JPH073414B2 - Method and apparatus for electrochemical deionization for ion chromatography - Google Patents

Description

TECHNICAL FIELD The present invention relates to ion chromatography, and more specifically, to an electrochemical deionization method in ion chromatography for the purpose of separating and quantifying anions or cations and a method thereof. Regarding the device.

[Conventional Technology] What is ion chromatography?
・ Analytical chemistry journals such as SMALL) [Anal. Chem., 47 , 1801 (1975
)], Mainly high-performance liquid chromatography for the separation and detection of inorganic ions.
It has already been put to practical use and is being used for analysis of environmental samples and biological samples and control analysis of various processes.

According to this technique, by using a high-pressure pump, the flow rate of the eluent passing through the separation column is increased, thereby shortening the separation time, and at the latter stage of the separation column, the deionized column packed with the ion-exchange gel is removed. By using an ion column, a so-called suppressor column, high sensitivity is achieved in the measurement of the target component. However, it has been pointed out that this technology is troublesome to regenerate the suppressor column.

Therefore, in ion chromatography using a conductivity meter, a diffusion dialysis instrument using an ion exchange membrane in the latter stage of a separation column, that is, a so-called membrane suppressor is now drawing attention. References include, for example, TS Stevens, Journal of Analytical Chemistry [Anal. Chem., 53 , 1488 (1981)], JP-B-62-36179.
(Ken Murayama)

Compared with the suppressor column described above, such a Maybren suppressor does not require a regeneration operation and can be continuously used. However, it has been clarified that the membrane suppressor has a wide peak width because the flow velocity in the ion exchange tube is not constant, and the baseline is unstable because the strong acid of the ion exchange solution flows backward.

On the other hand, the use of an ion exchange membrane as an electrodialysis device is a well-known technique that has been put to practical use on an industrial scale, as seen particularly in the production of sodium hydroxide and chlorine by salt electrolysis.

Also in ion chromatography, an electrochemical suppressor using an electrodialysis device equipped with such an ion exchange membrane has been proposed. References to such electrochemical suppressors include, for example, KH Jansen, U.S. Pat. No. 4,459,357 (1984).
Year) and ZWTian Chromatography magazine [J. Chromatogr., 439 , 159 (1988
Year)].

As shown in FIG. 6, the ion chromatography for anion analysis proposed by K.H.Jansen et al.
Eluent tank 1, pump 2 for pumping eluent to sample injection valve 3, separation column 4 filled with anion exchange gel, electrodialysis device 5, and electrodialysis device for strong acid from ion exchange solution tank 8. It comprises a pump 6 for pumping to 5 and an electric conductivity meter 7 for measuring the electric conductivity. The electrodialysis device in this ion chromatography is for removing the metal cations of the eluent and for lowering the background of the conductivity meter, and enhances the measurement sensitivity of the target ion.

As shown in FIG. 7, this electrodialysis apparatus comprises a cell including a channel 18 formed by being sandwiched between a pair of ion exchange membranes 10 and 12, and a pair of an anode 14 and a cathode located on both outer sides of the cell.
It has 16 and. The eluent and the target component from the separation column 4 are introduced into the flow path 18, while the ion exchange membrane 10 and the anode 14 are introduced.
And ion conversion solutions 20, 22 between the exchange membrane 12 and the cathode 16.
A strong acid such as H ₂ SO ₄ , HNO ₃ , or H ₃ PO ₄ is used for the above. Then, a high potential of about 220 V is applied between the anode 14 and the cathode 16, and a current of 120 mA flows.

[Problems to be Solved by the Invention] In the above-described conventional electrodialysis device, the ion exchange membrane is easily deteriorated due to the high applied potential, the risk of leakage is high, and the amount of bubbles generated due to side reaction can be increased. It has a drawback that it causes instability of the baseline due to its large property.

The present invention has been made to solve the above problems, and an object thereof is to provide an electrochemical deionization method and an apparatus thereof that stabilizes a baseline and enables highly sensitive detection of a target component. is there.

[Means and Actions for Solving the Problems] According to the first aspect of the present invention, when the eluent is continuously flowed between two ion exchange membranes having the same polarity, the eluent is applied to both outer sides of the ion exchange membrane. By continuously flowing the ion exchange solution and applying a constant potential through the ion exchange solution, the ion exchange membrane and the eluent, the non-target component is deionized from the eluent to be converted into water or a weak electrolyte and at the same time An electrochemical deionization method for ion chromatography that aims to increase the sensitivity of the measurement of the target component by converting the component to a strong electrolyte and measuring the conductivity of the eluent after conversion. The step of measuring the relationship between the corresponding applied potential and the deionization efficiency and the conversion efficiency of the target component to the strong electrolyte in advance, and determining the optimum applied potential that maximizes both the deionization efficiency and the conversion efficiency, and the measurement. An electrochemical deionization method for ion chromatography is provided, which comprises a step of examining the concentration of an eluent to be deionized and deionizing a non-target component of the eluent by an optimum applied potential.

According to the second aspect of the present invention, in a deionization device connected to a subsequent stage of a separation column in ion chromatography, an eluent spacer having a flow path through which eluents released in opposite two-face directions pass, A pair of ion-exchange membranes of the same polarity that are placed outside the eluent spacer so that they come into contact with the eluent in the flow path of the liquid spacer, and are arranged on both sides of the ion-exchange membrane, and the inside is in contact with the ion-exchange membrane A pair of ion-exchange solution spacers having a channel through which the ion-exchange solution passes, and on the outside of the ion-exchange solution spacer so as to come into contact with the ion-exchange solution in the channel of the ion-exchange solution spacer. There is provided an electrochemical deionization apparatus for ion chromatography, which is composed of a pair of electrodes arranged.

According to the present invention, the eluent continuously flows between the two ion exchange membranes having the same polarity, and the ion exchange solution continuously flows on both outer sides of the ion exchange membrane. A constant potential is applied by the pair of electrodes through the ion exchange solution, the ion exchange membrane, and the eluent, thereby performing ion exchange between the eluent and the ion exchange solution. That is, the non-target component is deionized from the eluent, the eluent is converted into water or a weak electrolyte, and at the same time the target component is converted into a strong electrolyte.

Regarding the potential applied by the pair of electrodes, the relationship between the applied potential corresponding to each eluent concentration and the deionization efficiency and conversion efficiency is measured in advance, and the deionization efficiency and conversion efficiency are both maximum for each eluent concentration. The optimum applied potential is determined. At the time of actual deionization, the concentration of the eluent to be measured is investigated, and deionization of the non-target component of the eluent and conversion of the target component into a strong electrolyte are simultaneously performed by the predetermined optimum applied potential.

<Example 1> With reference to FIG. 1, an electrodialysis apparatus which is an example of a deionization apparatus for ion chromatography according to the present invention will be described.

The electrodialysis device generally comprises an eluent spacer 42 and
Two cation exchange membranes 30 and 32 arranged on the outside,
Furthermore, it is composed of two ion-exchange solution spacers 34 and 36 arranged on the outer side thereof, and a pair of anode 38 and cathode 40 arranged on the outermost side to form an integrated electrode cell structure. There is.

The eluent spacer 42 has a hexagonal channel 42a through which the eluent released in the vertical direction passes. This channel 42a, for example, has a thickness of 0.3 to 2 mm, a channel width of 5 to 20 mm, and a length of 20 to 200 mm, and in the channel, a synthetic resin such as nylon or polypropylene may be used if necessary. A mesh 43 made of polyethylene is inserted. The eluent spacer 42 is made of, for example, a silicone rubber plate. This mesh has the effect of increasing the ion exchange rate by causing a turbulent flow in the separating / delivering flow path, and also having the effect of increasing the ion exchange rate by decreasing the cell internal volume.

The cation exchange membranes 30 and 32 have a thickness of 0.05 to 0.6 mm.
The membrane resistance of the cation exchange membranes 30 and 32 is 0.5 to 10 ohm · cm ² in 0.5M saline solution at 25 ° C. 25 ℃ 1M / 0.1M
It is calculated from the membrane potential in an aqueous potassium chloride system, but in the cation exchange membrane, the cation transport number is 0.95 to 1.00, and in the anion exchange membrane, the anion transport number is 0.95 to 1.00.
Is. There are two types of ion exchange membranes, fluorine-based and hydrocarbon-based. For example, Nafion membrane (trade name) of DuPont, which is a fluorine-based ion exchange membrane, RAIPORE membrane (trade name) of Pall-RAI, and Soda Tokuyama. NEOSEPTA membrane (trade name) manufactured by Tosoh Corporation and TOSFEX membrane (trade name) manufactured by Tosoh Corporation can be used.

Ion exchange solution spacers 34 and 36 have a thickness of 0.5 to 10 mm.
Yes, for example, manufactured from a silicone rubber plate. Inside the ion exchange solution spacers 34 and 36, flow paths 34a and 36a for passing the ion exchange solution are formed.
The ion exchange solution flowing in the flow paths 34a and 36a is in contact with the cation exchange membranes 34 and 36, the anode 38, and the cathode 40, respectively. In the preferred embodiment of the present invention, the flow path 34a,
The ion exchange solution in 36a is used as the flow path 4 of the eluent spacer 42.
It is designed to flow in the opposite direction to the eluent flowing through 2a.

The anode 38 is, for example, DSE (trade name: Permelek electrode)
And the cathode 40 is made of, for example, stainless steel.

As a result of an experiment conducted on the relationship between the applied potential and the conductivity of the eluent using the electrodialyzer having the above-described structure, the results shown in FIG. 2 were obtained. In this case, 2 mM NaHCO ₃ /1.6 mM Na ₂ CO ₃ as the standard eluent and 2 times, 3 times,
A carbonate-based aqueous solution having a concentration of 4 times was used. Then, the flow rate of the eluent was set to 1 m1 / min, the applied potential was changed in the range of 0 to 20 V, and the relationship with the conductivity (μS / cm) of the conversion liquid as the eluent after ion exchange was measured. The ion exchange solution used was 25 mM sulfuric acid and was passed at 2 ml / min.

From FIG. 2, when 2 mM NaHCO ₃ /1.6 mM Na ₂ CO ₃ was used,
The conductivity of the reference eluent itself was about 600 μS / cm, but it was about 42 μS / cm at an applied potential of 0 V (corresponding to a conventional membrane suppressor) and 13 μS / cm at 5 V. The dialyzer has been shown to have a high deionization capacity. The above results indicate that the carbonate in the eluent was efficiently converted to carbonic acid, which is a weak electrolyte. On the other hand, when sodium hydroxide is used as the eluent, the conductivity is almost zero due to the increase in the applied potential, as in the carbonate eluent.
It decreased to μS / cm. This indicates that sodium hydroxide was efficiently converted to water.

The potential at which the above-mentioned solid deionization was completed increased as the concentration of the eluent increased. This demonstrates that in order to introduce the eluent into the electrolytic cell and completely deionize it, it is necessary to increase the applied potential to increase the mobility of ions. As a result of the experiment, in the applied potential range of 1 to 20 V, 30 mM in the sodium hydroxide eluent
To the extent of 8% NaHCO ₃ /6.4 mM N in carbonate eluents.
It has been found that sodium removal up to a ₂ CO ₃ is possible.

FIG. 3 is a graph showing the relationship between the potential applied to the deionization apparatus according to the present invention and the peak height of solute (NaCl) (conversion efficiency to strong electrolyte).

In FIG. 3, the detection sensitivity of chloride ion in the applied potential range of 1 to 15 V by the reference carbonate eluent is
At the minimum applied potential (5 V) where the deionization effect was observed, the maximum value was shown, indicating that HCl was efficiently produced. However, it decreased almost linearly when the applied potential was higher than that. This shows that the minimum applied potential at which the highest deionization effect shown in FIG. 2 is recognized is the most appropriate optimum applied potential in terms of conversion efficiency.

Therefore, this minimum applied potential increases as the concentration of the eluent increases, indicating that the applied potential needs to be appropriately selected depending on the concentration of the eluent.

Regarding the stoichiometric relationship of the ion exchange reaction in the electrodialysis apparatus of the present invention when various cations are present as counterions of nitrate ion, alkali metal and alkaline earth metal ions are respectively used in a separation column. Without using sodium hydroxide as an eluent, the sample was injected, and the result of measurement with an electric conductivity meter is shown in Table 1.

When the peak area when nitric acid was injected as a sample was 1.00, the relative peak areas per equivalent of all the above cations were
It shows 0.95 to 1.04, and the cation exchange reaction between hydrogen ions from the ion exchange solution (nitric acid) and each metal cation proceeds in a nearly stoichiometric manner, converting each eluent to water and each metal nitrate. It was shown that all salts were converted to nitric acid. On the other hand, the stoichiometric relationship of the cation exchange reaction in the electrochemical deionization apparatus was measured using hydrochloric acid, nitric acid, sulfuric acid and its sodium salt, and the results are shown in Table 2.
In these samples, the peak area ratios per equivalent of sodium salt type / acid type were almost equal, indicating that the corresponding acids were produced. This indicates that the electrochemical cation exchange reaction proceeds stoichiometrically without being affected by the anion morphology.

Example 2 Next, in place of the electrodialyzer for ion chromatography shown in FIG. 7, an ion chromatograph equipped with the electrodialyzer according to the present invention was used, and a standard sample and an actual sample were used under the following conditions. Experiments were performed on the samples.

* Separation column: TSKgel IC-Anion-PWxL (manufactured by Tosoh Corporation),
Temperature 35 ° C. * eluant: 2mM NaHCO ₃ /1.6mM of Na ₂ CO ₃ in, 1 ml / min * ion exchange solution: 25 mM in H ₂ SO _4, 2ml / min * deionizer: applied potential 5V, temperature 35 ℃ * standard ^{_{sample: CI - = 0.1mM NO 2 -}} = 0.2mM NO 3 - = 0.15mM PO 4 3- = a ^{_{0.2mM SO 4 2- = 0.1mM S 2}} O 3 2- = 0.2mM ( all sodium salts 100 μl injection) * Experimental sample: Rainwater (pH = 5.75) and soil elution water (pH = 7.
60) was centrifuged, and the supernatant was passed through a membrane filter with a pore size of 0.22 μm, and the resulting solution was diluted 1/5 with pure water and 100 μl of the sample was injected. As shown in FIGS. The results of separation and detection were good, and the reproducibility of nitrate ion according to the peak height was good with a coefficient of variation of 0.5% or less. Detection limit (S
/ N = 3) means Cl ⁻ = 0.32 μM, NO ₂ ⁻ = 0.78 μM, NO ₃ ⁻ =
0.45 μM, PO ₄ ^3- = 0.71 μM, SO ₄ ^2- = 0.33 μM, S ₂ O ₃ ^2-
= 0.65 μM.

<Example 3> In order to demonstrate that the present invention can be applied to deionization in ion chromatography of cations instead of the two cation exchange membranes in the electrodialysis apparatus of the present invention shown in FIG. Two anion exchange membranes, for example, Tosoh Fluorine-based anion exchange membrane IE-SF34 (trade name) is attached,
Other than that, the configuration was the same as that in FIG. Using such an electrodialyzer, 20 mM sodium hydroxide (NaOH) was used as an ion exchange solution, and a 2 mM nitric acid aqueous solution (HNO ₃ ) and 2 times thereof which were commonly used in ion chromatography of cations were used as an eluent. A three-fold or four-fold concentration was used. Apply an electric potential of 0 to 20 while flowing the eluent at 1 ml / min.
V was changed to V, and the relationship with the conductivity (μS / cm) of the conversion liquid that was the eluent after ion exchange was measured,
The conductivity decreased to almost 0 μS / cm at 20 V, demonstrating that nitric acid was efficiently converted to water by the hydroxide ion from the ion exchange solution, and this method was applied to cation ion chromatography. I showed that I can do it.

Regarding the stoichiometric relationship of anion exchange reaction in an electrodialyzer when various anions are present as counterions of sodium ion, chloride, nitric acid, phosphate and sulfate ions are used without separation column. Table 3 shows the results obtained by passing sodium hydroxide as a solution for ion exchange, injecting the sample, and measuring with a conductivity meter.

When the peak area when sodium hydroxide was injected as the sample was set to 1.00, the peak relative intensity ratio was 0.97 to 1.00 for all the above equivalents. That is, the anion exchange reaction between the hydroxide ion from the ion exchange solution and each anion proceeds almost stoichiometrically, and therefore each sodium salt is completely converted into water while converting the eluent into water. It was shown that it was converted to sodium oxide.

On the other hand, Table 4 shows the results of the measurement of the stoichiometric relationship of the anion exchange reaction in the electrodialysis device using hydroxides of lithium, sodium, potassium and nitrates thereof. In these samples, the peak areas per equivalent of nitrate-type / hydroxide-type ions were approximately equal, indicating that all nitrates were each converted to the corresponding hydroxide. This shows that the anion exchange reaction proceeds electrochemically stoichiometrically without being affected by the cation morphology, and this deionization method is also applied to cation ion chromatography. It was proved that it was possible.

[Effects of the Invention] As described in detail above, according to the present invention, the relationship between the applied potential corresponding to each eluent concentration and the deionization efficiency and the conversion efficiency into a strong electrolyte is measured in advance, and the deionization efficiency and The optimum applied potential that maximizes both conversion efficiency (detection sensitivity) is set, and when deionization is actually performed, the optimum applied potential corresponding to the concentration of the eluent to be measured is set to determine the non-target component of the eluent. Since deionization is performed, deionization can be performed efficiently without lowering conversion efficiency. This stabilizes the baseline and enables highly sensitive detection of the target component.

Incidentally, in the electrochemical deionization method for ion chromatography and the apparatus thereof of the present invention, the applied potential is 1 to 20.
It is in the V range and can be operated well with extremely low power. Therefore, applied potential of 50-550V, 10-120m
There are problems such as heat generation and deterioration of the ion exchange membrane, violent generation of bubbles, reproducibility problems, durability problems, or increase in running cost that the conventional electrodialysis device that operates with the electrolytic current of A has. The present invention has solved the problems all at once.

[Brief description of drawings]

FIG. 1 is an exploded perspective view showing main components of an electrochemical deionization apparatus for ion chromatography according to the present invention. FIG. 2 is a graph showing the relationship between the applied potential and the conductivity of the eluent deionized by the electrochemical deionization apparatus according to the present invention. FIG. 3 is a graph showing the relationship between the potential applied to the deionization apparatus according to the present invention and the peak height of solute (conversion efficiency to strong electrolyte). FIG. 4 is a graph (ion chromatogram) showing a change in conductivity peak corresponding to various ions of the standard sample deionized by the deionization apparatus according to the present invention. FIG. 5 is a graph showing various peak values obtained by analyzing rainwater and soil runoff water. FIG. 6 is a block diagram showing the basic structure of a conventional ion chromatography for anion analysis. And FIG. 7 is an exploded perspective view showing an example of an electrodialysis apparatus used as a deionization apparatus in conventional ion chromatography. 30, 32 …… Ion exchange membrane 34, 36 …… Spacer 38, 40 …… Electrode 42 …… Eluent spacer 43 …… Mesh

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Hayao Matsushita 2-2-104 Tsutsujigaoka, Midori-ku, Yokohama, Kanagawa Prefecture Examiner Koji Kashiwazaki (56) Reference JP-A-58-21564 (JP, A) JP-A-62-62 -186904 (JP, A) JP 62-36179 (JP, B2) Anal. Chem. , Vol. 47, (1975), p. 1801 Anal. Chem. , Vol. 53, (1981), p. 1488

Claims (7)

[Claims] 1. An eluent is continuously flowed between two ion-exchange membranes of the same polarity, and an ion-exchange solution is continuously flowed on both outer sides of the ion-exchange membrane to obtain the ion-exchange solution and the ions. By applying a constant potential through the exchange membrane and the eluent, the non-target component is deionized from the eluent and converted into water or a weak electrolyte, and at the same time, the target component is converted into a strong electrolyte, which is the eluent after conversion. An electrochemical deionization method for ion chromatography, which aims to increase the sensitivity of measurement of a target component by measuring the conductivity of a conversion solution, which is applied potential and deionization efficiency corresponding to each eluent concentration, and a strong electrolyte. To determine the optimum applied potential that maximizes the deionization efficiency and conversion efficiency, and the concentration of the eluent to be measured. Non target component process and, for ion chromatography electrochemical deionization method is configured to include a performing deionized the eluant. 2. The electrochemical deionization method for ion chromatography according to claim 1, wherein the applied potential is 1 to 20 volts. 3. The electrochemical deionization method for ion chromatography according to claim 2, wherein the ion exchange solution flows in a direction opposite to that of the eluent. 4. The method for electrochemical deionization for ion chromatography according to claim 3, wherein the ion exchange solution is a strong acid in anion ion chromatography or a strong base in cation ion chromatography. . 5. The electrochemical deionization for ion chromatography according to claim ₄ , wherein the strong acid is H ₂ SO ₄ , HNO ₃ , H ₃ PO ₄ , or the strong base is LiOH, NaOH, KOH. Method. 6. A deionization apparatus connected to a subsequent stage of a separation column in ion chromatography, wherein an eluent spacer having passages through which eluents released in opposite two-faced directions pass, and a flow of the eluent spacer. A pair of ion-exchange membranes of the same polarity arranged outside the eluent spacer so as to come into contact with the eluent in the channel, and ions that are arranged on both outer sides of the ion-exchange membrane and contact the ion-exchange membrane inside. A pair of ion-exchange solution spacers having a channel through which the exchange solution passes, and outside the ion-exchange solution spacer so as to come into contact with the ion-exchange solution in the channel of the ion-exchange solution spacer. An electrochemical deionization apparatus for ion chromatography comprising a pair of electrodes arranged. 7. Nylon, polypropylene, in order to increase the deionization efficiency, in the channel of the eluent spacer,
The electrochemical deionization apparatus for ion chromatography according to claim 6, wherein a mesh made of synthetic resin such as polyethylene is inserted.