JPS63234147A - Phase difference detecting method for alternating-current application and polarization reaction - Google Patents

Abstract

PURPOSE:To obtain a stable measured value and to improve detection accuracy by supplying a fine AC current between detection electrodes dipped in liquid to be inspected and detecting the polarization state of the liquid to be inspected based on the phase difference between inter-electrode potential generated at this time and the current. CONSTITUTION:The detection electrodes 7 are supplied with the alternating current from an oscillator 111 through a constant current circuit 12. At this time, the potential e0 generated between the detection electrodes 7 is amplified by an amplifier 13 to obtain a voltage E0 and a voltage e1 in phase with the current I is amplified by an amplifier 16 to obtain a voltage E1; and those voltages are inputted to a phase difference detecting means 20, which detects the phase difference between both. The detected phase difference signal fS is inputted to a CPU 14, which sends an instruction based upon the signal fS to a current control circuit 16 to control a tritration current supplied between an anode 2 and a cathode 4. The memory of the CPU 14 is stored with a phase difference corresponding to a tritration end point and the tritration current is controlled to zero when the phase difference is realized.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for detecting the polarization state of a test liquid, and particularly to a method for determining the end point of titration.

[Prior art and its problems]

In coulometric and volumetric titrations, it is important to accurately detect the progress of titration, especially the end point.

One of the methods for detecting the end point of this titration is to immerse a pair of electrodes in the test liquid and apply a small current to them from a constant current device.
There is a polarization potential detection method that controls the titer based on the polarization potential generated at this time.

That is, it utilizes the phenomenon in which the polarization potential becomes large (changes) near the end point of titration, and is used in Karl Fischer moisture meters.

This polarization potential detection method includes a method of applying direct current and a method of applying alternating current. The method of applying direct current has the disadvantage that the measured potential is unstable due to the influence of stirring of the test liquid, making it difficult to determine the end point. In addition, the method of applying alternating current has a smaller effect on stirring than the method of applying direct current, but as shown in Figure 6, as the frequency increases, the change in polarization potential becomes smaller, resulting in lower accuracy. There is,
When trying to obtain highly accurate measurement results,
As disclosed in the '880 publication, it is necessary to use a low frequency in the range of several Hz to several tens of Hz.

However, electrical circuits for processing alternating current in such frequency bands require special consideration in design, and in particular, it is extremely difficult to eliminate noise, so the drawback is that the accuracy cannot be achieved as expected. be.

[Purpose of the invention]

This invention was proposed in view of the above-mentioned conventional circumstances, and uses alternating current in a frequency band that is electrically easy to handle. The purpose of this invention is to provide a method that can detect the polarization state of.

[Means to achieve the purpose]

In order to achieve the above object, the present invention employs the following means. In other words, a small alternating current is passed between detection electrodes immersed in the test liquid, and the polarization state of the test liquid is detected based on the phase difference between the interelectrode potential generated at this time and the current. It is.

The present invention can be applied to, for example, a Karl Fischer moisture analyzer. In this case, when the Ig concentration in the test liquid is low, the phase difference is large, and as the titration progresses and Ig becomes excessive, the phase difference increases. becomes smaller. Therefore, the predetermined 12
By stopping the titration at a phase difference corresponding to the concentration, the H20 concentration in the test liquid can be measured.

The above method can be applied to both coulometric titration and volumetric titration.

〔Example〕

FIG. 1 shows an outline of a Karl Fischer moisture meter using coulometric titration to which the present invention is applied, and since this device itself is well known, its construction will be briefly explained.

A cathode chamber 3 is inserted into the electrolysis chamber 1, and an anode 2 and a cathode 4 for electrolysis are disposed with a diaphragm 5 at the bottom of the cathode chamber 3 in between. A detection electrode 7 is further inserted into the electrolytic chamber 1 . The electrolytic chamber 1 is filled with the generated liquid A, and the cathode chamber 3 is filled with the counter electrode liquid B. By passing a current between the anode 2 and the cathode 4, 12 is generated from the generated liquid A, and the sample is injected. The moisture in the sample liquid injected from the inlet 8 selectively reacts with the generated I2.

Incidentally, the generated liquid A is stirred with a stirrer 6 so that the titration is performed evenly.

The electrical equivalent circuit in the detection electrode 7 is as shown in FIG. 2, where a resistor R1 and a capacitor C1, which vary depending on the water concentration (iodine concentration) and a capacitor C0 inherent between the electrodes, are connected in parallel. Then, the series resistor R0 is further connected. In this equivalent circuit, when there is excess water in the test liquid, it becomes polarized, and since both the resistance bone R1 and the capacitance C1 are large, the potential appearing between the detection electrodes has a phase delay greater than that of the current flowing between the two poles. become. Anode 2. As the titration progresses by passing a current between the cathodes 4 and 12 m degrees increases and becomes a negative state, the resistance bone R9 and the capacitance C
3 becomes smaller. From this, as shown in Figure 3 (al), as the titration progresses, the phase difference between the current (solid line) and potential (broken line) decreases. In Figure 3 (a), the interelectrode potential is The numbers ■■■■ attached to the curves (each broken line) indicate that the titration amount increases in this order.

Here, the potential decreases in the order of the broken lines I can understand different things.

Therefore, if you know in advance the phase difference when twit reaches a predetermined value and stop the titration when this phase difference is reached, it is possible to avoid using the polarization potential detection method (also known as It is possible to measure the moisture concentration of

FIG. 3(a) above shows the phenomenon when 30 Hz alternating current is applied between the detection electrodes 7, and 10001 (
When an alternating current near z is applied, a result as shown in FIG. 3(b) is obtained. That is, the phase difference between the potential curve between the detection electrodes 7 shown by the broken line and the current curve shown by the solid line gradually becomes smaller as the titration progresses.

FIG. 4 shows the frequency characteristics of the conventional method and the method according to the present invention. That is, the detection electrode 7 in the conventional method
The relationship between the frequency of alternating current applied to the electrode and the potential between the electrodes vl...
・V4 and frequency and phase difference F,...F according to the present invention
4 is expressed using the titer as a parameter. The titration is indicated by the 1...4 suffixes attached to the F and V symbols as shown schematically in Figure 6 above.
...corresponds to ■.

As is clear from this figure, in the conventional method, as the frequency increases, the change in the initial potential (potential when water is excessive) and final potential (potential when iodine is excessive) becomes higher in the high frequency region near 1000H2. 3 compared to 30Hz
%(E l 0f10112/E 31182=0.
03), and therefore highly accurate measurements cannot be made in high frequency ranges. On the other hand, in the present invention, even when the frequency approaches 1000 Hz, the change in phase difference due to frequency is around 70% of that at 30 Hz (Fl...nz/Fz.Equation 2=0.69
), it is possible to perform sufficiently accurate measurements even in high frequency ranges.

FIG. 5 shows a circuit diagram for implementing the invention. An alternating current supplied from an oscillator 11 is applied to the detection electrode 7 via a constant current circuit 12, and at this moment, the potential eo generated between the detection electrodes 7 is amplified by the amplifier 13, and the voltage E0 is in phase with the current I. The voltage E is amplified by the amplifier 16.

is input to the phase difference detection means 20, where the phase difference between the two is detected.

The detected phase difference S, is input to the CPU 4, and the detected phase difference S,
The r'U 14 gives an instruction to the current passing circuit 15 based on the signal Sf to control the titration current flowing between the anode 2 and the cathode 4. A phase difference corresponding to the titration end point is stored in the memory of the CPUI 4, and the titration current is controlled to become zero when the phase difference is reached.

As the phase difference detection means 20, a commonly used phase difference detection means can be used as is. To explain the illustrated example,
As shown in FIG. 3 (C1), the comparator 21 outputs a signal at the rHJ level when the voltage E1 corresponding to the applied current is positive and the voltage E0 corresponding to the interelectrode potential e0 is negative, and both are positive. Then, it becomes rLJ level. Also, it becomes rLJ level when both are negative. Therefore, this comparator 21 outputs a signal at rHJ level for a time corresponding to the phase difference between the applied current and the potential between the electrodes. , This rHJ level signal opens the gate 22 and allows the clock signal to pass through.The clock signal that has passed through the gate 22 is counted by the counter 23, and this counted value indicates the above-mentioned phase difference.

Although only coulometric titration has been described above, the present invention can of course be applied to volumetric titration, and can also be used to measure not only water content (but also other chemical quantities).

[Effects of the Invention] As explained above, in this invention, alternating current is passed through the detection electrode, and the polarization state of the test liquid is detected based on the phase difference between the interelectrode potential generated at this time and the above current. Because of this, it is possible to obtain stable measurement values against disturbances such as stirring, and because the applied AC current can be used at a frequency much higher than the commercial frequency, noise can be easily removed. In addition, since it does not require A/D conversion unlike the polarization potential detection method, it is possible to easily improve detection accuracy, and at the same time, the electrical design is extremely simple and inexpensive. be.

[Brief explanation of drawings]

Figure 1 is a schematic diagram of a moisture meter to which this invention is applied. Figure 2 is an equivalent circuit of the detection electrode. Figure 3 is a graph showing the phase difference between the applied current and the potential between the electrodes. Figure 3 (C1 is a timing chart of the circuit shown in Figure 5 below, Figure 4 is a graph showing the frequency characteristics of the conventional example and the present invention, Figure 5 is a block diagram showing a circuit for implementing the present invention, and Figure 6 is a graph showing the frequency characteristics of the conventional example and the present invention. The figure is a graph showing the electrode potential according to the conventional method. Figure 1 Figure 2 Figure 6 Figure ■■■ ■→Uraashi amount Q

Claims (1)

[Claims] A small alternating current is passed between the detection electrodes immersed in the test liquid, and the polarization state of the test liquid is determined based on the phase difference between the interelectrode potential generated at this time and the current. A method for detecting phase difference in polarization reaction with applied alternating current.