JP2005127721A - Ionic conductance measuring instrument - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an ionic conductance measuring method and a measuring instrument for performing measurement even with restrictions on the type of an electrode. <P>SOLUTION: This ionic conductance measuring instrument is equipped with a double-pipe structure formed out of two straight pipes being outer and inner pipes, and comprises an anode chamber within the inner pipe and a cathode chamber formed between the outer and inner pipes. By fixing the pipe diameters of straight pipe parts in an overlapping portion, the cross section area of a liquid part can be fixed. Further, by changing the extent to which the inner pipe is inserted into the outer pipe, the length of the straight pipe parts in the overlapping portion is changed. In keeping with the change, a cell constant can be changed at a definite rate. A method is provided for using the ionic conductance measuring method. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a method and apparatus for measuring conductivity (especially specific ionic conductivity) in a liquid containing several ionic species.

  Of the ionic conduction by the entire ionic species contained in the device, the contribution of the target ion that represents the performance of the device (for example, lithium ion conductivity in a lithium ion battery, hydrogen ion conductivity in a fuel cell using an acid solution, etc. ) Is important for device development.

Conventional methods for measuring ionic conductivity include the 4-terminal method (4-pole cell) and the 2-terminal method. There is no difference in the cell shape in any of these methods, and the total ion conductivity or specific ion conductivity can be measured by changing the measurement method. Here, the conductivity of only the ions of interest is expressed as “specific ionic conductivity”, and the ionic conductivity in the commonly used meaning is “total ionic conductivity”.

As shown in FIG. 1, the most basic measurement method of the four-terminal method uses two electrodes for current application (anode and cathode) and two electrodes for potential measurement (reference electrodes), and the flowing ionic current, There is a method of calculating the ionic conductivity from the relationship with the potential difference between the reference electrodes (the cell constant is actually obtained from the measurement of a standard solution having a known conductivity, not from the dimensions). At this time, an electrode in which only a single ion species is involved in the electrode reaction is used (in the example of FIG. 1, hydrogen ions are generated by hydrogen gas oxidation at the anode and hydrogen gas is generated by hydrogen ion reduction at the cathode), and a direct current is The specific ion conductivity (hydrogen ion conductivity in the example of FIG. 1) is measured by measuring after a sufficient time has elapsed since the start of flow and the entire system is in a steady state. On the other hand, immediately after starting to pass a direct current or when an alternating current is passed, all ion species are involved in the ion current in the liquid of the measurement unit, and thus the total ion conductivity is measured.

The four-terminal method is strict, but it has many electrodes and complicated cells. In addition, since the distance between the anode and the cathode cannot be shortened, it is necessary to apply a high voltage when the conductivity of the liquid to be measured is low.

  It is very common to obtain the total ionic conductivity using the four-terminal method, and [Non-Patent Document 1] describes the measurement principle by DC polarization.

As an example of measuring the specific ion conductivity using the 4-terminal method, [Non-Patent Document 2]
The hydrogen ion conductivity in the ambient temperature molten salt is measured.

In the two-terminal method, the measurement is performed using two parallel plate electrodes as shown in FIG. At this time, as in the four-terminal method, in order to measure the conductivity of only the target ions, it is necessary to create a DC steady state in which only the target ions are moving.

  Also, unlike the 4-terminal method, the resistance of the electrochemical reaction at the interface (charge transfer resistance) is also included in the measured voltage in the 2-terminal method, so in order to exclude this, the AC component is superimposed on the DC current and A method of separating the charge transfer resistance (impedance method) or a method of taking out only the ion transfer resistance by instantaneously interrupting a direct current (current interrupt method) is used.

  On the other hand, when measurement is performed by applying an alternating current that does not include a direct current component, the total ion conductivity is measured.

  Although it is very common to obtain the total ionic conductivity using the two-terminal method, for example, [Non-Patent Document 3] introduces an analysis by an AC impedance method.

An example of obtaining specific ion conductivity using the two-terminal method is measurement of lithium ion conductivity by impedance measurement in a steady DC polarization state using a lithium electrode in [Non-Patent Document 4].
・ Method of creating a DC steady state in which only the target ions are moving In the case of measuring lithium ion conductivity in the electrolyte solution for lithium batteries shown in Fig. 2, the anode and cathode should be the flat electrodes actually used in the battery. However, in order to measure the hydrogen ion conductivity in the electrolyte for a fuel cell as shown in FIG. 1, the anode must be a gas electrode, and gas generation occurs at the cathode. It was practically impossible to use an electrode, and measurement by the two-terminal method was difficult.
・ Principle As shown in Fig. 3, in a bipolar cell using the current interrupt method (or impedance method), if the length of the part with a constant cross-sectional area can be changed, the shape of the electrode part is complicated and the entire cell Even if the cell constant is unknown, the change in the cell constant can be calculated accurately. Therefore, the conductivity can be calculated from the change in the measurement result (solution resistance).
However, it is difficult to produce a plurality of cells that are exactly the same except for the “specified part” as shown in FIG. Therefore, in order to realize this method, it is necessary to produce a cell having a mechanism capable of freely expanding and contracting the “specified portion”. This is technically difficult, and when the ionic conductive liquid to be measured is a molten salt or the like, the cell material is limited to glass or the like because the measurement is performed at a high temperature, which is further difficult.

Total ion conductivity has few restrictions on the shape of the electrode, and there is already a simple measuring device.On the other hand, in order to measure specific ion conductivity, the electrode type (electrode reaction to generate ionic current) The electrode shape associated therewith is limited, which makes measurement difficult.
Izutsu Park “Electrochemistry of non-aqueous solutions” (Baifukan, 1995) p.128 "9th Fuel Cell Symposium Proceedings" (Fuel Cell Development Information Center, 2002.5.15) A1-23P The Electrochemical Society "Electrochemical Measurement Manual: Practice" (Maruzen, 2002) p.15 "Summary of the 67th Annual Meeting of the Electrochemical Society of Japan" (2000.4) 2B17

  An object of this invention is to provide the measuring method and measuring apparatus of an ionic conductivity which can be measured even if there are restrictions regarding the mode of the electrode.

The ion conductivity measuring device of the present invention comprises a double pipe structure formed by two outer pipes and an inner pipe, as shown in FIG. 4, and an anode chamber and an outer pipe inside the inner pipe, A cathode chamber is formed between the inner tubes. Further, by making the pipe diameter of the “overlapping portion of the straight pipe portion” represented by (a) in FIG. 4 constant, the cross-sectional area of the liquid portion can be made constant. Here, the cross-sectional area of the liquid portion refers to a value obtained by subtracting the cross-sectional area of the inner tube (outer glue) from the cross-sectional area of the outer pipe (inner glue). Furthermore, the cell constant is changed at a constant rate in accordance with the change in the length of the “overlapping portion of the straight pipe portion” by changing the degree of inserting the inner pipe into the outer pipe.
That is, in the straight pipe section, if the outer diameter of the inner pipe is r ₁ , the inner diameter r _{2 of the} outer pipe, and the length of the overlapping portion is d, the change in cell constant accompanying the change of d is theoretically

It is.

  FIG. 5 shows details of the ion flow in the “overlapping portion of the straight pipe portion”.

The size of each diameter of the outer tube and the inner tube of the ion conductivity measuring device according to the present invention is not particularly limited, but the size of the gap between the outer tube and the inner tube is somewhat smaller than other parts through which ions flow. Is preferred. Specifically, the inner diameter r ₀ of the inner tube is about ₂ mm to 50 mm, preferably about ₂ mm to ₂₀ mm, and the difference r ₂ -r ₁ between the inner diameter of the outer tube and the outer diameter of the inner tube is 1 mm or more. , R ₀ or less is preferable. Here, as can be seen from FIGS. 4 and 5, the length of the portion (b) below the `` turning point '' of the ion flow also changes as the double tube is taken in and out. The characteristics of this cell are lost if the change in the length of the) part affects the cell constant. However, if the length of the (b) part is sufficiently secured, the ion flow flowing into the (b) part can be assumed to be substantially zero,
It is thought that the cell constant is not affected even if the length of the (b) part is changed. Accordingly, in order to minimize the influence of the change in the length of the portion (b), it is essential to secure a sufficient length even when the length of (b) is the smallest. The length of the portion (b) to be secured is preferably about one digit longer than the gap length of the tube, although it depends on the measurement accuracy. Preferably, it is 10 times or more of r ₂ -r ₁ and 100 times or less.

  The cell of the present invention is preferably composed of a material that does not react with the measurement solution and has no electrical conductivity, such as glass, resin, rubber, or ceramic.

The voltage application electrode of the present invention is selectively used depending on the type of ions to be measured. For example,
・ Hydrogen ions in acidic aqueous solution: platinum + hydrogen gas or iridium + hydrogen gas
-Lithium ions in non-aqueous solution: Lithium, lithium cobaltate or lithium graphite-Hydroxide ions in alkaline aqueous solution: Platinum + hydrogen gas, cadmium hydroxide or nickel hydroxide-Chloride in aqueous solution In the case of ions: platinum + chlorine gas or silver chloride may be mentioned, but not limited to the above.

  The thickness of the electrode used in the measuring apparatus according to the present invention is about 0.01 mm to 5.00 mm, preferably about 0.05 mm to 2.00 mm.

  The amount of current used in the measurement apparatus according to the present invention varies depending on the type of measurement solution. In general, the larger the current value, the smaller the measurement error may be. However, when the current is increased, the voltage also increases, resulting in problems such as decomposition of the solution to be measured. Therefore, as a preliminary experiment, a constant voltage is applied at “voltage that does not cause decomposition of the measurement solution” and the current value is measured after a sufficient amount of time has elapsed. A voltage of about 50 to 90%, preferably about 70 to 80% of the voltage at which the solution begins to decompose is applied. The “voltage at which the measurement solution does not decompose” can be set in advance by examining the potential range that the solution to be measured can withstand by another electrochemical method.

  Since the power source used in the measuring apparatus according to the present invention cuts off the current, a function generator may be used in combination with a constant current power source, or a constant current device with a current breaking function (current interrupter) may be used. preferable.

  Further, it is preferable to install the cathode at the lower end of the cathode chamber so that the change in shape of the cathode chamber accompanying the insertion and removal of the double tube does not affect the ion flow.

The measurement object of the present invention may be an ion conductive liquid, and examples thereof include an aqueous electrolyte solution, a non-aqueous solvent solution, and an ionic liquid. Examples of the aqueous electrolyte solution include aqueous solutions of HCl, KCl, HNO ₃ , H ₂ SO _{4 and the} like. Examples of the electrolyte salt of the non-aqueous solvent solution include alkali metal salts such as lithium salt, sodium salt and potassium salt, alkaline earth metal salts such as magnesium salt and calcium salt, or aluminum salt, which are used alone or in combination. can do. Examples of the lithium salt include LiN (C ₂ F ₅ SO ₂ ) ₂ , LiN (CF ₃ SO ₂ ) ₂ , LiClO ₄ , LiPF ₆ , LiBF _{4 and the} like. As the solvent, carbonates, lactones, ethers, ketones, nitriles, amides, sulfone compounds, esters, aromatic hydrocarbons and the like can be used alone or in combination of two or more, and ethylene carbonate. , Propylene carbonate, butylene carbonate, diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, γ-butyrolactone, γ-dimethoxyethane, tetrahydrofuran, anisole, 1,4-dioxane, 4-methyl-2-pentanone, cyclohexanone, acetonitrile, propio Nitrile, dimethylformamide, sulfolane, methyl formate, ethyl formate, methyl acetate, ethyl acetate, propyl acetate, ethyl propionate and the like can be exemplified. Examples of the ionic liquid include 1-ethyl-3-methylimidazolium ion, 1-butyl-3-methylimidazolium ion, n-butylpyridinium ion, trimethylhexylammonium ion, tributylmethylammonium ion, and the like as cationic species, tetrafluoro Examples include general molten salts containing borate ions, hexafluoroborate ions, bis (trifluoromethylsulfonyl) imide ions, trifluoroacetate ions and the like as anionic species.

  The ion conductivity measuring apparatus of the present invention can be applied to measurement of hydrogen ion conductivity in an acid solution, hydroxide ion conductivity in an alkaline aqueous solution, hydrogen ion conductivity in a room temperature molten salt, and the like.

  The measurement of ion conductivity according to this experiment is performed according to the following procedure.

(Preliminary experiment)
1. The cell is connected to a constant voltage device as shown in FIG. 6A, and a constant voltage is applied at a voltage that does not decompose the solution to be measured. For example, in the case of an aqueous sulfuric acid solution, the “voltage at which the measurement target solution does not decompose” refers to a voltage lower than that (0.9 V in the embodiment of the present invention) since water electrolysis starts at 1.23 V or higher.
2. Read the current value when sufficient time has passed and the current value no longer changes.

ΔＲは電気抵抗[Ω]の変化分を表す。
σは伝導度[S/cm]を表し、その逆数1/σは、比抵抗[Ω・cm]を表す。
Δｄ／Ｓは、セル定数[cm^-1]の変化分を表す。
７．５の作業を測定対象溶液を用いて行うことにより、６で算出したセル定数変化分を用いて伝導度を算出することができる。 (This experiment)
3. As shown in FIG. 6B, a constant current device having a function capable of instantaneously interrupting the current is connected to the cell, and the constant current is supplied with a current value smaller than the current value obtained in the preliminary experiment, so that the voltage becomes constant. Wait until.
4). An oscilloscope is connected to read the instantaneous change in cell voltage, and the current is cut off instantaneously. The instantaneous voltage change at this time is read (for example, the current value at 1 μs after the cutoff is read).
5). The above 3 and 4 are repeated while changing the length of the “directly overlapping portion”.
By performing the operation of 6.5 using a standard solution with known conductivity (such as a KCl solution whose concentration is accurately determined), the change in cell constant can be calculated from the following formula (formula II).

ΔR represents a change in electrical resistance [Ω].
σ represents conductivity [S / cm], and its reciprocal 1 / σ represents specific resistance [Ω · cm].
Δd / S represents a change in the cell constant [cm ⁻¹ ].
By performing the operation of 7.5 using the measurement target solution, the conductivity can be calculated using the cell constant change calculated in 6.

  With the measuring device of the present invention, it is possible to measure the specific ion conductivity when the electrode type is limited.

In addition, in the measurement of total ion conductivity, when there is a restriction on electrode reaction and a simple electrode cannot be used (for example, when gas generation on the electrode surface is unavoidable when current is passed), The problem can be solved by using the present invention.

  Hereinafter, an example is given and demonstrated.

-Manufacture of cell As a materialization of the principle shown in Fig. 4, a glass cell for measuring hydrogen ion conductivity in an acid aqueous solution was manufactured as shown in Fig. 7. (a) is an outer tube, which consists of a “straight tube portion” having an inner diameter of 8 mm and a length of 40 mm, and a cathode chamber provided with a platinum electrode (cathode) at the upper portion thereof. (b) is an inner tube, which is a `` straight tube '' with an outer diameter of 6 mm, an inner diameter of 4 mm, and a length of 30 mm;
On top of that, it consists of an anode chamber with a platinum mesh electrode (anode) plated with platinum. The anode and cathode leads are platinum. Since the anode reaction is hydrogen gas oxidation, the anode chamber has a thin tube for bubbling hydrogen gas, and is arranged so that bubbles pass through the platinum mesh electrode. Further, a hydrogen gas supply port and an exhaust port are provided for gas bubbling. An acid aqueous solution as a solution to be measured was put in the outer tube, the inner tube was inserted, and the solution in the anode chamber was sufficiently dissolved through hydrogen gas for 30 minutes to obtain a hydrogen saturated state. The “straight tube portion overlapping portion” can be changed by moving the inner tube in and out as shown in (c) and (d).

・ Preparation of solution to be measured The concentration of hydrogen ions to be measured is the same, and the concentration of other ions is different.
(A) 0.05 MH ₂ SO ₄
(H ⁺ : 0.1 M, SO ₄ ^2- : 0.05 M)
(B) 0.05 MH ₂ SO ₄ + 0.05 MK ₂ SO ₄
(H ⁺ : 0.1 M, K ⁺ : 0.1 M, SO ₄ ^2- : 0.1 M)
(C) 0.05 MH ₂ SO ₄ + 0.1 MK ₂ SO ₄
(H ⁺ : 0.1 M, K ⁺ : 0.2 M, SO ₄ ^2- : 0.15 M)
Three types of solutions were prepared. Of these, the solution (A) is an aqueous solution containing only sulfuric acid, but since the hydrogen ion conductivity is known from the literature, the cell constant is determined from the measurement results of the solution (A), and the solutions (B) and (C) are measured. The validity of this measurement was examined using the results.
-Determination of cell constant The measurement results using the solution (A) are shown in FIG. 8 (a). Measurement uses the current interrupt method,
The current value was 0.4 mA. FIG. 8 is a plot of the voltage drop due to the length of the “straight pipe portion overlapping portion” and the solution resistance. From this result, it was found that a linear relationship with a slope of 61.0 mV / cm was established between the two quantities of “length” and “voltage drop”.

From the literature, the conductivity of 0.1MH ₂ SO ₄ is 0.0251 S / cm, and the H ⁺ transport number expected from the ultimate ionic molar conductivity is about 0.81, so the H ⁺ conductivity in 0.1 MH ₂ SO _{4 is} Is estimated to be 0.0204 S / cm (The Electrochemical Society "Electrochemical Handbook 4th Edition" (Maruzen, 1985) p. 81, 82). Therefore, the effective cross-sectional area of the double pipe part is
0.4 mA ÷ 61.0 mV / cm ÷ 0.0204 S / cm = 0.321 cm ²
Is required.

・ Measurement of mixed solution
The solutions (B) and (C) were measured in the same manner as the (A) solution. The results are shown in FIGS. 8 (b) and (c). For solution (B), the current value was 0.3 mA, and measurement was performed by the current interrupt method. As a result, a slope of 46.6 mV / cm was obtained. In solution (C), a slope of 43.9 mV / cm was obtained at 0.3 mA. Using the above results and the cell constants obtained in the previous section, the hydrogen ion conductivity of the (B) liquid and (C) liquid is determined.
(B) Solution: 0.3 mA ÷ 46.6 mV / cm ÷ 0.321 cm ² = 0.0201 S / cm
(C) Solution: 0.3 mA ÷ 43.9 mV / cm ÷ 0.321 cm ² = 0.0213 S / cm
It becomes.

FIG. 9 is a graph showing the above results. For comparison, the total ion conductivity measured using a normal conductivity meter (Toa Radio CV-40M) is also shown. Since the total ion content increases in the order of (A) → (B) → (C), the total ion conductivity increases, but the H ⁺ concentration is constant, so the measurement result of H ⁺ conductivity is almost constant. It can be seen that From the above, it was shown that hydrogen ion conductivity was measured in the cell of the present invention, and that this was possible even though the electrode shape was not simple. The simple electrode shape means that the surface is smooth, such as a metal flat plate, a plane perpendicular to the direction in which ions want to flow can be made, and the cross-sectional area S for flowing ions can be made the same area. An electrode that satisfies the condition that bubbles do not enter. On the other hand, when the electrode is made of powder, the surface cannot be made smooth. When gas supply is required, the electrode becomes a metal mesh or sponge-like electrode, and the above conditions are not satisfied.

FIG. 1 shows a cross-sectional view of a four-terminal method (four-pole cell) measuring apparatus. FIG. 2 is a cross-sectional view of a two-terminal method (bipolar cell) measuring apparatus and a graph showing the relationship between current and time or voltage and time in the current interrupt method. FIG. 3 shows a two-terminal method in a cell (virtual) having a variable-length connection part. FIG. 4 is a sectional view of a cell constant variable mechanism using a double pipe according to the present invention. FIG. 5 shows the flow of ions in the straight tube portion of the double tube according to the present invention. In the figure, r ₀ represents the inner diameter of the inner tube, r ₁ represents the outer diameter of the inner tube, and r ₂ represents the inner diameter of the outer tube. FIG. 6A represents an electric circuit used in a preliminary experiment in ion conductivity measurement. FIG. 6B shows the electrical circuit used in this experiment for measuring ionic conductivity. (A) Outer tube, (b) Inner tube, (c), (d) The cell which concerns on this invention, put the non-measurement liquid in them, and assembled. (D) shows the case where the straight pipe part overlap part is reduced by pulling up the inner pipe as compared with (c). It is a graph showing the relationship between the "straight-pipe part overlap length" and the voltage drop part by a current interrupt. (A): Liquid (A) was used, and the current value was 0.4 mA. (B): The liquid (B) was used, and the current value was 0.3 mA. (C): The liquid (C) was used, and the current was 0.3 mA. It is a graph showing the H ^<+> conductivity measurement result using the cell which concerns on this invention, and the conductivity measurement result by all the ion species using a normal conductivity meter.

Claims (2)

A double tube structure comprising an inner tube and an outer tube, comprising an anode chamber inside the inner tube and a cathode chamber formed between the outer tube and the inner tube, and the length of the overlapping portion of the two straight tubes An ion conductivity measuring device having a mechanism for changing the thickness. By making the diameter of the overlapping part of the two straight pipe parts of the ion conductivity measuring apparatus according to claim 1 constant, the cross-sectional area of the liquid part is made constant, and the length of the overlapping part of the straight pipe is changed. Thus, a method for measuring the ionic conductivity of a liquid contained in the liquid part from a change in cell constant that changes at a constant rate obtained from the following formula:

Here, r ₁ indicates the outer diameter of the inner pipe in the straight pipe portion, r ₂ indicates the inner diameter of the outer pipe, and d indicates the length of the overlapping portion.

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