JPH0778478B2 - Method for measuring free chlorine in sample water - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for measuring free chlorine existing in water and sewage water, industrial water, river water, etc., and in particular, free chlorine in sample water not affected by bound chlorine. It relates to a method for measuring chlorine.

[Conventional technology]

Generally, chlorine (Cl ₂ ) or sodium hypochlorite (NaClO) is used to disinfect water. When chlorine or sodium hypochlorite is injected into water, hypochlorous acid (HC
lO) and hypochlorite ions (ClO ^-) produce.

In the case of chlorine, the reactions of formulas (1) and (2) are performed.

Cl ₂ + H ₂ OHClO + HCl (1) HClO + ClO ⁻ + H ⁺ (2) In the case of sodium hypochlorite, the reactions of formulas (3) and (4) are performed.

NaClO + H ₂ OHClO + NaOH (3) HClOClO ⁻ + H ⁺ (4) Here, chlorine, hypochlorous acid and sodium hypochlorite are collectively called free chlorine, which has a strong bactericidal power.
Of these free chlorine, the equilibrium in the above formula (1) shifts to the right in the neutral sample water,
₂ does not exist.

Aqueous sewage water, industrial water, river water, etc. that require actual sterilization contain ammonia (NH ₃ ).
As shown in (6) and (7), monochloramine (NH ₂ C
l), dichloramine (NHCl ₂ ) and trichloramine (NCl ₃ ) are produced.

NH ₃ + HOCl → NH ₂ Cl + H ₂ O …… (5) NH ₂ Cl ＋ HOCl → NHCl ₂ ＋ H ₂ O …… (6) NHCl ₂ ＋ HOCl → NCl ₃ ＋ H ₂ O …… (7) These are called combined chlorine, Monochloramine and dichloramine have bactericidal activity, but trichloramine does not.

Now, the chlorine injection amount in this sterilization treatment is, for example, in the Water Supply Act, "
It is mandatory to sterilize the chlorine so that it is kept at pm or more. Therefore, monitoring of free chlorine is indispensable. The polarographic method is generally used to measure this free chlorine,
There are a reagent type and a reagentless type, but since a reagent and a reagent pump are unnecessary and maintenance is considerably small, the reagentless type is often used recently.

The measurement principle will be described. When a rotating electrode (gold electrode) and a counter electrode (silver electrode) are immersed in sample water and an appropriate voltage is applied between them, the reaction of formulas (8) and (9) Ag → Ag ⁺ + e ⁻ ... on the silver electrode. ^{... (8) Ag + + OCl} - → AgCl + 1 / 2O 2 ...... (9) on the gold electrode, the reaction HOCl + e of formula ^{(10) - → 1 / 2H} 2 + OCl - ...... (10) occurs, rotating electrode from the counter electrode An electric current proportional to the free chlorine concentration in the sample water flows toward. By measuring this current, the concentration of free chlorine can be obtained.

[Problems to be Solved by the Invention]

In the conventional polarographic method for measuring free chlorine, it was difficult to detect only free chlorine because the reduction voltages of free chlorine and bound chlorine were close to each other, and there was a problem that bound chlorine was affected. Especially, since the concentration of bound chlorine is about 1/4 of that of free chlorine, its influence is great.

Particularly in recent years, pollution of environmental water such as rivers and lakes has increased, and the concentration of ammonia has increased, so that the amount of bound chlorine produced is large. Therefore, there is a problem in that free chlorine cannot be accurately measured due to interference with bound chlorine, and it is not possible to determine whether or not sterilization is sufficiently performed.

This invention has been made in view of the above points, and its object is to use a sensor selective for hypochlorous acid in sample water,
An object of the present invention is to provide a method for measuring free chlorine that does not interfere with bound chlorine.

[Means for Solving the Problems]

As a result of intensive studies to solve this problem, the present inventors have found that a metal oxide semiconductor gas sensor containing In ₂ O ₃ as a main component responds to hypochlorous acid in free chlorine in sample water. Based on this finding, the present invention has been completed.

The above-mentioned object is according to the present invention that the sample water containing hypochlorous acid is brought into contact with a carrier gas and then the carrier gas is replaced with In ₂ O 3.
It is achieved by measuring with a metal oxide semiconductor type gas sensor containing ₃ as a main component.

The sample water and carrier gas can be contacted by immersing a porous tube in sample water and passing the carrier gas through the tube, tubing method, headspace method in which the gas phase above the sample water makes contact, and sample water There is a bubbling method in which a gas is blown into.

The semiconductor gas sensor has an In ₂ O _{3 structure on} the surface of an electrically insulating substrate.
A metal oxide semiconductor thin film containing as a main component is formed, and the gas component is detected by the resistance change of the semiconductor thin film.

[Action]

Both hypochlorous acid and bound chlorine in the sample water move into the carrier gas upon contact with the carrier gas. The metal oxide semiconductor gas sensor based on In ₂ O ₃ does not respond to bound chlorine. The hypochlorous acid transferred from the sample water into the carrier gas is not absorbed in the form as it is or in the decomposed state.
It is presumed that it changes the resistance of the metal oxide semiconductor gas sensor whose main component is ₂ O ₃ .

〔Example〕

 Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a layout diagram showing a configuration of an apparatus according to an embodiment of the present invention. In FIG. 1, 1 is a filter filled with activated carbon and silica gel for obtaining clean dehumidified air, 2 and 3 are flowmeters for dry gas and carrier gas, respectively.
An extraction tank 4 has sample water inside, and a porous tube 5 is immersed in the sample water. This porous tube 5 has, for example, a pore diameter of about 0.2 to 5.0 μm and a porosity of about 20 to 80 μm.
% Of continuous micropores and hydrophobicity and made of continuous porous tetrafluoroethylene resin. 6 is a measurement chamber containing a metal oxide semiconductor type gas sensor containing In ₂ O ₃ as a main component for detecting free chlorine in the gas phase, and 7 is a metal oxide semiconductor type containing In ₂ O ₃ as a main component. A recorder for recording the output of the gas sensor, and a pump 8 for sucking the dry gas and the carrier gas.

The details of the metal oxide semiconductor type gas sensor containing In ₂ O ₃ as a main component built in the measurement chamber 6 are shown in FIG. 2 (a),
It is shown in (b). A metal oxide semiconductor type gas sensor containing In ₂ O ₃ as a main component is an electrically insulating substrate, for example, an alumina substrate 11 or an In substrate formed by vapor deposition on the surface of the alumina substrate 11.
Metal oxide semiconductor thin film 12 containing ₂ O ₃ as a main component, Pt film electrodes 13A and 13B for measuring the resistance change of the semiconductor thin film 12, and sensitivity due to water and oil in the vapor phase formed on the back surface of the alumina substrate 11. It is composed of a Pt film heater 14 for avoiding deterioration with time.

How the measurement of free chlorine is performed in the apparatus shown in FIGS. 1 and 2 will be described below.

Ambient air is sucked into the device by the pump 8. This air is dehumidified and cleaned by the filter 1, and one is flow meter 2
Through to the dry gas and the other through the flow meter 3 to become the carrier gas. The carrier gas reaches the measurement chamber 6 via the porous tube 5 immersed in the sample water in the extraction tank 4, but when passing through the porous tube 5, free chlorine in the sample water is slightly separated from the porous tube 5. It penetrates the pores and diffuses into the carrier gas.

Not only free chlorine but also water vapor and bound chlorine diffuse into the carrier gas. If the temperature of the pipe from the porous tube 5 to the measuring chamber 6 is low, this water vapor will condense and become droplets. If droplets are formed in this way, the free chlorine extracted by the porous tube 5 causes vapor-liquid equilibrium again in the droplet portion and the free chlorine concentration in the gas phase portion changes.
It causes measurement error. In this device, in order to prevent water vapor from condensing, it is mixed with the dry gas that has passed through the flowmeter 2 in the middle of the pipe from the porous tube 5 to the measurement chamber 6. It is desirable that this mixing position be as close as possible to the porous tube 5.

The carrier gas mixed with the dry gas is then introduced into the measurement chamber 6 and the free chlorine concentration is measured by the metal oxide semiconductor gas sensor whose main component is In ₂ O ₃ , and the output is recorded in the recorder 7. .

A calibration curve is shown in FIG. Measurement conditions are dry gas flow rate 30
ml / min, carrier gas flow rate 120 ml / min, the porous tube has a maximum pore diameter of 2.0 μm, porosity of 50%, inner diameter of 2.0 mm, wall thickness of 0.4 mm,
It has a length of 600 mm. The sample water used was prepared by appropriately diluting sodium hypochlorite and adjusting the pH to 6.93 with a phosphate buffer. The sensor output is obtained according to the hypochlorous acid concentration.

In addition, regarding the interference of bound chlorine, monochloramine 4.9 mg / 1,
When dichloramine 7.0 mg / 1 solution was prepared and measured with the above device, the output of the metal oxide semiconductor gas sensor containing In ₂ O ₃ as the main component was not obtained at all, and these components were not disturbed. It became clear.

A part of hypochlorous acid is dissociated into both hypochlorite ion and hydrogen ion. The solid line in Fig. 4 theoretically shows this dissociation state, and most of it exists as hypochlorous acid at pH = 6 or less, and conversely most of it exists as hypochlorite ion at pH = 9 or more. . The circles in Fig. 4 indicate 0.65 as chlorine.
Figure 3 shows the voltage output of the metal oxide semiconductor gas sensor containing In ₂ O ₃ as the main component when the sample water is prepared by changing the pH of the mg / 1 sodium hypochlorite solution. The HOCl abundance ratio was experimentally obtained by converting the concentration using a line. Since the experimental value obtained using the metal oxide semiconductor gas sensor containing In ₂ O ₃ as the main component is very close to the theoretical value (solid line) showing the abundance ratio of hypochlorous acid, It can be seen that the metal oxide semiconductor gas sensor containing In ₂ O ₃ as a main component responds to hypochlorous acid in the sample water.

The measurement method using the tubing method as the extraction method of free chlorine in the sample water has been described above, but the extraction method of the present invention is not limited to the tubing method, and hypochlorous acid in the sample water is vapor-phased. Any method may be used as long as it is a method of diffusing it inside and extracting it.

FIG. 5 shows the apparatus structure when the headspace method is used as the extraction method. Since hypochlorous acid in the sample water in the extraction tank 4 diffuses from the water surface to the gas phase, free chlorine in the sample water is measured as in the device shown in FIG. Since the porous tube of the heads base method is not used, there is an advantage that the structure is simple and there is no change with time due to dirt on the tube surface.

FIG. 6 shows an apparatus configuration when the bubbling method is used as the extraction method. When the gas is sucked by the pump 8, the pressure inside the extraction tank 4 is reduced, so that the carrier gas is sucked into the sample water from the ball diffuser 21 and the hypochlorous acid in the sample water is extracted into the gas phase portion. Therefore, the free chlorine in the sample water is measured as in the device shown in FIG. Similar to the headspace method, this bubbling method has the advantage that it does not change over time due to dirt on the tube surface.Because the carrier gas is forcedly blown into the sample water, the response speed is fast and low concentrations can be measured. There is an advantage.

〔The invention's effect〕

According to this invention, sample water containing hypochlorous acid is brought into contact with a carrier gas, and then the carrier gas is measured using a metal oxide semiconductor gas sensor having In ₂ O ₃ as a main component. Only the hypochlorous acid in the sample water can be accurately measured without being affected by chlorine.

[Brief description of drawings]

FIG. 1 is a layout diagram showing the configuration of an apparatus according to an embodiment of the present invention, and FIG. 2 shows a metal oxide semiconductor type gas sensor containing In ₂ O ₃ as a main component according to the embodiment of the present invention. (A)
Is a perspective view thereof, FIG. 2 (b) is a sectional view thereof, FIG. 3 is a diagram showing a calibration relationship according to an embodiment of the present invention, and FIG.
FIG. 6 is a diagram showing the theoretical value (solid line) and the experimental value (◯) of the Cl dependence in relation to pH, and FIG. 5 is a layout diagram showing the configuration of an apparatus according to another embodiment of the present invention. FIG. 6 is a layout view showing a configuration of an apparatus according to still another embodiment of the present invention. 1: Filter, 4: Extraction tank, 5: Porous tube, 6: Measuring chamber, 7: Recorder, 8: Pump, 11: Alumina substrate, 12: Semiconductor thin film, 13A, 13B: Pt film electrode, 14: Pt film Heater, 21: Ball diffuser.

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Hiroshi Hoshikawa 1-1 Tanabe Nitta, Kawasaki-ku, Kawasaki-shi, Kanagawa Fuji Electric Co., Ltd. (72) Inventor Katsugu Kono 1 Tanabe Nitta, Kawasaki-ku, Kawasaki-shi, Kanagawa No. 1 in Fuji Electric Co., Ltd. (72) Inventor Yoshitaka Takada 2-5-4 Mitsuyachu, Yodogawa-ku, Osaka-shi, Osaka Inside New Cosmos Electric Co., Ltd. (72) Inventor Sakai, 2 Mitsuya-chu, Yodogawa-ku, Osaka-shi, Osaka Chome 5-4 New Cosmos Electric Co., Ltd. (72) Inventor Toyoaki Aoki 3-37-32 Kusuhada, Hirakata-shi, Osaka (72) Inventor Yukio Hama 2-453 Nonomiya, Ibaraki-shi, Osaka

Claims (1)

[Claims] 1. A sample characterized in that sample water containing hypochlorous acid is brought into contact with a carrier gas, and then the carrier gas is measured using a metal oxide semiconductor gas sensor containing In ₂ O ₃ as a main component. Method for measuring free chlorine in water.