JP2013015506A - Ammoniac nitrogen measurement method of coulometric titration system, and device thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an ammoniac nitrogen measurement device of coulometric titration system that can measure up to 0.05 mg/L as a lower limit of measurement.SOLUTION: A surface area of a cathode used as an electrolysis electrode is set to 1/50 to 1/100 of a surface area of an anode, and current density of the cathode is set to 0.1 to 0.2 A/cm. Consequently, the ammoniac nitrogen with 0.05 mg/L can be measured.

Description

  The present invention relates to a method and apparatus for measuring ammonia nitrogen by a coulometric titration method.

  Ammonia nitrogen measuring equipment that measures trace amounts of ammonia nitrogen discharged from environmental water such as seawater, lake water, river water, sewage treatment plants, and industrial wastewater by coulometric titration has high measurement accuracy and requires a short time. It has the features that can be measured with, and has been widely used. In addition, the improvement of this apparatus is a technique for which a research report (Patent Document 1) has been made.

JP 2000-1111521 A

“Water Supply Test Method, Explanation Edition 2001”, Japan Waterworks Association, 2001, p. 5

  The lower limit of quantification in a conventional coulometric titration-type ammoniacal nitrogen measuring device is about 1 mg / L, and it is difficult to accurately measure this concentration or less. On the other hand, the upper limit of measurement is measurable up to several thousand mg / L, but there are few high-concentration samples such as several thousand mg / L in actual samples, and practicality is low. In a sewage treatment plant or the like that actually measures ammonia nitrogen, the ammonia nitrogen concentration of raw water is about 50 mg / L to 100 mg / L, and the ammonia nitrogen concentration in the final treated water is 1 mg / L or less. There are many. Therefore, an apparatus capable of easily and accurately measuring a lower concentration in a conventional coulometric titration type ammoniacal nitrogen measuring apparatus is desired. However, there is no coulometric titration type ammoniacal nitrogen measuring device that can accurately measure 1 mg / L or less at present.

  In the coulometric titration type ammoniacal nitrogen measuring device, as described in Patent Document 1, platinum is used for the anode and the cathode which are electrolytic electrodes. In the present invention, the surface area and current density of platinum are defined in more detail, and the ammonia nitrogen concentration can be measured up to 0.05 mg / L.

The present invention relates to an electrolytic electrode of a coulometric titration-type ammonia nitrogen measuring apparatus, wherein the surface area of the cathode is set to 1/50 to 1/100 of the surface area of the anode, and the current density of the cathode is 0.1 A / cm ^{2 to} 0. .2 A / cm ² .

  By using the surface area and current density of the electrolytic electrode described above, it was possible to measure ammonia nitrogen concentration up to 0.05 mg / L.

Schematic diagram of coulometric titration ammonia nitrogen measuring device Schematic diagram of electrolytic electrode Graph showing the relationship between the electrolysis current and the measured value of 2ppm ammonia nitrogen standard solution Titration curve when the electrolysis current is 4 mA Titration curve when electrolytic current is 10 mA

  The process of reaching the surface area and current density of the electrolytic electrode as described above will be described based on the results of each experiment.

First, the point of focus that has led to the extremely small area of the cathode clothing will be described. The surface area of the anode and cathode of the electrolytic electrode used in a coulometric titration ammoniacal nitrogen measuring device that is generally marketed is about 1.6 cm ² , and the electrolytic current is about 30 mA. Because the electrolysis current is large, high concentration ammoniacal nitrogen can be measured quickly. On the other hand, for low concentrations, the electrolysis current is too large and the measurement time becomes extremely short, resulting in poor accuracy. There are also drawbacks. Therefore, it is conceivable to improve the accuracy of low concentration by reducing the electrolysis current. However, if the surface area of both electrolysis electrodes is large, the current density becomes small, resulting in electrolysis other than the original purpose on the cathode surface. Electrolytic efficiency becomes worse. The electrolysis other than the original purpose generated on the cathode surface may be reduction of bromine described later. In other words, in order not to cause undesired electrolysis on the cathode surface, the surface area of the cathode may be reduced and the current density increased. The current density of the electrode in the above-mentioned commercially available apparatus is about 0.02 A / cm ² , and the present invention has led to the idea of extremely reducing the surface area of the cathode from the viewpoint of increasing this current density. It is.

As a result of experiments on the surface area of the cathode and the current density in the present invention, the surface area of the cathode is set to 1/50 to 1/100 of the surface area of the anode, and the current density of the cathode is 0.1 A / cm ^{2 to} . It came to the conclusion that by making 2 A / cm ² , ammoniacal nitrogen up to 0.05 mg / L can be measured.

  Details of the experiment in which the surface area and current density of the electrolytic electrode were determined as described above will be described below.

First, the surface area of the electrolytic electrode will be described. The platinum used for the electrolytic electrode was a platinum wire of φ0.8 mm. The anode was prepared by cutting a platinum wire into 13 cm and making it into a spiral shape. This surface area is 3.27 cm ² . On the other hand, in order to conduct comparative experiments on the surface area of the cathode, (1) the same surface area as the anode, (2) 1 cm cut (surface area 0.26 cm ² ), (3) 0.5 cm cut (surface area 0.13 cm ^2), we were prepared four (4) which was cut into 0.1 cm (surface area 0.03 cm ^2). The electrolytic current was 4 mA, and a 2 ppm ammoniacal nitrogen standard solution prepared from ammonium chloride was measured. The electrolyte used for the measurement was a potassium bromide concentration of 0.4 mol / L and a pH buffering agent adjusted to about pH 9 with sodium carbonate and sodium bicarbonate. 10 mL of the present electrolyte and 10 mL of 2 ppm ammoniacal nitrogen standard solution were collected in an electrolytic cell and measured. The measurement results are shown in Table 1. As a result, almost 100% as the electrolytic efficiency is that obtained was found to be the surface area of the cathode is 0.13 cm ² and 0.03 cm ^2. The cathode surface area of 0.13 cm ² is about 1/25 of the anode surface area, and 0.03 cm ² is about 1/109. Since it was found that approximately 100% electrolysis efficiency was obtained within this range, the surface area of the cathode was determined to be appropriate from 1/50 to 1/100 of the anode in consideration of the margin.

  The following factors can be considered as factors that can achieve 100% electrolysis efficiency by reducing the surface area of the cathode side of the electrolytic electrode.

  First, the principle of this measurement will be described. The principle of this measurement is that bromine ions contained in the electrolyte are oxidized on the anode side by passing an electrolysis current to generate bromine, and ammonia is measured by the quantitative reaction of the bromine with ammonia. Based on the principle. That is, 100% electrolysis efficiency cannot be achieved unless all the bromine generated reacts with ammonia.

  A factor that reduces the electrolytic efficiency is that a part of bromine generated at the anode is reduced again to bromine ions on the cathode side. Since the electrolysis is performed at a constant current, if the surface area on the cathode side is large, the resistance between the anodes becomes small and the voltage applied between both electrodes becomes small. Normally, water (hydrogen ions) is reduced on the cathode side to generate hydrogen. However, under conditions where the voltage is low, bromine is first reduced at a voltage lower than water, so some bromine It is speculated that they will also be reduced. For this reason, it is considered that by reducing the surface area on the cathode side, the voltage between both electrodes is increased, and the reduction of bromine is suppressed.

  On the other hand, from the viewpoint of the reaction rate of ammonia and bromine and the contact frequency of bromine to the cathode, it is presumed that the smaller the surface area of the cathode, the more effective the suppression of bromine reduction. First, immediately after the start of electrolysis, ammonia in the titrant is in excess of bromine, so the reaction rate between bromine and ammonia is fast. Therefore, bromine is not easily reduced immediately after the start of electrolysis because the frequency of contact with the cathode is low. However, since the amount of ammonia decreases as electrolysis progresses and approaches the end point, the reaction rate with bromine gradually decreases. For this reason, the frequency of contact of bromine with the cathode increases. It can be easily estimated that the contact frequency of bromine with the cathode increases as the surface area of the cathode increases. Therefore, it is presumed that the smaller the surface area of the cathode, the lower the bromine contact frequency and the less the reduction of bromine at the cathode.

  As an additional factor, it is speculated that reducing the surface area of the cathode makes it difficult for bromine to reach the cathode because the entire cathode is covered with hydrogen gas, which is also a factor that suppresses the reduction of bromine. . These factors described above are considered to be factors that have achieved 100% electrolysis efficiency when the surface area of the cathode is reduced. That is, the reduction of bromine at the cathode is the main reason why it was not possible to measure 1 ppm or less in coulometric titration ammoniacal nitrogen measurement, and improving the cathode surface area as shown in Table 1 accurately measures 1 ppm or less. Is a prerequisite for.

Next, it experimented about the optimal electrolysis current. In the above experiment, the electrolysis current was 4 mA, but the electrolysis current was changed in the range of 1 mA to 25 mA, and a 2 ppm ammonia nitrogen standard solution was measured in the same manner. As the surface area of the cathode, 0.03 cm ^{2, at} which electrolysis efficiency of 100% was obtained in the above-described experiment, was used. The measurement results are shown in Table 2, and the graphs are shown in FIG. As a result of conducting this experiment, a measurement error within 3% was obtained when the electrolysis current was in the range of 1 mA to 15 mA. In particular, within the range of 2 mA to 5 mA, the error was within 1%, indicating that it was the optimum condition for this measurement.

  The following factors can be considered as factors causing slight measurement errors when the electrolytic current is other than 2 mA to 5 mA.

  First, at an electrolytic current of 1 mA, the value was lower than the true value. As this factor, it is considered that a part of ammonia was volatilized during the measurement. When a 2 ppm ammonia nitrogen standard solution is measured under the condition of an electrolytic current of 1 mA, it takes about 8 minutes as the measurement time. Since the measurement time is long, it is presumed that a measurement error due to volatilization of a part of ammonia has influenced.

  Next, under the condition of electrolytic current of 6 mA to 15 mA, a value higher than the true value was shown. This is considered to be due to the reduction of bromine at the cathode shown in the previous section. When the electrolytic current is increased, more bromine is generated at the anode. Generated bromine reacts immediately with ammonia, but if the bromine concentration is high, bromine generated from the anode is somewhat more excessive than bromine consumed by reaction with ammonia. Under the condition of excessive bromine, bromine is slightly reduced at the cathode, that is, bromine is consumed except for the reaction with ammonia, and as a result, the measured value is considered to be high.

  As a basis for this estimation, titration curves with electrolytic currents of 4 mA and 10 mA are shown in FIGS. When the electrolysis current is 4 mA, the current instruction value is approximately 0 μA during the measurement, and the current rapidly increases immediately before the end point (EP) and reaches the end point. On the other hand, the titration curve of the electrolytic current of 10 mA shows that the current instruction value gradually increases during the measurement and reaches the end point. The indicator electrode is a constant voltage current detection method. When bromine becomes excessive, the current value increases, and a point where the current value is equal to or higher than a predetermined current value is detected as an end point. An increase in current value during measurement means that a slight amount of bromine is always present in the titrant. Since this slightly existing bromine is reduced to bromine ions on the cathode side, the measured value is estimated to be high.

  Finally, the measured value showed a low value under the condition of an electrolytic current of 20 mA to 25 mA. This is considered to be due to blank measurement errors. As a procedure for the main measurement, it is necessary to first measure a blank contained in the electrolytic solution and subtract it from the main measurement. The blank value is about 0.2 ppm. That is, it is indispensable to accurately measure the ammoniacal nitrogen that the blank can be measured accurately. Under the condition of an electrolytic current of 20 mA to 25 mA, a problem occurs in the measurement of this blank. As described above, the normal blank is about 0.2 ppm, but bromine is generated abruptly at an electrolysis current of 20 mA to 25 mA, so that the electrolysis current is detected by detecting the end point with the indicator electrode and detecting the end point. There is a slight time difference between the stops. The excess of bromine increases due to the time difference. Actually, the blank value at an electrolysis current of 20 to 25 mA was 0.4 to 0.5 ppm. That is, it is presumed that the measured value has decreased because the blank subtraction has increased.

From the experimental results up to the previous section, in order to measure low concentration ammoniacal nitrogen, the surface area of the cathode is set to 1/50 to 1/100 of the surface area of the anode, and the electrolytic current is set to 2 mA to 5 mA. making the current density 0.1A ^/ cm ² ~0.2A ^/ cm 2 was found to be the best conditions. Based on this result, the lower limit of determination of ammonia nitrogen in the apparatus according to the present invention was determined.

  The test method followed the calculation method of the lower limit of quantification described in “2 Self-accuracy control” of Non-Patent Document 1. A parallel test was conducted to measure the ammonium chloride standard solution prepared stepwise so that the concentration of ammoniacal nitrogen was 0 to 0.1 ppm, and the concentration at which the coefficient of variation was 10% was calculated by calculation of power regression and hyperbola regression. Calculated. Table 3 shows the parallel test results of 0-0.1 ppm ammonia nitrogen standard solution.

  Based on the results in Table 3, when power regression calculation is performed, the concentration of the coefficient of variation of 10% is 0.0483 ppm, and when hyperbolic regression calculation is performed, it is 0.0416 ppm. Since the higher concentration is adopted as the lower limit of quantification, the value (0.0483 ppm) calculated by power regression is the lower limit of quantification. From this result, it was demonstrated that 0.05 ppm of ammoniacal nitrogen can be quantified.

  As described above, according to the present invention, the lower limit of quantification of the coulometric titration-type ammoniacal nitrogen measuring apparatus could be lowered from about 1 mg / L to 0.05 mg / L. As a result, in addition to the simplicity of measurement, which is an advantage of the conventional coulometric titration-type ammoniacal nitrogen measuring device, it is possible to measure in a low concentration region that better matches the market demand. Practicality and versatility were dramatically improved.

  An embodiment of the present invention will be described with reference to FIG. FIG. 1 is a schematic view of a coulometric titration ammoniacal nitrogen measuring apparatus of the present invention. This apparatus includes a control unit 1 for controlling an electrolysis current and a detector, a display unit 2 for displaying measurement results, a keyboard 3 for operating the control unit, an electrolytic solution and a sample, and performing electrolysis. It consists of a titration cell 7 to perform. In the titration cell 7, an indicator electrode 8 connected to the detection unit 5, an electrolytic electrode 9 connected to the electrolytic current power supply control unit 4, and a stirrer 10 connected to the motor 6 are arranged. The indicator electrode 8 has a gold wire 11 on one side and a silver wire 12 on the other side, and serves to detect the titration end point. On the other hand, the electrolytic electrode 9 comprises an anode 13 and a cathode 14 and plays a role of generating bromine by flowing an electrolytic current.

  The actual measurement operation will be described. When the electrolytic solution and the measurement sample are put into the titration cell 7 and measurement is started by an input operation from the keyboard 3, the electrolysis current power supply control unit 4 is turned on and electrolysis is started. At the same time, the motor 6 connected to the stirrer 10 rotates to agitate the liquid in the titration cell, and the purpose of performing the chemical reaction in this measurement quickly and reliably is achieved. The electrolytic current is controlled to 2 mA to 5 mA by the control unit 1. At the anode 13, bromine ions contained in the electrolytic solution are oxidized to generate bromine. The generated bromine immediately reacts with ammonia contained in the sample. When the reaction proceeds in this way and ammonia disappears in the titration cell and bromine becomes excessive, the indicator electrode 8 detects excess bromine, stops electrolysis, and the measurement is completed. The ammonia nitrogen concentration in the sample is calculated based on the amount of electricity passed until the measurement is completed, and the measurement result is displayed on the display unit 2.

The structure of the electrolytic electrode in the present invention will be described with reference to FIG. The electrolytic electrode includes an electrolytic electrode body 20, an anode 21 in which a platinum wire is spirally formed, and a cathode 22 in which the platinum wire is covered with a resin tube 23. First, the anode 21 needs to be manufactured with a surface area 50 to 100 times larger than that of the cathode 22. As a method of increasing the surface area, a method of forming platinum in a plate shape or a net shape is also conceivable, but in this embodiment, a special processing is unnecessary, and a method of forming a platinum wire that is the easiest to manufacture in a spiral shape. Selected. On the other hand, the cathode 22 has a shape in which a platinum wire is covered with a resin tube, and has a structure in which platinum is exposed so that the tip portion has a surface area of 0.03 cm ² . By adopting a shape covered with a resin tube, it is possible to adjust the exposed area of platinum only by changing the length of the tube, which is suitable for the present invention.

DESCRIPTION OF SYMBOLS 1 Control part 2 Display part 3 Keyboard 4 Electrolytic current power supply control part 5 Detection part 6 Motor 7 Titration cell 8 Indicator electrode 9 Electrode electrode 10 Stirrer 11 Gold wire 12 Silver wire 13 Anode 14 Cathode 20 Electrolytic electrode trunk | drum 21 Anode 22 Cathode 23 Resin tube

Claims (3)

  In a coulometric titration type ammoniacal nitrogen measuring device, a method capable of measuring 0.05 mg / L as a lower limit of quantification of ammoniacal nitrogen. In the coulometric titration type ammoniacal nitrogen measuring device, the surface area of the cathode of the electrolytic electrode is set to 1/50 to 1/100 of the surface area of the anode, and the current density of the cathode is 0.1 A / cm ^{2 to} 0.2 A / cm. ^2. A coulometric titration ammoniacal nitrogen measuring device characterized in that it is ² .   3. The electrolytic electrode according to claim 2, wherein platinum is used for both the anode and the cathode, the anode is formed of a platinum wire in a spiral shape, the platinum wire is coated with a resin or the like, and the exposed area of platinum is 1/50 of the anode. A coulometric titration ammoniacal nitrogen measuring device comprising an electrolytic electrode having a structure of 1/100 to 1/100.

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