DETERMINATION OF THE CONTENT OF Cu$^{+2}$ AND Ag$^{+}$ IN SOLID SOLUTIONS BASED ON LEAD AZIDE BY AMALGAM POLAROGRAPHY WITH ACCUMULATION

V. P. PICHUGINA, S. M. RYABYKH

(Presented by the scientific seminar of the Department of Radiation Chemistry and the Laboratory of CTT)

In studying the decomposition of PbN$_6$ under the action of heat, light, and ionizing radiation, it was established in a number of works that, when Ag$^{+}$ and Cu$^{+2}$ cations are introduced into the crystal lattice of PbN$_6$, the stability of PbN$_6$ changes substantially [1, 2, 3]. Since the stability of PbN$_6$ changes nonlinearly with concentration, according to a complex law, exact knowledge of the amount of Ag$^{+}$ or Cu$^{+2}$ incorporated into the PbN$_6$ crystal lattice is highly desirable.

PbN$_6$ is usually prepared by a precipitation reaction when solutions of Pb(NO$_3$)$_2$ and NaN$_3$ are mixed. To introduce Ag$^{+}$ or Cu$^{+2}$ cations into the PbN$_6$ crystal lattice, the corresponding amount of AgNO$_3$ or Cu(NO$_3$)$_2$ was added to the initial Pb(NO$_3$)$_2$ solution. In the works mentioned, investigators usually limited themselves to this information, assuming that all Ag$^{+}$ or Cu$^{+2}$ cations from the initial solution are coprecipitated into PbN$_6$. Such a point of view cannot be accepted unconditionally. However, in attempts to analyze the indicated solid solutions for Cu$^{+2}$ and Ag$^{+}$ content, researchers encountered serious experimental difficulties. Attempts made in the CTT laboratory of the Tomsk Polytechnic Institute to analyze these systems by chemical methods and by emission spectral analysis have so far not yielded the desired results. Other methods of analysis of the indicated systems also appear complicated [4—5].

In the present work, the concentration of Ag$^{+}$ and Cu$^{+2}$ additives in PbN$_6$ was determined by the method of amalgam polarography with accumulation. In comparison with other methods of analysis, this method is distinguished by high accuracy, simplicity, and sensitivity in determining microconcentrations.

Experimental Part

Since by the APA method it is possible to analyze only cations present in solution, PbN$_6$ was dissolved beforehand. As the solvent, either a 1 N HNO$_3$ solution or a saturated ammonium acetate solution was used. The dissolved sample usually did not exceed 50 mg. In analyzing PbN$_6$ for Cu$^{+2}$ and Ag$^{+}$ content, the selection of the supporting electrolyte is of great importance. For determining Cu in PbN$_6$, the best supporting solution is 0.25 M ammonium acetate + 0.1 M tart-

nitric acid, and for the determination of Ag\(^+\)—0.1 N HNO\(_3\) + 0.1 N KCNS. During electrolysis a potential was applied to the stationary mercury electrode (cathode) at which no lead deposition occurred; therefore Pb\(^{+2}\) did not affect the accuracy of the determination.

The procedure for determining Cu\(^{+2}\) in PbN\(_6\) is as follows. An aliquot of the analyzed sample is added to the supporting solution (\(V = 3\) ml), and accumulation is carried out for 3–5 min at \(\varphi = -0.4\) V relative to the saturated calomel electrode, while the solution is stirred with nitrogen. After accumulation is completed, the passage of N\(_2\) through the solution is stopped and the anodic oxidation current is recorded at a linearly varying potential. The Cu\(^{+2}\) concentration is determined by the method of additions; analysis for Ag\(^+\) is performed analogously, except that, as indicated above, another supporting solution is used and accumulation is carried out at \(\varphi = -0.4\) V relative to the saturated mercurous sulfate electrode (to avoid precipitation of silver by Cl\(^-\) ions). The results obtained are given in Table 1. The procedure was developed using synthetic mixtures. For each composition no fewer than three determinations were made. The discrepancy did not exceed 5%.

Table 1

The results presented in Table 1 show that, when Cu\(^{+2}\) and Ag\(^+\) additives are introduced into PbN\(_6\), the concentration of the impurity cations actually coprecipitated is considerably lower than that introduced with the initial nitrates. The discrepancy is especially large in the case of the Cu\(^{+2}\) additive. Apparently, this is explained by the selective solubility of the azides formed in the mother liquor.

The analysis of PbN\(_6\) for the content of Cu\(^{+2}\) and Ag\(^+\) has been carried out for the first time. Since the true concentration of the Cu\(^{+2}\) and Ag\(^+\) additives differs from that previously assumed, the results obtained earlier for the thermal, photochemical, and radiation-chemical stability of solid solutions of Cu\(^{+2}\) and Ag\(^+\) in PbN\(_6\) require correction.

REFERENCES

1. Yu. A. Zakharov, G. G. Savel’ev. In: Chemical Reactions Involving Solids, 1969, (in press).

2. Yu. A. Zakharov, S. M. Ryabykh, G. G. Savel’ev. In: Chemical Reactions Involving Solids, 1969, (in press).

3. Yu. A. Zakharov, S. M. Ryabykh, A. P. Lysykh. Kinetics and Catalysis, 9, 725, 1968.

4. Yu. I. Ustenko, G. V. Zamorskaya. Zavodskaya laboratoriya, 30, 270, 1964.

5. M. Cygańkowska, V. Downarowicz. Chemia Analityczna, 11, 55, 1966.