ELECTRONIC ATTACHMENT FOR IMPROVING THE RESOLVING POWER OF THE METHOD OF AMALGAM STRIPPING POLAROGRAPHY

Yu. A. IVANOV

(Presented by the Scientific-Methodological Seminar of the CTF)

In analytical practice, when using the method of amalgam stripping polarography, one often has to deal with the analysis of multicomponent systems. At a certain ratio of concentrations and difference in the potentials of the anodic peaks of these components, their mutual overlap occurs, which sharply increases the error in determining the concentrations of the impurities being analyzed [1].

On average, it may be assumed that, with identical concentration and reversibility of the electrode process, the absence of mutual influence of the waves is observed only when the difference between the peak potentials of neighboring elements is more than 0.2 V.

Various methods are known for taking into account and reducing the mutually interfering influence of closely spaced waves [2]. However, the use of these methods is not always possible and in a number of cases is associated with an additional expenditure of time and labor. Another solution to the stated problem is to carry out an automatic stopping of the change in the potential applied to the cell at the peak maximum, so that dissolution of the metal takes place at a constant potential.

The aim of the present work is the technical implementation of a device that automatically stops the change in the voltage applied to the cell when the peak potential for the recording polarograph is reached. In what follows, we shall call this device, for short, the resolving attachment.

The block diagram of the resolving attachment consists of the following units (Fig. 1).

1) The input circuits \((C_1 C_2 C_3 R_1 R_2 R_3 R_4 K_1 U_p)\) serve to set the actuation current of the resolving attachment and are connected directly to the polarograph amplifier in parallel with the recording instrument RP.

2) The modulator \((T_1 D_2 R_5 R_6 R_7 R_8 R_9)\) [3] serves to convert the direct-current signal into an alternating voltage with a frequency of 50 Hz for subsequent amplification.

3) The amplifier \((T_2 T_3 T_4 C_4 C_5 C_6 R_{10} R_{11} R_{12} R_{13} R_{14} R_{15} R_{16} R_{17})\) serves to amplify the signal to the magnitude necessary for operation of relay P1.

4) The output stage \((T_5 D_3 D_1 P_1 C_7 C_8 R_{18} R_{19})\) [4] controls the operation of the actuating mechanism IM, which is an electromagnetic clutch.

The operating principle of the resolving attachment is based on the fact that, at the moment when the current passes through a maximum, its derivative

\[ \frac{d I_{\mathrm{я}}}{dt} \]

changes its sign, passing through zero. For the voltage \(U_{\mathrm{у}}\), which controls the operation of the highly sensitive semiconductor relay RP, consisting of a modulator, an amplifier, and relay R1 (Fig. 1), one may write:

\[ U_{\mathrm{у}} = R_4 I_c + I_{\mathrm{п}} R_4, \tag{1} \]

where \(I_c\) is the current through capacitor \(C_3\); \(I_{\mathrm{п}}=\dfrac{U_p}{R_4+R_2}\). The current \(I_c\) is related to the current flowing through the cell by the following relation:

\[ I_c = K' R C_3 \frac{d I_{\mathrm{я}}}{dt}, \tag{2} \]

where \(K'\) is the current amplification factor of the polarograph; \(R\) is the resistance of the measuring mechanism of the recording instrument.

Fig. 1. Schematic diagram of the resolving attachment. RP—self-recording instrument, D—motor, IM—electromagnetic clutch, GN—voltage generator.

Triodes: \(T_1\)—MP103A, \(T_2\)—P16B, \(T_3\)—P11A, \(T_4\)—P16B, \(T_5\)—P11A.
Diodes \(D_1\)—D7Zh, \(D_2\)—D9V, \(D_3\)—D7Zh.
Capacitors: \(C_1\), \(C_2=500\,\mu\text{F}\times 12\,\text{V}\), \(C_3=100\,\mu\text{F}\times 12\,\text{V}\) (ETO),
\(C_4=10\,\mu\text{F}\), \(C_5\), \(C_6=80\,\mu\text{F}\), \(C_7=50\,\mu\text{F}\), \(C_8=100\,\mu\text{F}\times 60\,\text{V}\).
Relay R1 of type RFO-474 (RSM-2).

Resistances: \(R_1=1\,\text{k}\Omega\), \(R_2=(10\div20)\,\text{k}\Omega\),
\(R_3=3\,\text{k}\Omega\), \(R_7=2\,\text{k}\Omega\), \(R_5, R_{14}, R_{17}=1.5\,\text{k}\Omega\),
\(R_6=360\,\Omega\), \(R_{10}=1.3\,\text{k}\Omega\), \(R_8=1\,\text{k}\Omega\),
\(R_9=20\,\text{k}\Omega\), \(R_{13}=510\,\Omega\), \(R_{11}=16\,\text{k}\Omega\),
\(R_{12}=12\,\text{k}\Omega\), \(R_{16}=150\,\Omega\),
\(R_{15}=3.3\,\text{k}\Omega\), \(R_{19}=47\,\Omega\),
\(R_{18}=5.1\,\text{k}\Omega\).

Substituting relation (2) into expression (1) and denoting \(R_4 R C_3 K'\) by \(K\), we obtain:

\[ U_{\mathrm{у}} = K \frac{d I_{\mathrm{я}}}{dt} + R_4 I_{\mathrm{п}} = U_c + U_{\mathrm{п}}, \tag{3} \]

where \(U_c = K \dfrac{d I_{\mathrm{я}}}{dt}\) is the control-signal voltage; \(U_{\mathrm{п}} = R_4 I_{\mathrm{п}}\)—bias...

threshold voltage. Relation (3) makes it possible to conclude that this circuit can be used to solve the problem posed. At \(U_c = 0\), by means of potentiometer \(R_2\), the control voltage \(U_y\) is set equal to the release voltage \(U_{\text{rel}}\) of the semiconductor relay RP. Through its normally closed contacts 1P1, a voltage of 20 V is applied to actuator IM, which provides the drive for the voltage generator.

The circuit is assembled in such a way that, when the current through the cell increases

\[ \left(\frac{dI_{\text{cell}}}{dt} > 0\right), \]

the quantity \(U_c\) is subtracted from \(U_p\), which causes a further decrease in \(U_y\), i.e., no switching occurs in the circuit.

After the maximum has been reached, the current through the cell begins to decrease, and its derivative changes sign.

The voltage \(U_y\) begins to increase and, at a certain value \(U_y = U_{\text{op}}\), relay P1 operates, disconnecting the drive of the voltage generator (GN). At a constant cathode potential, as the element being determined continues to dissolve, the current \(I_{\text{cell}}\) gradually falls to some constant value \(I_0\), and relay P1 releases, since \(U_y = U_{\text{rel}}\). A further decrease in potential takes place. Thus, stopping and starting of the linearly varying voltage occur automatically, without operator intervention.

The maximum coincidence of the stopping potential with the peak potential can be obtained with a large value of the signal \(U_c\) at the initial moment of decrease of the current \(I_{\text{cell}}\), and with a small value of the threshold voltage \(U_{y.p} = U_{\text{op}} - U_{\text{rel}}\) of relay RP.

Practically complete coincidence of the stopping potential with the peak potential occurs when the semiautomatic operating mode of the resolving attachment is used, in which starting is performed by pressing button \(K\) (Fig. 1), while stopping is performed automatically by means of the attachment. In this mode the reference voltage is set in the interval \(U_{\text{rel}} < U_p \leq U_{\text{op}}\). When button \(K\) is pressed, \(U_y = U_p < U_{\text{rel}}\). The threshold voltage of the semiconductor relay circuit developed is \((1.5 \div 3)\) mV.

The threshold value with respect to the derivative of the current through the cell depends on the gain factor \(K'\) (2).

For an OH-101 polarograph (Hungary), at a measurement limit of \(1 \cdot 10^{-7}\) A, the value

\[ \frac{dI_{\text{cell}}}{dt}, \]

calculated from expression (3), is \(1 \cdot 10^{-10}\) A/s for the automatic mode and not more than \(1 \cdot 10^{-11}\) A/s for the semiautomatic mode.

The use in the polarograph of an electronic integrator as the voltage generator makes it possible to avoid using an actuator, which worsens the operating parameters of the resolving attachment. Starting and stopping the change of potential when the integrator is used is accomplished by switching on and off the constant voltage applied to the input of the integrator through contacts 1P1 of relay P1.

Figures 2 and 3 show polarograms recorded with an ordinary polarograph (polarograms 1) and with the use of a polarograph with the resolving attachment (polarogram 2). In both cases, an electronic integrator was used as the voltage generator.

It is seen from Fig. 2 that, when the attachment is used, there is practically no overlapping of the Sb and Bi peaks, whose peak potentials differ by no more than 0.05 V, i.e., the resolving power with respect to potential-

Fig. 2. Anodic peaks in a solution of \(0.1\ \gamma/\text{ml}\) \(Bi^{3+}\) and \(0.16\ \gamma/\text{ml}\) \(Sb^{3+}\) against a background of \(1\ \mathrm{M}\ \mathrm{HCl}\). Polarogram 1 was recorded with an ordinary polarograph. Polarogram 2 was recorded using the resolving attachment. Experimental conditions: rate of potential change \(W = 200\ \text{mV/min}\); accumulation time \(\tau_{\mathrm{e}} = 4\ \text{min}\); solution volume \(V = 5\ \text{ml}\); cathode—a mercury-film electrode with area \(S = 0.16\ \text{cm}^2\); anode—a saturated calomel electrode.

Fig. 3. Anodic peaks in a solution of \(50\ \gamma/\text{ml}\) \(Sn^{2+}\) and \(0.1\ \gamma/\text{ml}\) \(Pb^{2+}\) against a background of \(2\mathrm{M}\ \mathrm{CH_3COONa} + 2\mathrm{M}\ \mathrm{CH_3COOH}\). Polarogram 1 was recorded with an ordinary polarograph. Polarogram 2 was recorded using the resolving attachment.

at the same concentration of the components increases severalfold.

In addition, stopping the change in potential at the peak of the first, more electronegative element, which greatly exceeds the second element in concentration, makes it possible, before recording, to switch the sensitivity of the instrument to a higher setting in order to obtain a clearly defined peak of the second element.

Fig. 3 shows a polarogram of Sn and Pb at a concentration ratio of 1 : 500. From polarogram 1 it is practically impossible to determine lead, whereas polarogram 2 was recorded without superposition of the peaks; i.e., the concentration ratio may be even greater.

In conclusion, the author expresses gratitude to Docent M. S. Roitman for valuable guidance in designing the resolving attachment, and to Professor A. G. Stromberg for his attention to the work.

Conclusions

1. A resolving attachment for an OH-101 polarograph has been developed, assembled, and tested in operation.

2. Tests of the attachment confirmed a significant increase in the resolving power and accuracy of analysis by amalgam stripping polarography with accumulation when using the device considered, which is of great practical importance for the determination of microimpurities in high-purity materials.

Literature

1. A. G. Stromberg, E. A. Zakharova. Zavod. labor., 30, 261 (1964).

2. T. A. Kryukova, S. I. Sinyakova, T. V. Aref’eva. Polarographic Analysis. Goskhimizdat, 1959.

3. V. I. Anisimov, A. P. Golubev. Transistor Modulators. Energiya, 1964.

4. I. P. Stepanenko. Fundamentals of the Theory of Transistors and Transistor Circuits. Gosenergoizdat, 1963.