ concentration is calculated by subtracting the previously obtained Se ⁴⁺ concentration from the calculated total concentration of Se ⁴⁺ and Se ⁶⁺ .

In addition, taking into account the change in the volume of the sample solution due to the pH adjusting agent added during pH adjustment, the total concentration of Se ⁴⁺ and Se ⁶⁺ and the volume of the sample solution during the second oxidation elution step were used as analysis targets. determine the total content of ^{Se 4+} and ^{Se 6+} included in the sample solution, ^{Se 4+} and ^Se from the total content of ^{6+, Se} determined in consideration of the volume of the sample solution at the first oxidation elution step similarly ⁴⁺ The content of Se ⁶⁺ can also be calculated by subtracting the content of. In this way, more accurate measurement can be performed. This point is the same in the first embodiment and the 2-1 embodiment.

9. Post-electrolysis step Post-electrolysis is performed in the same manner as in Embodiment 2-1.

According to the present embodiment configured as described above, the apparatus configuration can be made simpler than that of the first embodiment. When the amount of the sample solution is small, both the detection of Se ⁴⁺ at pH ≧ 9 and the detection of Se ⁴⁺ and Se ⁴⁺ at pH <9 are performed using the same sample solution. It can be carried out.

  The present invention is not limited to the above embodiment.

  For example, the electrodeposition of gold on the surface of the boron-doped diamond electrode 22 may not be performed before the reduction deposition of selenium ions, and the electrodeposition of gold and the reduction deposition of selenium ions may be performed in parallel. .

  In addition, when the pH of the sample solution and the sampling amount are almost known, the pH of the sample solution can be easily adjusted by adjusting the addition amount of the pH adjusting agent. The apparatus 100 may not include the pH sensors 124a and 124b.

  Furthermore, in the above embodiment, the analysis by the three-electrode method using the working electrode (boron-doped diamond electrode) 22, the counter electrode 23 and the reference electrode 42 is performed, but only the working electrode 5 and the counter electrode 3 are provided. Analysis by an electrode method may be used.

  In addition, it is needless to say that some or all of the above-described embodiments and modified embodiments may be appropriately combined, and various modifications can be made without departing from the scope of the invention.

  The present invention will be described in more detail with reference to the following examples. However, the present invention is not limited to these examples.

<Production of boron-doped diamond (BDD) electrode>
After using the silicon substrate {Si (100)} to texture the substrate surface, the substrate was set in a holder of a microwave CVD film forming apparatus (manufactured by ASTeX). As a film forming source, acetone was used as a carbon source, and B (OCH ₃ ) as a boron source was dissolved so as to have a boron / carbon (B / C) molar ratio of 10 ⁴ ppm.

Then, after passing pure H ₂ gas as a carrier gas through this film-forming source, it is introduced into the chamber, and hydrogen (532 mL / min) is preliminarily flowed through a separate line to a predetermined pressure (115 Torr = 115 × 133.322 Pa). It adjusted so that it might become. Subsequently, after injecting and discharging 2.45 GHz microwave power, the power was adjusted to 5 kW, and when stable, pure H ₂ gas (15 mL / min) was used as a carrier gas for the film formation source. The film was formed by a microwave plasma assisted CVD method at a film formation rate of 1 to 4 μm / h. A BDD electrode was obtained in which a diamond thin film having a standard diamond crystal size of about 5 μm and a thickness of about 40 μm was formed in a reaction time of about 10 hours.

<Production of gold-modified boron-doped diamond (BDD-Au) electrode>
The BDD electrode obtained as described above is immersed in a 0.1 M hydrochloric acid aqueous solution containing 50 μM of HAuCl ₄ and held at a potential of −0.4 V for 30 seconds with respect to the Ag / AgCl electrode. Gold particles were electrodeposited to prepare a BDD-Au electrode.

<Measurement system>
Electrochemical measurement was performed in a single cell using a potentiostat (manufactured by ALS / CH) at room temperature (23 ° C.). As the working electrode, the BDD-Au electrode (estimated surface area 0.28 cm ² ) obtained as described above was used. Further, an Ag / AgCl electrode was used as the reference electrode, and a Pt electrode was used as the counter electrode.

<Test 1> Cyclic Voltammetry A selenium aqueous solution (pH 1) prepared by dissolving Se ⁴⁺ and Se ⁶⁺ in 0.1 M HClO ₄ at different concentrations was prepared, and cyclic voltammetry was performed. At this time, electrodeposition (reduction deposition) of selenium was performed at -1.2V. Among the obtained results, a voltammogram is shown in FIG. 6, and the relationship between the potential and the peak current value associated with the oxidation elution of selenium ions is shown in the graph of FIG.

As shown in FIG. 6, the peak current value accompanying oxidation elution was observed in the vicinity of 0.9 V for any selenium ion of Se ⁴⁺ (a) and Se ⁶⁺ (b). In addition, as shown in FIG. 7, the peak current value was proportional to the selenium ion concentration regardless of whether Se ^{4+ or} Se ⁶⁺ .

<Test 2> Cyclic Voltammetry Similar to Test 1, Se ⁴⁺ (1 ppm) alone, Se ⁶⁺ (1 ppm) alone, and an aqueous selenium solution containing both Se ⁴⁺ (1 ppm) and Se ⁶⁺ (1 ppm) ( pH 1) was prepared and cyclic voltammetry was performed. The obtained results are shown in the voltammogram of FIG.

As shown in FIG. 8, the peak current value associated with oxidation elution is observed at around 0.9 V for both Se ⁴⁺ and Se ⁶⁺ selenium ions, and Se ⁴⁺ and Se ⁶⁺ Even when both of these were included, the current generated with the oxidative elution of selenium ions was confirmed as a single peak.

<Test 3> Effect of pH in cyclic voltammetry of Se ⁴⁺ and Se ⁶⁺ (1)
0.1 M Briton-Robinson buffer containing Se ⁴⁺ (10 ppm) or Se ⁶⁺ (10 ppm) was prepared with varying pH and cyclic voltammetry was performed at a scan rate of 100 mV / s. Of the obtained results, the Se ⁴⁺ voltammogram is shown in FIG. 9, the Se ⁶⁺ voltammogram is shown in FIG. 10, and the relationship between the pH and the peak current value associated with the oxidative elution of selenium ions is shown in the graph of FIG.

For both Se ⁴⁺ and Se ⁶⁺ selenium ions, as the pH increased, the position of the current peak accompanying oxidation elution moved in the negative potential direction. However, in Se ⁶⁺ , when the pH was 9 or more, the current peak accompanying the selenium ion oxidation elution was not observed.

<Test 4> Effect of pH on cyclic voltammetry of Se ⁴⁺ and Se ⁶⁺ (2)
Se ⁴⁺ (10 ppm) alone, Se ⁶⁺ (10 ppm) alone and 0.1 M Briton-Robinson buffer containing both Se ⁴⁺ (10 ppm) and Se ⁶⁺ (10 ppm) at a pH of 3.55 or 10 .22, and cyclic voltammetry was performed at a scanning speed of 100 mV / s. The obtained results are shown in the voltammogram of FIG.

At pH 3.55 (FIG. 12 (a)), in all the sample solutions, a current peak accompanying selenium ion oxidation elution was observed in the vicinity of 0.8 to 0.9V. On the other hand, at pH 10.22 (FIG. 12 (b)), when Se ⁴⁺ alone and Se ⁴⁺ (10 ppm) and Se ⁶⁺ are mixed, a current peak associated with selenium ion oxidation and elution is present in the vicinity of 0.5V. Although observed, Se ⁶⁺ alone did not observe a current peak accompanying selenium ion oxidation elution.

12 ... pH adjusting part 22 ... Boron doped diamond electrode (working electrode)
25 ... Analysis cell (cell)
26 ... Potentiogalvanostat (potential fluctuation part, current detection part)
100: Electrochemical analyzer

Claims (11)

A method for electrochemically analyzing selenium ions having different ionic values contained in a sample solution by voltammetry using a carbon electrode as a working electrode,
Adjusting the pH of the sample solution to 9 or higher,
Varying the potential of the working electrode in the negative potential direction to reduce and precipitate Se ⁴⁺ on the surface of the working electrode; and
A method for electrochemical analysis, comprising: sweeping the potential of the working electrode in a positive potential direction to oxidize and elute selenium ions from the surface of the working electrode. Adjusting the pH of the sample solution to less than 9;
Varying the potential of the working electrode in the negative potential direction to reduce and deposit Se ⁴⁺ and Se ^{6+ on the} surface of the working electrode; and
The electrochemical analysis method according to claim 1, further comprising a step of sweeping the potential of the working electrode in a positive potential direction to oxidize and elute selenium ions from the surface of the working electrode.   3. The electrochemical analysis method according to claim 2, wherein the sample solution for adjusting the pH to 9 or more and the sample solution for adjusting the pH to less than 9 are respectively separated from the same sample solution.   3. The electrochemical analysis method according to claim 2, wherein the sample solution for adjusting the pH to 9 or more and the sample solution for adjusting the pH to less than 9 are the same. First, the pH of the sample solution is adjusted to 9 or more to perform Se ⁴⁺ reduction precipitation and oxidation elution, and then the pH of the sample solution is adjusted to less than 9 to reduce the Se ⁴⁺ and Se ⁶⁺ reduction precipitation and oxidation elution. The electrochemical analysis method according to claim 3 or 4, wherein:   The electrochemical analysis method according to claim 1, wherein the carbon electrode is a conductive diamond electrode.   A method in which gold is electrodeposited on the surface of the conductive diamond electrode before the selenium ions are reduced and deposited on the surface of the conductive diamond electrode or in parallel with the reduction and precipitation of selenium ions on the conductive diamond electrode. 7. The electrochemical analysis method according to 6. An apparatus for electrochemically analyzing selenium ions having different ionic values contained in a sample solution by voltammetry,
A first pH adjuster for adjusting the pH of the sample solution to 9 or more;
A second pH adjuster for adjusting the pH of the sample solution to less than 9;
A working electrode made of a carbon electrode, and a cell for containing the sample solution;
Varying the potential of the working electrode in the negative potential direction, supplying a potential for reducing and precipitating selenium ions on the surface of the working electrode, and then sweeping the potential of the working electrode in the positive potential direction, A potential fluctuation portion for supplying a potential for oxidizing and eluting selenium ions from
An electrochemical analyzer comprising: a current detection unit that detects a current change associated with a change in potential of the working electrode.   The electrochemical analyzer according to claim 8, wherein the first pH adjusting unit and the second pH adjusting unit are separately provided.   The electrochemical analyzer according to claim 8, wherein the first pH adjusting unit also serves as the second pH adjusting unit. The electrochemical analysis device according to claim 10, wherein the cell serves as both the first pH adjusting unit and the second pH adjusting unit.