JPS5841343A - Voltammetry analysis - Google Patents

Abstract

PURPOSE:To make analysis without developing error due to the change in sensitivity of the surface condition of an electrode by applying instantaneously for a short time a specified voltage and estimating the steady limit current from a current curve in the unsteady state that is obtained before the electrode reaction reaches a steady state. CONSTITUTION:In voltammetry analysis, a specified voltage E1 is applied for a time t1 (within 1 second) to seek ifinfinity (steady limit current) by the curve-fitting of a current curve in the unsteady state before the electrode reaction reaches the steady state. Because in this way the time is not given in which the electrode surface changes by a substance to be measured, obstructing substance and substances produced by electrode reaction and the sensitivity changes to develop error, the analysis is exact and the measurement is made in a very short time.

Description

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

Recently, the portammetry method and nine different electrochemical analysis methods using it have been developed, and many of them have been used in dormitories.
This allows for faster analysis compared to other analytical methods.

However, in most of these methods, the steady-state %L< obtained by applying a constant applied voltage or a low-speed sweep applied voltage directly measures the diffusion-limited current in a quasi-steady state, and performs the entire analysis.

In case of this method, #Ic is caused by changes in the electrode surface state due to the substance to be measured, interfering substances, and substances generated from these substances by electrode reactions, resulting in changes in sensitivity. This has the disadvantage of causing errors due to In this respect, the AC pump 7 similarly has errors due to electrode contamination.

To avoid these problems, methods have been used to mechanically polish the electrode surface or physically renew the electrode surface, such as with dripping mercury electrodes, but these methods have mechanical problems and are difficult to handle. The disadvantage is that it is inconvenient.

The present invention has been made to solve the above-mentioned problems, and it provides a method that allows accurate analysis to be carried out in a short period of time without changing the surface condition of the electrode. be.

A feature of the present invention is that a predetermined applied voltage is instantaneously applied for a short period of time, and the steady-state limit current is predicted from the S unsteady-state current curve caused by the applied voltage before the electrode reaction reaches a steady state. Then, the analysis value was determined and it came in second place.

The present invention will be described below with reference to the accompanying drawings.

Figure 1 shows a schematic circuit diagram of the portammetry method, where 1 is a potentiostat, 2 is an electrolytic cell, 3 is a working electrode, and 4t
The equivalent circuit of the working electrode 3 of the potentiostat constructed as shown in FIG. 1, where f is the counter electrode and 5 is the reference electrode, is similar to that shown in FIG.

where %, Fta is electrode resistance, Re is electrode reaction resistance,
Cdt is electric double layer capacitance, 1f is 7 Allada current, Id
td electric double layer charging current. Voltage 3 shown in FIG. 2 is regulated to be equal to voltage E in FIG. 1 by the action of a potentiostat.

By the way, by appropriately adjusting the voltage E and the solution to be measured, and by allowing sufficient time after applying the voltage, conditions are created so that the component of R@ approximates the resistance Rf due to the movement process of the reactive species, and then It is a general polarographic method to find If, that is, the limiting current ω.

In contrast, in the method of the present invention, as shown in the third factor, a predetermined applied voltage TA1, generally a voltage at which the target reaction is diffusion-limited, that is, a plateau voltage lz, is applied for a short time,
By curve fitting the current curve (range of tl) of the unsteady current 1 shown in FIG. 3B obtained thereby, the limiting current 1fao is estimated and the solution to be measured is analyzed.

Here, the current 1 shown in FIG. 3B is 1f shown in the second garden.
and ldA.

1-1f+ldA (+) On the other hand, from Figure 2, these are E-η+ (Idt 10th) for B and W.
na υ)B −# +(Cdt(ty/dt)+
v/'Re) R# (2').

Generally, the electric double layer capacitance Cdt changes rapidly, and the electrode reaction resistance R@%ma and the time 1 (1
+ range). Although it is difficult to solve the equation (2) analytically, for reactions where the electrode reaction rate is sufficiently faster than the diffusion rate, a uniform constant constant is applied so that the thickness of the concentration boundary layer becomes thinner. If the solution to be measured has a certain flow rate (for example, flowing at a constant flow rate through an electrolytic cell), it is possible to estimate the critical current value Φ using a simple approximation formula. 1. For example, when measuring the residual chlorine concentration in a solution to be measured, a flow rate of approximately α51E/sec is sufficient.In such cases, generally
! Regarding pHB current lK, quantity (@) - eye ■ ten o * xp (-bt) (3
) can be applied, and by fitting the above equation (3) with data for a predetermined time (range of tz), it is possible to find the limit current. Since there is generally a linear relationship with the concentration of
By creating a calibration curve between So and concentration, the concentration of the solution to be measured can be easily determined. Note that the above approximate formula is
It is also possible to replace it with a more appropriate approximation formula depending on the current curve.

In the method of the present invention, the measurement time (1<) is rounded to within 1 second, which is sufficient, and there is no change in the electrode surface condition during this time, and even if there is, it is 1 degree that can be ignored. Therefore, there is no longer a problem in the conventional method that sensitivity changes due to changes in the electrode surface condition and errors caused by this change occur.

Note that the applied voltage 1+ may be set positive or negative depending on the substance to be measured in the solution to be measured.

In general, although it varies depending on the type of substance to be measured, the applied voltage E
1 is in the range of -2V to +800mV (however, this voltage is shown in Figure 2 as the voltage it, and the voltage on the potentiostat is of the opposite polarity), and the actual measurement #IC
When the voltage is low, the voltage lx that does not cause an electrode reaction may be switched to the voltage B1.

According to the method of the present invention, the concentrations of many substances such as residual chlorine, sodium thiosulfate, hydrazine, sodium sulfite, silane, phosphine, arsine, etc. in a solution to be measured can be quantitatively and accurately measured.

Next, the relationship between the estimated limiting current obtained by the method of the present invention and the concentration of the solution to be measured will be explained.

For a solution to be measured with residual chlorine of about 100 ail/l to IQ f/l, a platinum electrode with an area of about 1.2-m was used as the working electrode, and an AF-mu aCt reference electrode with an area of 12 dS was used as the working electrode.
Using U8316L counter electrode, flow velocity α5 of the fixed solution
m/sec, B s-α6V vs, AFCt
It was measured under the condition of %tt-500ms. Obtained current 1
(t), t -50ms -500m-
Regarding the data of 1 (t)-1fce ten o exp(-bt)
(!D) was used to find the value ψ, and excellent linearity was obtained for the residual chlorine concentration as shown in Figure 4.

As is clear from FIG. 4, the concentration of the solution to be measured can be determined extremely accurately from the limiting current value estimated by the method of the present invention. Another advantage is that the estimated limiting current value is not affected by the pH of the solution to be measured.

In this manner, according to the method of the present invention, accurate analysis is possible through extremely short measurement times.

Furthermore, since the applied voltage is instantaneous, there is no change in the electrode surface condition, there is no need to horizontally polish the electrode surface, and there is also the advantage that there is almost no change in sensitivity due to measurement.

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

[Brief explanation of drawings]

Figure 1 is a schematic circuit diagram of the portammetry method, Figure 2 is an equivalent circuit of the working electrode of the potentiostat shown in Figure 1, and Figure 3 shows the applied voltage (Figure 3) and its FIG. 4 is a graph showing the relationship between the estimated limit current measured by the method of the present invention and the residual chlorine concentration. lFi potentiostat, 2 is an electrolytic cell, 3 is a working electrode, 4 is a counter electrode, and 5 is a reference electrode. Applicant Bionics Shoki Co., Ltd. Agent Patent Attorney Masaaki Hihara Patent Attorney Tadashi Hamamoto

Claims (1)

[Claims] A portammetry analysis method characterized by applying a predetermined applied voltage for a short time and estimating the steady-state limit current by curve fitting the obtained unsteady-state current curve.