POLAROGRAPHIC STUDY OF DITHIZONE COMPLEXES OF LEAD AND CADMIUM IN A CHLOROFORM–METHANOL MIXTURE AND DIETHYLDITHIOCARBAMATE COMPLEXES OF LEAD AND CADMIUM IN AN ETHYL ACETATE–METHANOL MIXTURE

Yu. A. Karbainov, A. G. Stromberg

Studies by a number of authors \[1–7\] show that the solvent does not remain indifferent in the process of complex formation, but exerts a substantial influence on the stability and composition of complex compounds. The study of this influence, especially the influence of a nonaqueous solvent on the composition and instability constants of complex compounds, is of great importance both for analytical chemistry and for the theory of solutions. In this connection, and also in connection with the development of an extraction-APN method, the objective of the work was set as follows: to study the composition and stability of dithizone complexes of lead and cadmium in a chloroform–methanol mixture and of diethyldithiocarbamate complexes of lead and cadmium in an ethyl acetate–methanol mixture over a wide range of changes in ligand concentration. In the case of successive complex formation, determination of the composition and instability constants from polarographic data in the presence of a reversible electroreduction process is carried out by the calculation methods of DeFord and Hume \[8\] or Yatsimirskii \[9\].

The equation derived by DeFord and Hume has the following form:

\[ (\varphi_{1/2})_c = (\varphi_{1/2})_u - \frac{R_{\mathrm{T}}}{zF}\ln\left(1+\frac{c_l}{K_1}+\frac{c_l^2}{K_2}+\cdots+\frac{c_l^n}{K_n}\right), \]

where \((\varphi_{1/2})_c\) is the half-wave potential of the complex ion;
\((\varphi_{1/2})_u\) is the half-wave potential of the simple ion;
\(c_l\) is the ligand concentration;
\(K_i\) is the instability constant of the complex ion.

The solution of this equation is obtained graphically by Leden’s method \[10\], the essence of which is as follows: from the equation

\[ (\Delta\varphi_{1/2})_c = -b \lg F_0 \]

the value of the function \(F_0\) is found. Having found \(F_0\) from the experimental data, a plot \(F_0=f(c_l)\) is constructed, from which the value of the instability constant \(K_1\) is found, proceeding from the relation

\[ \left(\frac{dF_0}{dc_l}\right)_{c_l=0}=K_1. \]

Analogously, plots are constructed in other coordinates, \(F_1=f(c_l)\); \(F_2=f(c_l)\), etc., where

\[ F_1=\frac{F_0-1}{c_l}; \qquad F_2=\frac{F_1-\kappa_1}{c_l}, \]

and the values \(\kappa_2\), \(\kappa_3\), \(\kappa_4\), etc., are found.

In our work we used the DeFord and Hume method, which is more accurate than the Yatsimirsky method. As already stated above, application of the DeFord and Hume method to study the composition and instability constants of complex ions is possible only when there is a reversible process of electroreduction of the latter at a dropping mercury electrode. We checked reversibility by means of the equation for a reversible polarographic wave, by plotting graphs of

\[ \varphi - \lg \frac{i}{I-i}. \]

Processing the polarograms in these coordinates showed that the slopes of the straight lines \(\varphi - \lg \dfrac{i}{I-i}\), although they increased somewhat with increasing ligand concentration, nevertheless remained close to the theoretical value, equal to \(\dfrac{0.059}{z}\). When the concentration of dithizone in the mixture was varied in the range \(6.24 \cdot 10^{-4} \div 1 \cdot 10^{-3}\) mol/l, the slopes \(\varphi - \lg \dfrac{i}{I-i}\) varied from 0.031 to 0.036; in the case of sodium diethyldithiocarbamate \((0.0045 \div 0.12\) mol/l) \(\operatorname{tg}\upsilon = 0.031 \div 0.033\). Overall this indicates the reversible character of the electroreduction of the dithizone complexes of lead and cadmium from the chloroform–methanol mixture, as well as of the diethyldithiocarbamate complexes of lead and cadmium from the ethyl acetate–methanol mixture, and that two electrons participate in the electrode reaction.

The dependence between the limiting current \((I_k)\) of lead and cadmium ions and the square root of the height of the mercury reservoir is proportional [11], which indicates the diffusion nature of the limiting current.

Solutions with a definite ligand concentration were prepared as follows. Solutions of dithizone in chloroform and of sodium diethyldithiocarbamate in ethyl acetate of various concentrations were prepared separately, as was a 0.2 N solution of \(\mathrm{NH_4NO_3}\) in methanol (the latter was used in the experiments as the indifferent electrolyte). Dithizone, diethyldithiocarbamate, chloroform, and ethyl acetate were first thoroughly purified according to the procedures described in [12–13]. The separately prepared solutions were mixed in a 1:1 ratio. An addition of a standard solution of lead or cadmium was introduced beforehand into the resulting mixture so that the concentration of these elements in the mixture was equal to \(1 \cdot 10^{-3}\)—\(9.8 \cdot 10^{-4}\) mol/l. Before the experiment, a 10 ml volume of the working solution was transferred to the polarographic cell and intensively stirred with nitrogen for 20–30 minutes, after which the cathodic polarographic wave was recorded. The experiment was repeated three to four times, and the mean value of the half-wave potentials and wave heights was calculated. The concentration of sodium diethyldithiocarbamate was varied in the range 0.0045—0.154 mol/l, and the concentration of dithizone in the range \(6.24 \cdot 10^{-4}\)—\(4.5 \cdot 10^{-3}\) mol/l. We did not take a broader range of changes in dithizone concentration because, at dithizone concentrations above \(5 \cdot 10^{-3}\) mol/l, maxima appeared on the polarographic waves, and suppressing them presented great difficulty. In our case, however, the investigations were carried out in the absence of surface-active substances that suppress maxima.

Over the entire range of changes in ligand concentration it was possible to obtain distinct polarographic waves without maxima, with clearly expressed-

…by the diffusion-current region. With increasing ligand concentration, the half-wave potentials shift in the negative direction. The dependence \((\Delta \varphi_{1/2})_c — f(\lg c_l)\) (Figs. 1 and 2) is expressed by a characteristic curve indicating the stepwise nature of complex formation.

Fig. 1. Dependence of \(\Delta \varphi_{1/2}\) on \(\lg c_l\). Dithizone. Curve 1—lead; 2—cadmium

Fig. 2. Dependence of \(\Delta \varphi_{1/2}\) on \(\lg c_l\). Sodium diethyldithiocarbamate. Curve 1—lead; 2—cadmium

The functions \(F_0(x)\), \(F_1(x)\), and \(F_2(x)\), both in the case of lead and in the case of cadmium, are expressed by curved lines, while the function \(F_3(x)\) is a straight line at a certain angle to the \(c_l\) axis. This character of the curves corresponds to the presence of four complex compounds with coordination numbers \(P = 1, 2, 3, 4\), respectively.

Table 1 presents the values of the instability constants of the dithizone complexes of lead and cadmium in a chloroform–methanol mixture, as well as

Composition and instability constants of lead and cadmium complexes
Table 1

diethyldithiocarbamate complexes of these elements in an ethyl acetate–methanol mixture over the investigated range of changes in ligand concentrations. From a comparison of the data on the instability constants of the lead and cadmium complexes it is evident that the stability of the cadmium complexes is somewhat higher than the stability of the corresponding lead complexes. On the other hand, the stability of the oxine [11] and DDC complexes of lead and cadmium is approximately the same, but lower than the stability of the corresponding dithizone complexes of lead and cadmium.

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