Toward the Polarographic Determination of Zirconium and Hafnium

M. S. Zakharov, A. G. Stromberg

In connection with the rapid growth in industrial consumption of zirconium and hafnium, the need is becoming ever more acute for the development of rapid and precise methods for the quantitative determination of these elements in various substances and especially in natural materials.

Literature data from reviews of polarographic determinations of many elements and substances show that, in many cases, the polarographic method of determination surpasses chemical, colorimetric, and other methods both in accuracy and in speed of determination. However, up to now comparatively little attention has been given to the polarographic determination of zirconium and hafnium. Therefore, the study of the polarographic behavior of zirconium and hafnium is of considerable interest.

According to investigations by some authors [1, 2], in a 0.001 M solution of $\mathrm{ZrOCl}_2$ on a background of 0.1 M KCl ($\mathrm{pH}\sim 3$), a reduction wave of $\mathrm{Zr}^{4+}$ to $\mathrm{Zr}^{3+}$ was observed. The half-wave potential is $-1.65$ V vs. S.C.E.

In the work of other authors [3] it is indicated that, in aqueous solution, the solubility of zirconium salts is low and the wave of tetravalent zirconium merges with the hydrogen wave. In methyl alcohol, however, on a background of 0.1 N LiCl, a distinct wave is obtained. Moreover, according to the data of these authors, in aqueous solutions salts of tetravalent zirconium undergo strong hydrolysis.

Korshunov and Malyugina [4], who studied the behavior of zirconium at the dropping mercury electrode in solutions of acids, simple salts, and in the presence of various complexing agents, obtained well-defined diffusion waves of zirconium in solutions of KCl, KNO$_3$, and LiNO$_3$.

These authors established that the latter have a catalytic character.

Thus, there is no unified opinion in the literature concerning the composition of the supporting electrolyte on which an accurate polarographic analysis of tetravalent zirconium can be carried out.

With regard to hafnium there is only information [1] that the reduction potential of the latter is very negative and that hafnium salts undergo very strong hydrolysis. In the present work, results are given of a polarographic study of zirconium and hafnium on various supporting electrolytes, in various solvents, and in the presence of some elements with which zirconium occurs under natural conditions.

Results of the Investigation and Their Discussion

The investigation was carried out with an PV-1 polarograph. The sensitivity of the mirror galvanometer used by us was \(2.5 \cdot 10^{-9}\) A/cm/m. For the investigation, solutions of zirconyl chloride were prepared in water, methanol, and ethanol (absolute).

In methanol, against a background of \(0.1\ \mathrm{N}\ \mathrm{LiCl}\), the zirconium wave is distinct, and the half-wave potential is approximately \(-1.4\ \mathrm{V}\) (Fig. 1). In order to determine the optimum concentration of methanol in water, studies were carried out on the determination of zirconium in methanol containing 20, 40, 60, and 80% water. As can be seen from Figs. 2 and 3, with increasing water concentration the height of the zirconium wave increases; moreover, the increase in the latter at first occurs almost rectilinearly, and then (from 60% \(\mathrm{H_2O}\)) the increase becomes sharper.

The observed fact is apparently due to the catalytic character of the zirconium wave.

Fig. 1. Polarogram of \(\mathrm{ZrOCl_2}\) against a background of \(0.1\ \mathrm{M}\ \mathrm{LiCl}\) in methanol.
Curve 1 — residual current; 2 — \(10^{-3}\ \mathrm{M}\ \mathrm{ZrOCl_2}\).

A distinct zirconium wave is also obtained in absolute ethanol against a background of \(0.1\ \mathrm{N}\ \mathrm{LiCl}\); the diffusion current is directly proportional to the zirconium concentration (Fig. 4).

An excellent zirconium wave is also obtained in an aqueous solution of \(0.1\ \mathrm{N}\ \mathrm{KCl}\); in this case no hydrolysis phenomena were observed (Fig. 5). The same figure also shows the zirconium wave obtained in ethanol (abs.) under otherwise identical conditions. It is evident from it that the zirconium wave obtained in water is approximately four times higher than the latter, obtained in ethanol. A study of the influence of pH on the height of the zirconium wave in aqueous solutions showed that, at the same zirconium concentration, with increasing acid concentration in the solution the wave height increases. These two facts, again, can apparently be explained by the catalytic character of the zirconium wave obtained in aqueous and aqueous-acid solutions.

It should be noted that analogous results were also obtained by Korshunov and Malyugina [4].

Studies carried out in order to clarify the behavior of metals occurring in natural ores together with zirconium (\(\mathrm{Mg}\), \(\mathrm{Ca}\), \(\mathrm{Ti}^{4+}\), \(\mathrm{Fe}^{2+}\), \(\mathrm{Fe}^{3+}\), \(\mathrm{Al}\)) showed that Mg and Ca in methanol against a background of \(0.1\,N\) LiCl do not interfere with the determination of zirconium, whereas \(\mathrm{Ti}^{4+}\), \(\mathrm{Fe}^{2+}\), \(\mathrm{Fe}^{3+}\), and \(\mathrm{Al}^{3+}\) are reduced at more positive potentials than zirconium and, consequently, interfere with the determination of the latter.

Fig. 2. Polarogram of \(\mathrm{ZrOCl_2}\) against a background of \(0.1\,M\) LiCl in water–methanol mixtures:
1—20%; 2—40%; 3—60%; 4—80% \(\mathrm{H_2O}\).

Fig. 3. Dependence of the zirconium wave height against a background of \(0.1\,M\) LiCl on the water content in water–methanol mixtures.

Fig. 4. Polarograms of ZrOCl₂ against the background of 0.1 M LiCl in ethanol:
1—\(1 \cdot 10^{-3}\) M ZrOCl₂; 2—\(2 \cdot 10^{-3}\) M ZrOCl₂.

Fig. 5. Polarograms of \(1 \cdot 10^{-3}\) M ZrOCl₂:
1—against the background of 0.1 M LiCl in ethanol; 2—against the background of 0.1 M KCl in water.

Of course, the determination of zirconium can also be carried out in the presence of the last four ions, but their concentration must be considerably lower than the concentration of zirconium.

Fig. 6. Polarogram of \(1 \cdot 10^{-3}\) M hafnium in an aqueous solution of 0.1 M LiCl.

Studies carried out with hafnium showed that the latter is hydrolyzed very strongly. Therefore, its polarographic study was carried out in strongly acidic media (\(\mathrm{pH} \sim 3\)); under such conditions the hafnium wave merges with the hydrogen wave. A polarogram of this element could be obtained only at \(\mathrm{pH} \sim 3.5\) (Fig. 6) (LiCl served as the supporting electrolyte), but in this case partial hydrolysis was observed, which, naturally, does not permit quantitative determination of the hafnium content. At the same time, as can be seen from Fig. 6, the height of the hafnium wave is approximately 6 times greater than the zirconium wave, which is apparently due to its strongly catalytic character.

Conclusions

1. It has been shown that zirconium gives distinct waves against a LiCl background in methanolic and ethanolic solutions, proportional to the zirconium concentration.

2. A good zirconium wave is also obtained in aqueous solutions, but in this case partial hydrolysis was observed, which, naturally, apparently can be explained by the catalytic character of the wave.

3. The polarographic determination of zirconium in methanol is interfered with by \(\mathrm{Ti}^{4+}\), \(\mathrm{Al}^{3+}\), \(\mathrm{Fe}^{2+}\), and \(\mathrm{Fe}^{3+}\). The presence of Ca and Mg does not interfere with the determination of zirconium.

4. The polarographic study of hafnium, because of the strong hydrolysis of its salts, presents considerable difficulty.

Literature

1. Kolthoff and Lingane. Polarography, Khimizdat, 1948.
2. Laubengager A. W., Eaton R. B., J. Am. Chem Soc., 62, 2702, 1940.
3. Colichman E., Ludewig W., Anal. Chem., 25, 1909, 1953.
4. Korshunov I. A., Matyugina N. I. ZhNKh IV, 1707, 1959.