JPS5841344A - Voltammetry analysis - Google Patents

Abstract

PURPOSE:To prevent the development of error in the analysis by changing over an applied voltage between a specified measurement voltage and a specified cleaning voltage that are different with a specified temporal interval and estimating the steady current from the current curve for the unsteady state by means of applying the measurement voltage and by cleaning the electrode by applying the cleaning voltage. CONSTITUTION:In voltammetry analysis, an applied voltage is applied by changing-over with a specified temporal internal t1 and t2 to a specified measurement voltage E1 and a specified cleaning voltage E2 that are respectively different. The steady limit current curve (ifinfinity ) is estimated by curve fitting of a current curve in the unsteady state obtained by the measurement voltage E1. On the other hand the substance that crystallizes on the electrode by the cleaning voltage E2 is made to dissolve. With this arrangement the measurement time t1 can be within 1sec and the cleaning time within about 2sec, and one cycle can be within a few seconds. Furthermore, exact analysis can be made with error due to pollution of the electrode prevented.

Description

[Detailed description of the invention] The present invention relates to a portammetric analysis method.

Recently, the portammetry method and various electrochemical analysis methods that apply it have been developed and put into practical use in large numbers.
This allows for faster analysis compared to other analytical methods.

However, most of these methods directly measure and analyze the diffusion-limited current in a steady or quasi-steady state obtained by applying a constant applied voltage or a low-speed sweep applied voltage.

Therefore, in the case of this method, the electrode surface condition changes due to the substance to be measured, interfering substances, and substances generated from these substances by electrode reactions, resulting in changes in sensitivity and errors due to this. There are drawbacks.

In this respect, AC polarography similarly has errors due to electrode contamination.

In order to avoid these problems, methods of mechanically polishing the electrode surface or physically renewing the electrode surface such as dripping mercury electrodes have been used, but these have mechanical problems. However, it has the disadvantage that it is inconvenient to handle the p% compost.

In addition, in order to remove contamination from the electrode surface, a method is known in which a voltage with the opposite polarity to the voltage applied during measurement is applied and the contaminants are eluted by electrolysis.
The measurement must be frequently interrupted, making continuous measurement difficult, and there is a problem in that there is a difference between the data immediately before and after the electrolysis, which reduces the reliability of the data.

The present invention was made for the purpose of solving the above-mentioned problems, and it makes the measurement of the limiting current and the cleaning of the electrode surface extremely easy.
By continuously dropping lines in a short period of time, accurate analysis can be performed in a short period of time, and a method capable of continuous measurement is provided.

A feature of the present invention is that the applied voltage is switched between predetermined different measurement voltages and cleaning voltages at predetermined time intervals, and an unsteady state current curve is obtained by applying the measurement voltage, and an electrode surface @ 1) is obtained by applying the cleaning voltage. Another feature of the present invention is that the steady-state limit current is estimated by performing cleaning and using the above-mentioned non-steady-state current self-contained curved fitting. The point KTo is that a voltage for elution is further applied to more effectively wash fIpt when the electrode surface is contaminated with two or more types of substances.

The present invention will be described below with reference to Attachment 11m.

Figure 1 shows a schematic circuit diagram of the polmenmetry method, with tr
i potentuostat, 2 is electrolytic electrode, 3 is working electrode, 4
The equivalent circuit of the working electrode 3 of the potentiostat constructed as shown in FIG. 1 is p as shown in FIG. 2, where 5 is a counter electrode and 5 is a reference electrode.

Here, Rj2 is electrode resistance %R@ is electrode reaction resistance, Cdt
F! Electric double layer capacitance, dA is Faraday current, and ldA is electric double layer charging current. The voltage E shown in FIG. 2 is regulated to be equal to the voltage l shown in FIG. 1 by the operation M of a potentiostat.

By adjusting the voltage E and the temperature of the solution tube to be measured, and after a sufficient period of time has passed after applying the voltage, conditions are set so that the component of Re approximates the resistance Bf due to the migration of reactive species. A general polar graph is used to determine If, that is, the limit current 1ftxs at that time.

On the other hand, in the first method according to the present invention, as shown in FIG. Third
@Current curve of unsteady current i shown in B (Range of 11! 2I)
By curve fitting, the limiting current 1teat
This is used to estimate and analyze the solution to be measured. Here, the measuring voltage generally refers to the voltage at which the desired reaction becomes diffusion-limited, that is, the plateau voltage, and the cleaning voltage and the voltage at which the substance deposited on the sample electrode elutes. Here, as shown in Figure 3B, The current shield shown is the sum of the circle and idt shown in Figure 2.

H-th+t ab (1) On the other hand, according to FIG. 2, these are 8. E-ma + (IdA tenth) for v
Rj (2) 1=v+[Cdj(ty/dt)+w/Re)Rg
It has the relationship (2').

Generally, the electric double layer capacitance Cdt changes depending on the electrode reaction resistance R,%! It is difficult to solve equation (2') analytically, but for reactions where the electrode reaction rate is sufficiently faster than the diffusion rate, the concentration is If the solution to be measured is kept at an appropriate constant flow rate so that the boundary layer becomes thinner (for example, flowing the solution to be measured at a constant flow rate in an electrolytic cell), a simple approximation formula can be used. The limiting current 1foo is calculated using
becomes possible to estimate. For example, when measuring the degree of residual chlorine immersion in a solution to be measured, a flow rate of approximately α5 m/sec is sufficient. In such a case, generally for the current 1 in Figure 3B, 1(t) -1fco + Rj6 @xp(
-bt) The approximation formula (3) can be applied, and the predetermined time (1
tfaot" can be obtained by fitting the above equation (3) to the data in the range of 1). This limiting current 1fo.

Since there is generally a linear relationship between If66 and the concentration of the substance to be measured in the solution to be measured, the concentration of the solution to be measured can be easily determined by creating a calibration curve between If66 and the concentration in advance.

The above approximation formula can also be replaced with a suitable approximation formula depending on the current curve. In the method of the present invention, the applied voltage switching cycle time is completed within a few seconds, so in practice, continuous This can be regarded as a measurement, and the measurement time (tl) is sufficient within 1 second, and electrode changes during this time can be ignored.

Note that the measurement voltage E1 may be set to positive or negative depending on the substance to be measured in the solution to be measured.*
The tl cleaning voltage Et may be set to have the same polarity as the measurement voltage, as well as the opposite polarity. In general, the measurement voltage He varies depending on the type of substance to be measured, but the measurement voltage He is -2v to +
1! OO mV, the interference voltage E1 is in the f 3 VC range (provided that this voltage represents voltage 8 shown in FIG. 2, and the voltage on the potentiostat has the opposite polarity).

According to the method of the present invention, the concentrations of residual chlorine, sodium thiosulfate, hydrazine, sodium sulfite, silane, phosphine, arsine, and many other substances in a solution to be measured can be quantitatively and accurately measured251. Next, the relationship between the estimated limit current obtained by the method of the present invention and the concentration of the solution to be measured, and the relative sensitivity will be explained by showing examples.

Example 1゜Residual chlorine too q/1-rot/l @ Fltv subject fl
For J'r constant solution, the area is 1.2 wm as a working electrode.
” using a platinum electrode, an As+ -APC4 reference electrode,
Using a counter electrode with an area of 1278 Us and 316 L, the measurement was carried out by applying the applied voltage shown in FIG. 4 under the conditions of the flow velocity of the solution to be measured α5V) and t+ -500 mSlow 11. Obtained current 1
Among the data of (t), t-5011s 4500
For the data in mm, 1(t)-j fao + l f s
*xp (-bt n (:I)t fitting was performed to find 1fao, and excellent linearity was obtained for the residual chlorine concentration as shown in Figure 5.

As is clear from Fig. 5, it is possible to know the limit current value of the solution to be measured very accurately from the limit current value estimated by this method.
It also has the advantage of not being affected by H.

! l! Under the same conditions as gastric flow side 1, add hypochlorite to general tap water and adjust by adding IJum to 1000QB/leq.
Regarding the solution to be measured with C1t, 1foo was continuously measured for a long period of time in the same manner as in Example C, and the change in relative sensitivity with respect to the number of days t-m was plotted. For comparison, the conditions were the same as above except that the applied voltage was not changed and B -600yaVv*APCl was used, and the limit current 1foo that reached a steady state was determined directly, and the change in relative sensitivity with respect to the number of days elapsed was investigated. Ta. The results are shown in FIG. In the case of the method of the present invention (curve T), almost no change in sensitivity occurs, but the steady state limit current t[I! In the case of the conventional method measured (curve n), the relative sensitivity decreased steadily as the number of days elapsed. In addition, the song ill
(It is assumed that the reason why the increased relative sensitivity is not observed is due to the peeling off of deposits on the surface of the working electrode, the adhesion of the catalytic substance, etc.)

As described above, the method of the present invention enables accurate analysis in a short time. Furthermore, there is no need to mechanically polish the electrode surface, making analysis operations extremely simple. Furthermore, since there is almost no change in sensitivity due to constant sensitivity, continuous measurement is possible over a long period of time, which is an advantage.

As described above, according to the method of the present invention, ionic contaminants (mainly metal I
I) is precipitated and the relative sensitivity changes,
These contaminants t-eluted voltage 82: Therefore, the electrodes are regularly washed for one period, and the electrodes are always maintained in a constant state, making it possible to stably measure 7.However, the voltage Bs# applied to the solution to be measured is Conversely, if a substance that would deposit on the electrode coexists, the above method is inappropriate because it changes the state of the electrode and affects the sensitivity.

/ In such a case, the second method according to the present invention! (1 can be suitably adopted.

That is, in the second method according to the present invention, the applied cleaning voltage Im
The voltage is maintained at a constant state by further applying an applied voltage that causes the precipitated substance to elute.

The present method will be explained in more detail with reference to Examples below.

Example 1 Add sodium thiosulfate (Nax8*Oi) to tap water so that αO and υyts* are obtained as sulfur (8),
Measurements were carried out in the same manner as in Example 2, except that sodium hypochlorite was further added and the applied voltage shown in the residual snow was used. The obtained relative sensitivity change over time is shown in FIG. 6 (curve mark). For comparison, the case where the method of Example 2 is applied (curve 1) and the case where the steady state limit current is directly determined without changing the applied voltage (curve v) are combined. Shown.

As is clear from Figure 6, the cleaning voltage H! Conversely, if there is a substance to be measured that precipitates on the electrode, applying a voltage that causes the substance to elute will effectively clean the electrode surface and reduce the relative sensitivity. Don't change. This means that the ionic contaminants deposited on the working electrode surface during the measurement are removed by the cleaning voltage E! On the other hand, contaminants such as dangerous sulfur precipitated by application of the second washing voltage IIs are eluted by the second washing voltage ml.

Therefore, the cleaning voltage]! In the case of a solution to be measured in which two or more types of substances that precipitate according to 1 are present, it is also possible to apply 92 or more types of applied voltages Th, Bs2, etc. depending on each substance.

Incidentally, in the method of the present invention as well as in other portammetry methods, the sensitivity changes somewhat depending on temperature, but conventional methods can be applied to correct this as is.

[Brief explanation of drawings]

Figure 1 is a schematic circuit diagram of the portammetry method, Figure 2 is an equivalent circuit diagram of the potentiometer O working electrode shown in Figure 1,
Figure 3 shows switching of applied voltage by the method of the present invention (Figure 3 person)
and the accompanying change in current (Figure 3 is a schematic explanatory diagram showing Bat-, Figure 4 is a graph showing the switching tube of the applied voltage used in Example 1, Figure 5 is the estimated limit current measured in Example 1) Fig. 61 is a graph showing the relationship between FE @ field current and the residual chlorine concentration, Fig. 61 is a graph showing the relative sensitivity of the field current measured in Example 2, and Fig. 7 is a graph showing the relationship between the value and the residual chlorine concentration. FIG. 8 is a graph showing the change in relative sensitivity of the limiting current measured in Example 3. 1 is a potentiostat, 2 is an electrolytic cell, a working electrode from 3 companies,
4 is the opposite pole, 5 is the welfare reference pole.

Claims (1)

[Claims] (+) The applied voltage is switched between a predetermined different measurement voltage and a cleaning voltage at predetermined time intervals, and a current mill S in an unsteady state is obtained by applying the measurement voltage, and at the same time, the application of the cleaning voltage is A portammetry analysis method characterized by estimating a steady-state limit current by curve-finting of the unsteady-state current curve obtained by cleaning the stained electrode surface. (2) Switch the applied voltage between predetermined different measurement voltages and washing voltages at predetermined time intervals, and estimate the steady-state limit current by curve fitting the current curve in the unsteady state obtained by applying the measurement voltages. A portammetry analysis method characterized in that the electrode surface is effectively washed by further applying a voltage that causes substances precipitated on the electrode to elute by the washing voltage.