RU2382356C1 - Method for switching chronoamperometry - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: invention relates to electrochemical methods of determining concentration of elements in aqueous solutions and can be used in industry in analysing solutions, in monitoring environmental objects, food products and other objects, especially in continuous and automatic measurements, as well as for amperometric detection in liquid chromatography. According to the invention, the method involves measuring electrochemical reaction current at constant potential of the working electrode. The current loop of the electrochemical cell is periodically switched, and amplitude of the first harmonic of the resultant pulsed current which is proportional to concentration of the determined element is measured.

EFFECT: invention provides high sensitivity to concentration of the determined element with high efficiency at the same time.

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Description

The invention relates to electrochemical methods for determining the concentration of elements in aqueous solutions, can be used in industry in the analysis of solutions, in the control of environmental objects, food products and other objects, especially in continuous and automatic measurements, as well as for amperometric detection in liquid chromatography.

In chronoamperometry, there is a wide variety of methods for both stimulation and measurement of the useful signal, aimed primarily at reducing the influence of the charging current of the double layer.

A known method of amperometric detection (US Patent No. 6413398, IPC G01N 027/26, publ. July 2, 2002). The method consists in applying a sequence of voltage pulses to the working electrode and registering the current arising from this. Due to the presence of voltage pulses for preliminary preparation of the working electrode, the method has high sensitivity, but the capacitive current resulting from the application of pulses does not allow to achieve the desired sensitivity. In addition, the need for exposure of the working electrode under the preparation potentials reduces the analysis performance.

A known method of square-wave voltammetry (Budnikov G.K. Fundamentals of modern electrochemical analysis / G.K. Budnikov, V.N. Maistrenko, M.R. Vyaselyov. - M.: World: Binom LZ, 2003, S.359- 371), which consists in the fact that an alternating impulse voltage with a small amplitude is superimposed on a constant or slowly changing polarizing voltage. The emerging pulsed current is measured at the end of each half-cycle when the capacitive current is minimal. This selection method is not effective enough, since the need for a measurement delay leads to a decrease in the useful signal, while the influence of the capacitive current remains significant.

The closest in technical essence and the achieved result to the claimed one is a two-stage chronoamperometry method, which consists in registering the current first at a potential jump from E ₁ to E ₂ , and then at a reverse jump from E ₂ to E ₁ (Electroanalytical methods. Theory and practice / Under Edited by F. Scholz; - M .: BINOM LZ, 2006, S.144-145). The resulting current-time dependencies are integrated and mutually subtracted. Integration reduces noise, and subtraction leads to the suppression of the capacitive component of the residual current. The method is effective for the case of using micro- and ultramicroelectrodes, since the value of the capacitive current for such electrodes is insignificant in comparison with the capacitive currents of macroelectrodes. However, in the general case, in the method of two-stage chronoamperometry, the complete elimination of the capacitive current cannot be achieved, since the value of the capacitance depends on the potential and can significantly differ at potentials E ₁ and E ₂ . The fundamental presence of capacitive current as interference is a consequence of a change in the potential of the working electrode.

The main task to be solved by the claimed invention is directed is the creation of a chrono-amperometric method of analysis combining high sensitivity to the concentration of the element being determined and productivity.

The problem is solved due to the fact that in the method of switching chronoamperometry, as well as in the prototype, the current of the electrochemical reaction is measured. According to the invention, the potential of the working electrode, at which the electrochemical reaction proceeds, is kept constant, the current loop of the electrochemical cell is periodically switched and the amplitude of the first harmonic of the emerging pulse current is measured, which is proportional to the concentration of the element being determined.

In order to determine the capabilities of the proposed method, we analyze the processes occurring in the electrochemical cell as a result of a simple redox reaction of some reversible element with a concentration of Cox. To simplify the analysis and avoid cumbersome calculations, we make the following assumptions:

- the measurement is carried out in conditions of intense forced convection;

- the potential of the working electrode is the potential of the limiting current of the element being determined;

- the electrochemical reaction of the element being determined is reversible;

- the residual current is the result of a parallel independent irreversible reaction;

- Faraday processes are independent of the process of charging the capacity of a double layer.

The assumptions made are satisfied in most practical cases and do not distort the essence of real processes. Let us draw up a cell model based on the equivalent Randles scheme (Fig. 1) (Electroanalytical methods. Theory and practice / Edited by F. Scholz. - M.: BINOM LZ, 2006, P.158). In the diagram of figure 1 are indicated: 1 - total ohmic resistance of the electrochemical cell Rom, 2 - charge transfer resistance through the electrode – solution interface Rs, 3 — Warburg diffusion impedance W, 4 — double layer capacitance Cd.

To reduce the order of the model, we replace the Warburg diffusion impedance W with an impedance consisting of a capacitor Cw and a resistance Rw connected in parallel. Due to the small thickness of the diffusion layer and its constancy, such a replacement does not cause a significant error. Not taking into account the capacity of the double layer, as well as considering the total ohmic resistance of the electrochemical cell to be negligible, we get the equivalent circuit of the Faraday process in conditions of forced convection (figure 2).

In the diagram of figure 2 are indicated: 5 - source of constant voltage E, 6 - active diffusion resistance Rw, 7 - capacitive diffusion resistance Cw, 8 - switch K. The parameters of the equivalent circuit are determined based on the assumption that they are linearly dependent on the concentration of the reagent:

,

,

,

,

,

,

Where

,

,

,

,

,

,

in its turn,

n is the number of electrons involved in the reaction;

F is the Faraday number, C / mol;

A is the area of the working electrode, m ² ;

D is the molecular diffusion coefficient, m ² / s;

R is the universal gas constant, J / (K · mol);

T is the absolute temperature, K;

δ is the thickness of the diffusion layer, m;

Ks is the rate constant of the electrochemical reaction, m / s.

After the beginning of the periodic switching by the switch 8 (Fig. 2) with a period Θ (the time of the closed state is equal to the time open) during the first 5-10 periods, the current oscillations are established. Using the p-conversion method designed to analyze pulsed systems with a finite closing time (E. Dzhuri. Pulse automatic control systems. M .: Fizmatgiz, 1963, p. 318-330), we calculate the dependence of the Faraday current I _F on time when closed vein:

From equation (2) it is obvious that any integral value of the current I _F (t) linearly depends on the concentration of the determined element Cox. In order to maximize the suppression of interference that is not synchronous with the switching period, caused, for example, by network interference, fluctuations in the fluid flow in the electrode space, and electrochemical noise, it is advisable to use the selective integral Fourier transform. We apply the Fourier transform for expression (2), we obtain the dependence of the first harmonic of the Faraday current oscillation on the concentration Cox:

Where

,

,

,

,

,

,

,

,

.

.

A similar dependence can also be obtained for the amplitude of the first harmonic of the double layer charging current S _d (Cox), if in the circuit of Fig. 2 we replace Cw with Cd and Rs with Rom. In accordance with the assumption that the processes of charging the capacitance of the double layer and the Faraday one are independent, we find that the total signal is proportional to the determined concentration:

Thus, the switching current of the cell has a new property: the proportional dependence of not only the steady-state current value, but also the charging currents of capacities, including the capacity of the double layer, on the concentration of the element being determined. The ability to use as an analytical signal not only the Faraday current, but also the charging current leads to an increase in the detection sensitivity compared to the prototype 10-100 times.

Another new property of the switching method is more intense, in comparison with the prototype, the suppression of the signal of the parallel background reaction, usually less reversible than the reaction of the element being determined. A decrease in the level of residual current leads to a decrease in the detection limit of the detected impurity compared with the prototype in some cases by 10 times.

The calculation of the dependence of the signal value S _F on the switching period Θ according to formula (3) shows that this dependence is insignificant in the range of values of the switching period from 100 μs to 1 second, therefore, the switching period Θ can be selected based on the speed of the equipment used, optimally 1-10 ms The measurement result, proportional to the concentration of the determined element, is obtained after one switching period, which leads to high productivity of the method.

Figure 1 presents the equivalent circuit of the Randles electrochemical reaction.

Figure 2 presents a diagram of a model of an electrochemical reaction under forced convection.

Figure 3 presents an exemplary diagram of a chromium analyzer that implements the proposed method.

Table 1 presents the results of comparative determinations of the concentration of chromium in some objects.

The implementation of the method of switching chronoamperometry on the example of a chromium concentration analyzer is presented in figure 3. The analyzer contains a cup 9 with a capacity of 25 ml, in which two electrodes are immersed: a silver chloride electrode 10 (CSE) as a reference electrode and a gold-carbon electrode 11 (ZUE) as a working electrode. Silver chloride electrode 10 (CSE) is connected to the output of a constant voltage source 12 (IPN). A gold-carbon electrode 11 (ZUE) is connected to the analog input of the switch 8 (K), the output of which is connected to the input of the current to voltage converter 13 (PTN). The output of the current to voltage converter 13 (PTN) is connected to the analog input of the analog-to-digital converter 14 (ADC). The output of the analog-to-digital converter 14 (ADC) is connected to the input bus of the microprocessor 15 (MP), the output bus of which is connected to the input of the indicator 16 (I). The first pulse output of the microprocessor 15 (MP) is connected to the control input of the analog-to-digital converter 14 (ADC). The second pulse output of the microprocessor 15 (MP) is connected to the control input of the switch 8 (K).

For analysis, a test solution with a volume of V _ol = 10 ml with a concentration of chromium (VI), for example, 1 μg / dm ³ in 0.05 M nitric acid, is placed in a beaker 9. The constant voltage source 12 (IPN) generates a constant voltage of -250 mV, measured relative to the gold-carbon electrode 11 (ZUE). In the circuit: DC voltage source 12 (IPN), silver chloride electrode 10 (CSE), the analyzed solution, gold-carbon electrode 11 (ZUE), switch 8 (K), the input of the current to voltage converter 13 (PTN) causes an electrochemical reaction current. The microprocessor 15 (MP) generates a sequence of start pulses of the analog-to-digital converter 14 (ADC) at the first pulse output, and a sequence of control pulses of the switch 8 (K) at the second pulse output, the frequency of which, for example, is 64 times lower than the frequency of the start pulses analog-to-digital Converter 14 (ADC), and the duty cycle is two. In accordance with the control pulses, the switch 8 (K) carries out current switching, as a result of which a periodic sequence of current pulses occurs, which is converted into a sequence of voltage pulses using a current to voltage converter 13 (PTN). At times determined by the start pulses of the analog-to-digital converter 14 (ADC), the voltage is sampled and converted into binary code by the analog-to-digital converter 14 (ADC). In accordance with the program recorded in the memory of the microprocessor 15 (MP), the sequence of binary codes from the analog-to-digital converter 14 (ADC) is multiplied with the accumulation of the result by the sequence of values of the sinusoidal function recorded in the memory of the microprocessor 15 (MP) in the form of a table. The number of values of the sinusoidal function is selected based on the available memory, the speed of the microprocessor 15 (MP). The microprocessor program 15 (MP) provides the calculation of the amplitude of the first harmonic of the current by the rectangle method according to the formula:

,

,

where S is the result of the transformation,

K _i - the current code from the analog-to-digital Converter 14 (ADC),

W _i - the current value of the sinusoidal function, calculated by the formula:

.

.

After the period of switching of the switch 8 (K), the calculations are stopped and the accumulated result S, proportional to the concentration of the element being determined, is displayed on the indicator 16 (I).

The proportionality coefficient of the result S depends on many factors, for example, the effective area of the working electrode, the convective diffusion coefficient, therefore, to calculate the concentration of chromium (VI) in the sample, the standard addition method is used, according to which the obtained result S is fixed as the result of the sample S _pr , in cup 1 make a standard addition of chromium (VI) with a volume of V _add = 10 μl of a concentration of C _add = 10 mg / dm ³ and after a time interval of 200 ms necessary to establish the concentration of chromium (VI) in the volume of the solution, fix the cut ltate S as a result of additive S _ext . The desired concentration of chromium (VI) is calculated by the formula:

.

.

Table 1 shows the results of the determination of chromium (VI) in a model solution containing 1 μg / dm ^{3 of} chromium (VI), technological and waste water with a high content of organic substances, obtained using the prototype, the proposed method and an independent method of atomic absorption spectrometry ( AAS). A comparison of the results obtained by the claimed method and the prototype method, indicates a higher sensitivity of the proposed method in the field of concentrations of less than 10 μg / DM ³ . The results obtained by the method of AAS confirm the accuracy of the determination by the claimed method. Since the applied standard addition method assumes a proportional dependence of the signal S on the concentration of the element being determined, good agreement of the results indicates such proportionality in a wide range of concentrations of the element being determined.

METHOD OF COMMUTATION CHRONOAMPEROMETRY Table 1 Try Found concentration, mcg / dm ³ According to the prototype According to the claimed method AAS method Model solution 1 μg / dm ³ Cr (VI) <10 1.05 ± 0.08 1.02 ± 0.07 Process water <10 0.52 ± 0.09 0.47 ± 0.08 Process water + 0.5 μg / d ³ Cr (VI) <10 1.1 ± 0.1 1.2 ± 0.1 Wastewater ^*) (without preliminary preparation) not found 50 ± 9 52 ± 7 Wastewater ^*) 54 ± 8 ^**) 52 ± 6 ^**) 53 ± 5 Technological solution (after electrolysis) 1246 ± 80 1315 ± 70 1410 ± 60 ^* ) permanganate oxidation in the sample is 12 mg O ₂ / dm ₃ ^** ) the results were obtained after the ashing of the sample at 450 ° C

Claims (1)

A method of switching chronoamperometry, including measuring the current of an electrochemical reaction, characterized in that the potential of the working electrode at which the electrochemical reaction proceeds is kept constant, the current loop of the electrochemical cell is periodically switched and the amplitude of the first harmonic of the emerging pulse current is measured, which is proportional to the concentration of the element being determined.

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