A. S. NAUMOVA

DETERMINATION OF MICROQUANTITIES OF ZINC AND CADMIUM IN ALCOHOLIC SOLUTION BY THE METHOD OF AMALGAM POLAROGRAPHY

(Presented by the scientific seminar of the Department of Physical Chemistry)

The present study was undertaken in order to investigate the possibility of determining microquantities of metals in alcoholic solutions by the method of amalgam polarography.

A fairly large number of works [1] have been devoted to the study of the polarography of nonaqueous solutions; however, the literature on the application of amalgam polarography in nonaqueous solutions is not known to us.

In the works of a number of authors [2, 3] it has been shown that, with a few exceptions [3, 4], the basic regularities (the general form of the polarogram, the half-wave potential, the dependence of the limiting current on concentration, and others) established for aqueous solutions are also observed in the case of nonaqueous solutions. It is further noted [5] that oxygen is considerably more soluble in organic solvents (and especially in alcohols) than in water; therefore, to remove oxygen from the solution by passing an inert gas through it, a longer time is required (up to one hour).

As the nonaqueous solvent we used ethyl alcohol that was not absolute, but 96.8% ($d_{2}^{2} = 0.800$), since there are indications in the literature [5, 6] that the presence of small amounts of water in the solvent does not produce noticeable changes in the polarograms. Zinc and cadmium, which had been studied in ethyl alcohol by the method of ordinary polarography by I. Ya. Turyan and E. I. Grinberg [7], were chosen as the objects of polarographic analysis. For comparison, we also carried out an analysis of aqueous solutions of these elements under otherwise identical conditions. Reagents of analytical-reagent and chemically pure grades were used for preparing the solutions.

As the indifferent electrolyte (supporting background), both in aqueous and in alcoholic solutions, $0.09\ M$ $NH_4Cl$ was used. The cathode was an electrode with a stationary mercury drop. An ordinary calomel electrode (sat.) was used as the reference electrode. The auxiliary electrode was connected with the alcoholic solution by means of an agar-agar bridge prepared with a dilute solution of $NH_4Cl$. The electrolytic cell had an ohmic resistance of 4500 ohm [8] with the alcoholic background solution, and 1400 ohm with the aqueous background solution. All measurements were carried out at room temperature. Oxygen was removed from the solution by passing nitrogen through it; in the case of aqueous solutions the nitrogen had first been purified with an alkaline solution of pyrogallol and a solution of alkali (KOH), while in the case of alcoholic solut-

boron, and nitrogen; in addition, it was dried with concentrated sulfuric acid and saturated with alcohol vapors. The solution was stirred only by the inert gas.

Measurements were carried out on an M-103 microphotopolarograph with a maximum galvanometer sensitivity of \(9.16 \cdot 10^{-3}\ \mu\text{A}/\text{mm}\). The experimental conditions were as follows. The volume of solution in the electrolyzer was \(10\ \text{ml}\); the mercury drop was isolated by electrolysis of a saturated solution of mercuric nitrate with a current of \(2.5\ \mu\text{A}\) for one minute on a platinum wire \(0.5\ \text{mm}\) long. In each experiment a new mercury drop was taken. Nitrogen was passed through the solution for one hour. Electrolysis of the solutions analyzed lasted 8 min, at a constant potential (for zinc \(-1.4\ \text{V}\) and for cadmium \(-1.0\ \text{V}\)); the galvanometer sensitivity was \(1:1\ \mu\text{A}/\text{mm}\) of scale. The change in current upon decreasing the voltage was recorded on photographic paper. Compensation of the residual current was equal to 5 divisions of the compensator scale for zinc and 7 for cadmium. The concentration was varied from \(1 \cdot 10^{-6}\) to \(5 \cdot 10^{-6}\ \text{M}\).

Fig. 1. Polarograms of the anodic dissolution of a zinc amalgam from an alcoholic solution of \(0.09\ \text{M}\) \(NH_4Cl\) (background); electrolysis duration 8 min; galvanometer sensitivity \(1:1\ \mu\text{A}/\text{mm}\). Curve: 1—background; \(2—1 \cdot 10^{-6}\); \(3—2 \cdot 10^{-6}\); \(4—3 \cdot 10^{-6}\); \(5—4 \cdot 10^{-6}\); \(6—5 \cdot 10^{-6}\) \(ZnCl_2\).

Polarograms of the anodic dissolution of a zinc amalgam from an alcoholic solution of \(0.09\ \text{M}\) \(NH_4Cl\) are shown in Fig. 1. Similar polarograms were obtained for cadmium in the same solution; moreover, with other conditions the same, the depth of the notches in alcoholic solutions is lower by 25% for cadmium and by 14% for zinc in comparison with aqueous solutions.

Table 1 gives the potential (in volts) of the notch peak in alcoholic and aqueous solutions in \(0.09\ \text{M}\) \(NH_4Cl\) for zinc and cadmium.

From the polarograms obtained, calibration curves were constructed, shown in Fig. 2 for zinc and in Fig. 3 for cadmium; these show that, in the concentration interval studied, the notch depth of cadmium and zinc in alcoholic solution is proportional to their concentration.

In order to determine the possibility of analyzing zinc and cadmium by the amalgam polarography method when they are present together—

Table 1

Potential
(in volts) of the notch peak in alcoholic and aqueous solutions in \(0.09\ \text{M}\) \(NH_4Cl\) for Zn and Cd

Fig. 2. Calibration graph for zinc in aqueous solution (straight line 1) and in alcoholic solution (straight line 2) within the concentration range \(1 \cdot 10^{-6}\)—\(5 \cdot 10^{-6}\) M \( \mathrm{ZnCl_2} \).

Fig. 3. Calibration graph for cadmium in aqueous solution (straight line 1) and in alcoholic solution (straight line 2) within the concentration range \(1 \cdot 10^{-6}\)—\(5 \cdot 10^{-6}\) M \( \mathrm{CdCl_2} \).

Fig. 4. Polarogram of the anodic dissolution of zinc amalgam (left peak) and cadmium amalgam (right peak) from an alcoholic solution; background \(0.09\) M \( \mathrm{NH_4Cl} \), electrolysis duration 20 min; galvanometer sensitivity \(1:1\ \mu\mathrm{A}/\mathrm{mm}\); the concentration scales of zinc and cadmium are identical and equal to \(1 \cdot 10^{-6}\) M.

... in an alcoholic solution, a polarogram was recorded for the anodic dissolution of these metals in an alcoholic solution of \(0.09\ M\ \mathrm{NH_4Cl}\) (Fig. 4). It follows from Fig. 4 that the cadmium peak is smaller than the zinc peak.

Conclusions

1. Anodic peaks of cadmium and zinc were obtained in a solution of ethyl alcohol (96.8 wt %) against a background of \(0.09\ \mathrm{NH_4Cl}\), at cadmium and zinc concentrations in the range from \(1 \cdot 10^{-6}\ M\) to \(5 \cdot 10^{-6}\ M\).

2. In the concentration range studied, the peak height of cadmium and zinc in alcoholic solution is proportional to their concentrations.

3. Other conditions being equal, the peak height in alcoholic solutions is lower by 25% for cadmium and by 14% for zinc in comparison with aqueous solutions.

4. The peak potential in alcoholic solution for zinc shifts toward positive potentials by 0.11 volt, while for cadmium it remains practically unchanged in comparison with aqueous solutions against a background of \(0.09\ M\ \mathrm{NH_4Cl}\).

Literature

1. N. A. Izmailov. Electrochemistry of Solutions. Publishing House of Kharkov University, 928 (1959).

2. Ya. I. Tur’yan and M. A. Filiman, ZhFKh, 24, 781 (1955).

3. A. M. Zan’kov and F. A. Manusova, Zh. obshch. khimii, 10, 1171 (1940).

4. H. A. Zaitinen und C. I. Nyman. Polarography in liqud ammonia. J. Am. Chem. Soc., 70, 3002 (1948).

5. G. B. Bachman and M. J. Astle. Amer. Chem. Soc., 64, 1303 (1942).

6. A. P. Toropov and A. M. Yakubov. ZhFKh, 30, 1702 (1956).

7. Ya. I. Tur’yan, E. M. Grinberg. ZhFKh, 27, 2152 (1954).

8. P. Arthur and G. Zions. Anal. Chem., 24, 1422 (1952).