JPH07113745A - Concentration detector for inorganic chemical containing aqueous solution - Google Patents

Abstract

PURPOSE:To provide a concentration detector for inorganic chemical containing aqueous solution, which is formed in a simple structure and at a low cost to detect the concentration of inorganic chemical containing aqueous solution or the amount of water at a real time. CONSTITUTION:A detecting tube 20 is set up through both partition walls 3 of a pipe-shaped maintframe 2. A projector 5 and a receiver 6 are provided opposing each other across the detecting tube 20. The projector 5 is provided with a light source 5A to generate a specified wavelength band of near infrared light, 0.75mum-2.5mum, which is photoabsorbed by water, and the receiver 6 is formed with a light detector 6A to detect light from the projector 5 which is transmitted through inorganic chemical containing aqueous solution flowing in the detecting tube 20. The quantity of light detected by the light detector 6A is converted into an electric signal via a detection circuit 9 and a controller and displayed as the amount of water or the concentration of the aqueous solution by a display or output as a print by a printer.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention generally relates to an apparatus for detecting the concentration of various inorganic chemicals-containing aqueous solutions, and in particular, it is discharged through a drain line in a semiconductor manufacturing process or an LSI manufacturing process. The present invention relates to a concentration detecting device for an aqueous solution containing an inorganic chemical, which can be used to detect and monitor the concentrations of a cleaning liquid, an etching removing liquid, a resist stripping liquid, etc. in real time.

[0002]

2. Description of the Related Art For example, a semiconductor manufacturing process or an LSI
In the manufacturing process, cleaning of Si wafer, Al, S
For removing i or SiO ₂ by etching, and for removing resist, etc., sulfuric acid (H ₂ SO ₄ ), hydrochloric acid (HCl),
An aqueous solution containing inorganic chemicals such as nitric acid (HNO ₃ ), hydrofluoric acid (HF), buffered hydrofluoric acid (BHF), and ammonia (NH ₄ OH) is used. It is drained from the tank to the external drain tank via the drain line. The treatment tank in the clean room is washed with water after the drainage is delivered. Therefore, the concentration of the drainage liquid stored in the external drainage tank through the drainage line varies variously.

Of the drainage delivered into the external drainage tank,
High-concentration drainage is collected by a drainage treatment company and processed.
On the other hand, a low-concentration drainage such that 90 to 99% is water is subjected to neutralization treatment in the factory, sewage treatment, or discharged as clean water to a river or the like.

Therefore, it is extremely important to detect and monitor in real time the concentration of the effluent delivered to the external drain tank via the drain line.

[0005]

However, as far as the present inventor is aware, there is no apparatus suitable for this purpose, that is, capable of detecting the concentration of the aqueous solution containing an inorganic chemical in real time.

The object of the present invention is to detect the amount or concentration of water in an aqueous solution containing an inorganic chemical in real time,
Moreover, it is an object of the present invention to provide a low-cost concentration detecting device for an aqueous solution containing an inorganic chemical, which has a simple structure.

Another object of the present invention is to realize in real time the amount or concentration of water such as a cleaning liquid, an etching removal liquid and a resist stripping liquid which are discharged through a drain line in a semiconductor manufacturing process or an LSI manufacturing process. It is an object of the present invention to provide a concentration detecting device for an aqueous solution containing an inorganic chemical, which can be used for detecting and monitoring the temperature.

[0008]

The present inventor has conducted many research experiments to achieve the above object.

First, the present inventor dilutes concentrated hydrochloric acid and concentrated sulfuric acid with distilled water as an aqueous solution containing an inorganic chemical,
The acid concentration was adjusted to 0%, 2.5%, 5%, 7.5%, 10%, and the water concentration was 100%, 97.5%, 95%, 9%.
2.5% and 90% samples were obtained and the absorption spectra of these samples were measured with a conventional near infrared analyzer. The results are shown in FIGS. 4 and 5. From this absorption spectrum, the wavelength of 1.
It was found that remarkable light absorption by water was observed in the vicinity of 48 μm and 1.9 μm, and that the light absorption changed measurablely depending on the water concentration even though the water concentration was extremely high.

Based on this result, the present inventor next
The sample is made to flow in a transparent hollow tube cell made of quartz glass, and a light projecting section and a light receiving section are arranged so as to face each other in a direction orthogonal to the axis of the hollow tube cell. Near-infrared light of 1.48 μm was projected onto the hollow tube cell, and the amount of light passing through the sample flowing in the hollow tube cell at the light receiving portion was measured. The result is shown in FIG.

From FIG. 6, there is a correlation coefficient of 0.99 between the amount of light detected by the light receiving section and the water concentration (or acid concentration).
It was found that there is a good correlation such as 47.

The present invention was made based on such novel findings of the present inventor.

In summary, according to the present invention, a detection tube in which an aqueous solution containing an inorganic chemical can flow in an inner flow passage, and near-infrared light in a specific wavelength band, which is absorbed by water, with respect to the axial direction of the detection tube. A light projecting unit for projecting light in a perpendicular direction, a light receiving unit for sensing light transmitted from the aqueous solution flowing from the light projecting unit and flowing in the detection tube, and an amount of light sensed by the light receiving unit are electrically converted. A concentration detecting device for an aqueous solution containing an inorganic chemical, comprising: a detection circuit unit for converting into a signal; and a control unit for calculating an electric signal from the detection circuit unit to obtain the amount or concentration of water in the aqueous solution. Is.

Preferably, the detection tube is a circular tube made of fluorine resin. Particularly preferably, the fluororesin is a tetrafluoroethylene-hexafluoropropylene copolymer resin, where D ₁ / D _{0 where} D _{1 is} the inner diameter and D ₀ is the outer diameter of the detection tube.
D ₀ ≧ 0.5.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The concentration detecting apparatus for an aqueous solution containing an inorganic chemical according to the present invention will be described in more detail below with reference to the drawings.

1 to 3 show an embodiment of the concentration detecting apparatus for an aqueous solution containing an inorganic chemical according to the present invention. According to the present embodiment, the concentration detecting device of the present invention will be described as being installed in a drain line or a bypass line of a semiconductor manufacturing process. Therefore, an aqueous solution containing an inorganic chemical is, for example, sulfuric acid (H ₂ SO ₄ ), hydrochloric acid (HCl), nitric acid (HNO ₃ ).
Hydrofluoric acid (HF), buffered hydrofluoric acid (BH
F), ammonia (NH ₄ OH), etc.

The concentration detecting device of the present invention is installed directly in the drainage line or integrally with the drainage line by providing a bypass line. Therefore, as shown in FIGS. 1 and 2, the concentration detecting device 1 of the present invention has a pipe-shaped main body portion 2 for connecting to a drain line or the like. Since the above-described aqueous solution containing an inorganic chemical having a strong corrosiveness is flowed through the drain line or the like, the main body portion 2 is preferably made of a fluororesin showing high chemical resistance. As the fluorine resin, PFA (tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer resin), FEP (tetrafluoroethylene-hexafluoropropylene copolymer resin), ETFE (tetrafluoroethylene-ethylene copolymer resin), ECTFE (trifluoroethylene chloride-ethylene copolymer resin), PTFE (tetrafluoroethylene resin), PCTFE (trifluoroethylene chloride resin), PVdF (vinylidene fluoride resin), VDF (vinyl fluoride resin), etc. It can be used preferably.

In the present embodiment, the main body portion 2 has its both ends 2A made somewhat larger in order to connect to the drain line or the like, but the main body portion 2 is not limited to this configuration. Partition walls 3 are formed substantially parallel to each other in the substantially central portion of the main body portion 2. A central hole 3 is formed in the center of the partition wall 3.
A is bored, and the elongated detection tube 20 is installed by fitting into both the center holes 3A. Center hole 3A of partition wall 3 and detection tube 20
And are integrally fixed by welding.

The detection tube 20 is not limited to this, but is preferably a hollow tube whose inner and outer shapes are circular, that is, a circular tube. Further, from the viewpoint of durability against the above-mentioned aqueous solution containing an inorganic chemical, like the main body portion 2,
It is preferably formed of a fluorine resin, but FEP (tetrafluoroethylene-hexafluoropropylene copolymer resin) is particularly preferable for the reason described later.

[0020] The detection tube 20, the inner diameter D _1, when the outer diameter is D _0, preferably in the D ₁ / D ₀ ≧ 0.5. This point will also be described in detail later.

In the space portion of the main body portion 2 which is partitioned by the both partition walls 3, the light projecting portion 5 and the light receiving portion are arranged so as to face each other across the detection tube 20 in a direction orthogonal to the axis of the detection tube 20. And 6 are provided. The light projecting unit 5 includes a light source 5A that generates near-infrared light in a specific wavelength band that is absorbed by water, that is, light of 0.75 μm to 2.5 μm. Light is projected in a direction orthogonal to the axial direction. This light source 5
The space including A has a lid 7 fitted to its outer opening. This lid 7 can also be made of fluorine resin,
It is preferable to fix the body portion 2 integrally by welding.

Various kinds of light sources may be used as the light source 5A, but the light emission wavelength range is 1.1 to 1.6 μm, for example.
m light emitting diode such as InP / InGaAsP (InP series), or 1.1 to 1.67 μm InP / In
A GaAsP (InP based) semiconductor laser diode or the like can be preferably used. In some cases, 0.1
A xenon lamp (including a pulse wave type) that emits light having a wavelength of 6 to 2.0 μm can also be used as the light source 5A of the light projecting unit 5 by also using a spectral filter.

On the other hand, the light receiving section 6 is provided with a photodetector 6A for sensing the light transmitted from the light projecting section 5 through the aqueous solution flowing in the detection tube 20. As the photodetector 6A, a Ge photodiode having a wavelength sensitivity range of 0.8 to 1.8 μm,
0.8-1.7 μm Ge avalanche photodiode, 0.7-1.7 μm InGaAs-pin photodiode, 1.0-1.6 μm Ge-pin photodiode, 1.0-3.6 μm PbS Photoconductive element,
1.0-3.1 μm InAs photovoltaic element, 1.0-
A 5.5 μm ZnSb photovoltaic element or the like can be used.

According to the present embodiment, an annular wall 8 is formed integrally with the main body portion 2 so as to surround the outer opening of the light receiving portion 6, and the detection circuit portion 9 is attached to the inside thereof. The detection circuit unit 9 has an electronic circuit that converts the amount of light detected by the light receiving unit 6 into an electric signal.

In the present embodiment, as this electronic circuit, a voltage detection circuit for frequency-converting the light quantity detected by the light receiving section 5 as disclosed in JP-A-4-324328 proposed by the present applicant is suitable. Can be used for. This voltage detection circuit will be briefly described with reference to FIGS. 7 and 8.

FIG. 7 is a block diagram showing the basic structure of the voltage detection circuit 21. In the present embodiment, a capacitor C is connected in series with a photodiode PD as a photodetector 6A of the light receiving section 6, and a current I _P output from the photodiode PD is accumulated in the capacitor C in proportion to the amount of incident light. , Voltage V _C. The charging voltage V _C is detected by the voltage detection circuit 21 and compared with a preset reference voltage. When the charging voltage reaches the reference voltage, the voltage detection circuit 21
Changes the output signal level. Due to this change in the signal level, the electric charge accumulated in the capacitor C is discharged, and the accumulation of the output current I _P from the photodiode PD is started in the capacitor C again. In this way, the detection circuit unit 9, that is, the voltage detection circuit 21, outputs a frequency signal according to the light intensity received by the light receiving unit 6.

FIG. 8 shows a more specific example of the voltage detection circuit 21 shown in FIG. 7, in which the photodiode PD and the capacitor C are connected in series via the analog switch 23, and the amount of incident light is adjusted. Photodiode PD proportionally
The current I _P flowing through the capacitor is stored in the capacitor C via the analog switch 23 and converted into the voltage V _C.
The voltage V _C stored in the capacitor C is C−M
Two OS-type first and second Schmitt inverters 2
It is detected by the voltage detector 21 in which 4 and 25 are connected in series. The operation of this voltage detection circuit can be easily understood by those skilled in the art with reference to the above-mentioned Japanese Patent Application Laid-Open No. 4-324328, so further description will be omitted.

As will be understood with reference to FIG. 3, the frequency signal from the detection circuit section 9 thus converted according to the light quantity is arithmetically processed by the arithmetic and measurement means of the control section 10 and is discharged. The number of pulses is converted according to the concentration of the liquid (water concentration or acid concentration). This output pulse from the control unit 10 is transmitted to the display unit 11 and displayed as the amount or concentration of the water of the aqueous solution containing the inorganic chemical on the display device or printed and output by the printer. . If desired, the display unit 11 may be provided with an alarm device so as to issue an alarm when the drainage reaches a specified concentration.

Next, the detection tube in the concentration detecting device constructed as described above will be further described.

The detection tube 20 used in the present invention is a light projecting section 5.
From the near-infrared light of a specific wavelength band, which is absorbed by water,
Any material may be used as long as it can pass a light beam having a wavelength of 0.75 μm to 2.5 μm.
The measurement target is sulfuric acid (H ₂ SO ₄ ), hydrochloric acid (HCl),
Since it is an aqueous solution containing inorganic chemicals such as nitric acid (HNO ₃ ), hydrofluoric acid (HF), buffered hydrofluoric acid (BHF), and ammonia (NH ₄ OH), which are extremely corrosive, fluororesin is currently preferred. is there.

However, the fluororesin generally has extremely poor moldability and is usually formed into an arbitrary shape by cutting the raw material block. However, in general,
If cutting is performed, the surface smoothness of the product becomes rough, light rays are scattered, and the transmittance tends to decrease. Therefore, it is not preferable to fabricate the detection tube 20 by machining the fluororesin. For that purpose, use transparent fluororesin board,
Although it is possible to manufacture the detection tube 20 having a rectangular cross section by welding the plate members that have been modeled, the manufacturing process thereof is complicated and difficult, and even in use, it is not affected by external impact. It has the drawback of being weak.

According to the results of the research and experiment conducted by the present inventor, even in the case of the fluorine resin, if it is FEP (tetrafluoroethylene-hexafluoropropylene copolymer resin), it has a very good transparency to the near infrared light. (Similar to glass), excellent in weldability,
Moreover, it has been found that a hollow tube having a circular cross section, that is, a circular tube can be produced by molding. Therefore, as the detection tube 20, a circular tube made of FEP (tetrafluoroethylene-hexafluoropropylene copolymer resin) is extremely suitable. However, the detection tube 20 of the present invention is not limited to this, and a glass tube coated with a fluororesin can also be used. In some cases, although there is a problem as described above, transparent fluorine is used. It is also possible to use a detection tube 20 having a rectangular cross section that is manufactured by using a resin plate material.

Next, the case where a circular tube is used as the detection tube 20 will be further described.

The size of the detection tube 20 can be variously changed according to the concentration of the drainage, etc., but in the present embodiment, the detection tube 20 has a circular hollow shape having an outer diameter of 2 to 10 mm and an inner diameter of 1 to 8 mm. Tubes are preferred.

However, in the embodiment described below,
The detection tube 20 was manufactured by FEP having a refractive index of 1.35, an outer diameter (D _O ) of 5 mm, and an inner diameter (D _I ) of various sizes. Although sulfuric acid having various concentrations was used as the aqueous solution containing the inorganic chemical (detection object), its refractive index (n _S ) varied within the range of 1.33 to 1.43 corresponding to the concentration. It was something to do.

As the light source 5A of the light projecting section 5, In
p / InGaAsP light emitting diode is used, and the light receiving part 6
An InGaAs-pin photodiode was used as the photodetector 6A.

Next, the shape of the detection tube 20, that is, the inner diameter (D
_I ) to outer diameter (D _O ) ratio (D _I / D _O ), and detector tube 2
The relationship between the refractive index (n _C ) of 0 and the refractive index (n _S ) of the object to be detected will be considered.

The inner diameter (D _I ) and outer diameter (D) of the detection tube 20
_O ) impact:

FIG. 9 shows the state of the optical path from the light projecting section 5 through the detection tube 20 to the light receiving section 6. In the embodiment shown in FIG. 9, the detection tube 20 has an outer diameter (D _O ) of 5 mm,
The inner diameter (D _I ) is set to 2.5 mm, that is, the inner diameter (D _I )
The outer diameter (D _O ) ratio (D _I / D _O ) was 0.5.

In FIG. 9, the optical path indicated by the solid line is the detection tube 20.
The optical path that passes through the inside of the object to be detected, and the optical path shown by the broken line is the optical path that passes only through the detection tube 20.

In this embodiment, the light incident from the light projecting section 5 through the detecting tube 20 to the light receiving section 5 has a light intensity (light amount or illuminance) as shown in FIG. It was found to show a distribution. That is, the light traveling along the optical path shown by the solid line, that is, the light containing the concentration information of the detected object,
The light traveling along the optical path indicated by the broken line, that is, the light that does not include the concentration information of the detected substance can be completely separated and captured, and as shown in FIG. It can be seen that the light intensity containing information clearly changes with the concentration of the detected substance, but the light intensity not including the concentration information of the detected substance is substantially constant.

Therefore, in this embodiment, in the signal processing in the light receiving section 6, the light intensity not including the density information of the object to be detected can be processed as the DC noise component, and the signal processing becomes easy.

Further, as shown by the alternate long and short dash line in FIG. 9, by disposing the light shielding plate 100 between the detection tube 20 and the light receiving portion 6, the light portion which does not include the concentration information of the object to be detected, that is, It is also possible to completely block the DC noise component.

FIG. 12 is a view similar to FIG. 9, but the detection tube 20 has an outer diameter (D _O ) of 5 mm and an inner diameter (D _I ) of 1.
5 mm, that is, the ratio of the inner diameter (D _I ) to the outer diameter (D _O ) (D _I
/ D _O ) is set to 0.3. In this embodiment, the range of the light traveling along the optical path indicated by the solid line, that is, the range of light containing the concentration information of the detected object is reduced, while the light traveling along the optical path indicated by the broken line, that is, the concentration of the detected object. The range of light that does not contain information is expanding. In this state, it becomes difficult to separate the two.

FIG. 13 is a view similar to FIG. 9, except that the detector tube has an outer diameter (D _O ) of 5 mm and an inner diameter (D _I ) of 4.0 m.
m, that is, the ratio of the inner diameter (D _I ) to the outer diameter (D _O ) (D _I / D
_O ) is set to 0.8. In this embodiment, the light traveling along the optical path indicated by the solid line, that is, the range of light containing the concentration information of the detected object is expanded, while the light traveling along the optical path indicated by the broken line, that is, the concentration information of the detected object. The range of light that does not include is eliminated. Therefore, in this embodiment, the light intensity not including the concentration information of the detected object, that is, the DC noise component can be substantially eliminated.

However, the inner diameter (D _I ) of the detection tube 20
When is increased, the amount of attenuation of the light passing through the object to be detected in the detection tube 20 is increased, the amount of light received by the light receiving unit 6 is decreased, and the sensitivity of the photodetector 6A constituting the light receiving unit 6 is improved and the detection is performed. There arises a problem that the circuit unit 9 has higher performance.

As described above, it is extremely preferable that the detection tube 20 has a ratio (D _I / D _O ) of the inner diameter (D _I ) and the outer diameter (D _O ) of 0.5 or more. However, the maximum value of the inner diameter (D _I ) depends on the mechanical strength required for the detection tube 20 actually used, or as described above, the sensitivity of the photodetector 6A constituting the light receiving unit 6 And detection circuit section 9
There are themselves limitations in terms of performance, usually, the inner diameter (D _I) and the ratio of the outer diameter _{(D O) (D I /} D O) is suitably be 0.8 or less.

In the description of each of the above-described embodiments, the light rays incident on the detection tube 20 from the light projecting portion 5 are described as being parallel light rays. However, as shown in FIG. According to the result, although the individual light paths are different between the parallel light rays and the oblique light rays with an angle of about ± 10 °, there are light traveling along the light path indicated by the solid line and light traveling along the light path indicated by the broken line. The distribution tendency of the whole is substantially the same, the angle ± 10
It was found that the same operational effect can be obtained even when an oblique light beam of about ° is used.

The influence of the refractive index (n _C ) of the detector tube and the refractive index (n _S ) of the object to be detected:

It is preferable that the refractive index (n _C ) of the detection tube 20 and the refractive index (n _S ) of the object to be detected are substantially the same, and in this case, with reference to FIGS. 9 to 14. As described above, the light that has entered the detection tube 20 from the light projecting unit 5 is condensed by the lens effect of the detection tube 20 and the object to be detected flowing in the detection tube 20, and the inner diameter (D _I ) and the outer diameter ( Ratio of D _O ) (D _I
/ D _O ) is 0.5 or more, the light traveling along the optical path indicated by the solid line, that is, the light containing the concentration information of the object to be detected, and the light traveling along the optical path indicated by the broken line, that is, the object The light that does not include the concentration information of the detected substance can be completely separated and captured.

As a result of the research and experiment conducted by the present inventor, in order to completely separate and capture the light containing the concentration information of the detected substance and the light not containing the concentration information of the detected substance, the refractive index of the detection tube is determined. (N
_C ) and the refractive index (n _S ) of the object to be detected (n _C −n _S )
Has been found to be important to be +0.15 to −0.10.

Therefore, the detection tube 20 has a refractive index of 1.
35 manufactured by FEP, and used as an inorganic chemical-containing aqueous solution (detection object) in the semiconductor manufacturing process or LS.
I Sulfuric acid (H ₂ SO used in manufacturing processes, etc.
₄ ), hydrochloric acid (HCl), nitric acid (HNO ₃ ), hydrofluoric acid (HF), buffered hydrofluoric acid (BHF), ammonia (NH ₄ OH), etc. _{Since S} ) varies from 1.33 to 1.43, the above conditions are sufficiently satisfied.

[0053]

According to the concentration detecting apparatus for an aqueous solution containing an inorganic chemical of the present invention, which is constructed as described above, the aqueous solution containing an inorganic chemical flowing in the inner flow passage of the detection tube has a near-red light of a specific wavelength band in which water absorbs light. Since it is configured to project external light and measure the concentration of this aqueous solution from the amount of light transmitted through the aqueous solution,
It has the features that the amount or concentration of water in the aqueous solution containing an inorganic chemical can be detected in real time, and that the configuration is simple and the cost is low. Further, the concentration detecting device of the present invention is capable of measuring the amount or concentration of water such as a cleaning liquid, an etching removal liquid, and a resist stripping liquid discharged through a drain line in a semiconductor manufacturing process or an LSI manufacturing process in real time. It can be used to detect and monitor.

[Brief description of drawings]

FIG. 1 is a sectional view of an embodiment of a concentration detecting device for an aqueous solution containing an inorganic chemical according to the present invention.

2 is a cross-sectional view taken along the line II-II in FIG.

FIG. 3 is an overall configuration diagram of a concentration detecting device for an aqueous solution containing an inorganic chemical according to the present invention.

FIG. 4 is an absorption spectrum diagram of hydrochloric acid.

FIG. 5 is an absorption spectrum diagram of sulfuric acid.

FIG. 6 is a diagram showing the relationship between the concentrations of sulfuric acid and hydrochloric acid and the absorbance in the concentration detector of the present invention.

FIG. 7 is a configuration diagram of an embodiment showing a relationship between a light projecting unit, a light receiving unit, and a detection circuit unit.

FIG. 8 is a diagram showing details of a voltage detection circuit.

FIG. 9 is a diagram showing a state of an optical path from a light projecting unit to a light receiving unit through a detection tube.

FIG. 10 is a diagram showing a light intensity distribution in a light receiving section.

FIG. 11 is a diagram showing a light intensity distribution in a light receiving unit that varies with deterioration of an object to be detected.

FIG. 12 is a diagram showing a state of an optical path from a light projecting unit to a light receiving unit through a detection tube.

FIG. 13 is a diagram showing a state of an optical path from a light projecting unit to a light receiving unit through a detection tube.

FIG. 14 is a diagram showing a state of an optical path from a light projecting portion to a light receiving portion through a detection tube.

[Explanation of symbols]

 2 Pipe-shaped main body part 3 Partition wall 5 Light emitting part 6 Light receiving part 9 Detection circuit part 10 Control part 11 Display part 20 Detection tube

Front Page Continuation (72) Inventor Kenichi Koizumi, 3-17-35, Shinzonan, Toda City, Saitama Prefecture

Claims (4)

[Claims] 1. A detection tube in which an aqueous solution containing an inorganic chemical can flow through an inner flow passage, and near-infrared light in a specific wavelength band, which is absorbed by water, is projected in a direction orthogonal to the axial direction of the detection tube. A light projecting section, a light receiving section that senses light that is projected from the light projecting section and that has passed through the aqueous solution flowing in the detection tube, and a detection circuit that converts the amount of light sensed by the light receiving section into an electrical signal. And a controller for calculating the amount or concentration of water in the aqueous solution by arithmetically processing an electric signal from the detection circuit unit. 2. The concentration detecting device for an aqueous solution containing an inorganic chemical according to claim 1, wherein the detecting tube is a circular tube made of a fluororesin. 3. The concentration detecting device for an aqueous solution containing an inorganic chemical according to claim 2, wherein the fluororesin is a tetrafluoroethylene-hexafluoropropylene copolymer resin. 4. The concentration detecting device for an aqueous solution containing an inorganic chemical according to claim 3, wherein D ₁ / D ₀ ≧ 0.5 when the inner diameter of the detection tube is D ₁ and the outer diameter thereof is D ₀ .