vacuum grease is applied to the upper part. Thus, a comparatively simple methodological improvement that reduces the magnitude of interference can serve as a sufficiently effective means of increasing the sensitivity of the polarographic method with accumulation.

STUDIES ON THE DETERMINATION OF TUNGSTEN BY THE METHOD OF FILM POLAROGRAPHY WITH ACCUMULATION

L. F. Zaichko, A. A. Kalin, G. S. Shamanskaya

Film polarography with accumulation (FPA) expands the possibilities of the electrochemical determination of impurities by including elements that do not form amalgams, are not reduced to the metallic state, and are more noble than mercury. Among the elements for which the application of electrochemical concentration methods in the form of a metal of zero valence has a number of fundamental limitations is tungsten [1]. This is due to the fact that, in practically all solutions, according to literature data, tungsten is reduced on mercury electrodes to intermediate valence states. Certain possibilities for concentrating tungsten in the form of \(WO\) are afforded by the conditions of co-deposition with other elements. Data on the polarographic behavior of \(W^{+6}\) on solid electrodes are practically absent.

We have studied the possibility of applying the FPA method to the determination of microconcentrations of \(W^{+6}\) after its preliminary concentration on a solid electrode. The experiment was carried out on an ON-102 polarograph; the rate of change of potential was 500 mV/min. An electrolyzer with inserted beakers was used in the work; the indicator electrode was a graphite electrode, and the reference electrode was a saturated calomel electrode.

Standard tungsten solutions were prepared from the chemically pure reagent \(Na_2WO_4 \cdot 2H_2O\). Stirring of the solution and removal of oxygen were carried out with nitrogen.

In this work, for the first time, anodic peaks of tungsten were obtained by the FPA method against a background of 0.1 N \(NH_4Cl\) (\(pH = 7\)). The peaks are reproduced satisfactorily (relative error of determination \(\pm(10\text{–}15)\%\), and have a convenient for

changes in shape, increase linearly with increasing concentration of \(W^{6+}\) in the range \(1 \cdot 10^{-7}\) to \(1 \cdot 10^{-6}\) g-ion/l, and can be used for the quantitative determination of tungsten in solution. The dependence of the maximum electrooxidation current on the time of preliminary electrolysis is also linear. Under the experimental conditions, the potential of the maximum current for the electro-dissolution of the deposit is \(+0.1\) V.

The effect of the electroaccumulation potential \(\varphi\) on the height of the anodic tungsten peak was studied in the range \((-1.2)\) to \((-2.8)\) V. The curve of the dependence of the maximum anodic current on the electrolysis potential has the form characteristic of analogous dependences in the PNA method [2]. At \(\varphi_{\mathrm{e}} = (-1.2)\) to \((-2)\) V the peak height gradually increases, reaching a limiting value at \(\varphi_{\mathrm{e}} \sim (-2)\) V; in the potential region \((-2)\) to \((-2.8)\) V the peak height is constant. In the studied potential interval the shape of the anodic tungsten peak does not change.

To establish the optimal conditions for concentration and electro-dissolution, we also studied the effect of the solution pH on the height and shape of the tungsten peaks. The anodic current reaches its maximum value when the electrolysis and electrooxidation of tungsten are carried out in solution at pH = 7. With an increase or decrease in the pH of the solution, the anodic tungsten peaks decrease and then disappear completely. Alkalization of the solutions was carried out by adding 0.1 N KOH solution.

The effect of \(Hg^{2+}\), \(Ag^{+}\), \(Au^{3+}\), \(Pd^{4+}\), \(Pb^{2+}\), \(Cu^{2+}\), and \(Zn^{2+}\) on the anodic tungsten peak was studied. It was found that the presence in solution of \(Hg^{2+}\), \(Ag^{+}\), \(Au^{3+}\), \(Pd^{4+}\), and \(Cu^{2+}\) ions distorts the shape and height of the tungsten peak (concentration ratio 1:1); \(Pb^{2+}\) and \(Zn^{2+}\) ions do not exert a noticeable influence if their content in solution does not exceed the content of \(W^{6+}\).

The investigations carried out have for the first time demonstrated the possibility of determining microconcentrations of tungsten by the PN method.

It is still difficult to draw a definite conclusion about the nature of the precipitate formed on the electrode surface during the concentration stage. It is possible that a solid phase of oxygen-containing tungsten compounds is formed on the graphite electrode. It is not excluded that, in the presence of uncontrolled impurities in the supporting solutions, a process of reduction of \(W^{+6}\) to the metal of zero valence proceeds on the solid electrode. The resolution of this question requires additional investigations, including the use of physical methods of study.

Literature

1. A. T. Vas’ko. Electrochemistry of Tungsten. Kiev, Tekhnika Publishing House, 1969.

1. L. G. Baletskaya, Some physicochemical questions of the APP method and its application in monitoring the production of phosphors. Dissertation, Tomsk, 1967.

ON TAKING INTO ACCOUNT, IN THE EQUATIONS FOR ANODIC CURRENT, THE INTERACTION OF COMPONENTS OF COMPLEX SYSTEMS

A. A. Kaplin, V. A. Kolpakov.

The possibility of simultaneously determining several elements is one of the principal advantages of the APP and PPN methods. However, the joint deposition of two or more elements on a mercury or solid electrode may lead to the appearance of the effect of mutual influence (EMI), a special case of which is interaction in the system determined element–electrode material. The presence of EMI generally changes the analytical characteristics of the useful signal. In works 1, 2 we developed a semiempirical theory of EMI in the APP and PPN method. As a quantitative measure of EMI at the accumulation stage, due to facilitation or hindrance of the discharge of one element in the presence of another, to a change in the nature of the substrate and the magnitude of the active surface, to the mutual influence of elements on the stage of nucleation, growth of nuclei, etc., the value of the degree of depletion of the solution \(\gamma\) was adopted. In addition to this, it is expedient to introduce the relative degree of depletion, or relative distribution coefficient,

\[ \theta=\frac{\gamma_2}{\gamma_1}, \]

which characterizes the changed ratio of the amount \((m_i)\) or concentration \((C_{э,i})\) of elements on the surface or in the volume of the electrode as compared with the solution:

\[ \theta=\frac{\rho_{э,m}}{\rho_m}=\frac{\rho_{э,p}}{\rho_p} \quad \text{and} \quad \rho_{э,m}=\frac{m_2}{m_1},\quad \rho_{э,p}=\frac{C_{a,2}}{C_{a,1}},\quad \rho_p=\frac{C_{p,2}}{C_{p,1}} \tag{1} \]

In the presence of EMI the value \(\theta \ne 1\). In the general case, \(\theta\) depends on the concentration of the element in solution. To take EMI into account at the stage of anodic oxidation for binary systems, it was proposed [1, 2] to use the activity method. More complex is the derivation of the anodic-current equation for the components of ternary and more complex systems. Apparently, in these variants as well it is expedient to use the statistical-thermodynamic method. In the general case

\[ I=K_a S a_i(I). \]

The activity of an element on the surface or in the volume of the electrode is

\[ a_i=\exp\left[\frac{1}{kT}\frac{\partial \ln F}{\partial N_i}\right], \]

where \(F\) is the free energy of mixing of the system. When the free energy is represented as a sum of products by concentration, containing unknown parameters, one can obtain relations for \(a_i\) [3, 4]. Thus, for a ternary system (the regular-solution approximation),

\[ a_1=N_1\exp\left\{\left[\beta_{12}N_2^2+\beta_{13}N_3^2+(\beta_{12}+\beta_{13}-\beta_{23})N_2N_3\right]\right\} \]

\[ a_2=N_2\exp\left\{\left[\beta_{12}N_1^2+\beta_{23}N_3^2+(\beta_{12}+\beta_{23}-\beta_{13})N_1N_3\right]\right\} \tag{2} \]

\[ a_3=N_3\exp\left\{\left[\beta_{13}N_1^2+\beta_{23}N_2^2+(\beta_{13}+\beta_{23}-\beta_{12})N_1N_2\right]\right\} \]