JPH0543269B2 - - Google Patents

Description

[Detailed description of the invention] [Technical field to which the invention pertains] The present invention relates to ammonia nitrogen (hereinafter referred to as
The ammonia analysis method in water involves online measurement of the concentration (abbreviated as NH ₃ -N), and is especially easy to maintain and enables stable measurement over a long period of _time . This article relates to an applicable method for analyzing ammonia in water.

[Prior art and its problems]

In biological denitrification processes, ammonia nitrogen (hereinafter referred to as
It is necessary to measure the concentration (abbreviated as NH ₃ -N). On the other hand, at water treatment plants, it is used as an indicator of contamination of raw water.
The importance of _NH3 -N is known. in this way
Various fields have pointed out the need for a device that can measure NH ₃ -N concentration, especially an apparatus that can perform online automatic analysis over a long period of time.

In order to meet these demands, the applicant has proposed a gas barge type ammonia analyzer.
A patent application is currently being filed as Japanese Patent Application No. 59-281422 (see Japanese Patent Application Laid-open No. 61-151454). The basic principle of this device is to make the sample water alkaline, bubble it with air, and measure the NH ₃ concentration contained in the exhaust gas, and apply the standard addition method to this.

FIG. 3 shows a configuration diagram of the analyzer according to the patent application. In FIG. 3, a certain amount of test water 1 is sampled by a sampling pump P1 and injected into a gas-liquid contact tank 2. During this water test 1, air is injected through the diffuser 3 and aeration is performed. P2 is an air pump for supplying air, 4
5 is a valve for adjusting the air flow rate, and 5 is a back pressure valve. After sampling, alkali injection mechanism 6
Sodium hydroxide was added to test water 1 by
Make the pH of water test 1 11 or higher. NH ₃ −N in water is
It exists as NH ₃ and NH ₄ ⁺ , and the relationship between the two is expressed as NH ₃ +H ₂ ONH ₄ ⁺ OH ^- (1). However, at pH > 11, the reaction shifts almost completely to the left. That is, NH ₃ --N exists as dissolved NH ₃ gas. When the above-mentioned aeration is performed in this state, NH ₃ in the water is released into the rising bubbles, and NH ₃ is contained in the exhaust gas.
is included. Now, if the NH ₃ -N concentration in water is χ _L and the NH ₃ -N concentration in exhaust gas is χ _G , then χ _G =κχ _L (2). Here κ is a proportionality constant. The exhaust gas passes through an electrode housing 8 into which a gas-permeable HN ₃ electrode 7 is inserted and is discharged into the atmosphere. Electrode 7 therefore produces an output E corresponding to χ _G . The relationship between χ _G and E is E=E ₀ −Slnχ _G S=RT/nF (3). Here R is the gas constant, T is the absolute temperature, F
is the Faraday constant, n is the number of charges of ions involved in the reaction within the electrode, and E ₀ is E at χ _G =1.
This equation is generally called the Nernst equation.

Substituting equation (2) into equation (3), we obtain E=E′ ₀ −Slnχ _L E′ ₀ =E ₀ −Slnκ (4). In other words, the relationship between χ _L and E is similar to equation (3),
It can be expressed by the Nernst equation. Therefore, when the sample water 1 is made alkaline and aerated and the power output reaches a steady value, equation (4) holds true. The electrode output is input to the arithmetic unit 10 via the converter 9, and the arithmetic unit 10 holds the electrode output at this point in time. If this hold value is E ₁ , then E ₁ =E′ ₀ −Slnχ _L (5). Next, the standard solution injection mechanism 11 is activated, and a fixed amount of NH ₄ Cl solution with a known concentration is injected. As a result, the NH ₃ -N concentration in the sample water 1 increases, and the electrode output also changes. If the change in NH ₃ −N concentration is Δχ _L and the power output after the change is E ₂ , then E ₂ =E′ ₀ −Sln(χ _L +Δχ _L ) (6). Subtracting equation (5) from equation (6) yields △E=E ₂ −E ₁ =−Sln (1+△χ _L /χ _L ) (7). △E and △χ _L are known, and temperature sensor 1
2, S can also be determined by measuring the temperature inside the electrode housing.

The desired χ _L is measured by χ _L =Δχ _L /e−ΔE/S−1 (8). When the measurement is completed, the pinch valve 13 is opened, the test water 1 is discharged, and the next measurement is started.

The ammonia analyzer described above detects NH ₃ − in exhaust gas.
By measuring the N concentration, NH ₃ −N in water
Since the concentration is measured, there is no contact between the sample water and the sensor, which prevents problems caused by contamination of the electrode membrane. In addition, κ due to changes in electrode characteristics, water quality fluctuations, etc.
Changes in E′ ₀ appear as fluctuations in E′ 0 , but the standard solution addition method allows measurement without being affected by changes in E′ ₀ .

As mentioned above, the above-mentioned NH ₃
Although the analyzer has excellent features, it also has the following drawbacks.

First, as shown in equation (2), χ _G is proportional to χ _L , but χ _G is lower than the equilibrium concentration for χ _L .
That is, the _NH3 partial pressure in the exhaust gas is lower than that in water. This is disadvantageous when measuring near the lower limit of quantification with the NH ₃ electrode compared to the usual measurement method (measuring the NH ₃ partial pressure in water) in which the electrode is inserted directly into the liquid.

The second drawback is the problem of foaming in the gas-liquid contact tank due to air pollution. In particular, when measuring wastewater, bubbles are likely to occur, and the bubbles may accumulate on the liquid surface and even reach the inside of the electrode holder 8. Foaming wets and contaminates the gas-liquid contact tank 2 and the inside of the piping. Since NH ₃ is easily absorbed in the contaminated area, χ _L
The response of χ _G to changes in is significantly delayed and eventually becomes unmeasurable.

The third drawback is related to the fact that in order to perform accurate measurements using the standard addition method, it is necessary to adjust the amount of addition so that the concentration of NH ₃ -N doubles or triples by adding the standard solution. . That is, in order to perform highly reliable measurements using this method, it is necessary to predict the NH ₃ -N of the sample water in advance and set the amount of standard solution to be added based on the predicted value. Therefore, if the NH ₃ -N concentration fluctuates drastically, the amount added may deviate from the optimum value.

[Purpose of the invention]

The present invention was made in order to solve the drawbacks of the conventional ammonia analyzer as described above, and its purpose is to provide easy maintenance and stable measurement over a long period of time, as well as to enable low concentration measurement.
An object of the present invention is to provide an online ammonia analysis method in water that can also be applied to the measurement of NH ₃ -N concentration.

[Key points of the invention]

In order to achieve the aforementioned object, according to the present invention:
A measuring cell is configured such that a liquid to be measured consisting of a mixture of sample water and an alkaline agent is continuously introduced, and a gas phase of a constant volume is formed between the liquid to be measured and the measuring cell. an ammonia electrode attached to the cell so as to be in contact with the gas phase; and a standard connected to the measurement cell, in which a standard solution with a known ammonia nitrogen concentration is mixed into the liquid to be measured at an arbitrary mixing rate. and a solution mixing mechanism, first holding the electrode output while keeping the mixing rate of the standard solution constant, and then adjusting the mixing rate so that the electrode output changes by a fixed amount from the held value, It is characterized by determining the ammonia nitrogen concentration in the sampled water from changes in the mixing rate due to control operations.

The reason why the ammonia in water analyzer according to the above-mentioned prior application uses the aeration method is based on the idea that it takes a long time to bring NH ₃ into an equilibrium state between gas and liquid. That is, in the aeration method, the NH ₃ -N concentration in the exhaust gas does not reach an equilibrium concentration with respect to the NH ₃ -N concentration in water, but the proportional relationship of equation (2) holds true. Since it is not an equilibrium relationship, κ changes depending on water quality, bubble diameter, etc., but the idea is that it is sufficient to be able to measure it in a manner that does not depend on fluctuations in κ using the standard addition method.

On the other hand, as a result of repeated experiments and research, the present inventors found that in the case of NH ₃ , gas-liquid contact on the free water surface can be easily carried out without using means to increase the gas-liquid contact efficiency such as aeration or mechanical stirring. It was found that vapor-liquid equilibrium was reached. The present invention is based on such knowledge, and will be explained in detail below with reference to Examples.

[Embodiments of the invention]

FIG. 1 is a block diagram of an analyzer used in the present invention, and FIG. 2 is an enlarged sectional view of the measurement cell in FIG. 1. In Figure 1, the solid line indicates the flow of material,
Each dashed line represents a signal flow. Test water is pumped up by a water sampling pump P1 and introduced into a water sampling tank 21 installed outside the analyzer lock. A small amount of test water is introduced into the rocker and enters the measurement cell 22 through the pinch valve V1 and tube pump P3. In the process, NH ₄ Cl standard solution and NaOH are mixed. NaOH is injected from the alkali tank 23 into the test water channel on the discharge side of the tube pump P3 by the metering pump P4. The injection flow rate is determined when the pH of the liquid to be measured is 11.
or more. In this way, the liquid to be measured, which is a mixture of test water and an alkaline agent, is continuously introduced into the measurement cell 22.

On the other hand, A in Figure 1 is the standard solution mixing mechanism,
The standard solution is transferred from the standard solution tank 24 to the pinch valve V2.
and enters the suction side flow path of the tube tank P3.

The liquid to be measured is introduced into the measurement cell 22 from the liquid inlet 31 shown in the second diagram, passes through the liquid reservoir 32, and flows out from the liquid outlet 33. The liquid to be measured in the liquid reservoir 32 is slowly stirred by a stirring magnet 35 which rotates due to magnetic coupling with an external magnet 34.

Further, in FIG. 2, reference numeral 25 denotes an ammonia electrode, for example, a partition type ammonia electrode, which is installed in the measurement cell 22 so as to be in contact with a gas phase portion 42, which will be described later. For example, as shown in the second figure, it is inserted into an electrode holder 36 made of stainless steel and supported by a spacer 37 made of fluororesin. The electrode holder 36 is heated by a band heater 38, and the temperature of the electrode holder 36 is kept higher than the temperature of the liquid to be measured using a platinum resistance thermometer 39 and a temperature controller not shown in this figure. It's starting to be preserved. This is the inner surface of the spacer and the electrode measurement surface 40.
This is to prevent water vapor from condensing and absorbing _NH3 , and to prevent measurement errors due to temperature drift.

Inside this measurement cell 22, a gas phase portion 42 with a constant volume is formed by a water surface 41, a spacer 37, and an electrode measurement surface 40. When the liquid to be measured is passed through the measurement cell 22, the NH ₃ -N concentration χ _G in the gas phase portion 42 is in equilibrium with the NH ₃ -N concentration χ _L in the liquid to be measured.

That is, χ _G =1/Hχ _L (9). Here H is Henry's constant. Therefore, the output E of the ammonia electrode 25 is E=E" ₀ -Slnχ _L E" ₀ =E ₀ +SlnH (10), and this signal is input to the arithmetic unit 26 in FIG. The calculation device 26 generates on/off signals for alternately opening the pinch valves V1 and V2, and adjusts the standard solution mixing rate r by adjusting the on/off ratio. In this embodiment, one measurement period is one hour. The first 30 minutes is the standardization mode, and the second 30 minutes is the measurement mode. In the standardization mode, r is fixed to a constant value _r1 . r ₁ is of the liquid to be measured.
The NH ₃ -N concentration is determined to be higher than the measurement limit of the ammonia electrode. Therefore, the _NH3 of the sample water
- If the N concentration is above the measurement limit, set r ₁ = 0,
In the following, we will proceed with the discussion assuming that r ₁ ≠ 0, but r ₁ in the formula
Of course, by setting 0 to zero, the case where r ₁ =0 can be handled in the same way.

In the standardization mode, the NH ₃ −N concentration χ _L1 of the liquid to be measured
becomes χ _L1 = (1-r ₁ ) χ ₀ + r ₁ χ _S = χ ₀ + r ₁ (χ _S −χ ₀ ) (11). Here, χ ₀ is the NH ₃ −N concentration of the test water, and χ _S is the NH ₃ −N concentration of the standard solution.

However, since χ _S is set sufficiently higher than χ _L (χ _S ≫ χ _L ), χ _L1 χ ₀ +r ₁ χ _S (12). Then, the electrode 25 generates the output E ₁ =E″ ₀ −Slnχ _L1 (13). At the end of the standardization mode, this value is held and the measurement mode is entered. In the measurement mode, the electrode output is constant from E ₁ . The amount △E changes and E ₂ = E ₁ +
The standard solution mixing rate r is controlled so that ΔE.
r when the control operation reaches equilibrium is r ₂ = r ₁ + △
r, the NH ₃ −N concentration of the liquid to be measured is χ _L2 = χ _L1 + △r (χ _S − χ ₀ ) χ _L1 + △rχ _S (14), and E ₂ = E″ ₀ = Sln( χ _L1 +△rχ _S ) (15) Therefore, subtracting (13) from equation (15), △E=−Sln(1+△rχ _S /χ _L1 ) (16) Therefore, χ _L1 = △rχ _S /e-△E/S-1 (17) χ _L1 is found.Furthermore, from equation (12), that is, χ ₀ = χ _L1 −r ₁ χ _S (18), the desired χ ₀ The arithmetic unit 26 performs the above calculation and transmits χ ₀ as a measured value.

〔Effect of the invention〕

While the conventional NH ₃ analyzer uses the proportional relationship between the NH ₃ -N concentration in the exhaust gas and the NH ₃ -N concentration in water, that is, the equation (2),
In the present invention, equation (9) of vapor-liquid equilibrium is used. Therefore, since κ>1/H, the latter χ _G is larger than the former χ _G for the same χ _L . Furthermore, in the present invention, by setting r ₁ ≠0 in the standardization mode, it has become possible to measure low concentration regions that were impossible to measure with conventional ammonia electrodes.

Further, in the present invention, since no aeration or rapid stirring is used, there is little contamination of the inner surface of the measurement cell due to foaming or splashing, and there is almost no delay in response due to absorption of NH ₃ .

Another difference between the conventional NH ₃ analyzer and the present invention is that in the former, △χ _L in equations (7) and (8) is kept constant and △E
In contrast, in the latter case, △rχ _S , that is, △χ can be calculated by keeping △E in equations (16) and (17) constant. The advantages of this are as follows.

(1) If △E is set, l−△E/S is treated as a constant. There is no need to calculate an exponential function to

(2) Since △rχ _S /χ _L1 is determined by determining △E, the conditions of the standard addition method, that is, the condition that the NH ₃ -N concentration changes by 2 to 3 times due to the addition of the standard solution, should always be met. ΔE can be set to .

[Brief explanation of the drawing]

FIG. 1 shows a configuration diagram of a specific example of the device used in the present invention, FIG. 2 shows an enlarged sectional view of the measurement cell in FIG. 1, and FIG. 3 shows a configuration diagram of a conventional device. . 22...Measurement cell, 23...Alkaline tank,
24... Standard solution tank, 25... Ammonia electrode, 26... Arithmetic device, 31... Liquid inlet, 33
...Liquid outlet, 34...External magnet, 35...
...Stirring magnet, 40... Electrode measurement surface, 41...
... Water surface, 42 ... Gas phase section, A ... Standard solution mixing mechanism, P1 ... Water sampling pump, P3 ... Tube pump, V1, V2 ... Pinch valve.

Claims (1)

[Claims] 1 A measuring cell configured such that a liquid to be measured consisting of a mixture of test water and an alkaline agent is continuously introduced, and a gas phase of a constant volume is formed between the liquid to be measured and this liquid to be measured; An ammonia electrode attached to the measurement cell so as to be in contact with the gas phase part is connected to the measurement cell, and a standard solution with a known ammonia nitrogen concentration is mixed into the liquid to be measured in the measurement cell at an arbitrary mixing rate. a standard solution mixing mechanism, first holding the electrode output while keeping the mixing rate of the standard solution constant, and then adjusting the mixing rate so that the electrode output changes by a fixed amount from the held value; A method for analyzing ammonia in water, which is characterized by determining the ammonia nitrogen concentration in sample water from changes in the contamination rate caused by control operations.