POLAROGRAPHIC SETUP WITH COMPENSATION FOR THE NONLINEAR COMPONENT OF THE RESIDUAL CURRENT

Yu. A. Ivanov, Yu. P. Belousov

(Presented by the scientific seminar of the MP laboratory)

In determining trace impurities on the order of \(10^{-8}\)—\(10^{-10}\) percent by weight by the method of amalgam polarography with accumulation, the masking effect of the nonlinear component of the residual current of the polarographic sensor cell on the oxidation current of the impurity being determined—which informs us of its quality and quantity—increases sharply.

The nonlinear component of the residual current of a polarographic sensor may be a function only of the magnitude and sweep rate of the electrolyzing voltage; therefore, in what follows, for brevity, we shall call it the stationary interference of the cell.

The stationary interferences of the cell, which cannot be compensated by means of existing direct-current and alternating-current polarographs, include interferences caused by the oxidation of impurities contaminating the reagents, solutions, and glassware used.

Thus, at present, careful repeated purification of the reagents and glassware used is the only means of reducing stationary interference; this, in the main, determines the sensitivity, labor intensity, and duration of the analysis.

Below a setup is described consisting of a direct-current polarograph and an electronic compensator, which makes it possible to compensate stationary interference of both capacitive and faradaic character.

The principle of operation of the setup is that the stationary interference of the cell is first recorded in the form of a graphical image, which subsequently, when the volt-ampere characteristics of the cell are taken, is converted into a direct compensation current and introduced into the measuring circuit of the polarograph in opposition to the cell current.

Thus, a necessary condition for the effective use of the proposed device is a high degree of reproducibility of the stationary interference with respect to the electrolyzing voltage and synchronization of this voltage with the compensation current.

The device and principle of operation of the setup are easily understood from consideration of the block diagram shown in Fig. 1.

As the measuring unit, a Hungarian OH-101 polarograph was used, comprising standard resistances \(R_k\), input circuits, a direct-current amplifier, a recording instrument, and a voltage generator in which the single-turn setting potentiometer was replaced by a multi-turn one.

Compensation of the stationary interference is carried out by means of an electronic compensator containing a dc measuring bridge, a linear rheochord \(R_0\), switch \(T_1\)—“recording”—“compensation,” a modulator using an MP 103A transistor [2], an ac amplifier,

Fig. 1. Block diagram of a polarographic setup with compensation of the nonlinear component of the residual current

consisting of a preamplifier using a 6N2P tube and a power amplifier using a 6P14P tube [3], an actuating reversible motor \(D_1\), a tracking bridge using SF3-1 photoresistors, a universal carriage, and a tape-transport mechanism.

The universal carriage is made in the form of a light rigid frame 1 (Fig. 2) of sheet aluminum, with internal dimensions \(20 \times 330\), through which a tape 2, 160 mm wide and cut from tracing paper, is drawn.

The frame is attached by its lower side, at the center, to bracket 3, which moves freely on six rollers along guide axis 4, positioned perpendicular to the motion of the tape. Mounted here as well are roller 5 of the slider of rheochord \(R_p\), on contact spring 6, and two SF3-1 photoresistors. The photosensitive surfaces of the photoresistors are arranged on an axis perpendicular to the motion of the tape and in the same plane with it, at a distance of 0.5 mm from each other. On the upper side of the frame, likewise at the center, an illuminator is mounted, made in the form of a vertical aluminum tube 7; in the upper base of this tube are mounted incandescent lamp 8, rated at 12 V and 21 candlepower, and a removable conical pen 9 with a line thickness of 0.8–1 mm. Recording is carried out with black ink.

The tape-transport mechanism contains reversible motor \(D_2\) (Fig. 2), reduction gear \(P\), driving roller 11, pressure roller 12, and the reels for forward 13 and reverse 14 winding, with a soft friction drive from reduction gear \(P\).

Fig. 2. Kinematic diagram of the electronic compensator

For adjustment of the setup (toggle switch \(T_1\) in the “recording” position), at the output of the polarograph amplifier, with the aid of the diffusion-current compensator, zero and maximum values of the output voltage are set successively; in this process the zero and maximum positions of the compensator carriage are adjusted by potentiometers \(R_3\) and \(R_2\), respectively. After this, the maximum voltage value is set at the amplifier output, toggle switch \(T_1\) is switched to the “compensation” position, and the next bridge is balanced by adjusting potentiometer \(R_5\). Potentiometer \(R_4\) is set so that, after the carriage is set to the zero mark, the amplifier output voltage remains maximal. Finally, after the compensator carriage is set to the mark of maximum deflection, the amplifier output voltage is set equal to zero with the aid of potentiometer \(R_1\), and toggle switch \(T_1\) is switched to the “recording” position. Thanks—

owing to the fact that the cell currents \(i_{\mathrm{ya}}\) and the compensation currents \(i_{\mathrm{k}}\) pass through the same reference resistances \(R_{\mathrm{k}}\), the adjustment of the circuit is not disturbed when selecting the sensitivity of P1.

The operating procedure for the setup is as follows: by conventional methods, the compensation currents of the polarograph are selected \((i_{\mathrm{k}}, i_{\mathrm{k}}^{\wedge}\) with the aid of compensators for the diffusion and capacitive currents) and the sensitivity of the polarograph (P1) is adjusted so that the nonlinear component of the residual cell current—the stationary interference over the required range of electrolyzing voltage—is recorded with maximum use of the compensator scale.

To read the compensation current, toggle switch T1 is moved to the “compensation” position; the universal carriage is set so that the beginning of the graphic image of the stationary interference lies over the gap between the photoresistors; and illuminator 8 and motor D2 are switched on. As the tape moves, the line of the graphic image, because of its curvature, begins to shift relative to the gap between the photoresistors, changing their illumination, which causes an imbalance of the tracking bridge. The actuator motor D1, to whose control windings the imbalance voltage of the tracking bridge—converted to alternating voltage of frequency 50 Hz and amplified—is applied, moves the carriage in the direction that reduces the imbalance. Thus, as the tape moves, the carriage and the slider of rheochord \(R_{\mathrm{k}}\) attached to it will follow the curve of the graphic image of the stationary interference.

Combining the recording and reading devices in one unit, with a rigid coupling between the driving roller of the tape-drive mechanism and the slider of the setting potentiometer of the voltage generator, makes it possible to ensure a high degree of synchronization between the supply of the compensating current and the voltage applied to the cell.

On the basis of the foregoing, the degree of compensation of the stationary interference by means of the proposed setup can be estimated from the formula

\[ \beta=\frac{i_{\mathrm{p}}}{\Delta i} =\frac{i_{\mathrm{p}}^{0}\pm \delta_{\mathrm{p}}}{i_{\mathrm{p}}^{0}(\delta_{\mathrm{p}}\pm \delta_{\mathrm{k}})} \gg \frac{1}{\delta_{\mathrm{p}}+\delta_{\mathrm{k}}}. \tag{1} \]

where \(\beta\) is the number showing by how many times the stationary interference is reduced.

\[ i_{\mathrm{p}}=f(u) \]

is the interference current.

\[ i_{\mathrm{p}}^{0}=f(u) \]

is the interference current recorded by the compensator.

\[ \delta_{\mathrm{p}}=\frac{i_{\mathrm{p}}-i_{\mathrm{p}}^{0}}{i_{\mathrm{p}}^{0}} \]

is the nonreproducibility of the interference current with respect to the electrolyzing voltage.

\[ \delta_{\mathrm{k}}=\frac{i_{\mathrm{k}}-i_{\mathrm{p}}^{0}}{i_{\mathrm{p}}^{0}} \]

is the error of the compensation current.

\[ \Delta i=i_{\mathrm{p}}-i_{\mathrm{k}} \]

(the compensating current).

Laboratory tests of the setup gave positive results. When a circulating electrolyzer [4] was used as the cell, which makes it possible to obtain a high degree of reproducibility \((\delta_{\mathrm{p}}=0.006)\), a value of 100 was obtained for \(\beta\).

In conclusion, the authors express their gratitude to Professor A. G. Stromberg for his attention to the work.

References

1. V. I. Anisimov, A. P. Golubev. Transistor Modulators, “Energiya,” Leningrad, 1964.

2. F. F. Kotchenko. Tracking Systems of Automatic Compensators, “Nedra,” Leningrad, 1965.

3. V. I. Kuleshov. Factory Laboratory (in press).