JPS5937464B2 - Coulometric titration method - Google Patents

Description

[Detailed description of the invention] The present invention relates to a coulometric titration method.

Coulometric titration, which is used for neutralization titration, oxidation-reduction titration, etc., is a process in which a titration reagent is generated by electrolysis, and this is reacted with the analyte in an electrolytic solution to perform titration. This method measures the amount of electricity that has flowed until the end of the titration, and calculates the amount of reagent required for the titration from Faraday's law of electrolysis. Therefore, the amount of electrolyzed electricity in coulometric titration and the amount of the analyte titrated must have a constant relationship based on Faraday's law, but the relationship between the two may not necessarily be constant due to a decrease in electrolytic efficiency. No.

The decrease in electrolytic efficiency is caused by ions leaking through the diaphragm between the electrolytic cell and the counter electrode cell due to diffusion and electrophoresis.5 To reduce this, a diaphragm is used to separate the two cells. A titration cell device equipped with an intermediate tank has been proposed. However, even when using this titration cell device, there is a drawback that as the number or amount of titration increases, the leakage of ions also increases and the electrolytic efficiency decreases. The decrease in electrolytic efficiency is due to
This can be alleviated by using a diaphragm with low ion diffusion and increasing its thickness, but in this case, the electric resistance of the electrolyte is large and a large current cannot be passed, so the titration takes a long time. For this reason, conventional methods have given up on repeatedly measuring highly concentrated samples in a short period of time with high precision.Therefore, the present inventors have developed a titration cell device with an intermediate tank as described above. As a result of intensive studies on measures to suppress the decrease in electrolytic efficiency, we learned that a method of electrolytically adjusting the ion concentration in the intermediate tank to a predetermined concentration is extremely effective, and based on this knowledge, we completed the present invention. did.

Hereinafter, the present invention will be explained in detail with reference to the drawings.

FIG. 1 is a schematic diagram of an example of an apparatus used in the method of the present invention. In the figure, 1 is a titration cell device, 2 is an electrolytic tank,
3 is a first counter electrode tank, 4 is a second counter electrode tank, 5 and 6 are diaphragms, 9
is the first electrolytic electrode, 10 is the second electrolytic electrode, and 11 is the third electrolytic electrode.
12 is an end point detection electrode, 13 is an ion concentration detection electrode, 14 is an electrolysis control device for adjusting ion concentration, 15 is an electrolysis control device for coulometric titration, and 16 is a display device for displaying the amount of electricity for coulometric titration. The titration cell device 1 is composed of three tanks: an electrolytic tank 2 made of glass or acrylic resin, a first counter electrode tank 3, and a second counter electrode tank 4, and these three tanks are made of porous sintered glass, Well-known diaphragm 5, such as ceramic or glass filter coated with agar or silica gel.
6, the electrolytic cell 2 and the first counter electrode cell 3, the first counter electrode cell 3
and the second counter electrode tank 4 are in communication with each other.

A first electrolytic electrode 9 and an end point detection electrode 12 are arranged inside the electrolytic cell 2 . The first electrolytic electrode 9 is an electrode for producing a titration reagent, and a well-known electrode can be used, but one made of a platinum wire mesh is recommended. The end point detection electrode 12 is appropriately selected depending on the purpose of titration, and for example, a PH measuring electrode is used for neutralization titration, and a platinum electrode is used for redox titration. Moreover, a rotor 7 for stirring is installed in the electrolytic cell 2. A second electrolytic electrode 10 and an ion concentration detection electrode 13 are arranged within the first counter electrode tank 3 . The second electrolytic electrode 10 plays the role of a counter electrode for coulometric titration or an electrode for generating ions for adjusting the ion concentration in the first counter electrode tank 3, so the second electrolytic electrode 10 is similar to the first electrolytic electrode 9. preferable. Since the ion concentration detection electrode 13 is for detecting the ion concentration of the solution in the first counter electrode tank 3, a PH measurement electrode, platinum electrode, etc. similar to the end point detection electrode 12 is used. Moreover, a rotor 8 for stirring is installed in the first counter electrode tank 3 . A third electrolytic electrode 11 serving as a counter electrode to the first electrolytic electrode 9 and the second electrolytic electrode 10 is installed in the second counter electrode tank 4 . This third electrolytic electrode 11 is also preferably similar to the first electrolytic electrode 9. The electrolysis control device 14 for adjusting ion concentration includes the ion concentration detection electrode 1
The electric signal corresponding to the ion concentration of the solution in the first counter electrode tank 3 obtained from 3 is set to a preset electric signal value (usually the electric signal value obtained from the end point detection electrode 12 at the end of coulometric titration is adopted). Ru.

), or the electric signal corresponding to the ion concentration of the solution in the electrolytic cell 2 obtained from the end point detection electrode 12 is compared with the set electric signal value, and the electrolytic current commensurate with the deviation is set as the second electric signal. electrolytic electrode 10 and third electrolytic electrode 1
1, and as shown in Figure 2,
It is composed of detection amplification sections 19 and 20, an ion concentration deviation detection section 21, an ion concentration setting device 23, and an electrolytic current control section 24. The detection amplification units 19 and 20 are connected to the ion concentration detection electrode 1
3 and the end point detection electrode 12, the ionic potential or polarization potential of the solution is detected, and this is converted and amplified into a DC voltage signal insulated from that potential. Various types of amplifiers are used depending on the purpose, such as a polarization potential amplifier using an AC constant current differential voltage detection method (see Japanese Patent Laid-Open No. 49-83495). A normal subtraction circuit and a potential setting device are used as the ion concentration deviation detection section 21 and the ion concentration setting device 23, respectively. Further, as the electrolytic current control section 24, a constant current output on/off control method, a continuous proportional output control method, etc., which are conventionally used as an electrolytic current control section for coulometric titration, are used. The coulometric titration electrolytic control device 15 and the coulometric titration electric test display device 16, as shown in Japanese Patent Laid-Open No. 49-83495, calculate the electric signal obtained from the end point detection electrode 12 in proportion to the deviation from the end point value. The first electrolytic electrode 9
and the third electrolytic electrode 11 or between the first electrolytic electrode 9 and the second electrolytic electrode 10, the supplied current value is integrated, and an analysis value corresponding to the amount of electricity is obtained. A well-known device configured to be displayed is used.

Next, the case where acid neutralization titration is performed using the apparatus configured as described above will be described.

As a titration method, the electrolytic current for coulometric titration is applied to the electrolytic electrode 9.
and 11 and between the electrolytic electrodes 9 and 10, the first counter electrode tank 3
There are four methods: adjusting the ion concentration of the solution in the electrolytic cell 2 to a preset concentration, and adjusting and maintaining it at the same concentration as the ion concentration of the solution in the electrolytic cell 2. will be applied to the electrolytic electrodes 9 and 11 to adjust and maintain the ion concentration of the solution in the first counter electrode tank 3 at a preset concentration. First, 1 for the three tanks 2, 3, and 4.
% of sodium sulfate, potassium chloride, or other electrolyte.

Next, put the sample into the electrolytic bath 2, close the switch 17 to the contact b side, close the switch 22 to the contact d side, and then switch 18.
close. Then, an electrolytic current is applied from the coulometric titration electrolysis control device 15 to the electrolytic electrodes 9 and 11, and coulometric titration is started. This amount of electrolytic electricity is supplied at an arbitrarily set current value until the electrical signal corresponding to the hydrogen ion concentration obtained from the end point detection electrode 12 reaches a preset end point value. Then, the amount of electricity used for the titration is converted into a number of coulombs or a number of gram equivalents and displayed on the titration amount display device 16 as a titration result. On the other hand, after the start of titration, when the hydrogen ion concentration of the solution in the first counter electrode tank 3 starts to rise, in the electrolytic control device 14 for adjusting ion concentration,
The signals from the detection amplification section 19 and the ion concentration setting device 23 are compared in the ion concentration deviation detection section 21, and an electrolysis current corresponding to the deviation is sent from the electrolysis current control section 24 to the electrolysis electrode 10,
11 and leaked into the first counter electrode tank 3 are electrolytically neutralized, so that the hydrogen ion concentration of the solution in the first counter electrode tank 3 is maintained at a predetermined concentration.

When adjusting the hydrogen ion concentration of the solution in the first counter electrode tank 3 to be the same as the hydrogen ion concentration of the solution in the electrolytic tank 2, the switch 22 may be closed to the contact c side. In this case, a signal to be compared with the signal obtained from the detection amplifier 19 is introduced from the detection section 20 to the ion concentration deviation detection section 21, and is thereafter controlled in the same manner as described above so that the deviation between both signals is always zero. Ru. Therefore, the hydrogen ion concentration of the solution in the first counter electrode tank 3 reaches its maximum value at the start of the coulometric titration, but as the titration progresses, the hydrogen ions are successively electrolytically neutralized.
The hydrogen ion concentration of the solution in the electrolytic cell 2 is maintained. Further, when applying electrolytic current for coulometric titration to the electrolytic electrodes 9 and 10, the same operation as described above may be performed with the switch 17 closed to the contact a side.

The case of acid neutralization titration has been described above, but in the case of alkali neutralization titration, the polarity of the electrolytic current applied to the electrolytic electrode may be reversed. Further, although it is preferable to detect the ion concentration electrically, it can also be detected by a change in color.

In this case, an indicator that changes color at a set hydrogen ion concentration is added to the first electrode tank 3, the color change is detected optically or with the naked eye, and electrolytic current is supplied to the electrolytic electrodes 10 and 11. do it. As described above in detail, the present invention prevents a decrease in electrolytic efficiency by a simple method of electrolytically adjusting and maintaining the ion concentration of the solution in the first counter electrode tank at a predetermined concentration.

Furthermore, since there is no need to use a diaphragm with particularly low diffusion or electrophoresis, there is an advantage that a large current can be passed and the titration time can be significantly shortened. Therefore, the present invention provides a coulometric titration method for neutralization titration of acids and alkalis, moisture measurement by Karl Fischer method, oxidation-reduction titration such as analysis of sulfur and nitrogen, especially when performing repeated titrations using highly concentrated samples. Are better. EXAMPLES Hereinafter, the present invention will be explained in more detail with reference to examples, but the present invention is not limited to the following examples unless it exceeds the gist thereof.

Example: The effective area of the diaphragm 5 of the device shown in FIG. 1 is about 10 cd,
Approximately 2 mm of agar is welded to a G4 glass filter (thickness: approx. 1.5 m 1 Lf), effective area as diaphragm 6 is approx. 10
cd, a ceramic filter with a thickness of about 1 mm, a platinum mesh as the electrolytic electrodes 9, 10, and 11, a PH composite electrode as the end point detection electrode 12 and the ion concentration detection electrode 13, and 1% Na2sO in each tank of the titration cell device 1. Neutralization titration of 0.1N hydrochloric acid (F-0.966) was performed using an apparatus containing a resting solution (switch 17 was closed to the contact b side).

During the titration, the pH of the solution in the first counter electrode tank 3 was automatically adjusted to 7.0. The results obtained are shown in Tables 1 and 2. For comparison, Table 2 also shows the results of neutralization titration performed in exactly the same manner as in the example except that the pH of the solution in the first counter electrode tank 3 was not adjusted.

As is clear from Table 2, in the comparative example (conventional method), the electrolytic efficiency gradually decreased as the number of measurements progressed, and it became impossible to measure at the 6th measurement, whereas in the method of the present invention, the amount of sample was increased. However, even if the number of measurements is increased, the electrolytic efficiency remains between 99.7 and 99.
9, indicating that measurement can be performed with high accuracy.

[Brief explanation of the drawing]

FIG. 1 is a schematic block diagram of an example of a coulometric titration device according to the present invention, and FIG. 2 is a →I circuit block diagram of an electrolytic control device for adjusting ion concentration. 1... Titration cell device, 2... Electrolytic cell, 3... First
Counter electrode tank, 4...Second counter electrode tank, ゜5, 6...Diaphragm, 9
．． DESCRIPTION OF SYMBOLS 10, 11... Electrolysis electrode, 12... End point detection electrode, 13... Ion concentration detection electrode, 14... Electrolysis control device for adjusting ion concentration, 15... Electrolysis control device for coulometric titration, 16 ...Coulometric titration electrical quantity display device.

Claims (1)

[Claims] 1. An electrolytic cell in which a solution containing a sample is housed, and a first counter electrode cell;
When carrying out coulometric titration by supplying an electrolytic current to the electrolytic cell using a titration cell device in which the first counter electrode cell and the second counter electrode cell are connected to each other via a diaphragm, changes occur as the titration progresses. A coulometric titration method, characterized in that the solution in the first counter electrode tank is electrolyzed by a second electrolytic current different from the coulometric titration electrolytic current, and adjusted to a predetermined ion concentration. 2 Coulometric titration is performed by applying an electrolytic current between the electrolytic electrodes installed in the electrolytic cell and the second counter electrode tank, and the ion concentration of the solution in the first counter electrode tank is electrically detected. , converting the obtained electric signal corresponding to the change in ion concentration into a second electrolytic current different from the electrolytic current, and converting the second electrolytic current into an electrolytic electrode installed in the first counter electrode tank. 2. The coulometric titration method according to claim 1, wherein the ion concentration of the solution in the first counter electrode tank is adjusted by applying voltage between electrolytic electrodes in the second counter electrode tank. 3 Perform coulometric titration by applying an electrolytic current between the electrolytic electrodes installed in the electrolytic cell and the first counter electrode tank, and electrically detect the ion concentration of the solution in the first counter electrode tank. Then, the obtained electric signal corresponding to the change in ion concentration is converted into a second electrolytic current different from the electrolytic current, and the second electrolytic current is connected to the electrolytic electrode in the first counter electrode tank and the electrolytic current. 2. The coulometric titration method according to claim 1, wherein the ion concentration of the solution in the first counter electrode tank is adjusted by applying voltage between electrolytic electrodes installed in two counter electrode tanks.