JPH0418262B2 - - Google Patents

Description

〔Technical field〕

The present invention relates to a sensor for measuring the concentration of dissolved carbon dioxide in a solution. [Prior Art and Problems] The carbon dioxide sensor conventionally used to detect the concentration of dissolved carbon dioxide in a solution uses a glass electrode. It has a structure in which a carbon dioxide gas permeable membrane is used to separate it from the solution. An electrolyte aqueous solution is contained between the glass electrode and the carbon dioxide permeable membrane. For measurement, this carbon dioxide gas sensor is immersed in a solution. Carbon dioxide molecules in the solution permeate the carbon dioxide permeable membrane, dissolve in the electrolyte aqueous solution, and then dissociate hydrogen ions through a reaction. CO ₂ +H ₂ OH ⁺ +HCO ₃ ^-The concentration of this dissociated hydrogen ion is measured using a glass electrode to indirectly determine the dissolved carbon dioxide concentration. Since such a conventional carbon dioxide sensor uses a glass electrode, its overall mechanical strength is low and it is difficult to manufacture. In particular, since it lacks flexibility, it is almost impossible to insert it into a living body by passing it through a flexible tube such as a catheter. Furthermore, the conventional carbon dioxide sensor described herein cannot also measure the dissolved oxygen concentration in a solution. Furthermore, in place of the glass electrode, it is possible to use a conductor such as platinum that generates an electromotive force when sensitive to hydrogen ions, using the same principle as above, but such a conductor is sensitive to oxygen. If the measurement solution contains dissolved carbon dioxide in addition to dissolved carbon dioxide, as most ordinary solutions do, it will be affected by the
It is difficult to accurately measure the desired carbon dioxide concentration. Purpose of the Invention Therefore, the purpose of the present invention is to have a structure that is flexible as a whole, and to not only accurately measure the concentration of dissolved carbon dioxide in a solution without being affected by dissolved oxygen gas, but also to An object of the present invention is to provide a gas sensor that can be constructed so that it can also measure concentration. According to the present invention, there is provided a gas sensor used for detecting the gas concentration of a solution to be measured by immersing it in the solution, the outer cylinder having a bottom and having a portion that comes into contact with the measurement solution permeable to carbon dioxide gas. an internal liquid contained in the outer cylindrical body and consisting of an aqueous electrolyte solution that reacts with carbon dioxide gas and dissociates an amount of hydrogen ions corresponding to the amount of carbon dioxide; a working electrode in which at least a portion in contact with the internal liquid is formed of platinum; a reference electrode installed in the outer cylinder in contact with the internal liquid; and the outer cylinder in contact with the measurement solution. a gold layer formed to cover the outer surface of the gold layer; means for reducing the gold layer by applying a voltage corresponding to the reduction voltage of oxygen to the gold layer; and a means for reducing the gold layer in response to the concentration of hydrogen ions. A gas sensor is provided, comprising means for measuring a potential difference generated between an electrode and a reference electrode. If a means for measuring the reduction current generated in response to the reduction of oxygen is incorporated into this sensor, the gas sensor can also measure the dissolved carbon dioxide concentration. Specific Description and Effects of the Invention The present invention will be described in detail below with reference to the drawings. FIG. 1 shows a gas sensor 10 according to a first aspect of the invention. This sensor 10
is equipped with a bottomed outer cylindrical body 11 that is permeable to carbon dioxide gas. The outer cylindrical body 11 may be a flexible bottomed tube made of a carbon dioxide permeable material such as silicone, fluororesin (eg, polytetrafluoroethylene), or polypropylene. Inside the outer cylinder 11,
Contained is an internal fluid 12 consisting of an aqueous solution of an electrolyte (eg, a carbonate such as sodium bicarbonate). The opening of the outer cylinder 11 is closed with a suitable closing member 12 such as a rubber stopper. The working electrode 14 passes through each of the closing members 13.
The reference electrode 15 reaches the internal liquid 12 inside the outer cylinder 11. At least the surface of the working electrode 14 that contacts the internal liquid 12 is made of platinum. Therefore, the working electrode 14 may be formed entirely of platinum, or may be formed of a suitable conductor (for example, stainless steel) with a portion of the surface covered with platinum (by sputtering method, etc.). be able to. The reference electrode 15 can be composed of a silver or silver-silver chloride (silver chloride deposited on a silver surface) electrode. In the preferred example shown, the working electrode 14 and the reference electrode 15 are both linearly configured, and the internal liquid 12
It is covered with an insulating material 16 such as fluororesin, except for the portion that comes into contact with. A thin gold film layer 17 covers the outer surface of the outer cylinder 11.
is formed. This gold layer 17 is preferably formed to a thickness of 0.01 μm to 0.1 μm by a vacuum deposition method such as a sputtering method. The gold layer 17 itself is permeable to carbon dioxide gas and oxygen gas. Note that the gold layer 17 does not need to cover the entire outer surface of the outer cylindrical body 11; it is sufficient to cover only the surface portion that comes into contact with the measurement solution described below. A protective film 18 permeable to carbon dioxide gas and oxygen gas may be formed to cover the gold layer 17, if necessary. This protective film 18 is, for example, an oxidative electropolymerized film obtained by electrolytically oxidizing and polymerizing a hydroxy aromatic compound such as phenol and/or a nitrogen-containing aromatic compound such as aminobenzene on the surface of the gold layer 17, or an externally It can be formed from the material used to form the cylindrical body 11 or the like. A counter electrode 19 is provided on the protective film 18, the circumference of which is covered with an insulating material 20 except for the tip surface which will be in contact with a measurement solution, which will be described in detail later. This opposite pole 19
can be formed from the same material as the reference electrode 15. Working electrode 14 and reference electrode 15 are connected to potentiometer 22 via lead wires 21a and 21b, respectively. The potentiometer 22 measures the potential difference between the reference electrode 14 and the working electrode 15, as will be described in detail below. Further, the gold layer 17 and the counter electrode 19 are connected to a voltage applying device 24 via lead wires 23a and 23b, respectively.
An ammeter 25 is provided within the lead wire 23b (or within the lead wire 23a) as required. The voltage application device 24 applies a constant voltage corresponding to the reduction voltage of oxygen molecules to the gold layer 17, as will be described in detail below. Also,
The ammeter 25 measures the reduction current value of oxygen molecules flowing when a voltage is applied to the gold layer 17 in this manner. To measure the concentrations of dissolved carbon dioxide and oxygen gas in a predetermined solution using the gas sensor 10 having the configuration shown in FIG. do. At this time, a voltage applying device 24 is applied to the gold layer 17.
A constant voltage (approximately -0.6 volts with respect to the counter electrode 19) corresponding to the reduction voltage of oxygen molecules is applied. Dissolved oxygen molecules in the measurement solution 26 are removed by the protective film 18
If the reduction voltage is not applied there, it will pass through the gold layer 17 and further through the outer cylindrical body 11 and reach the internal liquid 12, which will adversely affect the measurement of carbon dioxide concentration described later. However, as described above, since a voltage corresponding to the reduction voltage of oxygen molecules is applied to the gold layer 17, the reduction voltage causes reduction on the gold layer 17 according to the following equation. O ₂ +2H ₂ O+4e ^- 4OH ^- Therefore, oxygen molecules do not penetrate into the internal liquid 12 through the outer cylinder 11, and the measurement of carbon dioxide concentration, which will be explained later, can be performed reliably without being affected by oxygen. It is carried out. The value of the oxygen reduction current that flows in response to the reduction of oxygen molecules at this time corresponds to the dissolved oxygen gas concentration, so by measuring this with the ammeter 25, the dissolved oxygen concentration can be determined. On the other hand, dissolved carbon dioxide molecules in the measurement solution 26 are
passes through the protective film 18 and passes through the gold layer 17 without being affected by the voltage applied to the gold layer 17;
It penetrates into the internal liquid 12 through the outer cylinder 11. Therefore, an amount of hydrogen ions corresponding to the amount of dissolved carbon dioxide gas is dissociated by a reaction according to the following formula. CO ₂ +H ₂ OH ⁺ +HCO ₃ ^-The potential difference generated between the working electrode 14 and the reference electrode 15 depending on the concentration of this dissociated hydrogen ion eventually corresponds to the concentration of dissolved carbon dioxide in the measurement solution 26, so this By measuring with the potentiometer 25, the dissolved carbon dioxide concentration can be determined. The sensor shown in FIG. 1 has a generally flexible structure and is suitable for use by being inserted into tubes such as catheters or metal tubes such as stainless steel. FIG. 2 shows a sensor 30 according to a second aspect of the invention. In the figure, the same parts as in FIG. 1 are designated by the same reference numerals. The sensor 30 shown in FIG. 2 has a relatively short outer cylinder 11'. A spacer 31 having a large number of through holes, for example, a plastic mesh such as a nylon mesh is installed at the bottom of the outer cylinder 11'. The internal liquid 1 described in connection with FIG.
It is filled with an internal liquid (not shown) similar to No. 2. A working electrode 14 and a reference electrode 15 are inserted into the outer cylinder 11' while being fixed together with an insulating member 16. The tip surfaces of the working electrode 14 and the reference electrode 15 are exposed at the tip surfaces of the respective insulating materials 16 and are in contact with the internal liquid filled in the spacer 31 . The outer surface of the outer cylinder 11' is covered with a gold layer 17, and the gold layer 17 is coated with a protective film 1 if necessary.
Covered by 8. An insulating material 33 is formed so that the outer periphery of this sensor 30 is on the same surface. The rest of the structure is the same as sensor 10 of FIG. 1, except that lead 23b extends through insulating material 33. The measurement of carbon dioxide gas concentration and oxygen gas concentration by the sensor 30 shown in FIG. 2 follows the principle already described with respect to FIG. Example 1 In this experimental example, a sensor having the configuration shown in FIG. 1 was manufactured. First, stainless steel wire (diameter 1
mm) is coated with Teflon (registered trademark name),
Silicone carbide (particle size approx. 0.8μ) is applied to the exposed tip surface.
m) Grind and smooth with paper and alumina powder (particle size: approximately 0.3 μm), wash with water and methanol, and dry. On the tip surface of this stainless steel wire,
Sputter method using a dipole magnetron device (distance to platinum source 4 cm, under 10 Torr argon atmosphere,
A platinum thin film layer 17 with a thickness of 0.056 μm was formed by using a power supply of 200 watts and a sputtering time of 15 seconds.
4 was prepared. A silicone film (0.25 mm thick, Zefron manufactured by Fuji Systems) tube was used as the outer cylinder 11, and silver-silver chloride electrodes were used as the reference electrode 15 and the counter electrode 19. Internal solution 12 was a 5mM/1 aqueous sodium bicarbonate solution. (1) First, as the measurement solution 26, CO ₂ /
The working electrode 14 and the reference electrode 15 when the voltage applied to the gold layer 17 is varied using a standard phosphate buffer solution into which a mixed gas of O ₂ /N ₂ = 10/45/70 (volume ratio) is blown. The equilibrium potential value between Note that the correlation between the ratio of the carbon dioxide gas partial pressure to the oxygen gas partial pressure in the mixed gas and the ratio of the carbon dioxide gas partial pressure to the oxygen gas partial pressure in the phosphate buffer solution is a linear relationship with a slope of 0.502. The results are shown in Table 1 below. Table 1 Applied voltage (volt) Equilibrium potential value (mV) -0.8 180.0 -0.6 232.7 -0.4 259.3 -0.2 281.9 (2) Next, the mixed gas blown into the measurement solution 26
When the same experiment as above was conducted except that CO ₂ /N ₂ =10/115 (volume ratio), the equilibrium potential value was 230 mV. From this result and the results in Table 1 above, it can be seen that -0.6 volts (vs.
It can be seen that oxygen molecules are reduced by applying a voltage to the counter electrode (Ag/AgCl) 19). (3) In this way, it was confirmed that the voltage corresponding to the reduction voltage of oxygen molecules across the gold layer 17 was -0.6 volts, so with that voltage applied to the gold layer 17, as shown in Table 2 below, The dissolved carbon dioxide concentration in the measurement solution was changed, and the equilibrium potential value between the working electrode 14 and the reference electrode 15 at that time was measured. The results are also listed in Table 2.

[Table] Figure 3 is a graph showing the relationship between carbon dioxide concentration and equilibrium potential value based on the results of Table 2. This relationship is a linear relationship, and it is clear that it can be used as a carbon dioxide sensor. Potential response speed is also fast. Using a sensor manufactured in the same manner as in Example 1, an experiment was conducted to measure the O ₂ concentration in a mixed gas (CO ₂ , N ₂ , O ₂ ) using the current method. The electrolytic current value between the gold layer 17 and the counter electrode 19 of the electrode structure shown in FIG. 1 is read using an ammeter 25. And at this time
The CO ₂ concentration is also measured using the potentiometer 22 (the same method as in Example 1). In the experiment, a voltage of -0.6 volts (counter electrode (Ag/AgCl) 19) was applied to the gold layer 17 by a voltage application device (power supply) 24, and the oxygen reduction current value at this time was read by an ammeter 25. . O ₂ concentration 0mm
Hg ~ 273.6mmHg change (CO ₂ concentration 60.8mmHg constant,
Table 3 shows an example in which the total gas concentration was 713 mmHg (balanced with N ₂ gas).

[Table] From the results of Table 3, the relationship between oxygen gas concentration and oxygen reduction current value is illustrated in FIG. 4. It has become clear from this relationship that there is a linear relationship and that the gold layer 17 prevents oxygen gas from permeating into the tube and also functions as an oxygen sensor for measuring oxygen concentration. Therefore, it has become clear that by using this electrode, it is possible to simultaneously measure the concentrations of oxygen gas and carbon dioxide in a sample solution in which a mixed gas of oxygen and carbon dioxide is dissolved. Specific Effects of the Invention As described above, the gas sensor of the present invention can accurately measure the dissolved carbon dioxide concentration without being affected by the presence of dissolved oxygen in the measurement solution. Additionally, the dissolved oxygen gas concentration can also be measured.

[Brief explanation of drawings]

FIGS. 1 and 2 are sectional views of a gas sensor according to the present invention, and FIGS. 3 and 4 are graphs showing the characteristics of the gas sensor of the present invention. 11, 11'... Carbon dioxide permeable outer cylinder, 12
... Internal liquid, 14 ... Working electrode, 15 ... Reference electrode,
17... Gold layer, 19... Counter electrode, 22... Potentiometer, 24... Voltage application device, 25... Ammeter.

Claims (1)

[Scope of Claims] 1. A gas sensor used for detecting the dissolved gas concentration of a measurement solution by being immersed in the measurement solution, the sensor having a bottom and an outer surface whose part that comes into contact with the measurement solution is permeable to carbon dioxide gas. a cylindrical body, an internal liquid contained in the outer cylindrical body and consisting of an electrolyte aqueous solution that reacts with carbon dioxide gas and dissociates an amount of hydrogen ions corresponding to the amount thereof; a working electrode that is installed and has at least a portion in contact with the internal liquid made of platinum; a reference electrode that is placed in the outer cylinder in contact with the internal liquid; and the outer cylinder that comes into contact with the measurement solution. a gold layer formed to cover the outer surface of the body; a means for reducing the gold layer by applying a constant voltage corresponding to the reduction voltage of oxygen to the gold layer; and a means for reducing the gold layer in response to the concentration of hydrogen ions. A gas sensor comprising means for measuring a potential difference generated between the working electrode and the reference electrode. 2. A gas sensor used to detect the concentration of dissolved carbon dioxide in a solution to be measured by being immersed in the solution, which has a bottomed outer cylinder whose part that comes into contact with the measurement solution is permeable to carbon dioxide. , an internal liquid contained in the outer cylindrical body and consisting of an aqueous electrolyte solution that reacts with carbon dioxide gas and dissociates an amount of hydrogen ions corresponding to the amount thereof; A working electrode, at least a portion of which comes into contact with the internal liquid, is made of platinum; a reference electrode, which is placed in the outer cylinder in contact with the internal liquid; and an outer part of the outer cylinder, which comes into contact with the measurement solution. a gold layer formed covering the surface portion; a means for reducing the gold layer by applying a constant voltage corresponding to the reduction voltage of oxygen to the gold layer; and a means for reducing the gold layer by applying a constant voltage corresponding to the reduction voltage of oxygen to the gold layer; A gas sensor comprising: means for measuring a potential difference generated between a reference electrode; and means for measuring a reduction current generated in response to the reduction of oxygen.