JPH09236568A - Apparatus for measuring concentration of very small amount of oxygen - Google Patents

Abstract

PROBLEM TO BE SOLVED: To safely use an apparatus and to simplify the handling thereof by using a neutral salt or a salt near to neutrality as an electrolyte for preparing standard gas. SOLUTION: Sample gas is introduced into a humidifying tank 5 from an introducing pipe 6 to be raised through an electrolyte aq. soln. as air bubbles to be allowed to flow into a measuring chamber 10 and oxygen in the sample gas is electrolytically reduced by the detection electrode 2 of a measuring electrode 1 and the concn. of oxygen in the sample gas is measured from the current value at this time. At a time of calibration, a current is allowed to flow to an electrolytic electrode 7 to electrolytically generate oxygen and zero gas from which oxygen is perfectly removed is introduced into the humidifying tank 5 from the introducing pipe 6 and oxygen generated by electrolysis is mixed with the zero gas to prepare standard gas with desired concn. to calibrate a measuring apparatus. As an electrolyte preparing the electrolyte aq. soln. necessary for electrolytically generating oxygen in preparing the standard gas, a neutral salt or a salt near to neutrality, that is, sodium sulfate or potassium nitrate is used.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a trace oxygen concentration measuring device, and more particularly to a trace oxygen concentration measuring device for measuring the concentration of a small amount of oxygen contained in industrial gases requiring high purity. Is.

[0002]

2. Description of the Related Art Conventionally, various industrial gases with high purity have been used in various industrial fields. With the advancement of technology and its sophistication in recent years, the industrial gas mainly used in the semiconductor industry has been used. The demand for high purification is increasing more and more. And, it is required to guarantee the impurity concentration in the order of ppb for what is called ultra-high purity gas, and it is an important guarantee that oxygen (O ₂ ) is not contained in the impurities. It is one of the items. Therefore, in order to control the oxygen concentration in such gas,
Various types of trace oxygen concentration measuring devices have been proposed which can measure the concentration of oxygen contained in a trace amount as impurities quickly and accurately.

An example of such a measuring device is shown in FIG.
The open cathode type galvanic cell type as shown in FIG. This is a method for electrolytically reducing oxygen contained in a sample gas to obtain an output current according to the oxygen concentration, and in such a measuring device, in the measuring electrode 1, detection of silver, carbon, platinum or the like is performed. pole 2 by the following equation (1) _{O 2 + 2H 2 O + 4e} - → 4OH - the reaction of (1) is lead, the following (2) the counter electrode 3 made of cadmium formula 2Pb + 4OH ^- → 2Pb ^{_{(OH) 2 + 4e - ···}} (2) reaction proceeds each, whereby the oxygen reduction current in proportion to the oxygen concentration flows between the detecting electrode 2 and the counter electrode 3, to detect the current The oxygen concentration can be measured by.

Further, in the open cathode type galvanic cell type measuring device illustrated above, in order to stably proceed the reactions of the above formulas (1) and (2) without impairing the electrolysis efficiency, the detection electrode 2 and the counter electrode 3 are used. It is necessary that the electrolytic solution portion 4 between and is always holding a strongly alkaline electrolytic solution,
In addition, it is necessary to keep moisture on the surface of the detection electrode 2 at all times. As shown in the figure, a commonly used measuring apparatus introduces a sample gas into a humidification tank 5 through an introduction pipe 6, and raises water in the measuring chamber 10 by using the sample gas as bubbles to raise the water vapor. The measuring electrode 1 is configured to be humidified. In the figure, 11 is a sample gas discharge port.

Here, in order to stably measure the concentration of oxygen contained in a small amount in the sample gas, it is necessary to precisely calibrate the measuring device. In calibrating the measuring device, if a standard gas with a desired concentration can be obtained, it may be supplied from the outside and calibrated with the standard gas, but it is generally difficult to obtain a standard gas with an extremely small concentration. is there. For this reason, normally, oxygen is generated in the apparatus by electrolysis of water to prepare a standard gas having a trace oxygen concentration. In order to facilitate this electrolysis, an alkaline aqueous solution having a high concentration is used as the electrolytic aqueous solution. Specifically, as shown in FIG. 1, the alkaline aqueous solution stored in the humidifying tank 5 is added to the electrolytic electrode 7. Is immersed, and an electric current is passed through this electrode to electrolyze oxygen, and a zero gas from which oxygen has been completely removed by a deoxidizer or the like is introduced into the humidification tank 5 through the sample gas introduction pipe 6, and electrolysis is generated in this zero gas. A standard gas having a desired concentration is prepared by mixing the oxygen thus prepared.

Namely, the electrolytic generation of oxygen in an alkaline aqueous solution, following the anode 9 (3) _{^{4OH - → 2H 2 O + O}} 2 + 4e - The reaction of (3) is below the cathode 8 ( 5) The reaction of the formula 4H ₃ O ⁺ + 4e ⁻ → 4H ₂ O + 2H ₂ (4) respectively proceeds, and the concentration of the zero gas introduced into the humidification tank 5 at this time is F (cc / M
in. ), The concentration of the standard gas to be prepared is X (pp
m), assuming that the electrolysis efficiency in the electrolysis of oxygen is 100%, the current value I (μA) to be passed through the electrolysis electrode 7 is represented by the following equation (5) I = 0.267FX (5) A standard gas of a desired concentration can be prepared by introducing a predetermined amount of current calculated by the equation (5) into the electrolytic electrode 7 to generate oxygen and introducing zero gas into the humidification tank 5 at a predetermined flow rate. The measuring device is calibrated with standard gas.

[0007]

By the way, the electrolytic solution stored in the humidifying tank 5 in the measuring device as described above can increase the electrolysis efficiency in the electrolysis electrode 7 and generate a reactive gas. For the reason that there is nothing, strong alkaline electrolytes such as sodium hydroxide and potassium hydroxide are generally used, and careful handling has been required in handling the measuring device. That is, if the strong alkaline aqueous solution is splashed or adhered to the operator's body during maintenance or repair of the measuring device, the skin or the like may be damaged. I needed to pay attention. Further, as materials for the humidifying tank 5, the electrolytic electrode 7, etc., materials having resistance to a strongly alkaline aqueous solution must be thoroughly studied and selected, and usable materials are limited.

[0008]

The present invention has been made in view of the above problems, and a trace oxygen concentration measuring device of the present invention is a standard gas having a predetermined concentration by oxygen generated by electrolyzing an aqueous electrolyte solution. In an open-cathode galvanic cell type trace oxygen concentration measuring device that calibrates the standard gas, a neutral or neutral electrolyte is prepared to prepare the aqueous electrolyte solution necessary to prepare the low-concentration oxygen standard gas. It is characterized by using a salt close to the sex.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION
The sample gas is introduced into the humidification tank 5 in which the electrolyte aqueous solution is stored from the introduction pipe 6, and the sample gas is caused to rise in the electrolyte aqueous solution as bubbles to flow into the measurement chamber 10. 1 is humidified, oxygen contained in the sample gas is electrolytically reduced by the detection electrode 2 of the measurement electrode 1, and the current flowing at this time is detected to measure the oxygen concentration in the sample gas. An electrolytic electrode 7 for generating oxygen by electrolysis is immersed in the aqueous electrolyte solution. A current is passed through this electrode to generate oxygen electrolytically, and the oxygen is completely removed by a deoxidizer or the like. It is possible to prepare a standard gas having a desired concentration by introducing the zero gas into the humidifying tank 5 through the sample gas introduction pipe 6, and mixing the zero gas with the electrolyzed oxygen to calibrate the measuring device. It is configured to be capable (see FIG. 1).

In the present invention, when preparing a standard gas having a low oxygen concentration, a neutral or near-neutral salt is used as an electrolyte for preparing an aqueous electrolyte solution necessary for electrolytically generating oxygen. The electrolyte used is
It is necessary that the following conditions are satisfied.

That is, the electrolyte used in the present invention is 1) its aqueous solution is neutral or close to neutral,
2) High electrolysis efficiency, capable of generating a considerable amount of oxygen with respect to the electric current applied to the electrolysis electrode 3) 3) Solubility in water is large and precipitation is difficult. ) It is stable over a long period of time, and by electrolysis hydrogen chloride (HCl), chlorine (Cl ₂ ),
Reactive gas such as sulfurous acid (SO ₂ ) should not be generated, and 5) it should not react with the detection electrode 2 of the measurement electrode 1 to impair the characteristics of the electrode 1. ,is required.

The neutral or near-neutral salt used in the present invention is not particularly limited as long as it satisfies these conditions, but the present inventors have found that an electrolyte satisfying such conditions is used. As a result of studying some inorganic salts as the electrolyte, sodium sulfate (Na ₂₎ was selected as the electrolyte used in the present invention.
It was found from the following experiment that SO ₄ ), potassium nitrate (KNO ₃ ) and potassium hydrogen carbonate (KHCO ₃ ) are suitable. In addition, the concentration of the electrolyte aqueous solution is measured electrode 1
There is no particular limitation as long as the electrolyte does not precipitate due to evaporation of water or temperature change due to humidification and the current required for electrolysis can be passed. For example, sodium sulfate (N
When a ₂ SO ₄ ) is used as the electrolyte, its concentration is 20 to 1
It was preferred to be 00 g / l.

Experiment 1 Sodium sulfate (Na ₂ SO ₄ ) and potassium nitrate (KN
O ₃ ) and potassium hydrogen carbonate (KHCO ₃ ) were used as electrolytes to prepare an aqueous electrolyte solution at the concentration shown in Table 1, and this was electrolyzed. Then, the theoretical value of oxygen to be generated corresponding to the current passed through the electrolytic electrode was obtained, and this was compared with the value of oxygen actually generated to examine the electrolysis efficiency of each aqueous electrolyte solution. Table 1 shows the results.

[0014]

[Table 1]

Experiment 2 The relationship between the value of the electrolytic current flowing through the electrode when the 72 g / l Na ₂ SO ₄ aqueous solution was electrolyzed and the concentration of oxygen generated at this time was investigated. The result is shown by a graph in FIG. As can be seen from FIG. 2, the electrolysis current value and the oxygen concentration are in direct proportion, and the oxygen concentration generated can be known based on the value of the current flowing through the electrolysis electrode. Therefore, it was found that in order to generate oxygen at a desired concentration, the electrolytic current value corresponding to the oxygen concentration should be adjusted, and the concentration of oxygen generated can be arbitrarily changed. KNO
Similar results were obtained when ₃ and KHCO ₃ were used as electrolytes.

Experiment 3 The change in the electrolytic current value with the passage of time was investigated when electrolysis of a 72 g / l Na ₂ SO ₄ aqueous solution was carried out. The results are shown graphically in FIG. As can be seen from FIG. 3, the current flowing through the electrode during electrolysis was stable.
Similar results were obtained when KNO ₃ and KHCO ₃ were used as electrolytes.

Experiment 4 The change in electrolysis efficiency with the passage of time during electrolysis of a 50 g / l Na ₂ SO ₄ aqueous solution was examined. The results are shown in a graph in FIG. As can be seen from FIG. 4, the electrolysis efficiency was stable over a long period of time. Incidentally, KN
Similar results were obtained when O ₃ and KHCO ₃ were used as electrolytes.

[0018]

As described above, the trace oxygen concentration measuring apparatus of the present invention uses a neutral or near-neutral salt as an electrolyte for preparing an aqueous electrolyte solution. Compared with the above, the danger to the human body is drastically reduced, it can be used safely, and the handling is simple. Moreover, no particular restrictions are imposed on the selection of the material for the humidifying tank, the electrolytic electrode, and the like, and it is not necessary to neutralize the waste liquid when treating the waste liquid.

[Brief description of drawings]

FIG. 1 is a diagram schematically showing a galvanic cell type trace oxygen concentration measuring apparatus.

FIG. 2 is a graph showing the results of experiment 2.

FIG. 3 is a graph showing the results of experiment 3.

FIG. 4 is a graph showing the results of experiment 4.

Claims (1)

[Claims] 1. An open cathode type galvanic cell type trace oxygen concentration measuring device for preparing a standard gas having a predetermined concentration by oxygen generated by electrolyzing an aqueous electrolyte solution and calibrating with the standard gas, wherein a low concentration oxygen is used. A trace oxygen concentration measuring device, characterized in that a neutral or near-neutral salt is used as an electrolyte for preparing an electrolyte aqueous solution necessary for preparing the standard gas of.