JP3307827B2 - Potentiometric electrolytic ammonia gas detector - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an ammonia gas detector using a potentiostatic electrolysis method.

[0002]

2. Description of the Related Art For the detection of ammonia gas, a Severinghaus-type gas sensor in which a glass electrode is sealed in an electrolytic solution that takes in a gas to be detected through a diaphragm is used. Because it is proportional to the logarithm, not only does the measurement accuracy on the high concentration side decrease, but the internal impedance is several MΩ due to the use of glass electrodes.
, The noise is easy to pick up and handling is troublesome.

On the other hand, a working electrode having a platinum black film formed on a diaphragm and a counter electrode are brought into contact with an electrolytic solution, a constant potential is applied between these electrodes, and a gas dissolved in the electrolytic solution from the diaphragm acts on the working electrode. A potentiostatic gas detector that causes oxidation and reduction reactions at the poles and uses the electrolysis current at this time as a detection output is sensitive to most gases, and the detection output for gas concentration is a linear function. Therefore, it is characterized by high accuracy over a wide range of concentrations, low internal impedance, and high noise resistance, so that it is easy to handle.

However, when attempting to detect ammonia gas using a potentiostatic electrolytic gas detector having such excellent characteristics, it is difficult to use the strong oxidizing power of platinum black constituting the working electrode. However, in an environment where a large amount of hydrogen or an organic solvent for cleaning is used, such as in a semiconductor factory, an electrolytic current is generated by the hydrogen or the organic solvent in the environment, and it is impossible to detect ammonia gas. is there.

In order to suppress the sensitivity to hydrogen and organic solvents while making use of the noise resistance and the ease of maintenance of the potentiostatic gas detector, a neutral electrolyte and a working electrode made of a noble metal or a noble metal oxide are used. The ones used have also been proposed.

[0006]

According to the galvanostatic gas detector configured as described above, ammonia can be selectively detected even in an environment containing hydrogen or an organic solvent.
Since it shows sensitivity to carbon dioxide present in the atmosphere, there is a new problem that a measurement error occurs in the detection of ammonia gas. The present invention has been made in view of such a problem, and the object thereof is about several tens of ppm present in the environment regardless of the presence of hydrogen or an organic solvent and further regardless of the presence of carbon dioxide gas. To provide a novel potentiostatic ammonia gas detector capable of detecting ammonia gas with high accuracy.

[0007]

According to the present invention, there is provided a cell having a window sealed with a gas-permeable diaphragm having a working electrode formed on a liquid contact side. It contains a neutral electrolyte containing an alcohol having a vapor pressure lower than ethanol and a counter electrode, and the working electrode is 0 to 450 mV with respect to a silver / silver chloride electrode in a 0.1 mol potassium chloride electrolyte. Was maintained at the potential.

[0008]

The reaction in which the working electrode is oxidized by the hydroxyl ions generated by the dissociation reaction of ammonia proceeds simultaneously, and electrons are generated in proportion to the concentration of the permeated ammonia, thereby generating an electrolytic current serving as a detection output. On the other hand, hydrogen gas and organic solvents do not oxidize because the oxidizing action of the working electrode is weak, and the dissociation reaction of carbon dioxide gas and the reduction reaction at the working electrode are suppressed by ethylene glycol in the electrolytic solution. Therefore, the electrolysis current becomes extremely small.

[0009]

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram showing a first embodiment of the present invention. FIG. 1 shows an embodiment of the present invention, wherein reference numeral 1 denotes a cell container,
An electrolyte 2 to be described later is accommodated, and a through hole 3 is formed in one surface thereof.
Gas inlets 6a, 6a via packing 5 such as a ring
It is fixed in a liquid-tight manner by a holding lid 6 having a ‥‥.

The diaphragm 4 is made of a noble metal, such as Au (gold) or ruthenium (R), which is stable and has low oxidizing power on the electrolyte side.
u), a mixture of a fine powder of ruthenium oxide (RuO2) and a fluororesin powder is applied to a porous film 4a of a fluororesin or the like having air permeability and water repellency, sintered, reactive sputtering, etc. Thus, a working electrode 4b having a film formed is formed.

Further, references to the cell vessel 1 at a certain distance from the working electrode 4b, a counter electrode 7 made of silver wire, made of silver wire film salt silver is formed on the surface by potassium chloride solution A pole 8 is provided.

The working electrode 4b, the counter electrode 7, and the reference electrode 8
Is drawn out so as to maintain a liquid-tight state with the cell container 1 and connected to the measuring circuit 10. The working electrode 4b is located between the working electrode 4b and the reference electrode 8 in the 0.1 mol potassium chloride electrolyte. The potential is set to be 0 to 450 mV with respect to the silver / silver chloride electrode. Needless to say, if the components of the electrolyte solution of the reference electrode and the components of the electrode are specified, the potential of the working electrode is uniquely determined, so it is clear that the conversion can be made even when the components of the electrolyte solution and the components of the electrode are changed. is there. In addition,
In the case of a so-called bipolar type in which the counter electrode 7 also serves as a reference electrode, the counter electrode is maintained at a predetermined potential.

On the other hand, the electrolytic solution 2 is a neutral electrolytic solution, in this embodiment, about 0.1 mol of potassium chloride (KCl) or 0.1 mol of potassium hydrogen carbonate (KHCO 3).
It is prepared by adding a predetermined concentration of ethylene glycol to those containing 3). Reference numeral 11 denotes a through hole that also serves as an electrolyte injection port and an air communication port.

In this embodiment, when ammonia gas permeates through the diaphragm 4 and dissolves in the electrolytic solution 2,

Embedded image Reaction (formation of hydroxyl group) of ammonia in the electrolytic solution 2

Embedded image The oxidation reaction (consumption of hydroxyl groups) of ruthenium oxide forming the working electrode 4b proceeds to generate electrons in proportion to the concentration of permeated ammonia.

The electrons can be detected by a measuring device as an electrolytic current.

Investigation of the relationship between the concentration of ammonia gas and the electrolysis current revealed that the electrolysis current exhibited extremely high linearity with respect to the concentration of ammonia gas as shown in FIG.

Even if hydrogen or an organic solvent permeates through the diaphragm 4 and dissolves in the electrolyte solution 2, no electrolytic current is generated because the working electrode 4b has a weak oxidizing effect.

On the other hand, when the carbon dioxide gas permeates through the diaphragm 4 and dissolves in the electrolytic solution, generally,

Embedded image Dissociation reaction of carbon dioxide (production of hydrogen ions)

Embedded image A reduction reaction (consumption of hydrogen ions) of ruthenium oxide occurs.

However, the electrolyte 2 of the present invention
Contains ethylene glycol, so that these reactions are suppressed, and 2500 p
No electrolytic current is generated even for carbon dioxide gas of about pm.

That is, when a carbon dioxide gas having a concentration of 2500 ppm was measured without adding ethylene glycol to the electrolytic solution, a large electrolytic current flowed to the negative side as shown in FIG. 3A (region C in the figure).

On the other hand, ethylene glycol of 75 v
% By using an electrolyte solution containing
When the carbon dioxide gas at pm was measured, no change occurred in the electrolytic current as shown in FIG. 3B (region C in the figure).

After purging with high purity air, the concentration is 40 ppm.
When the ammonia gas was injected (region B in the figure), the electrolytic solution containing no ethylene glycol and the electrolytic solution containing ethylene glycol both produced an electrolytic current with respect to the ammonia gas. The contained detector had higher electrolysis current.

Subsequently, after purging with high-purity air, hydrogen gas having a concentration of 2% was injected (region H in the figure). As a result, in a detector using an electrolytic solution containing no ethylene glycol, an electrolytic current was generated. However, no generation of electrolytic current due to hydrogen gas was observed in the detector containing ethylene glycol.

[0024] On the other hand, the detector using an electrolytic solution containing 75v% ethylene glycol, in order to investigate the optimum setting the potential of the working electrode 4b, the potential of zero V to silver / salt and silver working electrode, and the negative Using a detector set at 200 mV, changes in the electrolytic current with respect to high-purity air and carbon dioxide gas were examined.
Although the baseline fluctuates slightly as seen in
No generation of electrolytic current correlated with the injection of carbon dioxide gas (region C in the figure) was observed.

On the other hand, in the detector set at minus 200 mV, as shown in FIG. 4B, not only the reduction current is generated in accordance with the injection of carbon dioxide gas (region C in the figure), but also It was found that a reduction current of oxygen was generated for nitrogen gas (region N in the figure).

The relationship between the potential and the electrolytic current was determined in order to determine the highest sensitivity for ammonia and the optimum potential of the working electrode which was insensitive to interference components such as carbon dioxide gas. However, ammonia gas (symbol ● in the figure), high-purity air (symbol □ in the figure), nitrogen (図 in the figure), carbon dioxide gas (symbol ○ in the figure), hydrogen gas (symbol ▲ in the figure)
Generated an electrolytic current as shown in FIG. 5 with respect to the potential of the working electrode 4b. From these facts, in order to minimize the influence of the coherent gas and detect the ammonia gas with high sensitivity, the potential of the working electrode 4b is set to about 100 mV to 350 mV with respect to the silver / silver chloride electrode. It is desirable to set.

In the above-described embodiment, about 75 v% of ethylene glycol is added as a carbon dioxide insensitive agent. However, it is sufficient to add 10 v% or more if only the electrolytic current due to the interference gas is suppressed. That is, FIG. 6 is a graph showing the relationship between ethylene glycol and the electrolytic current with respect to carbon dioxide gas having a concentration of 2500 ppm. As the content of ethylene glycol increases, the electrolytic current with respect to the coherent gas sharply decreases. At 10 v%, the electrolysis current without addition is almost 1/3.
To about 20 μA.

On the other hand, as is clear from FIG.
The electrolytic current for ammonia gas of about ppm is 20
Since it is about μA, if ethylene glycol is added in an amount of 10 v% or more, an extremely low concentration of ammonia gas present in the atmosphere can be detected.

By the way, since an alcohol-based substance having the hydrophilic property of ethylene glycol and having a lower vapor pressure than ethanol has a water absorption property, it is necessary to prevent an extreme increase in the electrolyte solution due to the water absorption property. 85v in temperate regions
% In an extremely dry environment.

Although ethylene glycol was used as the carbon dioxide insensitive agent, it was confirmed that alcohol-based substances having a lower vapor pressure than ethanol, such as propylene glycol and glycerin, also functioned as carbon dioxide insensitive agents. . For this reason, even when propylene glycol or glycerin is added at an optimum concentration in accordance with the water absorption of each and the humidity in the environment in which it is used, the same effect is exerted.

[0031]

As described above, according to the present invention, a cell having a window sealed with a gas-permeable diaphragm having a working electrode formed on the liquid contact side has a vapor pressure lower than that of ethanol. A neutral electrolyte containing alcohol and a counter electrode were accommodated, and the working electrode was maintained at a potential of 0 to 450 mV with respect to a silver / silver chloride electrode in a 0.1 mol potassium chloride electrolyte. Therefore, regardless of the presence of hydrogen, organic solvents, and carbon dioxide gas in the environment,
An extremely low concentration of ammonia gas of about ppm can be detected.

[Brief description of the drawings]

FIG. 1 is a sectional view of an apparatus showing an embodiment of the present invention.

FIG. 2 is a diagram showing the relationship between the concentration of ammonia gas and the electrolytic current in the above device.

FIGS. 3 (a) and 3 (b) are lines showing the relationship between a carbon dioxide gas, an ammonia gas, a hydrogen gas, and an electrolysis current in the conventional potentiostatic gas detector and the detector of the present invention, respectively. FIG.

FIG. 4 is a diagram showing a relationship between a potential set in the detector of the present invention, carbon dioxide gas and nitrogen gas, and an electrolytic current.

FIG. 5 is a diagram showing the relationship between the potential set at the working electrode of the detector of the present invention and the electrolytic current, taking the type of gas as a parameter.

FIG. 6 is a diagram showing a relationship between an ethylene glycol concentration and an electrolytic current with respect to carbon dioxide gas in the atmosphere.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Cell 2 Electrolyte 3 Window 4 Diaphragm 4a Porous membrane 4b Working electrode 7 Counter electrode 8 Reference electrode

──────────────────────────────────────────────────続 き Continuation of front page (58) Field surveyed (Int.Cl. ⁷ , DB name) G01N 27/416 JICST file (JOIS)

Claims (5)

(57) [Claims] 1. A cell having a window sealed with a gas permeable diaphragm having a working electrode formed on a liquid contact side, a neutral electrolyte containing an alcohol having a vapor pressure lower than ethanol, and a counter electrode. And the working electrode is maintained at a potential of 0 to 450 mV with respect to a silver / silver chloride electrode in a 0.1 mol potassium chloride electrolyte solution. 2. The alcohol whose vapor pressure is lower than ethanol is ethylene glycol, propylene glycol,
The potentiostatic ammonia gas detector according to claim 1, wherein the detector is a simple substance or a mixture of glycerin. 3. A controlled potential electrolysis type ammonia gas detector according to claim 1, wherein the working electrode is constituted by a noble metal or its oxides. 4. The potentiostatic ammonia gas detector according to claim 1, wherein the electrolytic solution contains 10 to 95% by volume of a simple substance or a mixture of ethylene glycol, propylene glycol, and glycerin. 5. The potentiostatic electrolytic ammonia according to claim 1, wherein the potential of the working electrode is maintained at 100 to 350 mV with respect to a silver / silver chloride electrode in a 0.1 mol potassium chloride electrolyte. Gas detector.