JP3443230B2 - Trace oxygen concentration measurement device - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an apparatus for measuring a trace amount of oxygen, and more particularly to a method for measuring the concentration of a trace amount of oxygen contained in an industrial gas or the like requiring high purity. The present invention relates to a trace oxygen concentration measuring device for performing [0002] Conventionally, various industrial gases of high purity have been used in various industrial fields, but with the recent technological advancement and sophistication, the industrial industry, especially the semiconductor industry, has been used. There is an increasing demand for higher purification of utility gas. It is required to guarantee the impurity concentration in the order of ppb for the so-called ultra-high-purity gas, and it is important to ensure that oxygen (O ₂ ) is not particularly contained in the impurities. One of the items. Therefore, in order to control the oxygen concentration in such gas,
Various trace oxygen concentration measuring devices capable of quickly and accurately measuring the concentration of oxygen contained in trace amounts as impurities have been proposed. As such a measuring device, for example, FIG.
An open cathode type galvanic cell type shown in FIG. This is to obtain an output current corresponding to the oxygen concentration by electrolytic reduction of oxygen contained in a sample gas. In such a measuring device, a detection electrode 1 made of silver, carbon, platinum, etc. 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. In the illustrated open-cathode galvanic cell type measuring apparatus, the detection electrode 2 and the counter electrode 3 are required to stably advance the reactions of the above-mentioned equations (1) and (2) without deteriorating the electrolytic efficiency. It is necessary that the strongly alkaline electrolyte is always held in the electrolyte part 4 between
Further, 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 device introduces a sample gas into a humidification tank 5 through an introduction pipe 6, raises the sample gas as air bubbles in water, and flows the water into the measurement chamber 10, thereby obtaining water vapor. Is configured so that the measurement electrode 1 is humidified. In the figure, reference numeral 11 denotes a sample gas outlet. Here, in order to stably measure the concentration of a small amount of oxygen contained in the sample gas, it is necessary to calibrate the measuring device precisely. When calibrating the measuring device, if a standard gas having a desired concentration can be obtained, it may be supplied from the outside and calibrated with the standard gas.However, it is generally difficult to obtain a standard gas having an extremely small concentration. is there. Therefore, usually, oxygen is generated in the apparatus by electrolysis of water to prepare a standard gas having a trace oxygen concentration. In order to facilitate the electrolysis, a high-concentration alkaline aqueous solution is used as the electrolytic aqueous solution. Specifically, as shown in FIG. And a current is passed through the electrode to generate oxygen electrolytically, and a zero gas from which oxygen has been completely removed by a deoxidizer or the like is introduced into a humidifying tank 5 through a sample gas introducing pipe 6, and the electrolytic gas is generated in the zero gas. A standard gas having a desired concentration is prepared by mixing the oxygen thus produced. That is, in the electrolytic generation of oxygen in an alkaline aqueous solution, the reaction of the following formula (3) at the anode 9: 4OH ⁻ → 2H ₂ O + O ₂ + 4e ⁻ (3) 5) Formula 4H ₃ O ⁺ + 4e ⁻ → 4H ₂ O + 2H ₂ (4) This is due to the progress of each reaction. At this time, the concentration of the zero gas introduced into the humidification tank 5 is represented by F (cc / M
in. ), The concentration of the standard gas to be prepared is X (pp
m), assuming that the electrolysis efficiency in the oxygen electrolysis generation is 100%, the current value I (μA) to be passed through the electrolytic electrode 7 is expressed by the following equation (5): I = 0.267FX (5) By flowing a predetermined amount of current calculated by equation (5) through the electrolytic electrode 7 to generate oxygen and introducing zero gas into the humidifying tank 5 at a predetermined flow rate, a standard gas having a desired concentration can be prepared. Calibration of the measuring device is performed with a standard gas. [0007] In the above-described measuring apparatus, the electrolytic solution stored in the humidifying tank 5 can increase the electrolysis efficiency of the electrolytic electrode 7 and contains a reactive gas. A strong alkaline electrolyte such as sodium hydroxide or potassium hydroxide is generally used because it does not occur, and careful handling of the measuring device has been required. In other words, in the maintenance and repair of the measuring device, if the strong alkaline aqueous solution splashes on the body of the worker and adheres, the skin etc. will be damaged, and especially when it adheres to the eyeball, there is a risk of blindness. We had to pay attention. In addition, the materials of the humidifying tank 5, the electrolytic electrode 7, and the like must be selected by carefully examining materials having resistance to a strongly alkaline aqueous solution, and usable materials are limited. SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems, and a trace oxygen concentration measuring apparatus of the present invention has a predetermined concentration by using oxygen generated by electrolyzing an aqueous electrolyte solution. In the open-cathode galvanic cell-type trace oxygen concentration measurement device that prepares a standard gas and performs calibration with such a standard gas, a neutral electrolyte is used to prepare the aqueous electrolyte solution required to prepare the low-concentration oxygen standard gas.
It is characterized by using a salt . DESCRIPTION OF THE PREFERRED EMBODIMENTS The trace oxygen concentration measuring apparatus of the present invention
The sample gas is introduced from the introduction pipe 6 into the humidification tank 5 in which the aqueous electrolyte solution is stored, and the sample gas is raised as bubbles in the aqueous electrolyte solution and flows into the measurement chamber 10 so that the measurement electrode is vaporized by the vapor of the aqueous electrolyte solution. 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 applied to this electrode to generate oxygen electrolytically, and oxygen is completely removed by a deoxidizer or the like. The standard gas having a desired concentration is prepared by introducing the zero gas into the humidifying tank 5 through the sample gas introducing pipe 6 and mixing the zero gas with the oxygen generated by electrolysis, and calibrating the measuring device with the standard gas. (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 required for electrolytically generating oxygen. The electrolyte used is
It is necessary to satisfy the following conditions. That is, the electrolyte used in the present invention is: 1) its aqueous solution is neutral or almost neutral;
2) exhibiting high electrolysis efficiency and generating a considerable amount of oxygen with respect to the current flowing through the electrolytic electrode 7; 3) having high solubility in water and hardly precipitating; ) is stable over time, the electrolysis by hydrogen chloride (HCl), chlorine (Cl _2),
It must not generate reactive gas such as sulfurous acid (SO ₂ ). 5) It must not react with the detection electrode 2 of the measurement electrode 1 and impair the characteristics of the electrode 1. ,is necessary. The neutral salt used in the present invention is not particularly limited as long as it satisfies these conditions. However, the present inventors have proposed some inorganic salts as an electrolyte satisfying such conditions. The following experiment showed that sodium sulfate (Na ₂ SO ₄ ) and potassium nitrate (KNO ₃ ) were suitable as electrolytes used in the present invention among those studied by the present inventors. I found it. In addition, the concentration of the aqueous electrolyte solution is determined by measuring electrode 1
There is no particular limitation as long as the electrolyte does not precipitate due to evaporation or temperature change of water due to humidification and a current required for electrolysis can be passed.
If the a ₂ SO ₄₎ electrolyte, its concentration is 20 to 1
It was preferably 00 g / l. Experiment 1 Sodium sulfate (Na ₂ SO ₄ ), potassium nitrate (KN
Using O ₃ ) as an electrolyte, an aqueous electrolyte solution was prepared at the concentrations shown in Table 1 and electrolyzed. Then, the theoretical value of oxygen to be generated corresponding to the current flowing through the electrolytic electrode was obtained, and the theoretical value of oxygen generated was compared with the theoretical value of oxygen generated to examine the electrolysis efficiency of each electrolyte aqueous solution. Table 1 shows the results. [Table 1] Experiment 2 The relationship between the value of the electrolytic current flowing through the electrode when a 72 g / l aqueous solution of Na ₂ SO ₄ was electrolyzed and the concentration of oxygen generated at this time was examined. The result is shown by a graph in FIG. As can be seen from FIG. 2, the electrolytic current value and the oxygen concentration are in direct proportion, and the concentration of the generated oxygen can be known based on the value of the current flowing through the electrolytic electrode. Therefore, in order to generate oxygen at a desired concentration, it suffices to adjust the electrolytic current value corresponding to the desired concentration, and it has been found that the concentration of generated oxygen can be arbitrarily changed. In addition, KNO
₃ Similar results when used as electrolyte was obtained. Experiment 3 A change in the electrolytic current value with time when the 72 g / l aqueous solution of Na ₂ SO ₄ was electrolyzed was examined. The result is shown by a graph in FIG. As can be seen from FIG. 3, the current flowing through the electrodes during the electrolysis was stable.
Incidentally, the same results when using the KNO ₃ as an electrolyte was obtained. Experiment 4 The change in electrolysis efficiency with time when electrolysis of a 50 g / l aqueous solution of Na ₂ SO ₄ was examined. The results are shown in a graph in FIG. As can be seen from FIG. 4, the electrolysis efficiency was stable for a long time. In addition, KN
O ₃ Similar results when used as electrolyte was obtained. [0018] As has been described in the foregoing, the present invention trace oxygen concentration measurement apparatus, in the electrolyte for preparing electrolytic solution
Since the alkaline salt is used, the danger to the human body and the like is drastically reduced as compared with the conventional one using a strong alkaline salt, and it can be used safely and is easy to handle. In addition, there is no special limitation in selecting the material of the humidifying tank, the electrolytic electrode, and the like, and there is no need to subject the waste liquid to neutralization or the like in treating the waste liquid.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a drawing schematically showing a galvanic cell type trace oxygen concentration measuring device. 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.

──────────────────────────────────────────────────続 き Continued on the front page (58) Fields surveyed (Int.Cl. ⁷ , DB name) G01N 27/26-27/49

Claims (1)

(57) [Claims 1] An open-cathode type galvanic cell-type micro-volume that prepares a standard gas of a predetermined concentration using oxygen generated by electrolyzing an aqueous electrolyte solution and performs calibration with the standard gas. A trace oxygen concentration measuring apparatus characterized in that a neutral salt is used as an electrolyte for preparing an electrolyte aqueous solution necessary for preparing a low concentration oxygen standard gas in the oxygen concentration measuring apparatus.