OSCILLOGRAPHIC POLAROGRAPH WITH A LOW POTENTIAL SWEEP RATE

V. Z. Bashkatov, Yu. A. Ivanov

(Presented by the scientific-methodological seminar of the Problem Laboratory of Microimpurities)

It has been shown theoretically that the rate of change of potential has a considerable influence on the magnitude and shape of cathodic and anodic peaks in polarography [1].

In polarographs in which various electronic self-recording instruments are used as automatic recorders, the peak shape is affected by the response speed of the instrument, i.e., by a quantity reciprocal to the time required for the measuring system to traverse the full scale of the instrument under the action of a maximum signal. This time can be determined from the formula [2]:

\[ \tau_{\text{scale}} = \frac{100}{\omega \cdot n}\ \text{sec}, \]

where \(\omega\) is the rate of change of potential in mV/sec; \(n\) is the number of electrons participating in the electrode process.

In self-recording polarographs the rate of change of potential is deliberately chosen to be small (1.8–20 mV/sec) so that it does not influence the shape of the polarogram, since under such conditions the maximum rate of change of the current will be less than the maximum rate of current measurement [2]. However, in precise investigations these measures prove insufficient, and additional measures have to be taken to eliminate the influence of the instrument response speed on the shape of the polarogram [3].

The influence of the rate of current measurement on the shape of the polarogram can be completely eliminated if a cathode-ray oscilloscope is used for measuring the current, as is done in an oscillographic polarograph. However, the lowest rate of change of potential on the OP-5122 oscillographic polarograph is 125 mV/sec. Increasing the capacitance of capacitor \(C_2\) in the sweep generator to 5.0 µF makes it possible to obtain a minimum rate of 30 mV/sec with a linear change in potential. With a further increase in the capacitance of capacitor \(C_2\), this condition is violated, especially in the region corresponding to the peak potential. Increasing the resistance of the \(R_5—R_{11}\) circuit in order to attain low rates does not appear possible because of the high resistance of this circuit—40 megohms.

The authors have constructed an attachment to the oscillographic polarograph that makes it possible to obtain low rates of change of potential, 0.0166–102 mV/sec. The attachment is a power amplifier-

DC amplifier, assembled according to a bridge-cascade circuit with series connection of the tubes. The attachment is powered from an autonomous unit. The attachment and the oscillographic polarograph must not have a common ground.

The output of the attachment is connected to the sweep generator of the oscillographic polarograph (Fig. 1). A linearly varying voltage is fed to the input of the attachment; this voltage can be taken from the terminals for connecting the polarographic cell of any self-recording or visual polarograph. As the source of such a voltage the authors used LP-55 and OH-101 polarographs. The driving polarograph is not grounded.

Fig. 1. Schematic diagram of the attachment

For operation with the attachment on an oscillographic polarograph in the low-speed mode, the toggle switch \(T\) connects the output of the attachment to the oscillographic polarograph, simultaneously disconnecting relay P1. The switches on the oscillographic polarograph are set to the positions “Sweep off, continuous,” “Sweep rate 0.125,” and “Initial voltage 0”; on the driving polarograph, to “Polarogram type—cathodic.” The remaining control knobs of the oscillographic polarograph are manipulated as in ordinary polarography.

After setting on the driving polarograph the potential-change rate required by us and \(0\) volts on rheostat \(R_2\) (zero setting) of the attachment, we set zero on the voltmeter of the oscillographic polarograph. Then, according to Table 1, we set the voltage on the rheostat of the driving polarograph (column 4), and with resistance \(R_2\) (calibration) of the attachment we set the corresponding voltage on the voltmeter of the oscillographic polarograph (column 5).

The authors obtained the following rates of potential change (Table 1).

When recording polarograms, we set \(0\) volts on the drum of the driving polarograph, and with the knob (Initial voltage) of the oscillographic…

Table 1

Notes: ¹⁾ When recording anodic polarograms, a positive potential is set [sign (+)]; for cathodic polarograms, a negative potential is set [sign (−)]. ²⁾ It is not difficult to see that the rate of change of potential on the oscillographic polarograph can be determined by the formula

\[ W_{\mathrm{osc}} = W_{z}\frac{b}{a}\ \mathrm{mV/sec}, \]

where \(W_z\) is the rate of change of potential on the master polarograph, mV/sec; \(a\) and \(b\) are the voltage on the rheochord of the master polarograph and the voltage on the oscillographic polarograph, set during calibration of the attachment.

… of the oscillographic polarograph—the initial potential. Having switched on the motor on the master polarograph, we record the polarogram.

To check the operation of the attachment, cathodic cadmium waves, \(Cd^{2+}\), were recorded at low rates of potential change on the oscillographic polarograph.

Conclusions

An attachment to an oscillographic polarograph has been designed and fabricated for obtaining low rates of potential change in the range 0.0166–102 mV/sec.

The possibility of its practical application for scientific research purposes has been demonstrated.

Literature

1. S. Shain, S. Lewinson, Anal. Chem., 33, 187, 1961.
2. S. B. Tsfasman. Electronic Polarographs. Metallurgizdat, 1960.
3. V. A. Igolinsky. Dissertation. Tomsk Polytechnic Institute. Tomsk, 1963.