JPH0466860A - Electrode for measuring concentration of carbon dioxide - Google Patents

Abstract

PURPOSE:To enable execution of accurate measurement even when dissolved oxygen is contained, by a method wherein the film thickness of a cation- exchange film having a terpyridine ligand - transition metal complex fixed, the exchange capacity thereof and the valence of a charge of the metal complex are made to be specific values. CONSTITUTION:In a solid film electrode 1 for measuring concentration of CO2 in a liquid, which is prepared by covering a conductive base 3 with a cation- exchange film 4 containing a terpyridine ligand - transition metal complex, the film thickness of the exchange film 4 is preferably 60 to 100mum. As for a metal complex coordination compound in the exchange film 4, the concentration of the transition metal complex is made to be 1mmol/cm<3> or below, or 0.1 to 0.5mmol/cm<3> preferably, for cation exchange capacity 1meq/g (dry film). It is preferable that the ion exchange capacity of the exchange film 4 is 0.1 to 10meq/g (dry film). For the base 3, graphite of which a reduction current for oxygen is small is preferable. For the exchange film 4, a film of fluorine resin is preferable. As to the terpyridine ligand, terpyridine and bipyridine may be mentioned. For a transition metal, Ru and Co are preferable.

Description

Detailed Description of the Invention [Field of Industrial Application] The present invention relates to an electrode for measuring the concentration of carbon dioxide in a liquid, and more particularly to an electrode for measuring the reduction current due to carbon dioxide using an amperometric method. .

[Prior Art] Accurately measuring the carbon dioxide concentration in blood is extremely important during surgery and the like.

Measurement of carbon dioxide concentration in such an aqueous medium involves absorbing carbon dioxide that has passed through a synthetic resin membrane into an aqueous sodium bicarbonate solution, and measuring the resulting change in hydrogen ion concentration in the aqueous sodium bicarbonate solution. However, it has been difficult to directly measure the concentration electrochemically using the current generated by the electrochemical reduction of carbon dioxide.

In other words, when attempting to reduce hydrogen ions in an aqueous solution using a carbonaceous electrode such as graphite, the reduction of hydrogen ions present in the aqueous solution will be -1.1V to -1.3V (vs. saturated chloride) when the pH is 70. This occurs at the potential of the sodium calomel electrode), but since the reduction of dissolved carbon dioxide to be measured occurs at a potential that is more base than the potential at which hydrogen ion reduction occurs, only the current generated by the reduction of carbon dioxide is observed. This was extremely difficult.

Therefore, the present inventors have developed a method for electrochemical reduction of carbon dioxide by forming a membrane containing a metal complex on a conductive substrate, and using a solid electrode that does not require an electrolyte inside, the coexistence of carbon dioxide in blood is achieved. We proposed in Japanese Patent Application No. 2-79573 ``Method for Measuring Carbon Dioxide Concentration'' that it is possible to measure carbon dioxide concentration unaffected by unavoidable dissolved oxygen, hydrogen ions, hydroxide ions, etc. ing.

[Problems to be Solved by the Invention] The electrode proposed by the present inventors for use in measuring carbon dioxide concentration is a method in which a cation exchange membrane containing a terpyridine-based ligand-transition metal complex is provided on a conductive substrate. This electrode is used as a working electrode, and a constant potential is applied between it and a counter electrode, and the current generated by the selective electrochemical reduction reaction of carbon dioxide that occurs at the working electrode is measured.

The reason why carbon dioxide concentration can be measured using an electrode in which a cation exchange membrane containing this terpyridine-based ligand-transition metal complex is provided on a conductive substrate is as follows: (a) When graphite is used as an electrode, the pH is The potential at which the reduction reaction of hydrogen ions occurs in an aqueous solution of 7.0 is -1°1v to -13v (vs. saturated sodium chloride calomel electrode 5SC
E) However, in contrast, in an electrode in which a cation exchange membrane containing a terpyridine-based ligand-transition metal complex is provided on a conductive substrate, the reduction reaction of hydrogen ions is largely shifted in the base direction. , -2,9V to -2,6V (vs. 5SCE
), the two-electron reduction of HCOOH production, which is an electrochemical reduction reaction of carbon dioxide, Co2+2H"+2 e--HCOOHEe = -0,
85V vs. 5SCE CO generation C02+ 2 H" +2 e-→CO+ H20E"
=-0,76V vs. 5SCE These reactions can be detected.

(b) Furthermore, since the terpyridine-based ligand-transition metal complex is fixed on the cation exchange membrane, hydroxide ions generated by water decomposition are eliminated by the action of the cation exchange membrane.

(C) Since the film containing the terpyridine-based ligand-transition metal complex provided on the conductive substrate is an organic film, it can alleviate the influence of oxygen that is inevitably dissolved in blood.

However, as in the case of measuring carbon dioxide concentration in blood, it was not sufficient in terms of the influence of dissolved oxygen.

[Means for Solving the Problems] The present invention was conceived through intensive study on improvements to the electrodes used in the previously proposed method for measuring carbon dioxide concentration.

In other words, the electrode used in the method for measuring the concentration of carbon dioxide proposed earlier has a terpyridine-based ligand-transition metal complex fixed in a cation exchange membrane, and has sufficient selectivity for the reduction reaction of hydrogen ions. However, the problem was that the selectivity for oxygen, which is dissolved together with carbon dioxide in blood, was not sufficient. Therefore, in order to solve this problem, we investigated various factors and found that, in particular, the membrane tail of the cation exchange membrane in which the terpyridine-based ligand-transition metal complex formed on the conductive substrate was fixed. It has been found that the objective can be achieved by setting the exchange capacity and the charge valence of the metal complex to predetermined values.

Various materials such as metals, metal oxides, and carbon-based materials can be used for the conductive substrate used as the base of the electrode for measuring carbon dioxide gas concentration of the present invention. Preference is given to using graphite.

The cation exchange membrane that fixes the terpyridine-based ligand-transition metal complex includes a cation exchange membrane made of a fluororesin-based perfluorocarbon and a hydrocarbon-based cation made of a copolymer of styrene and divinylbenzene. Although it is possible to use an exchange membrane, a fluororesin-based cation exchange membrane is preferred because it has excellent corrosion resistance against various drugs or substances to be measured. These cation exchange membranes are cation exchange membranes having chemical structures represented by the following (1), (2), and (3).

[(CF2CF2)-CF2CF2.

! F2 (CF2)? (However, R1 is SO3Na or cooNa and
m=5 to 135, n=-1000, X=1 to 100. Y=1 to 5) (However, R1 is SO3N a, R2 is COON
a or SO2N HCH2CH2N H2te, m=5 to 13.5, n=100. h=125 to 2700, X and i=1 to 100. Y and j=1 to 5) (However, R1+1cH2CF2CF2-R2 or C
H2(CF2CF2), -R2, R2 is 5o
3Na or COONa, m=5 to 13,5
, n=1000) Examples of the terpyridine-based ligand of the terpyridine-based ligand-transition metal complex used in the electrode for measuring carbon dioxide concentration of the present invention include terpyridine, bipyridine, or derivatives thereof.

Examples of the transition metal that forms the terpyridine-based ligand-transition metal complex include cobalt, nickel, ruthenium, iron, and the like, with ruthenium and cobalt being preferred.

The action in the cation exchange membrane containing the terpyridine-based ligand-transition metal complex of the present invention is illustrated in FIG. 2. The terpyridine-based ligand-transition metal complex is shown by the broken line surrounding the metal ion M2. , these appear to be trapped in the cation exchange membrane through weak interactions with the ion exchange groups in the cation exchange membrane. Hydroxyl ions in the aqueous solution cannot enter the membrane due to the selective permselectivity of the cation exchange membrane, but water and dissolved carbon dioxide HC
008 generated two-electron reduction CO2+ 2 H" +2 e --HCOOHEe
=-0,85V vs. S 5CE CO generation C02+ 2 H"+ 26 -CO+H20Ee=-
Since the reaction is shown as 0.76V vs. 5SCE, it can be observed as a change in current.

The ion exchange group (So3H-) and metal complex (divalent cation) of the ion exchange membrane have an ion exchange capacity of 0.9 meq.
/g (dry film): 10:3-10:
When the exchange capacity of the ion exchange membrane is large, the chance of contact between the ion exchange group and the metal complex increases; on the other hand, when the ion exchange capacity is small, the opposite is true.

In this way, the metal complex concentration in the ion exchange membrane is controlled by the ion exchange capacity of the ion exchange membrane, but the metal complex coordination compound in the cation exchange membrane has a cation exchange capacity of 1 meq/g (dry membrane ) per transition metal complex concentration of 1,0 mmol/cm3 or less, preferably 0,
1 mmol/cm3 or more 0, 5 mmol/cm3
・Also, by changing the film thickness, it is possible to pass or block ions in the aqueous solution to be measured, and in particular, by setting a specific film thickness, it is possible to prevent oxygen from interfering with the measurement of carbon dioxide concentration. It has been found that it is possible to prevent permeation and to prevent measurement errors caused by oxygen. When the exchange capacity of the ion exchange membrane is 0.91 meq/g (dry membrane), the thickness of the cation exchange membrane is preferably 60 μm to 100 μm.

Although the present invention has been explained assuming that the electrode of the present invention is used at around pH 7.0, the reduction reaction of carbon dioxide in water is
In the range of pH 5, 2 to 7.0, there is no adverse effect and there is no change in the reduction current.

FIG. 1 shows a cross-sectional view of the electrode of the present invention. The electrode 1 for measuring carbon dioxide concentration of the present invention has a conductive base 3 made of graphite attached to the tip of an insulating cylindrical body 2 made of plastic or the like, and a terpyridine-based ligand is attached to the conductive base 3. - A cation exchange membrane 4 containing a transition metal complex is coated. A lead wire 5 is attached to the conductive base and connected to an external circuit.

As shown in FIG. 3, the method for measuring the concentration of carbon dioxide using the electrode of the present invention is as shown in FIG.
Using a constant potential electrolyzer 9 in a three-electrode cell 8 with CE) as the reference electrode 7, a constant electric current value after applying a constant potential to the working electrode and performing electrolysis for a certain period of time flows due to the reduction of carbon dioxide. Measure as a current value. The electrolyte aqueous solution used in the measurement can be an aqueous solution of sodium chloride, potassium chloride, perchlorate, or a phosphate buffer. Also,
To measure the carbon dioxide concentration, a constant potential of -1.IV to -1.3V is applied to the working electrode.

[Function] The present invention provides a solid membrane electrode for measuring carbon dioxide concentration in a liquid in which a cation exchange membrane containing a terpyridine-based ligand-transition metal complex is coated on a conductive substrate. By setting the exchange membrane to a specific thickness, it is possible to increase the selectivity for oxygen dissolved in the liquid to be measured, making it suitable for measuring carbon dioxide concentration in clinical fields where dissolved oxygen is unavoidable. can be done accurately.

〔Example〕

Hereinafter, a more detailed explanation will be given with reference to Examples showing a method for synthesizing a complex used in the electrode for measuring carbon dioxide concentration of the present invention, a method for manufacturing the electrode, and results of measuring carbon dioxide concentration.

Complex synthesis example 1 2.2": 6", 2"-terpyridine (ter
py) and 75 mg of RuC13.3H20.
mg, 50 ml of ethanol with a water content of 20% by volume
was dissolved as a solvent. The solution was refluxed for 9 hours to form a complex. Next, acetone was gradually added to the obtained solution to precipitate the complex, and the yield of the complex was 40.
．． 6 mg, yield was 55%.

The obtained complex was identified as [Ru (terpy)2) C12 from the results of IR analysis, NMR analysis, and elemental analysis.

Subsequently, the obtained complex was dissolved in 2.5 ml of ethanol containing 20% by volume of water, and dissolved in 2.5 ml of saturated sodium perchlorate solution.
The perchlorate precipitated by adding ml of perchlorate was recrystallized with ethanol, and the yield of the obtained crystals was 73 mg, which was a yield of 55%.

The obtained crystals were analyzed using the same analytical method as above [Ru (
It was identified as (CI Oa) 2).

Electrode creation example 1 Cylindrical graphite electrode (EG-51 manufactured by Nippon Carbon Co., Ltd.) whose periphery is insulated and coated with epoxy resin and polyvinyl chloride resin (inner diameter 2 mm, area 3.14 x
A 5% solution of Nafion (registered trademark of DuPont), a perfluorocarbon-based cation exchange membrane (solution of 27474 Nafion 117 manufactured by Aldrich), was applied to the end face (the face opposite to the connection surface of the lead wire) of 10-2 cm”). 2
A 0 μm drop was applied and dried to coat a Nafion film with a thickness of about 50 μm.

The resulting Nafion-coated electrode was coated with [Ru
(terpy)2] (C1○4)2 8 in aqueous solution
[Ru (terpy)2
]”The complex was incorporated.

Electrode Creation Example 2 2 g/l of [Ru(terpy)2] (C104
) 5% Nafion solution containing 2 (Aldrich! 2
747-4) was dropped to a thickness of 20 μm and dried,
An electrode coated with a Nafion membrane containing a (terpy)2]" complex was obtained.

Example 1 Electrode Creation A three-electrode system using an electrode coated with an ion exchange membrane containing the complex prepared in Example 1 as a working electrode, a saturated sodium chloride calomel electrode (SSCE) as a reference electrode, and a platinum wire as a counter electrode. 50mM using a measurement cell! Add (1) carbon dioxide to the J acid buffer solution (pH = 7.38),
(2) Oxygen and (3) Nitrogen each at 15
Aerate for 1 minute to saturate with each gas, and after the liquid has settled,
FIG. 4 shows the results of cyclic portamentry measurement at 25° C. at a sweep rate of 50 mV per second.

Oxygen reduction current peak is -0.6V (vs. S S CE)
Nearby, the reduction current peak of carbon dioxide is -1.4V
(vs. 5SCE). From this result, the peak value of the reduction current between oxygen and carbon dioxide in an aqueous solution is approximately 800.
mV away.

Therefore, it can be seen that carbon dioxide concentration can be measured in the presence of oxygen.

Example 2 [Ru(terpy)2
]'' Using an electrode having a membrane incorporating the complex as a working electrode, the concentration of carbon dioxide in the presence of oxygen was measured as follows.

The applied voltage was set to -1.4 V (vs. 5SCE), which is not affected by the current due to the reduction of hydrogen ions, in a measurement cell with a three-electrode system consisting of a working electrode, a reference electrode made of a saturated sodium chloride calomel electrode, and a counter electrode made of platinum. The partial pressure of carbon dioxide in a gas mixture of carbon dioxide and nitrogen and the partial pressure of oxygen in a gas mixture of oxygen and nitrogen in a 50 mM phosphate buffer solution (pH = 7.38) were adjusted from OmmHg to 670 mmHg, respectively.
The reduction current was measured by cyclic portammetry by changing up to m m HE.

The results are shown in FIG. 5, and it is possible to measure the carbon dioxide concentration almost unaffected by dissolved oxygen.

Based on the change in partial pressure of carbon dioxide when the film thickness of the working electrode is 80 μm, the residual current value is 3 x 10-6 A, and the sensitivity is 3125 x 10
-"A/mmHg, while when the partial pressure of oxygen changes, the residual current value is 2.375xlO-'A, sensitivity 0.938xl
O-"A/mmHg. From this, the residual current values for both PCO2 and PO2 show current values on the order of 1O-6A.

However, the sensitivity is one order of magnitude higher for carbon dioxide.
Therefore, [Ru] supported on the Nafion membrane of the present invention
(tr repy) 2) By using a membrane, it becomes possible to measure the concentration of dissolved carbon dioxide in the presence of oxygen.

Example 3 Electrode Creation In the membrane electrode created by the method of Example 2, the thickness of the film formed on graphite was 10 pm and 40 pm.
Table 1 shows the residual current value (expressed in μA) when changing the thickness to μm, 80 μm, and 100 μm, and shows the sensitivity to carbon dioxide and oxygen (change in current value with respect to change in gas partial pressure) and film thickness. The relationship is shown in Figure 6 (A),
Figure (B) shows the selectivity ratio for carbon dioxide and oxygen with respect to film thickness.

Table M1 From these results, the sensitivity and selectivity to carbon dioxide concentration are selected to be large in the film thickness range of 60 μm to 100 μm. In particular, when the film thickness is 80 μm, carbon dioxide concentration is about 4.13 times more sensitive than oxygen concentration. It was confirmed that the carbon dioxide separation capacity was the highest in the presence of oxygen.

Example 4 As is clear from Example 3, [Ru (
ter py) 2] ”complexes respectively 0 and 2
5 mmol/c m3.0, 5 mmol/c
m3.1.0mmol/c m3.2.0mmol
Complex concentration and sensitivity (A
/ mm Hg) was investigated. The results are shown in FIG.

As is clear from this result, the supported amount of the complex is 0, 1
A range of 0.5 mmol/cm" or more is particularly preferable.

Example 5 Using the electrode of Electrode Creation Example 2, the pH was changed from 5.2 to 7.0 in a solution under a nitrogen atmosphere in the same manner as in Example 1, and the change in current value was 8th. It was as shown in the figure.

It is hardly observed at pH 6.0 to 7.0, and within this pH range, changes in hydrogen ion concentration due to the electrochemical reduction reaction of carbon dioxide have almost no effect. But p
When the temperature drops below H6.0, a change in current is observed.Although it was confirmed that acidic by-products were produced due to the electrochemical reduction reaction of carbon dioxide, it can be used without any problem for measuring carbon dioxide concentration in blood. Is possible.

〔Effect of the invention〕

The electrode for measuring carbon dioxide concentration of the present invention is a solid membrane electrode in which a cation exchange membrane containing a terpyridine-based ligand-transition metal complex with a specific membrane thickness is directly coated on a conductive substrate. When measuring the reduction current value of carbon using the amperometric method, the effect of dissolved oxygen along with carbon dioxide is small, so when measuring the carbon dioxide concentration in blood, it is It is an electrode that exhibits excellent characteristics for various measurements including clinical purposes, as it allows accurate measurements even if there is a problem.

[Brief explanation of drawings]

Figure 1 shows a cross-sectional view of the electrode of the present invention, Figure 2 shows the action of a cation exchange membrane containing a terpyridine-based ligand-transition metal complex, and Figure 3 shows the concentration of carbon dioxide. The measurement method is shown. Figure 4 shows the results of cyclic portamentry measurement at 25°C at a sweep rate of 50 mV per second. Figure 5 shows the sensitivity to changes in partial pressure. Figure 6 (A) shows the relationship between the sensitivity to carbon dioxide and oxygen (change in current value with respect to change in gas partial pressure) and film thickness, and Figure 6 (B) shows the relationship between film thickness and selectivity for carbon dioxide and oxygen. Figure 7 shows the relationship between complex concentration and sensitivity, and Figure 8 shows the relationship between complex concentration and sensitivity.
It shows the change in current value when H is changed from 52 to 7.0. Cylindrical arc 3... Conductive base 4... Cation exchange membrane containing terpyridine-based ligand-transition metal complex 5.
...Lead wire, 6...Counter electrode, 7...Reference electrode,
8... Three-electrode cell, 9... Constant potential electrolysis device

Claims (3)

[Claims] (1) In a solid membrane electrode for measuring carbon dioxide concentration in a liquid, in which a cation exchange membrane containing a terpyridine-based ligand-transition metal complex is coated on a conductive substrate, the membrane of the cation exchange membrane An electrode for measuring carbon dioxide concentration, having a thickness of 60 μm to 100 μm. (2) The metal complex coordination compound in the cation exchange membrane has a transition metal complex concentration of 1.0 mmol/cm^3 or less, preferably 0.0 mmol/cm^3 or less, per 1 meq/g of cation exchange capacity (dry membrane).
The electrode for measuring carbon dioxide according to claim 1, characterized in that the carbon dioxide concentration is 1 mmol/cm^3 or more and 0.5 mmol/cm^3 or less. (3) The ion exchange capacity of the cation exchange membrane is 0.1 to 10
Claim 1 characterized in that it is meq/g (dry film).
The electrode for measuring carbon dioxide concentration according to any one of items 1 to 2.