JPH032260B2 - - 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 they have a very short lifespan of 6 to 10 months. Another problem is that it cannot be used in a detection gas containing a relatively high concentration of carbon dioxide, or that 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, a negative electrode reaction product
is dissolved in the electrolyte, and the surface of the lead electrode is constantly renewed. However, when the electrolyte is saturated with PbO,
The surface of the negative electrode becomes passivated and the overvoltage of the negative electrode increases, which changes the current flowing between the positive and negative electrodes, disrupts the constant relationship between oxygen concentration and current, and ends the life of the oximeter.

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, instead of PbO being produced at the negative electrode as in equation (2) above, insoluble lead carbonate (PbCO ₃ ) or basic lead carbonate is generated. _{( 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 uses an acidic electrolyte mainly containing butyric acid to provide a galvanic cell-type oxygen concentration meter that has a long life and is not affected by carbon dioxide gas. This is what I am trying to do.

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 electrolyte that satisfies these conditions, the applicants of the present application have discovered a mixed aqueous solution of n-butyric acid, an alkali metal salt or ammonium salt of n-butyric acid, and a lead compound.

The alkali metals mentioned here are potassium, sodium and lithium, and the lead compound is lead salt of n-butyric acid or lead oxide.

Moreover, it is not necessary to limit the organic acid salt to one type, and a plurality of organic acid salts may be used. Note that iso-butyric acid cannot be used because of its low solubility in water.

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 n-butyric acid aqueous solution is
1.0 mol/, which is 10 times that for alkaline electrolyte. In other words, a concentration meter using n-butyric acid as an electrolyte has a lifespan ten times longer than that of a conventional one.

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 ...Equilibrium hydrogen generation potential at 25℃ P _H2 ...Partial pressure of hydrogen PH...PH of the electrolyte In other words, in equation (5), the smaller the PH, the more the equilibrium hydrogen generation potential of the positive electrode. becomes more noble, and the more hydrogen is generated from the positive electrode. In the case of an electrolytic solution with a low pH such as an aqueous n-butyric acid solution, 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, that is, an alkali metal or ammonium of the above-mentioned organic acid, is added to n-butyric acid (PH 2 to 3), the PH of the solution increases. If the PH of the solution becomes higher than 7 and shifts to the alkaline side, it will be affected by carbon dioxide gas, so the PH should be kept below 7, preferably in the range of 4 to 6.5. It is essential. 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 3 mol/
The pH of a mixed aqueous solution of n-butyric acid and 4 mol/potassium butyrate 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).

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 acetic acid, which has a high solubility of PbO, the life span 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, February issue, 63-71
(1972)], but in this case, since the mixing ratio of acetic acid and sodium acetate is 5.0M to 0.5M, the pH is 3 (page 65, right column), and hydrogen is also generated.

In order to improve this point, the inventors have already
We proposed an acetic acid mixed solution with a pH of 4 to 7.
No. 57-72131, Japanese Unexamined Patent Publication No. 187846/1983).

Compared to the n-butyric acid electrolyte according to the present invention, this acetic acid electrolyte has a higher solubility in the reaction products of the lead electrode, so the sensor life at room temperature is longer, but its viscosity is relatively low. , -5℃
It does not operate successfully at low temperatures below.

The n-butyric acid electrolyte is particularly suitable for operation at low temperatures of -5°C or lower.

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.

Figure 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. 3 mol/n-butyric acid and 4
A mixed aqueous solution of mol/potassium butyrate and 0.1 mol/lead oxide, 4 is a 20μ thick diaphragm made of tetrafluoroethylene-ethylene copolymer, and 5 is the diaphragm 4.
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 3 mol/n-butyric acid and 4 mol/potassium butyrate according to the present invention.
We manufactured two types of oxygen concentration meters, C and D, which use a mixed aqueous solution of 2 c.c. of 0.1 mol/lead oxide as the electrolyte.A and C are in air, B and D are in 21% oxygen, and 10
When a life test was conducted in a mixed gas of 69% 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 that uses an aqueous potassium hydroxide solution as the electrolyte has a lifespan A of only 6 months in air, and a lifespan B of just under 2 months when carbon dioxide gas is contained at 10%. In comparison, it can be seen that oxygen concentration meters C and D according to the present invention using a mixed aqueous solution as an electrolyte have a long life regardless of the presence or absence of carbon dioxide gas.

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. The oxygen concentration meter according to the present invention can also be applied to measuring 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 n-butyric acid, a lead compound, and an alkali metal salt or ammonium salt of n-butyric acid. A galvanic cell type oxygen concentration meter characterized in that the electrolyte is a mixed aqueous solution having a pH of 4 to 7.