JPH05232103A - Method for measuring partial pressure of carbon dioxde dissoved in water quickly and accurately - Google Patents

Abstract

PURPOSE:To make it possible to measure the partial pressure of carbon dioxide in water with a small amount of sample water by replacing the carbon dioxide dissolved into the water with the carbon dioxide in gas, and obtaining the concentration of the carbon dioxide in introduced gas, wherein the change in concentration of the carbon dioxide in the gas that is exhausted through an exhaust port is not present. CONSTITUTION:Air in a container 5 is gradually displaced with from a gas cylinder 1. The air, which has passed through the container 5, passes through a dryer 6 from an exhaust port 5c and passes through a measuring cell 7a of a carbon-dioxide-concentration measuring device 7. Then, the air is discharged into atmosphere. The concentration, which is measured with the measuring device 7, is recorded in a recorder 8. The air from the air cylinder 1 is made to flow until the concentration, of the carbon dioxide in the discharged gas and that in the air in the cylinder 1 become equal. When a three-way cock 4 is set at a liquid side 4b, the air becomes bubbles so as to pass in the water. The bubbles disappear at the surface of liquid. The air passes through an air flowout port 5c and the dryer 6. The air is discharged into atmosphere after passing the measuring cell 7a. The sample water is discharged at every one time, and the sample water is replaced. The cylinder 1 is used, and the operation is repeated.

Description

Detailed Description of the Invention

[0001]

It is an object of the present invention to measure the partial pressure of carbon dioxide dissolved in water with a small amount of water in a rapid and error-free manner.

[0002]

FIELD OF THE INVENTION The present invention relates to a method for measuring the partial pressure of carbon dioxide dissolved in water.

[0003]

2. Description of the Related Art As a conventional method for measuring the partial pressure of carbon dioxide dissolved in water, there is known a method having a structure as shown in FIG. 4 in the "Ocean Observation Guideline" issued by the Meteorological Agency.
In this example, the sample is pressurized and supplied by the liquid pump 3 and sprayed into the equilibrium from the spray 2 on the upper part of the equilibrium 1. The spray is constantly being supplied with sample. In the equilibrium, gas exchange takes place between carbon dioxide dissolved in water and carbon dioxide in the air in the equilibrium. The reason for using the spray is to increase the contact area of gas-liquid, promote gas exchange, and promote diffusion of carbon dioxide in the gas-liquid. The atomized water passes through the inside of the balancer, then collects at the bottom and is drained from the drain port. On the other hand, the gas system has a closed circuit, and is designed so that outside air does not enter. In this circuit, a balancer 1, a measuring cell 6a of a non-dispersive infrared absorption type carbon dioxide concentration measuring device 6, a dryer 5, and a gas pump 4 are connected. A volume of air in the gas circuit is circulated by the gas pump in the opposite direction to the water. When the gas is circulated while spraying water into the balancer, the concentration of carbon dioxide in the gas circuit, which is continuously measured by the carbon dioxide concentration measuring device 6, changes. This indicates whether carbon dioxide in the air and carbon dioxide in the water are gas-exchanged to dissolve carbon dioxide in the air into the water or carbon dioxide in the water is released into the air. When the partial pressure of carbon dioxide in the air in the equilibrium is higher than the partial pressure of carbon dioxide in water, the carbon dioxide concentration in air decreases, and vice versa. The partial pressure of carbon dioxide contained in water and in the air is a factor that determines whether carbon dioxide dissolves in or releases water and also its rate and amount.

The knowledge obtained so far regarding the concept of carbon dioxide partial pressure in water is as follows. When carbon dioxide is dissolved in water, in addition to carbon dioxide molecules physically dissolved in water, carbonate ions and hydrogen carbonate ions are generated. These ions quickly reach a chemically stable equilibrium with the carbon dioxide molecule. When the hydrogen ion concentration or temperature changes, the concentration of carbon dioxide molecules and the concentration of these ions change. When an ion becomes a carbon dioxide molecule, it first exchanges gas, and does not directly exchange gas with carbon dioxide in the gas phase. Therefore, carbon dioxide partial pressure is not related to ion concentration. Since it is not related to the ion concentration, the pH is low, and therefore, rainwater having a low ion concentration and seawater having a high pH can be measured by the same method. At the gas-liquid interface, the carbon dioxide molecules dissolved in the liquid phase and the carbon dioxide molecules in the gas phase are
It constantly moves from the liquid phase to the gas phase and from the gas phase to the liquid phase. As shown in Expression 1, the amount of movement is determined by the transport speed and the partial pressure difference between the liquid phase and the gas phase. When the vapor phase and the liquid phase are in equilibrium, Formula 2 holds between water, that is, between the carbon dioxide partial pressure in the liquid phase and the carbon dioxide partial pressure in the gas phase. here,
Pw is the physicochemical potential of carbon dioxide in the liquid phase, which is generally called the carbon dioxide partial pressure in the liquid phase. At the interface where gas and liquid are in contact, Pw is the gas from the liquid phase. It is the potential to move to the phase. Pa
Is the partial pressure of carbon dioxide in the vapor phase, and is proportional to the gas concentration in the vapor phase when the pressure in the vapor phase is constant. From the viewpoint of molecular movement, when Equation 2 holds, it can be said that the mutual movement amount of carbon dioxide to both phases at the interface is equal. At equilibrium, the carbon dioxide concentration in the liquid phase is determined by Henry's law from the carbon dioxide partial pressure in the gas phase. When the temperature is constant, the partial pressure in the liquid phase is proportional to the concentration, but even if the carbon dioxide concentration in the liquid phase is constant, the partial pressure changes when the temperature changes. As described above, the carbon dioxide partial pressure is a function of temperature and concentration, but the gas phase and liquid phase temperatures are the same and constant, and the carbon dioxide concentration in the liquid phase changes. If not, the carbon dioxide partial pressure in the liquid phase can be measured by measuring the carbon dioxide partial pressure in the gas phase, which is in equilibrium with the carbon dioxide in the liquid phase.

[0005]

[Equation 1]

[0006]

[Equation 2]

When the air is continuously circulated and the water is sprayed while keeping the temperature constant, the carbon dioxide concentration in the air in the circuit changes and becomes gradually constant, from 10 minutes to 2 minutes.
The carbon dioxide concentration in the air becomes constant in 5 minutes. When the concentration of carbon dioxide in the air became constant, the concentration was corrected by the pressure at that time, the partial pressure was calculated, and the partial pressure of carbon dioxide in water was used.

[0008]

By the way, it is said that the partial pressure of carbon dioxide in water and the partial pressure of carbon dioxide in air are in equilibrium. Even after 25 minutes, the concentration of carbon dioxide in air is little by little. , It turned out to be changing. Therefore, there is a problem that it is extremely difficult to determine whether or not the carbon dioxide in the air is in equilibrium. Further, since water is continuously sprayed, it is necessary to keep the water flowing at a flow rate of 100 cc / min or more for at least 10 to 25 minutes, which causes a problem that a large amount of water is required.

It has been found that there are three major reasons why it is difficult to determine whether or not equilibrium is achieved. First, as a structural problem, in the structure of FIG. 4, the air circuit cannot be in equilibrium unless it is disconnected from the outside. However, if water is not continuously drained from the drain port and water collects at the bottom of the balancer,
After the air pressure gradually increases and a certain amount of water accumulates,
The water went out all at once. This phenomenon is due to the air measurement system,
It happened because the drainage outlet was getting smaller to get closer to the closed circuit. As a result, water intermittently came out of the equilibrium and at the same time air came out as bubbles. Immediately after that, the inside of the balancer became a negative pressure, and the outside air was mixed in from the drain port. This shows that it is very difficult to make the air circuit a closed circuit. Secondly, there is a problem that the measurement time is long. Apparently, in order to know the time required to reach equilibrium, the time when almost no apparent change in carbon dioxide concentration was observed was measured, and it was about 1 hour. However, it is extremely difficult to keep the carbon dioxide concentration in the sample constant for a long time, so the carbon dioxide partial pressure in the sample changed and became equal to the carbon dioxide partial pressure in air, so there was no change. There was a doubt that it might have been. Even if the temperature does not change, the carbon dioxide partial pressure of the sample changes because the carbon dioxide concentration changes due to other factors such as photosynthesis and respiration of plankton in the sample, decomposition of organic carbon compounds, and changes in hydrogen ion concentration. , It turned out to change. For this reason, measuring the carbon dioxide partial pressure of seawater, river water, and rainwater must be rapid. Using seawater samples, we measured changes over time and found that the size was 2 to 5 μatm in winter and 5 in summer in 1 hour.
To 30 μatm. Therefore, it was found that the conventional method cannot continue the measurement for a long time. Thirdly, in the carbon dioxide concentration measuring instrument, the indicated value of the carbon dioxide concentration measuring instrument greatly changes depending on the pressure variation in the detector, and therefore the air circuit needs to keep the pressure constant. However, in the configuration of FIG. 4, it was found that the pressure cannot be kept constant because the gas is circulated by the gas pump 4 in the air circuit.

The large amount of sample required was problematic for the following reasons. That is, a sample collected in the deep sea with a depth of several thousand meters is valuable because a great deal of effort is required for sampling the sample. Also, from the viewpoint of transportation costs, samples that are processed from foreign countries and transported from abroad, or samples that are collected domestically or in mountainous areas are valuable. Therefore, it was desired to have a measuring method that can measure with a smaller amount of sample.

The object of the present invention is to solve the above-mentioned problems and to use a small amount of sample and a sparingly soluble gas containing carbon dioxide gas having different concentrations to more rapidly and accurately analyze the carbon dioxide content in water. It seeks to provide a method by which pressure can be measured.

[0012]

In order to achieve the above-mentioned object, the present invention provides a water inlet in a closed container provided with a water inlet and a water outlet and a gas inlet and a gas outlet. From
Put the sample water to measure the partial pressure of the dissolved carbon dioxide, do not come into contact with water, from the gas inlet provided at a high position in the container, containing carbon dioxide of a certain concentration, sparingly soluble in water After introducing the gas, measure the carbon dioxide concentration in the gas discharged from the gas outlet of the container with a carbon dioxide concentration meter, and confirm that the carbon dioxide concentration in the gas discharged from the gas outlet is It was installed at a low position for the purpose of releasing gas into the water after reaching the same concentration as the carbon dioxide concentration of
The gas is discharged into water from another gas inlet, carbon dioxide dissolved in the water is exchanged with carbon dioxide in the gas, and the maximum change in the carbon dioxide concentration in the gas discharged from the outlet, or It seeks to know the partial pressure of carbon dioxide dissolved in water by obtaining the carbon dioxide concentration in the introduced gas that does not change from the minimum. The present invention, the partial pressure of carbon dioxide in the air, and the partial pressure of carbon dioxide in water,
The main feature is that the partial pressure of carbon dioxide in water can be measured quickly with a small amount of sample.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail with reference to the embodiments shown in the drawings. FIG. 1 is a block diagram of a measuring device according to the present invention, and FIG. 2 is an example of a change in carbon dioxide concentration in gas when a sparingly soluble gas is caused to flow into water containing carbon dioxide at a constant concentration. FIG. 4 is a diagram showing a method for obtaining the measurement of the partial pressure of carbon dioxide in water from the maximum and minimum changes in carbon dioxide concentration. In these figures, in this example, the end of the gas flow path is open to the atmosphere, and therefore, the pressure change in the detector of the non-dispersive infrared absorption type carbon dioxide concentration measuring device can be prevented. , When the carbon dioxide is released from the water or absorbed in the water with a gas of a certain concentration, if the carbon dioxide concentration in the gas is different, the magnitude of the change is different. It is characterized by the fact that the partial pressure of carbon dioxide in water can be known by determining the gas concentration.

In FIG. 1, a sample bottle 9 is placed in a thermostatic chamber 11,
Container 5 is included. This is because the carbon dioxide partial pressure in water largely changes depending on the water temperature, so that the change due to temperature is prevented. In the dryer, the moisture prevents the non-dispersion infrared absorption type carbon dioxide concentration measuring device from interfering with the error, so that the mass flowmeter 3 apparently changes the carbon dioxide concentration depending on the change of the flow rate. The purpose is to monitor the gas flow rate in order to prevent it from happening and to keep the gas flow rate constant so that the magnitude of the change in the carbon dioxide concentration does not change depending on the flow rate. The sample water in the sample bottle 9 is transferred by the liquid feed pump 10 and put into the container 5 through the three-way cock 12. The amount of sample water is adjusted at the height of the drain port so as to be a constant amount between 10 cc and 15 cc. As a gas for releasing carbon dioxide in water or absorbing carbon dioxide in water, an air cylinder containing five carbon dioxide of known concentrations having different concentrations is used. First, one of them is connected to the cock 2 to open the cock 2. The air containing carbon dioxide passes through the mass flow meter 3 and reaches the three-way cock 4.
The flow rate measured by the mass flow meter is continuously recorded by the recorder 8. Mass flow meter reading is from 100 cc / min to 2
Keep constant between 00 cc / min. The three-way cock 4 is set on the air side 4a, and from the air introduction port 5a of the container 5,
Air containing carbon dioxide is placed in the container 5. The air in the container 5 is gradually replaced by the air from the cylinder. The air that has passed through the container passes through the outlet 5c, the dryer 6 and the measurement cell 7a of the carbon dioxide concentration measuring device 7, and then is discharged into the atmosphere. The carbon dioxide concentration in the container is continuously measured by the carbon dioxide concentration measuring device 7, and the concentration is recorded by the recorder 8. In order to improve the measurement accuracy, the carbon dioxide concentration measuring device 7 can detect the difference between the carbon dioxide of the gas flowing in the measuring cell by flowing the standard carbon dioxide of known concentration into the comparison cell 7b. There is. By doing so, the change in carbon dioxide concentration can be reduced by 0.1 pp.
It is possible to detect up to m. The air in the air cylinder is allowed to flow until the discharged gas and the carbon dioxide concentration in the air inside the cylinder become equal. Next, the three-way cock 4 is set on the liquid side 4b. Then, the air becomes bubbles in the water and passes through the water, and thereafter the bubbles disappear on the liquid surface, and the air passes from the air outlet 5c through the dryer 6 to the carbon dioxide concentration measuring device 7
After passing through the measuring cell 7a, the gas is released into the atmosphere. The operation described above is performed by discharging the sample water each time, passing a new sample water to wash the inside of the container, replacing the sample water, and containing another four carbon dioxide of known concentrations. Repeat using an air cylinder.

FIG. 2 is a change with time of the carbon dioxide concentration recorded by the recorder 8 as the gas passes through the measuring cell of the carbon dioxide concentration measuring device. When the three-way cock 4 in FIG. 1 is set to the air side 4a and the air in the container, the dryer, and the measurement cell is replaced with air from the cylinder, the air is replaced after about 2 minutes, and the same dioxide as that in the cylinder is replaced. Became the carbon concentration. Next, the three-way cock 4 was switched to the liquid side 4b.
Then, the carbon dioxide concentration reached its maximum or minimum after about 10 seconds and then slowly approached the carbon dioxide concentration in the air cylinder. When the pH of the rainwater sample was low, the concentration was early, and when the pH was high, the concentration of carbon dioxide in the air cylinder approached very slowly. With respect to the same sample water, the time change was shown by changing the air cylinder, but when the carbon dioxide concentration in the cylinder was 272 ppm, it showed a time change of 1, and when the carbon dioxide concentration in the cylinder was 294 ppm and 338 ppm, it showed a few time changes. Showed a change. When the carbon dioxide partial pressure in water is higher than the carbon dioxide partial pressure in air,
When carbon dioxide comes out from water and the carbon dioxide concentration in the air becomes high, on the contrary, when the carbon dioxide partial pressure in the water is low, the carbon dioxide is dissolved in the water and the carbon dioxide concentration becomes low. When the partial pressure of carbon dioxide in water is equal to the partial pressure of carbon dioxide in the air coming out of the cylinder, it does not change.

FIG. 3 shows a method for obtaining the carbon dioxide partial pressure in water from the difference in the magnitude of the change when the change after passing through the water becomes maximum for the case of containing five concentrations of carbon dioxide. It is the figure shown. When the difference between the partial pressure of carbon dioxide in the air coming out of the cylinder and the partial pressure of carbon dioxide in water is large, the change is large, and the smaller the difference, the smaller the change. , When there is no difference in the partial pressure of carbon dioxide in the air in the cylinder, the amount of carbon dioxide released from water and the amount dissolved in water become equal and do not change.

In FIG. 3, the vertical axis represents the maximum amount of change and the horizontal axis represents the carbon dioxide concentration in the air cylinder used. In order to obtain the data of 5 points in the figure, the amount of the sample water needs to be about 150 cc including the washing in the container. The measurement time is about 3 minutes to obtain one point of data and about 20 minutes including the time required for exchanging the sample. The width of carbon dioxide concentration in the used air cylinder is about 30p.
The linearity and reproducibility are good for each pm. Since the carbon dioxide partial pressure in water is equal to the carbon dioxide partial pressure in air that does not change, the concentration that does not change can be obtained by interpolation or extrapolation. This is the partial pressure of carbon dioxide in water. Regarding the error in the measured value, it is smaller when the concentration is obtained by interpolation than when it is extrapolated, and the measurement error is made smaller by reducing the width of the carbon dioxide concentration in the air cylinder. You can Further, since the linearity and the reproducibility are good, it is possible to obtain the carbon dioxide partial pressure in water at only one point, but naturally the measurement error becomes large.

The following comparative tests were conducted to verify this measuring method. As a result of measuring three times by the measuring method of the present invention, the carbon dioxide concentration was 393 ± 0.5 μatm, and the air having a carbon dioxide concentration of 396 ppm was continuously flowed for about 6 hours into the seawater. This is because the partial pressure of carbon dioxide in seawater is 3
The purpose is to obtain 96 μatm of seawater. The sample thus obtained was measured three times by the measuring method of the present invention,
It was 396 ± 0.5 µatm, and it was found that the measuring method of the present invention was accurate. Moreover, the obtained sample was measured using the conventional measuring method. In the conventional measurement method, the carbon dioxide partial pressure in the sample is predicted before the measurement is started, and air having a partial pressure close to the carbon dioxide partial pressure in the sample is used as the circulating air. Since the problems with the conventional measurement method have not been solved, the purpose here is to know the state of the concentration change of the circulating air, and in order to know the time to reach equilibrium, the partial pressure of carbon dioxide in the circulating air is , 0 ppm before starting spraying
And A few minutes after the measurement was started, a large change in the carbon dioxide concentration in the air was not seen, and after 10 minutes, it became about 370 ppm. After 1 hour, the carbon dioxide concentration increased little by little. This change over time is shown in FIG.
As shown in, when the gas containing a constant concentration of carbon dioxide is flowed into the seawater, the carbon dioxide partial pressure in the circulating air is slowly changed, similarly to the change with time of the carbon dioxide partial pressure in the seawater. It was shown to approach the partial pressure of carbon dioxide in seawater.

[0019]

As described above, in the present invention, it is not necessary to equilibrate the partial pressure of carbon dioxide in water and the partial pressure of carbon dioxide in air, so that the measurement can be performed more quickly and more accurately than the conventional measurement method. I can do it. Moreover, it has become possible to measure the carbon dioxide partial pressure in water with a small amount of sample water. Furthermore, since the gas circuit is open to the atmosphere, it is possible to avoid the disadvantage of the non-dispersive infrared absorption type carbon dioxide concentration measurement recorder in which the indicated value changes due to the pressure change in the detector. Now, the carbon dioxide partial pressure can be measured more accurately.

[0020]

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention.

[Fig. 2] When passing air containing carbon dioxide into water,
Diagram showing changes in the partial pressure of carbon dioxide in the air

FIG. 3 is a diagram showing a method for interpolating the carbon dioxide partial pressure in water from the rate of change in carbon dioxide concentration in air.

FIG. 4 is a block diagram showing a conventional example.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Eiji Yamashita 7 at 110 Urayasunishimachi, Okayama City, Okayama Prefecture

Claims (1)

[Claims] 1. A closed container which is provided with a water inlet and a water outlet and a gas inlet and a gas outlet and which can be maintained at atmospheric pressure, is provided with dissolved carbon dioxide gas from the water inlet. Insert the sample water whose partial pressure is to be measured, and introduce a sparingly soluble gas containing water with a certain concentration of carbon dioxide from the gas inlet provided at a high position in the container that does not come into contact with water. , The carbon dioxide concentration in the gas discharged from the gas outlet of the container is measured with a carbon dioxide concentration meter, and the carbon dioxide concentration in the gas discharged from the gas outlet is the carbon dioxide concentration in the introduced gas. For the purpose of releasing the gas into the water after reaching the same concentration as above, the gas was released into the water from another gas inlet provided at a low position, and the carbon dioxide dissolved in the water was converted into The carbon dioxide concentration in the gas discharged from the outlet after being exchanged with carbon A measuring method characterized by knowing the partial pressure of carbon dioxide dissolved in water by obtaining the carbon dioxide concentration in the introduced gas that does not change from the maximum or minimum change in temperature.