POLAROGRAPHIC STUDY OF OXYQUINOLINE COMPLEXES OF LEAD AND CADMIUM IN BENZENE–ETHANOL (METHANOL) AND CARBON TETRACHLORIDE–ETHANOL (METHANOL) MIXTURES

Yu. A. Karbainov, T. I. Burmistrova

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

To determine the composition and instability constants of complex ions from polarographic data, we used, as before [1], the DeFord and Hume method [2]. The application of the DeFord and Hume method to the study of the composition and instability constants of complex ions is possible only in the presence of a reversible process of reduction of the latter at a dropping mercury electrode. We checked reversibility by the equation of a reversible polarographic wave by constructing the dependence of

\[ \lg \frac{i}{i_d-i} - (-\varphi). \]

Processing of the polarograms in these coordinates showed that the slopes of the straight lines

\[ \lg \frac{i}{i_d-i} - (-\varphi) \]

are close to the theoretical value, equal to

\[ \frac{0.059}{n}. \]

When the concentration of oxyquinoline was varied in the range \(0.0035\text{–}0.0535\ \mathrm{mol/l}\) in a benzene–ethanol mixture for cadmium and lead, the slopes varied within the range \(0.026\text{–}0.036\). Under the same conditions, in a carbon tetrachloride–ethanol mixture, the slopes have values of \(0.026\text{–}0.034\). On the whole, this indicates the reversible character of the electroreduction of the oxyquinoline complexes of lead and cadmium in the systems studied.

The directly proportional relationship between the limiting current \((I_d)\) and the square root of the height of the mercury reservoir found in the work indicates the diffusion nature of the limiting current. Solutions with a specified ligand concentration were prepared as follows. A \(0.2\,N\) solution of \(\mathrm{NH_4NO_3}\) in ethanol (methanol) was prepared separately, as were solutions of oxyquinoline of a specified concentration in benzene or carbon tetrachloride. Oxyquinoline and carbon tetrachloride were first purified according to the procedures described in [3–4]. The prepared solutions were mixed in a ratio of \(1:1\). An addition of a standard solution of lead or cadmium was introduced into the resulting mixture so that their concentration in the mixture was equal to \(1 \cdot 10^{-3}\text{–}5 \cdot 10^{-3}\ \mathrm{mol/l}\). The prepared solutions were kept for 1–2 days. Before the experiment, the working solution, in a volume of 5 ml, was transferred to the electrolytic cell and vigorously stirred with nitrogen for 5–10 min, after which the polarographic wave was recorded. The experiment was repeated 3–4 times, and the mean value of the half-wave potentials was calculated.

Table 1

Composition and instability constants
of lead and cadmium complexes

Benzene–ethanol

Benzene–methanol

Carbon tetrachloride–ethanol

Carbon tetrachloride–methanol

In the studied interval of changes in ligand concentration, polarographic waves with a clearly expressed diffusion-current region were obtained. With increasing ligand concentration, the half-wave potentials shift in the negative direction. The dependence \(\Delta \varphi_{1/2} — f(\lg C_{\text{л}})\) is expressed by a characteristic curve indicating the stepwise nature of complex formation.

The Leden functions \(F_0(x)\), \(F_1(x)\), and \(F_2(x)\) for lead and cadmium are expressed by curved lines, while the function \(F_3(x)\) is a straight line at a certain angle to the \(C_{\text{л}}\) axis. This character of the curves corresponds to the presence of four complex compounds with coordination numbers, respectively, equal to 1, 2, 3, 4.

Table 1 presents the experimentally found values of the instability constants of the oxyquinoline complexes of lead and cadmium in benzene—ethanol (methanol) and carbon tetrachloride—ethanol (methanol) mixtures.

REFERENCES

1. Yu. A. Karbainov, A. G. Stromberg. Izvestiya TPI, vol. 174, Tomsk, TSU Publishing House, 1971, 24.

2. D. D. Deford, D. N. Hume. J. Amer. Chem. Soc., 73, 5321 (1951).

3. A. Weissberger, E. Proskauer, J. Riddick, E. Toops. Organic Solvents. Moscow, Foreign Literature Publishing House, 1958.

4. S. I. Sinyakova, A. G. Dudareva, I. V. Makarova, M. N. Talalieva. In: Methods of Analysis of Chemical Reagents and Preparations, Issue 5–6, Moscow, IREA, 1963.