JP2006177800A - Trace chlorine ion concentration analysis method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a trace chlorine ion concentration analysis method capable of analyzing accurately trace chlorine ions in a high-concentration alkaline sample, and organizing a hardly failed device. <P>SOLUTION: A standard solution 4 and the high-concentration alkaline sample are collected respectively as much as a prescribed quantity in a titration tank 8, and an acid for neutralization is added thereto to thereby acquire a weakly acidic property. Then, a constant current is made to flow by using a silver electrode 13 as a positive electrode and a metal electrode 14 as a negative electrode to perform electrolysis until included chlorine ions disappear. The quantity of the chlorine ions in the sample solution is calculated by subtracting the quantity of electricity required for electrolyzing only the standard solution 4 from the quantity of electricity required for the electrolysis. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a method for analyzing a chlorine ion concentration in a sample solution, and more particularly to a method for quickly and accurately analyzing a trace amount of chlorine ions in a highly concentrated alkaline sample solution.

  Conventionally, in order to analyze the sodium chloride concentration in a highly concentrated alkaline sample solution such as a sodium hydroxide solution having a concentration of about 32% by weight, the chloride ion concentration in the sample solution is determined by precipitation titration using a silver nitrate solution. It was common to quantify.

  In the precipitation titration method, a predetermined amount of a sample is taken, neutralized acid is added to make the sample solution weakly acidic, then a silver nitrate solution is used as a titration reagent, and the titration reagent is dropped into the sample solution and contained. Chlorine ions are consumed and disappeared by making silver chloride precipitates. Utilizing the fact that when the equivalent point is reached, the contained chloride ions disappear and the silver ions are present in excess, this excess silver ion is detected by the silver ion electrode, and the chloride ion is detected from the potential change of the silver ion electrode. And the equivalent point of silver ions is detected. From the amount of titration reagent dripped until this equivalence point is detected, the chloride ion concentration is obtained, and the sodium chloride concentration in the sample solution is calculated.

  In the precipitation titration method, it was necessary to use a burette or syringe for titration with a silver nitrate solution. Further, since the syringe is driven by a motor in order to automate the analysis, the apparatus becomes mechanically precise and large.

  In addition, since a syringe driven by a motor is used, liquid leakage and syringe volume change due to wear deterioration of the syringe wetted part, malfunction of the syringe drive motor, malfunction due to foreign matter biting in the transmission part (gear, etc.), liquid temperature Changes in the amount of liquid dropped due to the generation of bubbles due to dissolved air due to the change, air suction due to wear of the piston portion, etc. may cause analysis errors or make analysis impossible.

  As a method for analyzing a trace amount of chlorine ions, Patent Document 1 describes that electrolysis is performed until a chlorine ion disappears by passing a constant current between the electrodes in a titration cell in which tap water or the like is used as a sample and silver-made positive and negative electrodes are arranged. Describes a method for measuring the chlorine concentration to obtain the chlorine ion concentration in the sample from the amount of electricity required. However, there is no description about the analysis of trace chlorine ions in high-concentration alkaline samples.

JP-A-01-313754

  Accordingly, an object of the present invention is to provide a trace chlorine ion concentration analysis method that can accurately analyze trace chlorine ions in a high-concentration alkaline sample and can be an apparatus with few failures.

  The present inventors have intensively studied to solve the above-described problems. As a result, it was found that by analyzing the concentration of trace chlorine ions in a high-concentration alkaline sample by a coulometric titration method using a silver electrode, the analysis can be performed with high accuracy and the apparatus can be reduced in failure.

  That is, according to the method of the present invention, a standard solution and a high-concentration alkaline sample are respectively collected in a titration tank, neutralized acid is added to make it weakly acidic, and then the silver electrode is used as a positive electrode and the metal electrode is used as a negative electrode. Electrolyze until the contained chlorine ions disappear by supplying a constant current, and subtract the amount of electricity when only the standard solution is electrolyzed from the amount of electricity required for the electrolysis to calculate the amount of chlorine ions in the sample solution. This is a method for analyzing trace chlorine ion concentration in a high-concentration alkaline sample.

  According to the method for analyzing a trace amount of chlorine ions in a high-concentration alkaline sample of the present invention, it is not necessary to use a syringe by performing coulometric titration using a silver electrode, the analysis can be performed with high accuracy, and the apparatus is compact with few failures. Automatic analyzer.

  In the chlorine ion concentration measuring method of the present invention, a predetermined amount of a high concentration alkaline sample and a standard solution are collected in a titration tank.

  Although it is not particularly limited as a high-concentration alkaline sample to be subjected to chlorine ion concentration measurement, it is preferable that the chlorine ion concentration analysis method of the present invention targets so-called liquid caustic soda produced by an ion exchange membrane method. Liquid caustic soda produced by the ion exchange membrane method is a sodium hydroxide solution having a concentration of about 32% by weight, and is a highly concentrated alkaline solution. The liquid caustic soda contains 25 to 40 mg / l sodium chloride, and the chloride ion concentration is 15 to 25 mg / l. The high-concentration alkaline sample needs to be neutralized during the analysis, and the chlorine ion concentration in the sample after neutralization decreases. If the chlorine ion concentration in the sample after neutralization is 10 mg / l or less, the proportionality between the potential measured by the chlorine ion electrode and the chlorine ion concentration is lost and the chlorine ion concentration cannot be determined. In order to prevent this, a standard solution is added to the titration tank together with a highly concentrated alkaline sample.

  The standard solution used in the chlorine ion measurement method of the present invention is an aqueous solution having a known chlorine ion concentration. Although it does not specifically limit as a standard solution, Sodium chloride aqueous solution, hydrochloric acid aqueous solution, potassium chloride aqueous solution etc. can be used. The concentration of the sample in the titration tank is not particularly limited as long as the chlorine ion concentration of the sample is equal to or higher than the lower limit of determination of coulometric titration after the sample is neutralized with the neutralizing acid. The concentration of the standard solution should be neutralized with a large amount of acid when liquid caustic soda is used as the sample, and the sample may be diluted, or measurement errors may easily occur if the standard solution is too high. Considering this, the range of 50 to 500 mg / l is desirable.

  The collection amount of the high-concentration alkaline sample and the standard solution is not particularly limited. When the chlorine ion concentration is low, it is desirable to improve the accuracy by increasing the collection amount. However, particularly in a high concentration alkaline sample, the amount of acid necessary for neutralization also increases. In order to reduce the size of the measuring apparatus, it is preferable to set the collection amount to 1 to 10 ml.

  In the present invention, a predetermined amount of high-concentration alkaline sample and standard solution are collected in a titration tank, and then neutralized with a neutralizing acid. In order to perform coulometric titration using a silver electrode, it is necessary to make the liquid to be titrated weakly acidic. When the titrant is alkaline, silver ions generated by electrolyzing silver become silver hydroxide precipitates and do not react with chlorine ions. If the titrant is made weakly acidic, silver ions generated by electrolysis are present in the solution as ions and react quantitatively with chlorine ions. As the neutralizing acid, nitric acid, hydrochloric acid, acetic acid and the like can be used. If the amount of acid to be used is too thin, the amount of acid required for neutralization becomes large, and there is a possibility that the concentration of chloride ions decreases due to an increase in the amount of liquid during titration, making titration impossible. Concentrated acids must be handled with care. When targeting liquid caustic soda, it is desirable to use about 5 to 10 mol / l of nitric acid.

  In the present invention, coulometric titration is performed after making the mixed solution of the collected sample and standard solution weakly acidic. Instead of dropping the silver nitrate solution, the silver electrode is used as an anode and the metal electrode is used as a cathode. A constant current is passed between the two electrodes to dissolve and generate silver ions from the silver electrode. The metal electrode is not particularly limited as long as it can flow current without corrosion. Generally, a platinum electrode is used.

  The generated silver ions react with the chlorine ions in the titrant to be precipitated as silver chloride. Along with this, chlorine ions in the titrant are reduced, so the amount of electricity that has flowed until the chlorine ions disappear is measured, the chloride ion content is quantified from this amount of electricity, and the chloride ion concentration is calculated. Therefore, the dripping apparatus required for the precipitation neutralization titration method is unnecessary. In addition, it is preferable to use a measuring tube and a 6-way valve for collecting a sample and a standard solution, and to share these, since the apparatus can be made compact and failure can be reduced.

  Detection of the equivalence point between chlorine ions and silver ions in the titrant is performed by measuring the chlorine ion potential with a chlorine ion electrode or by measuring the silver ion potential with a silver ion electrode.

  In the present invention, when a comparison electrode having gelled potassium chloride as an internal liquid is used as a reference electrode when measuring the potential at the equivalence point, the amount of liquid junction from the reference electrode of chlorine ions is reduced to that of a normal reference electrode or internal electrode. Since the liquid is less than the reference electrode (double junction type reference electrode) made of potassium chloride and potassium nitrate, the equivalence point can be detected with high accuracy.

  In the present invention, an electric quantity corresponding to the amount of chlorine ions contained in the high-concentration alkaline sample and the standard solution is obtained by coulometric titration. Therefore, coulometric titration of only the standard solution using the same method as above, the amount of electricity necessary for coulometric titration of only the chlorine ions in the standard solution is obtained, and the sample solution and standard solution containing high-concentration alkaline trace chlorine ions are By subtracting from the amount of electricity required for titration, the chloride ion content in the high-concentration alkaline trace chlorine-containing sample is determined, and the chloride ion concentration is calculated.

  In the present invention, the current flowing between both electrodes of the silver electrode serving as the anode and the metal electrode serving as the cathode is preferably pulsed. When a constant current is applied in pulses, the amount of electricity required for coulometric titration is proportional to the number of pulses. Furthermore, the number of pulses is proportional to the chlorine ion content of the titrant in the titration tank. Therefore, the chlorine ion concentration in the high-concentration alkaline sample can be calculated from the number of pulses.

The calculation method will be described in detail. First, coulometric titration of only the standard solution, but after taking a predetermined amount of standard solution in a titration tank and making it weakly acidic with acid, a pulsed constant current is applied with the silver electrode as the positive electrode and the metal electrode as the negative electrode. Shed. After the acid addition, it is desirable to add pure water so that the amount of liquid in the titration tank becomes a predetermined amount. Electrolysis is performed until chlorine ions disappear, and the number of pulses _Pst1 required for the electrolysis is measured. Although chlorine ion concentration of the standard and pulse number P _st1 it is possible to calculate the chloride ion concentration, close to the coulometric titration conditions of the high-concentration alkaline samples, in order to reduce the measurement errors, then the Shizukujoso After washing, the standard solution is sampled twice in the titration tank, neutralized and electrolyzed again, the number of pulses _Pst2 is measured, and the chlorine ion concentration K per pulse is calculated by the following equation (1). It is preferable to calculate.

K = A / ( _{Pst2- Pst1} ) (1)
(Where A is the chloride ion concentration of the standard solution)
As a method of collecting the standard solution twice the predetermined amount, it may be collected twice by a sample loop capable of collecting the predetermined amount.

On the other hand, when the titration solution obtained by collecting the standard solution and the high-concentration alkaline sample once is similarly subjected to coulometric titration and the number of pulses P _{s + st1} required for electrolysis is measured, the high-concentration alkaline trace chlorine is _expressed by the following equation (2). The chlorine ion concentration C in the ion-containing sample solution can be calculated.

C = K × (P _{s + st1} −P _st1 ) (2)
Since the chloride ion concentration can be calculated from the number of pulses in this manner, calibration and analysis can be automatically performed.

  The method of flowing the constant current in a pulse shape is not particularly limited. For example, the constant current device may be a pulsed constant current by controlling it with a sequencer.

  The above-described measurement of the chlorine ion concentration in the sample solution containing high-concentration alkaline trace chlorine ions of the present invention can be performed by the following method.

  FIG. 1 is a diagram of an automatic analyzer according to the present invention.

  First, the coulometric titration process of the standard solution will be described. This step is for coulometric titration of chlorine ions in the standard solution with a pulsed constant current to determine the calibrated chlorine ion amount per pulse. In addition, the coulometric titration is performed twice: a step of collecting the standard solution once and a step of collecting the standard solution twice.

  First, the standard solution is sampled in a titration tank, and the process is as follows. The standard solution (4) is sampled by replacing the sample solution (4) into the sample loop (6) through the sampling six-way valve (5) by opening the standard solution solenoid valve (2) and operating the sampling pump (3). When the standard solution is collected, the sampling 6-way valve (5) is rotated 60 degrees as shown in FIG. 2 (one-sixth rotation of one round), and the flow path is changed to remove the pure water from the pure water solenoid valve (7). Water is fed into the sample loop (6) in which the standard solution is accumulated, and the standard solution in the sample loop is washed into the titration tank (8) with the pure water fed in. Thereafter, the 6-way valve (5) is rotated 60 degrees and returned to the original flow path of FIG.

  When the standard solution is collected in the titration tank (8) in this way, the collected solution is moved to a step of making it weakly acidic. Neutralizing acid (9) is added to the titration tank by an acid pump (10). The amount of neutralizing acid to be added may be determined in advance such that the sample becomes weakly acidic. Next, in order to keep the liquid amount in the titration tank (8) constant, the pure water electromagnetic valve (7) is opened while monitoring the liquid amount with the liquid level gauge (12), and pure water is injected into the titration tank (8), Let the amount of liquid be a predetermined amount.

When the liquid amount is set to a predetermined amount, the step of titration is started. Titration is performed by operating a stirrer (11) and flowing a constant current for silver electrolysis between the silver electrode (13) and the metal electrode (14) while stirring the liquid to be titrated in the titration tank (8). The constant current device (16) is controlled by the sequencer of the controller (18) and flows a pulsed constant current. At this time, the chlorine ion concentration is monitored by the potential with the chlorine ion electrode (20) and the comparison electrode (21) and the ion meter (17), and the point where the generated silver ion and the chlorine ion to be analyzed are equivalent is detected. At the equivalent point, silver ions become excessive, and the potential changes rapidly. Therefore, the equivalent point is detected based on the amount of change, and the titration is completed. When the titration is completed, the drainage solenoid valve (15) is opened to drain the titrated liquid. At this time, the number of pulses _Pst1 from the start of titration until reaching the equivalent point is measured.

The above is the process of collecting the standard solution once and performing coulometric titration. Subsequently, the standard solution is sampled twice, and coulometric titration is performed, and the number of pulses _Pst2 is measured. In this step, the step of collecting the standard solution is repeated twice, a neutralizing acid is added, and the subsequent steps are the same as those described above.

  Next, the titration process of a high concentration alkaline sample will be described.

First, a sample is sampled in a titration tank, and the process is as follows.
The sampling solenoid valve (1) is opened, the sampling pump (3) is operated, the sampling six-way valve (5) is passed through, and the sampling loop (6) is replaced. When the sample is collected, the sampling 6-way valve (5) is rotated 60 degrees as shown in FIG. 2 (one-sixth rotation of one round), the flow path is changed, and pure water is sampled from the pure water solenoid valve (7). The collected sample loop (6) is sent to the sample loop (6), and the sample in the sample loop is washed into the titration tank (8) by the sent pure water. Thereafter, the 6-way valve (5) is rotated 60 degrees and returned to the original flow path of FIG. Subsequently, the standard solution (4) is sampled in the same step as described above into the titration tank in which the sample is sampled. When sampling the standard solution, the standard solution solenoid valve (2) is opened and the sampling pump (3) is operated.

  Thus, if a sample and a standard solution are extract | collected to the titration tank (8), it will transfer to the process which neutralizes a sample and makes it weakly acidic. Neutralizing acid (9) is added to the titration tank by an acid pump (10). Since the concentration of the sample for neutralization is known in advance, the amount of neutralizing acid to be added may be determined in advance so that the sample is neutralized and becomes weakly acidic.

  When the sample and standard solution are collected in a titration tank and become weakly acidic, the pure water solenoid valve (7) is opened while monitoring the liquid level with the liquid level gauge (12) to keep the liquid level in the titration tank (8) constant. Then, pure water is poured into the titration tank (8) to make the liquid amount a predetermined amount.

  When the liquid amount is set to a predetermined amount, the step of titration is started. Titration is performed in the same manner as the titration of the standard solution.

The number of pulses Ps _{+ st1} measured by titration performed in this step corresponds to the total amount of the chlorine ion amount in the sample and the chlorine ion amount in the standard solution.

  Chlorine ions in the sample are calculated from the results of the above titrations.

First, the chloride ion concentration K per pulse is obtained from the value obtained by collecting and titrating only the standard solution. The calculation formula is based on the above-described formula (1).
K = A / ( _{Pst2- Pst1} ) (1)
(Where A is the chloride ion concentration of the standard solution)
Next, subtract the number of pulses when only the standard solution was sampled and titrated from the number of pulses when the sample and standard solution were sampled and titrated, and multiply by the previously determined amount of chloride ions per pulse. The chloride ion of is calculated.
C = K × (P _{s + st1} −P _st1 ) (2)

(Example)
Under the following conditions, the chloride ion concentration in the liquid caustic soda (concentration of about 32% by weight) solution produced by the ion exchange membrane was analyzed. Table 1 shows the measured number of pulses. Table 2 shows the analysis result of the chlorine ion concentration.
Coulometric titration conditions Highly concentrated alkaline sample solution: about 32% by weight liquid caustic soda 5ml
Standard solution: Sodium chloride aqueous solution having a sodium chloride concentration of 100 ppm 5 ml (when collected once), 10 ml (when collected twice)
Neutralizing acid: 6.6 ml of 8M aqueous nitric acid solution
PH after neutralization: 5
Total volume when titrating: 50ml
Silver electrode: 10 mmΦ × 40 mm silver bar Metal electrode: 6 mmΦ × 50 mm platinum bar Current: 50 mA DC current supplied at 20 Hz Chlorine ion electrode: HS-205S (manufactured by Toa Denki Kagaku Kogyo)
Reference electrode: MR501C (manufactured by ToKo)

(Reference example)
The same sample as in the example was also subjected to manual analysis by precipitation titration.

  About 20 g of high-concentration alkaline sample solution was accurately weighed, and after neutralizing caustic soda with 13M aqueous nitric acid using phenolphthalein as an indicator, an additional 5 ml of 13M aqueous nitric acid was added. After cooling the liquid temperature to 25 ° C., about 5 ml of 0.05M silver nitrate aqueous solution was added dropwise to completely precipitate chloride ions as silver chloride. To this solution was further added 3 ml of 10 wt% aqueous ferric ammonium sulfate solution as an indicator, and silver ions were titrated with 0.05M ammonium thiocyanate aqueous solution with stirring. The chloride ion concentration in the high concentration alkaline sample solution was calculated from the weight of the high concentration alkaline sample solution, the dropping amount of the 0.05M aqueous silver nitrate solution and the required titration amount of the 0.05M aqueous ammonium thiocyanate solution. The results of analysis by the titration method are shown in Table 2.

  According to the analysis method of the present invention, the chlorine ion concentration in a high-concentration alkaline sample solution can be measured with high accuracy as in the precipitation titration method by manual analysis. In addition, the sequencer can automatically analyze trace chlorine ions in a high-concentration alkaline sample, including calibration work by coulometric titration of a standard solution.

FIG. 1 is an automatic analyzer diagram as an example of an embodiment of the present invention. 6-way valve switching diagram

Explanation of symbols

(1) Sample solenoid valve (12) Level gauge (2) Standard solution solenoid valve (13) Coulometric titration Ag electrode (3) Collection pump (14) Coulometric titration metal electrode (4) Standard solution (15) Drainage Solenoid valve (5) Six-way valve for sampling (16) Constant current device (6) Sample loop (17) Ion meter (7) Pure water solenoid valve (18) Controller (8) Titration tank (19) Operation panel (9 ) Acid for neutralization (20) Cl ^- ion electrode (10) Acid pump (21) Comparative electrode (11) Stirrer

Claims (3)

Collect a predetermined amount of standard solution and high-concentration alkaline sample in a titration tank, add neutralizing acid to make it weakly acidic, and then supply a constant current with the silver electrode as the positive electrode and the metal electrode as the negative electrode Electrolysis until chlorine ions disappear, and subtract the amount of electricity when only the standard solution is electrolyzed from the amount of electricity required for the electrolysis to calculate the amount of chlorine ions in the sample, characterized in that the high concentration alkaline sample Of trace ion concentration analysis. A predetermined amount of standard solution is collected in a titration tank, acid is added to make it weakly acidic, a silver electrode is placed in the solution as a positive electrode, and a metal electrode is placed as a negative electrode. After measuring the number of pulses P _St1 required for the electrolysis by consuming and erasing the contained chlorine ions with the generated silver ions, the titration tank is washed and the standard solution is sampled twice as much as the predetermined amount in the titration tank , Weakly _acidify , electrolyze, measure the number of pulses P _St2 , and calculate the chloride ion concentration K per pulse by the following formula (1) in advance. The chlorine ion concentration C in the high-concentration alkaline sample according to claim 1, wherein the chlorine ion concentration C in the high-concentration alkaline sample is calculated from the number of pulses P _{S + St1} required for electrolyzing the sample by the following formula (2): Io Concentration analysis method.
K = A / (P _St2 -P _St1 ) (1)
(Where A is the chloride ion concentration of the standard solution)
C = K × (P _{S + St1} −P _St1 ) (2) The disappearance of contained chlorine ions is detected by an ion electrode using a reference electrode using gelled potassium chloride as an internal solution as a reference electrode. Trace chlorine ion concentration analysis method.

Images