DETERMINATION OF TRACE AMOUNTS OF IMPURITIES OF LEAD, ZINC, CADMIUM, AND COPPER IN NITRIC ACID OF SPECIAL PURITY

L. F. TRUSHINA, M. S. ZAKHAROV

(Presented by the scientific seminar of the Chemical-Technological Faculty)

In the production of high-purity materials, in semiconductor technology, and in the analysis of especially pure substances, nitric acid is widely used; the content of impurities in it must not exceed \(10^{-7}\)—\(10^{-8}\%\). In this connection, there arises the need to develop methods for determining trace amounts of substances. One of the most promising methods for determining microquantities of certain elements, including Cu, Pb, Zn, and Cd, is the APA method.

A. G. Stromberg [1] showed that, for the determination of \(10^{-7}\%\) and smaller amounts of impurities by the APA method, it is necessary to use the following procedures:

1) use of a film electrode,
2) an intensive mode of stirring the solution,
3) thorough purification of the solution being analyzed from oxygen,
4) use of high sensitivity of the polarograph.

Using the indicated procedures and carrying out polarography from a solution with a volume of \(1\ \mathrm{ml}\) in an electrolytic cell, we established the possibility of determining impurities of Pb, Zn, Cu, and Cd at a concentration of \(5 \cdot 10^{-9}\ M\). The accumulation time for the impurities was 30 min; the sensitivity of the polarograph was \(3 \cdot 10^{-9}\ \mathrm{a/mm}\).

To create an element concentration of \(5 \cdot 10^{-9}\ M\) in a solution of volume \(1\ \mathrm{ml}\), calculations show that it is necessary to use, for analysis, \(5\ \mathrm{ml}\) of nitric acid containing \(n \cdot 10^{-8}\%\). Removal of nitric acid from the impurities was carried out by us by evaporation.

In order to determine Zn, Pb, Cd, and Cu from a single weighed sample, several backgrounds were tested: \(0.1\,N\ \mathrm{HNO_3}\); \(0.1\,N\ \mathrm{KOH}\); \(0.1\,N\ \mathrm{Na_2SO_4}\); \(0.1\,N\ (\mathrm{NH_4})_2\mathrm{SO_4}\); \(0.1\,N\ \mathrm{C_4H_6O_6}\). Reagents of special purity were used to prepare these solutions. The peak potentials of the elements are presented in Table 1.

It is evident from the table that, against the background of \(0.1\,N\ \mathrm{C_4H_6O_6}\), the anodic waves of the named elements do not overlap with the peaks of other elements.

In addition, when working at low polarograph sensitivities, the magnitude of the residual current begins to play a rather important role; it becomes comparable with the signal and prevents measurement of the waves with the required accuracy. Therefore it is expedient to use, for polarography, a background possessing a minimum value of residual current. For this purpose we investigated a series of backgrounds. The results of the investigations are presented in Table 2.

Table 1

Peak potentials of elements on several backgrounds

Table 2

Comparative characteristics of the residual current for different electrolytes

As follows from the table, the 0.1 N C₄H₆O₆ background has the smallest residual current. This fact can apparently be explained by the following: (a) the oxygen present in dissolved form is removed more readily from a 0.1 N C₄H₆O₆ solution than from other electrolytes during the same time of nitrogen purging, and (b) the capacitive current measured at the peak potentials of Zn, Pb, Cd, and Cu in 0.1 N C₄H₆O₆ solution has a minimum value in comparison with other electrolytes. Therefore, in subsequent studies we used only this background.

The conditions of electro-dissolution, as is known, have a considerable effect on the resolving power (the possibility of simultaneous determination of elements with close peak potentials). To this end, we studied the influence of the thickness of the mercury film and the rate of change of potential on the height and shape of the anodic peaks. The results of the studies are shown in Figs. 1–4. Figures 1 and 2 give the dependences of the height of the anodic wave of Cd on the rate of change of potential and on the thickness of the mercury film. The experimental data show that, as the rate of change of potential increases, the wave height increases according to a linear law; moreover, for thicker films (12 μ and more), the wave height decreases at the same rate of change of potential. Figures 3 and 4 show that, as the rate of change of potential increases, the anodic peak of cadmium shifts toward more positive values (correspondingly, the value $(E_p - E_{1/2})$ increases). On the basis of these data, in subsequent studies on the development of the procedure we adopted the value $\omega = 0.009$ V/sec (at larger values of $\omega$ the Pb and Cd waves overlap), and the thickness of the mercury film $l = 10$ μ.

It is known that, when the bulk of the substance being analyzed is evaporated, trace amounts of elements behave differently from macro-

![Figure 1 graph: Effect of the rate of potential change on the height of the anodic peak. Vertical axis: \(I, a \cdot 10^6\). Horizontal axis: \(\omega \cdot 10^3\), V/sec. Curves 1–5 are shown for mercury-film thicknesses of 4, 6, 8, 10, and 12 μ, respectively.]

Fig. 1. Effect of the rate of potential change on the height of the anodic peak. Curve 1—for a mercury-film thickness of 4 μ; curve 2—6 μ; 3—8 μ; 4—10 μ; 5—12 μ.

![Figure 2 graph: Dependence of the height of the cadmium anodic peak on film thickness. Vertical axis: \(I, a \cdot 10^6\). Horizontal axis: \(\ell, \mu\). Curves 1–5 correspond to \(\omega = 0.002\), 0.006, 0.009, 0.012, and 0.016 V/sec.]

Fig. 2. Dependence of the height of the cadmium anodic peak on the film thickness. Curve 1—for \(\omega = 0.002\) V/sec; 2—0.006 V/sec; 3—0.009 V/sec; 4—0.012 V/sec; 5—0.016 V/sec.

amounts of the latter. Thus, trace amounts may volatilize with the evaporated substance at temperatures at which macroquantities do not yet evaporate [2]. Therefore, we carried out studies on

![Figure 3 graph]

Fig. 3. Dependence of \((E_n - E_{1/2})\) for cadmium on the rate of change of potential.
Curve 1—\(l = 4\,\mu\); 2—\(6\,\mu\); 3—\(8\,\mu\); 4—\(10\,\mu\); 5—\(12\,\mu\).

![Figure 4 graph]

Fig. 4. Effect of the thickness of the mercury film on the shift of the peak potential.
Curve 1—for \(W = 0.002\ \mathrm{V/sec}\); 2—\(0.004\ \mathrm{V/sec}\); 3—\(0.006\ \mathrm{V/sec}\); 4—\(0.009\ \mathrm{V/sec}\); 5—\(0.012\ \mathrm{V/sec}\); 6—\(0.016\ \mathrm{V/sec}\).

the possibility of volatilization of cadmium and lead during evaporation of nitric acid at various temperatures. In these studies, definite amounts of the elements were introduced into the sample being analyzed before its evaporation. The impurity content after evaporation was determined by the standard-addition method. The results of the studies are given in Table 3, from which it is seen that the optimal evaporation temperature is \(t = 135^\circ\mathrm{C}\).

Standard solutions of Zn, Pb, Cd at concentrations of \(10^{-6}\) M were stored in polyethylene vessels, and at concentrations of \(10^{-8}\) M—in quartz vessels, since our experiments to determine the stability of standard solutions of low concentrations as a function of the material of the vessel showed—

showed that solutions with low contents of Zn, Pb, and Cd are best stored in quartz vessels.

Washing of the beakers in which the test sample was evaporated was carried out by treating them in the washing solution with ultrasound (three 5-min sonications followed by a change of tridistillate after each series). A UZV-2.5A type generator was used. Comparison of the data (Table 4) on washing beakers by steam, proposed earlier [3], by steam with a water trap [4], and by ultrasound shows that the latter is the most effective.

Table 3

Study of the completeness of determination of Pb and Cd

On the basis of the investigations carried out, we propose the following procedure for the simultaneous determination of \(10^{-7}\)—\(10^{-8}\%\) impurities of Cd, Zn, Pb, and Cu. Nitric acid in an amount of 5 ml is placed

Table 4

Efficiency of washing beakers

Table 5

Results of determination of elements in nitric acid of special purity

in a fluoroplastic beaker for polarography, previously checked for purity, and is evaporated in a chamber isolated from

from the laboratory atmosphere, to moist salts, at \(t = 135^\circ\mathrm{C}\). The beaker is transferred into the box and 1 ml of background solution, 0.1 N \(C_4H_6O_6\), is poured into it. The solution in the beaker is purged for 10 min with oxygen-free nitrogen; electrolysis is carried out at a potential of \(-1.6\) V for 30 min. After electrolysis, polarography is performed in the potential range \(0.2\)—\(1.2\) V (sat. calomel electrode) at a polarograph sensitivity of \(3 \cdot 10^{-9}\) A/mm. The impurity content is evaluated by the method of additions.

Analyses of special-purity nitric acid were carried out according to the developed procedure; the results are presented in Table 5.

The coefficient of variation of the determination results is 12.1% for zinc, and 15.3% and 5.3%, respectively, for cadmium and lead.

LITERATURE

1. A. G. Stromberg. Izvestiya TPI (in press).
2. A. K. Babko. Zavodskaya laboratoriya, 29, 548 (1963).
3. V. A. Iglinskii. Candidate’s dissertation, Tomsk, 1963.
4. M. S. Zakharov, L. F. Trushina. Izvestiya TPI (in press).