JPH0543408Y2 - - Google Patents

Description

[Detailed Description of the Invention] Industrial Application Field The present invention relates to a sensor for measuring the concentration of an acid aqueous solution, and more specifically to a sensor that determines the concentration of an acid aqueous solution from its water vapor pressure.

Prior Art There are many requests to measure the concentration of aqueous solutions of acids such as sulfuric acid, hydrochloric acid, and acetic acid. In particular, since the concentration of sulfuric acid as an electrolyte in a lead battery is closely related to the charging/discharging state of the lead battery, it is eagerly awaited for an apparatus that can easily measure the concentration of sulfuric acid in a lead battery. Conventionally, the sulfuric acid concentration in a lead-acid battery is usually measured using a float type hydrometer or a photorefraction type hydrometer. Furthermore, as a special method, a method has been proposed (German Patent No. 2254207 (1973)) in which the sulfuric acid concentration is determined by measuring the humidity in the space above the sulfuric acid electrolyte of a lead battery. This method is based on the principle that since the water vapor pressure in the upper space, which is in vapor-liquid equilibrium with the sulfuric acid aqueous solution, changes depending on the concentration of the sulfuric acid aqueous solution, the concentration of the sulfuric acid aqueous solution can be determined by measuring the water vapor pressure in the space. Based on. On the other hand, by further developing this proposal, a humidity sensor coated with a porous polypropylene membrane was directly immersed in an aqueous sulfuric acid solution, and the humidity sensor measured the partial pressure of water vapor diffusing through the pores of the porous polypropylene. A method has been proposed to do so. (JL Weininger, JL
Briant.J.Electrochem.Soc.129, 2409 (1982)) Problems that the invention aims to solve The above-mentioned float-type hydrometers and photorefraction-type hydrometers are too large, so they cannot be inserted into a lead-acid battery. Another problem was that the concentration of the sulfuric acid aqueous solution could not be measured continuously. In contrast, when the humidity above the electrolyte of a lead-acid battery is measured using a humidity sensor, when the concentration of sulfuric acid changes, the water vapor pressure of the sulfuric acid aqueous solution is adjusted until the water vapor pressure in the space reaches equilibrium. There was a problem that it took a long time and the response speed was slow. In addition, in the case of the method of Weininger et al. mentioned above, porous polypropylene has insufficient water repellency, so not only water vapor but also the sulfuric acid aqueous solution itself passes through the membrane and reaches the humidity sensing part of the humidity sensor. Not only was it impossible to measure accurately, but it was also chemically attacked and had an extremely short lifespan.

Means for Solving the Problems The present invention uses a membrane that covers the humidity sensor as a membrane that has a water repellent layer made of porous fluororesin and a selectively permeable layer for water vapor. The above-mentioned problems are solved by disposing a solid acid vapor absorber mainly composed of carbonates or hydroxides of earth metals.

Operation The principle of operation of the present invention is basically the same as that proposed by Weininger et al. mentioned above. That is, the humidity sensor is covered with a porous water-repellent plastic membrane,
When immersed in an acid aqueous solution, the water in the acid aqueous solution becomes vapor and diffuses through the pores of the plastic membrane, and eventually the gas and liquid are balanced across the membrane. At this time, the water vapor pressure in the gas phase within the membrane is detected as humidity by the humidity sensor.

In general, the higher the acid concentration, the lower the water vapor pressure of an aqueous acid solution. The concentration of the acid aqueous solution can be determined from the relative humidity value obtained when the above-mentioned humidity sensor coated with a porous plastic film is immersed in the aqueous solution. Therefore, this humidity sensor covered with a porous plastic film functions as a concentration sensor for an acid aqueous solution. Also,
This porous plastic membrane has more than just a protective cover for the humidity sensor.

The coating membrane of the humidity sensor according to the present invention has a porous fluororesin membrane and a water vapor selectively permeable membrane. The porous fluororesin membrane has the function of preventing the acid aqueous solution itself from entering the interior and coming into direct contact with the humidity sensor, and allowing only water vapor to pass through. The fluororesin film has much higher water repellency than the polypropylene proposed by Wininger and others mentioned above, and exhibits excellent functionality for the purpose. Incidentally, although this porous fluororesin membrane prevents the intrusion of the acid aqueous solution, it does not prevent the permeation of the vapor of the acid itself. On the other hand, a water vapor selectively permeable membrane prevents the permeation of the vapor of the acid itself,
It has the ability to selectively transmit only water vapor. Perfluorocarbon sulfonic acid is suitable as the material for this water vapor selectively permeable layer, but it is not necessarily limited thereto. Further, as a method for forming this layer, perfluorocarbon sulfonic acid may be applied to one side of the porous fluororesin membrane.

If the membrane has such a double layer structure, the permeation of acid vapor will be extremely reduced. However, the concentration sensor coated with the membrane having the double layer structure also
If the sensor is used in an acid aqueous solution for a long period of time, a small amount of acid vapor will permeate the membrane and adversely affect the sensor.

Therefore, the inventors of the present invention proposed a membrane with a triple layer structure in which an acid vapor absorbing layer made of calcium carbonate was provided on a water vapor selectively permeable layer in order to capture the trace amount of acid vapor that permeated through the membrane with a double layer structure. proposed. This acid vapor is captured, for example, by the following chemical reaction.

H ₂ SO ₄ +CaCO ₃ → CaSO ₄ +CO ₂ +H ₂ O By using a membrane with a triple layer structure as described above, the permeation of acid vapor could be almost completely prevented. When used in the water vapor selective permeation layer, the amount of acid vapor that permeates through the water vapor selective permeation layer increases, and the calcium carbonate layer that traps the acid vapor becomes thicker, resulting in a problem that the response speed of the concentration sensor becomes slower. Therefore, in the present invention, as a method to extend the life of the concentration sensor without impairing its responsiveness, a solid acid vapor absorber, such as a block of calcium carbonate, is placed around the humidity sensing part of the humidity sensor. We propose a concentration sensor coated with a membrane having a resin layer and a water vapor selective permeation layer.
A small amount of acid vapor that has passed through the porous fluororesin layer and the water vapor selectively permeable layer reacts preferentially with calcium carbonate around the moisture sensitive area, so it does not attack the moisture sensitive area.

This concentration sensor has approximately the same lifespan and approximately twice the response speed as the concentration sensor having a triple layer structure. Note that the acid vapor absorber in the present invention is not limited to calcium carbonate, but any material may be used as long as it is mainly composed of carbonates or hydroxides of alkali or alkaline earth metals.

Example An embodiment of the present invention will be described in detail below.

First, a water vapor selectively permeable layer is formed by coating one side of a porous polytetrafluoroethylene membrane with a thickness of 0.1 mm and a porosity of 75%, which will serve as a water repellent layer, with an alcoholic solution of perfluorocarbon sulfonic acid resin. In this way, a membrane with a double layer structure is formed.

Next, fine powder of calcium carbonate (particle size approximately 20μ)
Add 0.1 ml of an aqueous suspension of polytetrafluoroethylene (60% solids) to 10 g, mix, then dry and crush. 0.5g of crushed powder is pressed at 500Kg/
It is pressed at cm ² and formed into a U-shaped block with a size of 15 x 20 mm and a thickness of 3 mm, and this is used as an acid vapor absorber.

The above U-shaped block of calcium carbonate is placed around the humidity sensing part of a humidity sensor in which a moisture-sensitive film made of a polymer electrolyte is formed on an alumina substrate, and this is combined with a porous fluororesin layer and a water vapor selectively permeable layer. A sensor for measuring the concentration of an aqueous solution is fabricated by coating it with a membrane consisting of a double layer structure.

The structure of the acid aqueous solution concentration sensor thus obtained is shown in FIG. 1, and its cross-sectional structure is shown in FIG.
In FIGS. 1 and 2, both sides and the bottom of a humidity sensor 1 are surrounded by an acid vapor absorber 2 mainly made of calcium carbonate molded as described above, and a water repellent layer 3 and a water vapor selectively permeable layer 4 are formed. It is coated and packaged with a double layer membrane composed of.

The detection method of the humidity sensor used for the concentration sensor is
This type of polymer resistance type has a moisture-sensitive film made of polymer electrolyte formed on an alumina substrate with interdigitated electrodes.It detects the impedance when 1KHz alternating current is applied, and detects the change in impedance by changing the voltage. It is designed to be detected as a change.

FIG. 5 is a diagram showing the relationship between the sulfuric acid aqueous solution and the voltage detected by the concentration sensor. As the concentration of the sulfuric acid aqueous solution increases, the voltage of the concentration sensor decreases,
The concentration of the sulfuric acid aqueous solution can be accurately detected by the measured voltage value.

When the response speed of the sensor is determined from the change in the voltage of the concentration sensor when the concentration of the sulfuric acid aqueous solution is changed, as shown in Figure 3, the sensor A of the present invention has a response rate of 90%.
The response time was 3 minutes, which was much faster than the 90% response time of concentration sensor B, which was coated with a thick three-layer calcium carbonate layer of 5 minutes. This is because the sensor A of the present invention does not have a layer of calcium carbonate on the surface of the detection part that prevents vapor transmission.

Furthermore, we investigated the characteristics of the aqueous solution concentration sensor when continuously measuring the concentration of sulfuric acid aqueous solution. That is,
The results of measurements over 1000 hours in a 40% sulfuric acid aqueous solution (temperature 30°C) regarding the sensor of this invention are the 4th.
As shown in the figure, the sensor of the present invention maintained a stable voltage of 57 mV, and had a lifespan comparable to that of a concentration sensor using a triple-layered membrane.

Effects of the invention As described above, the present invention provides a sensor for measuring the concentration of an acid aqueous solution that can be measured continuously, has good responsiveness, and has a long life, and its industrial value is extremely large. .

[Brief explanation of the drawing]

Fig. 1 is a front view showing the structure of a sensor for measuring the concentration of a sulfuric acid aqueous solution according to the present invention, Fig. 2 is a sectional view taken along the line A-A in Fig. 1, and Fig. 3 is a characteristic showing the response speed of the concentration sensor. 4 are diagrams showing changes in the characteristics of the concentration sensor when the concentration in an aqueous sulfuric acid solution is continuously measured. FIG. 5 is a diagram showing the relationship between the concentration of the sulfuric acid aqueous solution and the voltage of the concentration sensor. 1...Humidity sensor, 2...Acid vapor absorber, 3...
...Water repellent layer, 4...Water vapor selectively permeable layer.

Claims (1)

[Scope of utility model registration request] A solid acid vapor absorber mainly composed of carbonates or hydroxides of alkali or alkaline earth metals is placed around the humidity sensing part of the humidity sensor, and the acid aqueous solution itself enters the interior and comes into direct contact with the humidity sensor. The membrane is coated with a porous fluororesin layer that has the function of preventing the acid from passing through and allowing only water vapor to pass through, and a water vapor selective permeation layer that has the function of preventing the vapor of the acid itself from passing through and selectively allowing only water vapor to pass through. A sensor that measures the concentration of an acid aqueous solution.