CONCENTRATION DETECTION ELEMENT FOR SOLUTE IN AQUEOUS SOLUTION

Abstract: This invention provides a solute in an aqueous solution concentration detection element sealed airtight by a porous non-water-permeable fluororesin film or a two-layered film of a porous non-water-permeable fluororesin film and a selective vapor permeable layer or a three-layered film of a porous non-water-permeable fluororesin film, a selective vapor permeable layer and an acid vapor absorption layer, by utilizing a property that the concentration of a solute in an aqueous solution is closely associated with the vapor pressure of the aqueous solution.

Description: The present invention relates to a sensor for measuring solute concentration in an aqueous solution and more particularly, provides a sensor which is suitable for simple and continuous detection and measurement of the concentration of the solute in an aqueous solution from the water vapor pressure of the solution. As a means for measuring a solute concentration in an aqueous solution, chemical analysis, or instrumental analysis such as atomic absorption spectroscopy, ion chromatography, etc. has been adopted heretofore. Further as a simpler means, a hydrometer, an optical refraction type gravimeter, a conductivity meter, etc. may be used. There are many demands for simple measurement of aqueous solutions of acids such as sulphuric acid, hydrochloric acid, acetic acid, etc. but the measurement is extremely difficult. For example, a concentration of sulphuric acid which is an electrolyte of a lead-acid battery is closely related to a charge and discharge state of the lead-acid battery and therefore, it has been strongly desired to develop a simple method for measuring a sulphuric acid concentration. However, such an adequate measurement technique has not been attained yet. And it is general to determine the concentration with a hydrometer, an optical refraction type gravimeter, etc. described above. As a particular method, it is proposed in West German Patent No. 2,254,207 (1973) to determine a sulphuric acid concentration by measuring water vapor pressure of the upper space beyond a sulphuric acid electrolyte of a lead-acid battery. That is, this method comprises measuring water vapor pressure in the upper space in a gas-liquid equilibrated state with a sulphuric acid aqueous solution to determine a concentration of the sulphuric acid aqueous solution since the water vapor pressure of the space varies depending upon the sulphuric acid concentration. This proposal has further been advanced and it has been proposed in J.L. Weininger, J.L. Briant, J. Electrochem. Soc., VOL129,2409 (1982) to cover a humidity sensor with a porous polypropylene film, directly immersing the covered humidity sensor in a sulphuric acid aqueous solution and measuring partial pressure of water vapor diffused through pores of the porous polypropylene film. Conventional chemical analysis and instrumental analysis described above generally require equipments in a large scale and at the same time, operations are complicated. Even the operation with hydrometer, an optical retraction type gravimeter and a conductivity meter, are complicated and skill is required for measurement. Besides, it is generally inadequate for continuously monitoring a change in concentration of an aqueous solution. Upon measurement of a concentration of an aqueous solution of acids such as sulphuric acid, etc. described above, in particular, sulphuric acid as an electrolyte in a lead-acid battery, a hydrometer or an optical retraction type gravimeter has an excessively large size so that the meter cannot be inserted into the lead-acid battery, which makes continuous measurement impossible. According to the method of West German Patent supra, when a sulphuric acid concentration changes, it takes a long time until the water vapor pressure of the sulphuric acid aqueous solution and the water vapor pressure of the space reach equilibrium and a response speed is slow. Further according to the method of Weininger et al., the porous polypropylene film has a problem inherent in its material; that is, due to insufficient water repellency, not only water vapor but also the sulphuric acid aqueous solution itself permeates through the film to reach a moisture-sensitive part of the humidity sensor so that is is difficult to properly determine humidity. In addition, the sensor has the defect that its life is extremely short because of chemical corrosion by sulphuric acid. French Patent No. 2597978 describes a method and apparatus for determining the concentration of an aqueous solution in which a humidity sensor is arranged in a chamber having an inlet and an outlet for dry air, one wall of the chamber of the apparatus comprising a water-repellent polytetrafluoroethylene membrane having pores therein with about 0.2 µm diameter. U.S. Patent No. 4468948 discloses an apparatus for measuring the concentration of a gaseous or volatile substance in a liquid, comprising a sampling means with porous tubing leading from the sampling means to a detecting means. The pore size of the tetrafluoroethylene of the tubing is between 0.1 and 1.0 µm and the porosity is 20 to 80%. The present invention provides a sensor for measuring a concentration of a solute in an aqueous solution, comprising a humidity sensor which is covered with a porous liquid-water-impermeable, water vapor permeable polymer film characterised in that the polymer is a flourine polymer, and the film has a moisture vapor transmission rate (hereinafter MVRT) of from 500 to 150000g/m² per 24hr. Preferably, said fluorine polymer film may have a selective water vapor-permeable layer associated therewith. Preferably said fluorine polymer film may have both a selective water vapor-permeable layer and an acid vapor-absorbing layer associated therewith. Advantageously, the fluorine polymer film may be polytetrafluoroethylene. When the humidity sensor is covered with a porous liquid-water-impermeable, water vapor permeable fluorine polymer film having an MVTR of from 500 to 150000g/m².24hr, preferably 500 to 10000g/m².24hr and immersed into an aqueous solution, water in the aqueous solution permeates through the polymer film in the form of vapor and liquid and vapor are equilibrated at the both sides of the film, whereby a water vapor pressure in the gaseous phase inside of the film is detected as humidity by the humidity sensor. Accordingly, by previously measuring relationship between a solute concentration and a water vapor pressure of an aqueous solution to be measured or by previously determining the relationship between a known concentration of the solute and a relative humidity value of the humidity sensor, a solute concentration of the aqueous solution can be determined by a relative humidity value when the humidity sensor for measuring the solute concentration of the aqueous solution covered with the porous fluorine resin film is immersed in an aqueous solution having an unknown concentration to be measured. The porous fluorine polymer film described above should provide a high response speed of the sensor for the aqueous solution concentration. For this purpose, the polymer film should be as water vapor permeable and as porous as possible and have a thickness as thin as possible; on the other hand, in view of mechanical strength and life of the sensor, the polymer film should be less porous and thicker. To satisfy such contradictory requirements, the present inventors have found that the moisture vapor transmission rate (measured by Japanese Industrial Standards L-1099) of the polymer film is an important factor for determining response speed and life of the aqueous solution concentration sensor and have determined a relationship between such a moisture vapor transmission rate of the film. The relationship between the moisture vapor transmission rate and the response time of the sensor is depicted in Fig. 1. That is, in case that the moisture vapor transmission rate is less than 500g/m².24hr, the aqueous solution concentration sensor exhibits too late response speed and is not generally applicable for change in concentration of the aqueous solution. Further in case that the moisture vapor transmission rate is larger than 150000g/m².24hr, not only water vapor but also water permeate through the film to cause an error in the humidity sensor; or, depending upon kind of solute, the solute might permeate through the film to corrode the humidity sensor. It is thus mandatorily required to set the upper limit at 150000g/m².24hr. Covering the humidity sensor with a double-layered film obtained by additionally coating or laminating the porous fluorine polymer film described above with a selective water vapor-permeable layer such as perfluorocarbon sulphonic acid makes more accurate measurement possible since water vapor alone permeates and solute vapor does not permeate. This double-layered film shows effectiveness especially in an aqueous solution of a solute having high vapor pressure. Moreover, life of the humidity sensor is prolonged. Further by forming an acid vapor-absorbing layer using calcium carbonate, for example, together with the selective water vapor-permeable layer described above, a trace amount of acidic vapor that cannot be intercepted by the selective water vapor-permeable layer can be trapped. That is, this trapping is effected by chemical reaction as: H₂SO₄ + CaCO₃ → CaSO₄ + CO₂ + H₂O The acid vapor-absorbing layer described above can easily be formed by coating a mixture of finely divided calcium carbonate and a binder such as fluorine-contained urethane resin or fluorine resin, etc. Specific embodiments of the present invention are explained by referring to the accompanying Figures. The Figures indicate technical contents of the present invention. Fig. 1 shows a relationship between a moisture vapor transmission rate of covered film of the humidity sensor adopted in the present invention and a sensor response time. Fig. 2 is a front view of the sensor having a basic structure in the present invention. Fig. 3 is a cross-sectional view of the sensor. Fig. 4 shows a relationship between a concentration of magnesiumchloride aqueous solution and a value obtained by an aqueous solution concentration sensor. Fig. 5 is a front view of the second embodiment of the present invention as shown in Fig. 2. Fig. 6 is a cross-sectional view of Fig. 5. Fig. 7 shows a relationship between a concentration of sulphuric acid aqueous solution and a voltage detected by the aqueous solution concentration sensor. Fig. 8 shows a change with voltage depending on time under constant concentration condition of the sulphuric acid aqueous solution. Fig. 9 is a front view of the third embodiment of the present invention as shown in Fig. 2 and Fig. 5. Fig. 10 is a cross sectional view of Fig. 9. Fig. 11 shows a relationship between a concentration of sulphuric acid aqueous solution and a voltage detected by the aqueous solution concentration sensor. Fig. 12 shows a change with voltage depending on time under constant concentration condition of the sulphuric acid aqueous solution. In these figures, numeral 1 shows a humidity sensor, 2 denotes a porous liquid-water-impermeable fluorine polymer film, 3 denotes a selective water vapor-permeable layer, and 4 denotes a acid vapor-absorbing layer. Basic embodiments of the present invention are as shown in Figs. 2 and 3. For example, a concentration detector element 1 composed of a commercially available polymer type humidity sensor or ceramic type humidity sensor is covered and is heat-sealed with a porous liquid water impermeable, water vapor permeable fluorine polymer film having a moisture vapor transmission rate of from 500 to 150000g/m².24hr. That is, the fluorine polymer described above has a water repellency showing an advanced contact angle of about 110°; this film is made by roll-pressing and stretching and have the maximum pore diameter of 0.5 µm, preferably 0.01 to 0.2 µm and porosity of 25 to 85%, preferably 50 to 80%. Thus, the film is obtained as having an appropriate liquid-water-impermeable property while it has a preferred moisture vapor transmission rate. In addition, the film has excellent resistivity to acid and other chemicals. As a specific embodiment, the aqueous solution concentration sensor was prepared by using the concentration detection element 1 consisting of the polymer type humidity sensor whose out-put signal was designed by electric circuit to express a relative humidity by voltage, covering the humidity sensor with a porous polytetrafluoroethylene film 2 having the maximum pore diameter of 0.5 µm and porosity of 75% and having a thickness of 0.1 mm and heat sealing the covering. The sensor was immersed in aqueous solutions of magnesium chloride hexahydrate (MgCl₂.6H₂O) adjusted to various concentrations and output voltages by the aqueous solution concentration sensor were measured. The results are shown in Fig. 4. That is, as the concentration of magnesium chloride aqueous solution becomes high, the voltage of the aqueous solution concentration sensor becomes low. It is thus clear that the concentration of magnesium chloride can be accurately detected by the voltage value measured. Further in this case, a response rate of the aqueous solution concentration sensor was determined based on the change in voltage when a concentration of the magnesium chloride aqueous solution was changed, whereby 90% response took 3 minutes which was a considerably rapid response and it was confirmed that the measurement could be sufficiently practically usable. In addition, the concentration of magnesium chloride was continuously measured using this aqueous solution concentration sensor, while changing the concentration of magnesium chloride. The detection characteristic as described above did not change even after 1000 hours passed. It was confirmed that the sensor showed satisfactory long life and the method exhibited excellent in reproductivity. A second embodiment of the present invention is shown in Figs. 5 and 6. A double-layered plastic film was obtained by coating an alcohol solution of perfluorocarbon sulphonic acid on one surface of the same porous liquid-water-impermeable film as in Fig. 2 and 3 to form a selective water vapor-permeable layer 3. The humidity sensor 1 was covered and heat-sealed with the plastic film at its periphery as in Figs. 5 and 6. The sensor was immersed in aqueous solutions of sulphuric acid adjusted to various concentrations and output voltages were measured. The results are shown in Fig. 7. It was confirmed that preferable measurement was possible as in Fig. 4. Further, a response rate was determined as in Figs. 2 and 3. Rapid response as 90% response for 3 minutes was obtained and it was confirmed that the measurement was sufficient for practical use. In addition, when the concentration of sulphuric acid was continuously measured using this aqueous solution concentration sensor, change in the sensor characteristics was continuously monitored. The results are shown in Fig. 8, together with those of the sensor shown in Figs. 2 and 3 having no selective water vapor-permeable layer 3. The sensors showed preferable long life under the severe condition of 40% sulphuric acid and temperature of 30°C. In particular, with the sensor of Figs. 5 and 6, no abnormality was noted in characteristics even after 500 hours being passed. It was confirmed that the measurement method showed excellent reproductivity. A third embodiment of the present invention is shown in Figs. 9 and 10. A sensor was prepared by further forming an acid vapor-absorbing layer 4 obtained by binding calcium carbonate with a binder onto a water vapor selectively permeable layer 3 formed on a porous liquid-water-impermeable layer 2. More specifically, a mixture of calcium carbonate powders having a particle diameter of 20 µm or less and fluorine-modified urethane resin was used to form an acid vapor-absorbing layer 4. This sensor was immersed in aqueous solutions of sulphuric acid adjusted to various concentration and relationship between the concentrations of the solutions and output voltages of the sensor was examined. The results are shown in Fig. 11. It is clear that the concentration of the sulphuric acid aqueous solution can be adequately determined by voltages measured. Substantially rapid response at 90% response for 3 minutes was obtained and it was confirmed that the response was sufficient for practical use. In addition, when the concentration of sulphuric acid was continuously measured using this aqueous solution concentration sensor, change in the sensor characteristics was continuously monitored. That is, the sulphuric acid aqueous solution having a concentration of 40% was measured over 1100 hours (at a temperature of 30°C). The results are shown in Fig. 12. A measured voltage of the aqueous solution concentration sensor indicates continuously the stable value of 60mV. It was confirmed that continuous and accurate detection measurement could be made on a sulphuric acid concentration in a lead-acid battery over a sufficiently along period of time. It is apparent that in further embodiments other than humidity sensors as described above may be used and almost similar results can be obtained. According to the present invention as described above, a concentration of the solute in an aqueous solution can be accurately determined by simple operations for measuring a voltage of the aqueous solution concentration sensor. In addition, the sensor is of small size and easily applicable to any portions to be measured. Furthermore, the present invention can provide the aqueous solution concentration sensor having a good response and a long life. Besides, even when a solute has a high vapor pressure, the present invention enables to measure effectively the concentration of the aqueous solution by the selective water vapor-permeable layer and/or the acid vapor-absorbing layer. Thus, the great effects of the present invention from an industrial viewpoint is clearly noted.

Claims: A sensor for measuring a concentration of a solute in an aqueous solution, comprising a humidity sensor (1) which is covered with a porous liquid water-impermeable, water-vapor permeable polymer film (2) characterised in that the polymer is a fluorine polymer, and the film has a moisture vapor transmission rate (MVTR) of from 500 to 150000g/m² per 24hr. A sensor according to claim 1,characterized in that said fluorine polymer film (2) has a selective water vapor-permeable material (3) attached thereto. A sensor according to claim 2, characterized in that said selective water vapor-permeable material (3) has an acid vapor-absorbing layer (4) attached thereto. A sensor according to any one of the preceding claims, characterized in that the fluorine polymer film (2) is porous polytetrafluoroethylene.

Inventors: FUJITA YUKO JAPAN STORAGE BATT, IWANAMI YOSHIHARU JAPAN STORAG, KUDO HISASHI JAPAN STORAGE BAT, NOMI HARUO OKAYAMA PLANT JAPAN, SHIBATA YOSHIHIKO OKAYAMA PL J, UENO RYOZO, FUJITA, YUKO, JAPAN STORAGE BATTERY CO., LTD., IWANAMI, YOSHIHARU, JAPAN STORAGE BATTERY CO., LTD, KUDO, HISASHI, JAPAN STORAGE BATTERY CO., LTD., NOMI, HARUO, OKAYAMA PLANT, JAPAN GORE-TEX INC., SHIBATA, YOSHIHIKO, OKAYAMA PL.,JAPAN GORE-TEX INC, UENO, RYOZO
Applicants: JAPAN GORE TEX INC, JAPAN STORAGE BATTERY CO LTD, JAPAN GORE-TEX, INC., JAPAN STORAGE BATTERY COMPANY LIMITED
Application year: 1989
Main classification: G01N7
IPC codes: G01N27, G01N7