JPH032259B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an improvement of a galvanic cell type oxygen concentration meter, and its purpose is to provide an oxygen concentration meter that is not affected by carbon dioxide contained in the detected gas and has a long life. It is not intended to be provided.

Galvanic cell oxygen concentration meters are generally easy to use, inexpensive, and operate at room temperature, so they are used in a wide range of fields.

The principle is that the positive electrode of an oxygen-lead battery consists of a positive electrode made of a metal that is effective for electrochemical reduction of oxygen, a negative electrode made of lead, an electrolyte, and a diaphragm to restrict the permeation of oxygen to the positive electrode. This takes advantage of the fact that when a certain resistance is connected between the electrode and the negative electrode, there is linearity between the current flowing there and the oxygen concentration.

Conventional galvanic cell oximeters have two drawbacks. One is that it has a very short lifespan of 6 to 10 months, and the other is that it cannot be used in detection gases that contain relatively high concentrations of carbon dioxide, or its lifespan is extremely short. .

This drawback stems from the fact that conventional oximeters use alkaline electrolytes such as potassium hydroxide or sodium hydroxide.

This point will be explained below.

When an alkaline electrolyte is used in a galvanic cell oxygen concentration meter, at the positive electrode O ₂ +2H ₂ O+4e ^- →4OH ^- ...(1) At the negative electrode 2Pb+4OH ^- →2PbO+2H ₂ O+4e ^-
...(2) A reaction occurs. PbO, which is a negative electrode reaction product, is
The surface of the lead electrode is constantly renewed by dissolving it in the electrolyte.

However, when the electrolyte becomes saturated with PbO, the surface of the negative electrode becomes passivated and the overvoltage of the negative electrode increases, causing a change in the current flowing between the positive and negative electrodes.
The fixed relationship between oxygen concentration and current breaks down, and the life of the oxygen concentration meter comes to an end.

The short lifespan of conventional oxygen concentration meters using alkaline electrolytes is precisely because the solubility of PbO, the negative electrode product, in alkaline aqueous solutions is as low as 0.1 mol/at most.

On the other hand, when the detected gas contains a relatively large amount of carbon dioxide gas, at the negative electrode, instead of producing PbO as shown in equation (2), insoluble lead carbonate (PbCO ₃ ) or basic lead carbonate ( _{Pb2CO3 _}
(OH) ₂ ) is generated and the overvoltage at the negative electrode increases significantly, making it impossible to measure the oxygen concentration.

In contrast to conventional concentration meters that use such alkaline electrolytes, the present invention aims to provide a galvanic cell type oxygen concentration meter that uses an acidic electrolyte and has a long life and is not affected by carbon dioxide gas. .

The conditions required for the electrolyte in such a concentration meter are as follows:
The solubility of lead oxide, a reaction product, is high;
It is acidic, and there is no hydrogen generation from the positive electrode.

As an electrolytic solution that satisfies these conditions, the applicants of this application have developed a mixed aqueous solution of a mixture of two or more selected from the group of propionic acid and n-butyric acid, a lead compound, and an alkali metal salt or ammonium salt of an organic acid. discovered.

The alkali metals mentioned here are potassium, sodium, and lithium, and the organic acids include formic acid,
Acetic acid, such as acetic acid, propionic acid, butyric acid, maleic acid, and glutamic acid, whose alkali metal or ammonium salts are soluble in water, and the lead compound is a lead salt of the above-mentioned organic acid or lead oxide.

Furthermore, it is not necessary to limit the organic acid salt to one type;
May be used in combination. The mixture of acids is acetic acid-
Propionic acid, acetic acid-n-butyric acid, propionic acid-
Any combination of n-butyric acid and acetic acid-propionic acid-N-butyric acid may be used, and the mixing ratio is arbitrary.

This mixed aqueous solution will be explained below.

When an acidic electrolyte is used in a galvanic cell oxygen concentration meter, at the positive electrode O ₂ +4H ⁺ +4e ^- →2H ₂ O ......(3) At the negative electrode 2Pb + 2H ₂ O → 2PbO + 4H ⁺ +4e ^-
...(4) The following reaction occurs, and lead oxide (PbO) is produced at the negative electrode, just as in the case of alkaline electrolyte.

The solubility of lead oxide in the above organic acids varies depending on the type, combination and mixing ratio, but for example, 2.1 mol/in the case of acetic acid, 1.5 mol/in the case of propionic acid, and 1.5 mol/in the case of n-butyric acid. In the case of , it is 1.0 mol/, which is 10 to 21 times that for alkaline electrolyte. In other words, the oxygen concentration meter using the organic acid-based aqueous solution as the electrolyte has a lifespan 10 to 20 times that of conventional oxygen concentration meters. in this way,
The inventors of the present application have discovered that the life of an oxygen concentration meter is determined by the solubility of lead oxide in the electrolytic solution.

From the viewpoint of viscosity, the order of viscosity is n-butyric acid>propionic acid>acetic acid, and the higher the viscosity, the better the properties as an oxygen concentration meter at low temperatures. Therefore, it is advisable to appropriately select a mixture of these organic acids in relation to the desired life span and operating temperature.

Next, considering hydrogen generation from the positive electrode, the hydrogen generation equilibrium potential of the positive electrode is given by the following equation (5).

Here, E _H ...Hydrogen generation equilibrium potential at 25℃ P _H2 ...Hydrogen partial pressure PH...PH of the electrolyte In other words, in equation (5), the smaller the PH, the hydrogen generation equilibrium potential of the positive electrode The higher the concentration of hydrogen, the more likely it is that hydrogen will be generated from the positive electrode. In the case of an electrolytic solution with a low pH such as the aqueous solution of the acid mixture in the above combination, hydrogen is likely to be generated because the potential of the positive electrode becomes quite base, especially at low oxygen concentrations.

Conversely, as the pH increases, the hydrogen generation equilibrium potential of the positive electrode becomes less noble, making it difficult to generate hydrogen.

Therefore, when a salt consisting of a weak acid and a strong base, ie, an alkali metal or ammonium salt of the organic acid, is added to the acid (PH 2 to 3) consisting of the above combination, the pH of the solution increases.

If the pH of the solution becomes greater than 7 and shifts to the alkaline side, it will be affected by carbon dioxide gas, so the pH should be 7 or less, preferably 4 to 4.
It is important to keep it within the range of 6.5. However, even if the pH of the solution is kept within the above range, there is still a risk of hydrogen generation.

On the other hand, the equilibrium potential of lead is expressed as follows: E ^Pb /Pb = -0.367 + 0.0296log [Pb]
(VvsSCE) ...(6) where E ^Pb /Pb ... Equilibrium potential of lead at 25℃ [Pb] ... Activity of lead ions in the electrolyte The larger the amount of lead ions added, the more the lead electrode In other words, it can be seen that the potential of the positive electrode becomes more noble.

That is, if lead ions are added to the above-mentioned mixed solution until the equilibrium potential of lead becomes nobler than the equilibrium potential for hydrogen generation, hydrogen will no longer be generated. Lead ions can be added in the form of lead oxide or organic acid salts, but
The amount added should be the minimum amount that can avoid hydrogen generation. If the amount is too high, the solubility of lead oxide, which is a reaction product, will decrease, resulting in a shortened lifespan.

The mixed electrolyte thus obtained, for example 1.5 mol/1 mol of acetic acid/2 mol of propionic acid/
The pH of a mixed aqueous solution of potassium acetate - 2 mol/potassium propionate and 0.1 mol/lead oxide is:
6.2, the hydrogen generation equilibrium potential is -0.61V (vsSCE), and the equilibrium potential of lead is -0.60V (vsSCE).

Also, 5 mol/acetic acid-4 mol/potassium acetate
In the case of a mixed aqueous solution of 0.1 mol/lead acetate, the PH is
The pH of a mixed aqueous solution of 6.2, 2 mol/propionic acid - 3.5 mol/potassium propionate - 0.1 mol/lead oxide is 6.25, and its hydrogen evolution equilibrium potential is -0.615V.
(vsSCE).

In this mixed solution, the equilibrium potential of lead is nobler than the hydrogen generation equilibrium potential, so hydrogen is not generated from the positive electrode. Furthermore, since the solution is acidic, it will not be affected by carbon dioxide gas, and since the solution uses an acid with a high solubility of PbO, the life will be extended.

An example of using a mixed aqueous solution of acetic acid, sodium acetate, and lead acetate as the electrolyte [R, Ellsworth, "The Chemical Engineer"]
The Chemical Engineer) (258), pp. 63-71 (1972)], but in this case, the mixing ratio of acetic acid and sodium acetate is 5.0M to 0.5M, so the PH
is 3 (page 65, right column), and hydrogen is also generated.

In the example above, the sensor is not sealed,
It is clearly stated that it has an open structure (No.
Page 65 left column, Figure 1 and the bottom paragraph of Figure 1). Therefore, even if hydrogen is generated, it is presumed that it is not recognized and does not pose much of a problem. This is not unrelated to the fact that the oxygen sensor described in this reference is intended only for measuring dissolved oxygen. In any case, it is generally desirable to use a closed type oxygen sensor, including when measuring oxygen concentration in the gas phase, but in the case of a closed type oxygen sensor, hydrogen is generated at the pH of the electrolyte described above. And that becomes a decisive issue.

One of the major features of the present invention is that it solves these problems.

The electrolytic solution of the galvanic cell type oxygen concentration meter according to the present invention has been described above, and in order to further explain the present invention, one embodiment will be described in detail below with reference to the drawings.

FIG. 1 shows a schematic diagram of the cross-sectional structure of a galvanic cell type oxygen concentration meter according to an embodiment of the present invention. 1.5 mol/acetic acid as electrolyte
A mixed aqueous solution of 1 mol/propionic acid and 2 mol/potassium acetate-2 mol/potassium propionate and 0.1 mol/lead oxide, 4 is a 20μ thick diaphragm made of tetrafluoroethylene-ethylene copolymer, 5 is the above diaphragm 4 is an O-ring for fixing it to a holder 6 made of polyvinyl chloride resin, and 7 is a resistor interposed between the positive electrode 1 and the negative electrode 2.

Oxygen in the detection gas permeates through the diaphragm 4 and reaches the positive electrode 1.
When the oxygen reaches the surface of the positive electrode, a reaction according to the above-mentioned equation (3) occurs, and a current corresponding to the amount of oxygen that has passed through flows from the positive electrode 1 to the negative electrode 2.

Therefore, by measuring the voltage across the resistor 7, the amount of oxygen permeation, in other words, the oxygen concentration can be determined.

Next, in order to confirm the effect of the mixed electrolyte according to the present invention, four oxygen concentration meters of the same type as those described above were prepared, and one using the conventional 4 mol/potassium hydroxide aqueous solution 2 c.c. as the electrolyte A. , B and 2 c.c. of a mixed aqueous solution of 1.5 mol/acetic acid - 1 mol/propionic acid and 2 mol/potassium acetate - 2 mol/potassium propionate and 0.1 mol/lead oxide according to the present invention is used as an electrolyte. Two types of oxygen concentration meters, C and D, were manufactured; A and C were in air, and B and D were in air.
When a life test was conducted in a mixed gas of 21% oxygen, 10% carbon dioxide, and 69% nitrogen, the results shown in Figure 2 were obtained. From Figure 2, it can be seen that the conventional oxygen concentration meter using potassium hydroxide aqueous solution as the electrolyte can be used even in air.
If the lifespan A is only a few months and contains 10% carbon dioxide gas, then the oxygen concentration meters C and D using the mixed aqueous solution as the electrolyte according to the present invention have a lifespan B of just under two months. It can be seen that it has a long lifespan regardless of the presence or absence of.

As detailed above, the present invention provides a galvanic cell type oxygen concentration meter that has a long life and is not affected by carbon dioxide gas, and has extremely high industrial value.

Note that the electrolytic solution may be used after being gelled. Further, the oxygen concentration meter of the present invention cannot be applied to measurement of dissolved oxygen concentration.

[Brief explanation of the drawing]

FIG. 1 shows a schematic cross-sectional structure of a galvanic cell type oxygen concentration meter according to an embodiment of the present invention, and FIG.
A comparison of life test results between a conventional product and a product of the present invention is shown. 1... Positive electrode, 2... Negative electrode, 3... Electrolyte, 4
...Diaphragm, 5...O-ring, 6...Holder,
7...Resistance, A, B...Conventional products, C, D...Products of the present invention.

Claims (1)

[Claims] 1. A positive electrode made of a metal or metal oxide with high activity in reducing oxygen, and a negative electrode made of lead,
In a galvanic cell oxygen concentration meter mainly composed of an electrolytic solution and an oxygen permeable diaphragm, the electrolytic solution is a mixture of two or more types selected from the group of acetic acid, propionic acid, and n-butyric acid; Using a mixed aqueous solution of a lead compound and an alkali metal salt or ammonium salt of an organic acid selected from formic acid, acetic acid, propionic acid, butyric acid, maleic acid, and glutamic acid, and whose pH is 4 to 7. A galvanic cell type oxygen concentration meter that is characterized by