JPH0772728B2 - Device for measuring concentration of chemical species in liquid - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an apparatus for measuring the concentration of chemical species such as oxygen in a fluid such as water or air, and particularly to a dissolved oxygen measuring apparatus used in pollution control equipment. Here, the chemical species concentration is the concentration of the electrochemically active chemical species dissolved in the test fluid.

2. Description of the Related Art Membrane carbani cell type measuring instruments have been conventionally used to measure the concentration of chemical species such as dissolved oxygen concentration. With this method,
Dissolved chemical species in the test solution, for example oxygen, diffuse through the diaphragm into the electrolytic solution separated from the test solution by the diaphragm. A cathode current flows in proportion to the chemical species concentration in the electrolytic solution, and the chemical species concentration is measured by reading the cathode current.

The distribution of the chemical species concentration near the diaphragm in the galvanic cell system is shown in FIG. 2 for the case of the dissolved oxygen meter, but similar distributions also occur for other chemical species. Oxygen diffused into the electrolyte is immediately reduced, so the cathode surface 1 in the electrolyte is
The oxygen partial pressure or concentration at is almost zero. When the test solution is agitated, the oxygen concentration gradient 4 from the dissolved oxygen concentration level 3 of the test solution to the cathode surface 1 is a dash-dotted line in FIG. 2 with the interface 2 between the test solution and the diaphragm as a contact point. Occurs in the diaphragm like. Such an oxygen meter has the following problems.
At the same time as the consumption of the chemical species, the electrolyte is also changed in quality, so that it is necessary to replace the electrolyte, and the anode is consumed as the electrolyte is changed.

When the stirring is stopped and the test solution is stationary, a boundary layer is formed in the test solution as shown by the portion sandwiched between the interface 2 and the line 5 in FIG. In the boundary layer, the sample liquid 17 is stationary due to the fluid viscosity. On the other hand, a steady flow of species towards the cathode causes a drop in species concentration within this boundary layer. This is called a local consumable layer. As a result, the pressure of the diaphragm apparently increased, and when the mixture was stirred again, the local consumable layer disappeared and the diaphragm thickness returned to a normal value. Since the thickness of the local consumable layer changes depending on the strength of stirring, a measurement error occurs. This error is called a flow velocity error. Due to this local consumable layer, the concentration gradient decreases as shown by the broken line 6, and when it is remarkable, it causes an error of nearly 50%. For the same reason, the dirt on the surface of the diaphragm causes an error, and the temperature characteristics of the diaphragm physical properties directly affect the measurement of the dissolved concentration.
Since it is the deviation method, the response, that is, the response time is slow.

One solution to the above problem is the so-called zero method described in Japanese Patent Publication No. 56-51582 (hereinafter referred to as the Connery method by taking the inventor name of the invention of the same publication). Referring to the plan view of FIG. 3, the cross-sectional view of FIG. 4, the perspective view of FIG. 5, and the enlarged view of the essential part of FIG. 6 showing the Connery method, a plurality of comb-shaped anodes connected to the anode base 7 are shown. 7a and a plurality of comb-shaped cathodes 8a connected to the cathode base 8 are arranged on the substrate 6 in such a position that one is between the other in a side-by-side positional relationship with each other. Hereinafter, the electrodes shown in FIGS. 3 to 5 are referred to as “comb-shaped electrodes”. Substrate 9 and diaphragm 10
And the electrolyte solution 11 is filled. The anode base 7 and the cathode base 8 are connected to the power source through the lead wires 12 and 13 and the ammeter 14.
Connected to 15. The enlarged view of the main part of FIG. 6 shows the insulator 16 interposed between the anode 7a and the cathode 8a.

The cathode 8a consumes oxygen in the reaction of O ₂ + 4H ⁺ + 4e ⁻ → H ₂ O, and the anode 7a generates the same amount of oxygen in the reaction of 2H ₂ O → O ₂ + 4H ⁺ + 4e ⁻ . A current (hereinafter referred to as an output current) accompanying the reaction flows. As shown by ΔQo ₂ in Fig. 6, test solution 1
When oxygen flows from 7 to the electrolytic solution 11, the amount of oxygen in the electrolytic solution increases by that amount, and the output current increases until the increased total oxygen amount is consumed. As the output current increases, the amount of oxygen generated in the anode also increases.

If the average oxygen concentration in the electrolytic solution 11 exceeds the dissolved oxygen concentration of the test solution 17, oxygen flow from the electrolytic solution 11 to the test solution 17 occurs, and the average oxygen concentration in the electrolytic solution 11 decreases. , The output current also decreases. Therefore, when the average concentration of oxygen generated in the anode in the electrolytic solution and the dissolved oxygen concentration in the test solution become equal, the output current reaches equilibrium. At this time, the oxygen inflow ΔQo ₂ from the test solution 17 to the electrolytic solution 11 through the diaphragm 10 is apparently zero. Further, since the output current at the time of equilibrium is proportional to the dissolved oxygen concentration of the test liquid, it is used as it is for measuring the dissolved oxygen concentration.

However, the change in the output current when there is a change in the dissolved oxygen concentration of the test solution 17, because the oxygen concentration of the electrolytic solution takes time until it becomes equal to the dissolved oxygen concentration of the test solution after the change, It will be delayed by that amount. To meet this delay, the diaphragm 10
It is effective to make thin. On the other hand, the dissolved oxygen meter is used to measure the flowing test liquid in water purification equipment and others.
When the diaphragm 10 is thin, a part of oxygen generated at the anode leaks to the test liquid 17 in a large amount as described later, which causes a flow velocity error. Further, an extremely thin diaphragm is not suitable for industrial use because it may damage the membrane.

Therefore, the Connery method has an essential problem that the flow velocity error increases when the diaphragm is thinned to improve the response, and conversely when the diaphragm is thickened, the flow velocity error decreases but the response deteriorates. Actually, the description of the Japanese Patent Publication No. 56-51582, column 17, line 30 regarding the Connery method is 90%.
An example in which the response time is 5 seconds is shown, but the flow velocity error is large. It is said that the 90% response time is limited to about 40 if the flow velocity error is reduced to the extent that it does not hinder practical use due to an increase in the diaphragm thickness.

Therefore, the problem to be solved by the present invention is to suppress the flow rate error due to the presence or absence of stirring, etc. while maintaining a fast response in the concentration measuring device for dissolved chemical species such as oxygen. is there.

Means for Solving the Problems In order to solve the above problems, the present inventor has focused on the chemical species concentration distribution in the diaphragm 10 in the Connery method. It is known that it is advantageous to use a thin diaphragm for quick response. By suppressing the leakage of chemical species from the thin diaphragm to the test solution, it is considered that the influence of the test solution flow rate can be prevented.
We have developed a structure that limits as much as possible to the vicinity of the electrode related to the generation of chemical species, for example, near the anode that generates oxygen.

Therefore, the apparatus for measuring the concentration of chemical species in a fluid according to the present invention is characterized by the arrangement of electrodes, and as shown in FIG. 1, an anode is placed in the electrolyte solution 11 separated from the fluid 17 to be tested by the diaphragm 10. Anode 7a as a positive electrode and a cathode 8a as a negative electrode are arranged, and an amount of chemical species proportional to the current flowing through the electrodes 7a, 8a is generated in one of the electrodes and consumed in the other electrode. , The average concentration of the chemical species in the electrolyte solution 11 is made equal to the chemical species concentration of the fluid 17 to be tested, and the current is measured when the amount of the chemical species passing through the diaphragm 10 becomes substantially zero. A device for measuring the dissolved chemical species concentration of a fluid 17, in the electrolytic solution parallel to the diaphragm 10.
A plurality of the anodes 7a and a plurality of the cathodes 8a are arranged at lattice points of a virtual lattice 17 intersecting in two directions so that the anodes 7a and the cathodes 8a alternate in two directions of the lattice, and the adjacent anodes 7a The center electrode pitch L between the cathode 8a and the cathode 8a (Fig. 17) is measured from the electrode surface to the test liquid 17
The distance to the contact surface between the membrane and the diaphragm 10 is several times or less, and the error due to the flow velocity of the fluid to be detected is suppressed while maintaining the response speed above a certain level.

Preferably, the electrode pitch is approximately equal to or less than the distance from the electrode surface to the contact surface between the test liquid and the diaphragm.

Action The action of the present invention will be described below with reference to an embodiment in which the test fluid is the test liquid 17, but the test fluid may be a gas such as air. As shown in FIG. 5, since the anode 7a and the cathode 8a of the conventional Connery method are parallel and elongated, the distribution of the chemical species concentration ρ in the diaphragm 10 is as shown in FIG. 7 when the thickness of the electrolyte is ignored. In addition, the anode and the cathode are symmetrical with respect to the center, and the concentration distribution pattern on the anode becomes similar to that on the cathode.

Referring to FIG. 6, when the chemical species concentration ρ of the test liquid 17 is low and the chemical species concentration ρ is high on the surface portion of the diaphragm 10 close to the test liquid 17, the chemical concentration of the test liquid 17 is increased. Seeds easily leak. Since this leakage is proportional to the flow velocity of the test liquid 17, the measurement is easily affected by the flow velocity of the test liquid depending on the presence or absence of stirring. As a measure of the easiness of leaking of the chemical species, a constant concentration of the test liquid is used as a reference concentration, and a difference between the chemical species concentration ρ on the diaphragm surface and the reference concentration of the test liquid is obtained to obtain a distribution of the concentration difference. Fig. 15 is calculated under certain conditions, assuming 50% as the standard concentration of the test liquid in the concentration distribution study, in the case of the conventional Connery method structure with the chemical species concentration ρ distribution of Fig. 7. The distribution of the concentration difference Δρ between the chemical species concentration ρ and the reference concentration of the test liquid on the test liquid side surface of the separated diaphragm 17 is schematically shown. In the case of the conventional Connery method, this concentration difference distribution is
It becomes a "kamaboko" shape. In the case of the structure of the present invention shown in FIG. 1,
The agricultural depression distribution corresponding to the conventional concentration difference distribution of the "kamaboko" shape is the "fushicha bowl" shape as shown in FIG. It was found that the density difference of the "Fushichawan" type distribution according to the present invention is much smaller than the density difference of the "Kamaboko" type distribution by the Connery method. Comparing the integrated values of the concentration difference on the square diaphragm surface of L × L in FIGS. 15 and 17, under the condition that the diaphragm thickness t is equal to the electrode pitch L (t = L), The integrated value in the case of the “Fushichawan” shape according to the present invention is reduced to one fifth of that in the case of the conventional “kamaboko” shape. Therefore, if the responses are made equal, it is expected that the influence of the flow rate of the test liquid in the device of the present invention will be reduced to one fifth of that in the conventional Connery method. On the contrary, if the same flow velocity error is allowed, it is expected in the present invention that the response is about 5 times faster by reducing the diaphragm thickness.

Description of Principle Hereinafter, one example of the dissolved oxygen meter will be described in the following order, but the present invention is not limited to the dissolved oxygen meter.

(A) Analysis of oxygen concentration distribution by a conventional comb-shaped electrode (B) Analysis of oxygen concentration distribution by an island electrode according to the present invention (C) Distribution of oxygen concentration difference between a test liquid and a diaphragm (D) Effective coefficient η (E) Experimental Example (A) Analysis of Oxygen Concentration Distribution by Comb-shaped Electrodes The x-axis, y-axis and z-axis are selected as shown in FIG. Each electrode is infinitely long in the y-axis direction, and on the xy plane the electrodes 7a,
Assume that an infinitely thick diaphragm 10 is placed in contact with 8a,
Ignore the presence of electrolyte 11. If the oxygen concentration on the anode 7a is ρ = 1 and the oxygen concentration on the cathode 8a is ρ = 0,
The concentration curve is as shown in FIG. The oxygen concentration ρ = 0.5 is x
-A straight line perpendicular to the x-axis on the z-plane. Oxygen is transferred from the anode to the cathode so that it is orthogonal to these isoconcentration curves.
It flows like the dotted line in the figure.

Since the shapes of the concentration curves are similar on the anode and the cathode, only the anode is first considered as shown in FIG. For convenience of calculation, it is assumed that the curve in FIG. 8 represents the potential u (x, z), the anode potential is 1, and u = 0 at the point of x = 0 and x = L on the x-axis and is grounded. Suppose L is the pitch between the centers of the electrodes 7a and 8a. At this time, the following Laplace equation is established in the diaphragm 10.

u (0, z) = 0 (2) u (L, z) = 0 (3) The potential u (x, 0) on the x-axis at z = 0 is symmetrical as shown in FIG. Yes f (x) shall be given.

u (x, 0) = f (x) (4) The boundary condition f (x) does not have to be trapezoidal as in FIG. 9, but the potential is constant between P and Q corresponding to the electrode width W. Must have a value of 1.

The shape factor α of the electrode is defined as follows, and it is assumed that the anode and the cathode have the same value.

α = W / L (5) When f (x) of the equation (4) is given by the trapezoid of FIG. 9, the solution of the equation (1) is as follows.

b / L = (1-α) / 2 (8) However, n is an odd number. If the results of equations (6) to (8) are plotted, the potential diagram of Fig. 10 is obtained. The result of converting the potential u into the concentration ρ by the diagram is also shown in the same figure.

ρ (%) = (u + 1) 100/2 (9) In FIG. 5, when α = 0.5, the diaphragm thickness in the z-axis direction is defined as t, and the distribution of the oxygen concentration ρ inside the diaphragm 10 is shown in FIG. Please consider.

(I) t / L <1 For example, when t / L = 0.1, the oxygen concentration directly above the anode 7a reaches 90%. Therefore, when such a thin diaphragm is used, the reference oxygen concentration ρ It is expected that a large amount of oxygen leaks to the test liquid 17 of 50%. Since this leakage is proportional to the flow velocity of the test liquid 17, it is considered that the flow velocity of the test liquid 17 is largely influenced by the presence or absence of stirring.

(Ii) t / L = 1 For example, when the thickness is t / L = 1, the oxygen concentration ρ at the contact surface between the test liquid 17 having the reference oxygen concentration ρ = 50% and the diaphragm 10 is directly above the center of the anode 7a ( (X / L) = 0.5) 50 + 2.5%, just above the center of the cathode 8a ((x / L) = -0.5) 50-2.5%, oxygen to the test solution 17 with a standard oxygen concentration of 50% It is thought that there are few leaks. Therefore, the flow velocity error becomes small.

(Iii) t / L> 1 As the thickness t of the diaphragm 10 increases, the oxygen concentration in the diaphragm in the vicinity of the test solution 17 becomes closer to 50%, and the test solution with the reference oxygen concentration ρ = 50%.
Oxygen leakage to 17 is reduced, making it less susceptible to the flow rate of the test solution. However, increasing the thickness of the diaphragm 10 will lead to a decrease in response. (B) Analysis of oxygen concentration distribution using island electrodes Coordinate axes x, y and z are selected as shown in FIG. The electrodes are arranged at a pitch L, and the electrode shape is a square with one side W (W <L). As in the case of the comb-shaped electrodes, it is assumed that the potential on the side surface of the square pole on the square OABC having the electrode pitch L as one side is 0 and is grounded. The potential u (x, y, 0) on the bottom surface, that is, the xy plane is assumed to be a pyramidal trapezoid as shown in FIG. Under these assumptions, the three-dimensional Laplace equation corresponding to equation (1) is as follows.

However, u shows u (x, y, z). The boundary conditions are u (x, 0, z) = 0 (11) u (x, L, z) = 0 (12) u (0, y, z) = 0 (13) u (L, y, z) = 0 (14) The solution to this equation is given by

However, m and n are odd numbers.

However, m and n are odd numbers and m ≠ n.

In case of m = n, m → n, Plotting the result of Eq. (16) results in Fig. 13. Further, the result of converting the potential u into the concentration ρ by the equation (9) is also shown in the same figure.

The concentration potential line in FIG. 13 is lower than that in the case of the comb-shaped electrode in FIG. This point will be considered further.

(C) Distribution of oxygen concentration difference between the test liquid and the diaphragm The oxygen concentration of the test liquid 17 is set to 50%, and this is referred to as the reference oxygen concentration. Further, the difference between the membrane surface concentration and the reference oxygen concentration is called a concentration difference Δρ. FIG. 14 shows the concentration difference Δρ on the vertical axis and the width direction x / L on the horizontal axis when the thickness of the diaphragm 17 is t / L = 0.5 and t / L = 1 for the comb-shaped electrode. It is a density difference distribution map. It was confirmed by numerical calculation that this distribution can be handled practically as a sinusoidal waveform distribution.

FIG. 15 shows a three-dimensional view of the concentration difference Δρ in the case of the comb-shaped electrode. The concentration difference Δρ in this case is convex upward on the anode and convex downward on the cathode as shown in FIG.
On the anode with a positive concentration difference Δρ,
Oxygen flows out to 17, and oxygen flows in reverse on the cathode where the concentration difference Δρ is negative. This local inflow and outflow of oxygen is strongly controlled by physical factors such as the presence or absence of agitation outside the diaphragm and local fouling, which causes a flow velocity error. Therefore, it is better that oxygen does not flow in and out even locally. In other words, it is necessary that the average value of the absolute value of the density difference Δρ per unit area is small. The amount of outflow ΔQ1 of oxygen from the area of width L and depth L on the anode is the area L
It is considered to be proportional to the volume of "kamaboko" on xL.

If the volume, the outflow amount ΔQ1 and the proportional constant are K, the following equation holds.

Here, h ₁ is the maximum value of the concentration difference Δρ at the comb electrode, and is the value of Δρ at the position (x / L) = 0.5.

With respect to the island-shaped electrode according to the present invention, the concentration difference distribution similar to that shown in FIG. It was confirmed by numerical calculation that this distribution curve can be handled as a sinusoidal waveform distribution for practical purposes.

FIG. 17 shows a three-dimensional view of the concentration difference Δρ of the island electrode of the present invention. On the anode, an upward convex “bowl bowl” shape is formed, and on the cathode, a downward convex “bowl bowl” shape is formed. In FIG. 17, the outflow amount ΔQ2 of oxygen flowing out from the area L × L on the anode to the test liquid 17 is considered to be proportional to the volume of the “Fushichawan” on the anode, and therefore the following equation holds.

Here, h ₂ is the maximum value of the concentration difference Δρ at the island electrode, and is the value of Δρ at the position (x / L) = 0.5, (y / L) = 0.5.

(D) Effective coefficient η As a coefficient showing the effectiveness of the island-shaped electrode according to the present invention, the ratio of the oxygen outflow amounts ΔQ1 and ΔQ2 to different electrode shapes is defined as the effective coefficient η, and the value is calculated as follows. Like

η = ΔQ1 / ΔQ2 = (πh ₁ ) / (2h ₂ ) (20) When the shape factor α is given and the thickness t / L of the diaphragm 17 is determined, the maximum values h ₁ and h _{2 of the} concentration difference Δρ are obtained. Can be calculated. FIG. 18 shows the effective coefficient η for the diaphragm 17 and the thickness t / L.
And the changes in the maximum values h ₁ and h ₂ of the concentration difference.

As is apparent from FIG. 18, when the diaphragm thickness t / L is 1, that is, when the diaphragm thickness is equal to the electrode pitch, the effective coefficient η is 5. This means that with this diaphragm thickness, the island-shaped electrode of the present invention can reduce the influence of the specific test solution flow rate to one-fifth as compared with the conventional comb-shaped electrode. Further, if the influence of the specific test solution flow rate is made equal, the island-shaped electrode of the present invention is
It is expected that the response can be greatly improved.

(E) Experimental example A comb-shaped electrode dissolved oxygen meter with a conventional structure was used with an electrode pitch L = 0.25.
mm, electrode width W = 0.1 mm, shape factor α = W / L = 0.4, and polyethylene diaphragm thickness 30 μm. The dissolved oxygen concentration was measured with this comb-shaped electrode dissolved oxygen meter. The 90% response time is 20 seconds, and the indicated value is 13 depending on whether or not the test solution is agitated.
% Fluctuated.

A dissolved oxygen meter having a square island-shaped electrode of FIG. 1 according to the present invention was prepared by using an electrode pitch L = 0.25 mm, an electrode width W = 0.1 mm, a shape factor α = W / L = 0.4, and a polyethylene diaphragm thickness of 30 μm. Made by the original. The dissolved oxygen concentration was measured with this island electrode dissolved oxygen meter. The 90% response time was 18 seconds, and the variation in the indicated value with or without stirring the test solution was 8%. As a result, it was confirmed by the measured value that the suppression effect against the influence of the test liquid stirring, that is, (1 / η) was reduced to about two-thirds as the theoretical value. The island-shaped electrode was manufactured by a known IC manufacturing technique.

In the above description, the island electrode is assumed to be square, but the shape of the island electrode in the present invention is not limited to square. For example, round island electrodes can be used. Further, it is not necessary that all the island electrodes are independent.

Example 19 FIG. 19 shows that the anode 7a has a plurality of independent circular electrodes, the centers of the electrodes are aligned with the lattice points, and the cathode 8a has a plurality of independent annular electrodes, one in each of two directions of the lattice. An example in which the anode 7a is arranged so as to surround the anode 7a is shown below.

It was confirmed by an experiment that, as the diaphragm 10, a polyethylene diaphragm having the same thickness as in the experimental example, or a fluororesin-based material or a silicon rubber-based material can be used.

Although the electrodes of the anode 7a and the cathode 8a are all flat in the above embodiment, the electrodes do not necessarily have to be flat and may be cylindrical or other curved surfaces.

Effect of the Invention Since the chemical species concentration measuring device in the fluid according to the present invention has the configuration described above, the flow velocity error can be significantly increased as compared with the conventional device while maintaining the high response while keeping the thickness of the diaphragm as the conventional device. It can be reduced. For example, when the electrode pitch L is equal to t (L = t) for a diaphragm having a thickness of t (L = t), a remarkable effect of reducing the flow velocity error by up to one fifth of that of the conventional device is achieved. On the other hand, if the flow velocity error is kept constant, the electrode configuration of the present invention can significantly improve the response.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an explanatory view of an electrode arrangement according to the present invention, FIGS. 2 to 6 are explanatory views of a conventional dissolved oxygen meter, and FIGS.
11 and 12 are explanatory views of conditions used for explaining the present invention, FIGS. 10 and 13 are distribution diagrams of oxygen concentration in the diaphragm, and FIGS. 14 to 18 are oxygen concentration difference. Illustration of the 19th
The drawing is an explanatory view showing another embodiment of the electrode arrangement according to the present invention. 1 ... Cathode surface, 2 ... Interface between test liquid and diaphragm, 3 ...
... concentration level, 4 ... concentration slope, 5 ... line showing the end of the local consumable layer, 7a ... anode, 8a ... cathode, 9 ...
Substrate, 10 ... diaphragm, 11 ... electrolyte, 12, 23 ... lead wire, 14 ... ammeter, 15 ... power supply, 17 ... test solution.

Claims (1)

[Claims] 1. An anode, which is an anodic electrode, and a cathode, which is a cathodic electrode, are arranged in an electrolytic solution separated from a fluid to be measured by a diaphragm, and an amount of chemical species proportional to the current flowing through the electrode is contained in the electrode. Generated in one electrode and consumed in the other electrode, the average concentration of the chemical species in the electrolyte is equal to the chemical species concentration of the test fluid,
In a device for measuring the concentration of a dissolved chemical species in a test fluid by the electric current when the amount of the chemical species passing through the diaphragm becomes substantially zero, a virtual lattice of two-way crossings in the electrolyte solution parallel to the diaphragm. A plurality of the anodes and a plurality of the cathodes are placed at the grid points so that the anodes and the cathodes alternate in two directions of the grid, and the center electrode pitch between the adjacent anodes and the cathodes is set from the electrode surface. An apparatus for measuring the concentration of chemical species in a fluid, wherein the distance to the contact surface between the test liquid and the diaphragm is several times or less and the error due to the flow velocity of the test fluid is suppressed while maintaining a response time of a certain time or longer.