Study of the Mechanism of Formation of a New Liquid Phase on Solid Electrodes
Abstract Generated abstract
This paper examines the mechanism by which a new liquid phase forms on solid electrodes, addressing the unresolved question of whether phase formation proceeds through nuclei or monolayer growth. Using polarographic measurements during electrolysis of divalent mercury in 0.1 M hydrochloric acid, the authors analyze how anodic peak height, peak potential, and half-peak width vary with electrolysis time and mercury concentration on solid electrodes. Breaks and inflection points in logarithmic current time and potential time dependences indicate a change in reaction mechanism, with an initial stage attributed to nucleus formation and a later stage showing a cubic time dependence consistent with nucleus growth. Similar behavior on platinum and gold electrodes supports the conclusion that the new phase forms as nuclei rather than as monolayers.
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STUDY OF THE MECHANISM OF FORMATION OF A NEW LIQUID PHASE ON SOLID ELECTRODES
B. F. Nazarov, L. D. Vishtakalyuk
(Submitted by the scientific seminar of the Department of Physical and Colloid Chemistry)
The question of the nature of the nuclei formed on the electrode surface and of the mechanism of their appearance remains, to this day, not definitively resolved. This is explained by the difficulty of experimentally studying the process of formation of a new phase: direct observation and counting of nuclei is practically impossible because of the extremely small size of these nuclei and their rapid change with time. Therefore the changes that occur in supersaturated solutions before the onset of visible crystallization are still explained by various hypotheses [1—5]. There is no complete clarity concerning the nature of the nuclei, in particular their structure [6—7].
The aim of our work is to study the mechanism of formation of a new liquid phase on solid electrodes.
Experimental conditions: electrolysis potential — 0.6 V, volume of the working solution 10 ml, rate of change of potential \(16.6 \cdot 10^{-4}\) V/s, background — 0.1 M HCl. The work was carried out on an LP-7 polarograph (Czechoslovakia). Divalent mercury was taken as the element being determined.
The dependence of the height of the anodic peaks of divalent mercury on the duration of electrolysis was studied. Fig. 1 presents the dependence of the anodic peak of mercury on the electrolysis time in logarithmic coordinates. It is seen from Fig. 1 that, as the electrolysis time increases, the dependence \((\lg h — \lg t)\) forms a bend at point \(b\), i.e., it forms two sections: \(ab\) and \(bc\). From the tangent of the angle of inclination for both sections, the exponents of the power-law functional dependence \((i—t)\) were found.
Fig. 1. Dependence of the height of the anodic peak of mercury on the duration of electrolysis in logarithmic coordinates:
\(C_{\mathrm{Hg}} = 0.005\gamma\)
For the section \(ab\), \(\tg \Theta_1 = 0.3\); at point \(b\) the curve sharply changes its slope, which indicates a change in the reaction mechanism.
For the section \(bc\), \(\tg \Theta_2 = 3\), i.e., in this case the \(i—t\) dependence obeys an equation of the type
\[ i = kt^3. \]
This indicates that here the growth of nuclei proceeds by the mechanism proposed in [8]. In the section \(ab\), nuclei are formed.
Figure 2 presents the dependence of the peak potential and of the half-peak width on the electrolysis time and on the mercury concentration in the solution. As can be seen from Fig. 2, on all the curves, inflection points are regularly observed at definite points; moreover, the electrolysis time at which inflection points are observed on the curves \((\varphi_{\mathrm{p}}—t_{\mathrm{e}})\) corresponds to the electrolysis time at which inflection points are observed on the curves \((\Delta \varphi^{n/2}—t_{\mathrm{e}})\), and corresponds to the electrolysis time at which the inflection in Fig. 1 occurs.
This once again confirms the assumption that at point b (Fig. 1) a change in the reaction mechanism occurs.
The same experiments were repeated on platinum and gold electrodes. The results agree approximately with those described above.
The dependence \((h—t_{\mathrm{e}})\) obtained on the gold electrode in logarithmic coordinates also has a break, and the tangent of the angle of inclination is equal to: \(\operatorname{tg}\Theta_1 = 0.6;\ \operatorname{tg}\Theta_2 = 2.9\).
Fig. 2. Dependence of the peak potential and half-peak width on electrolysis time and mercury concentration in solution: curve 1 — \(C_{\mathrm{Hg}} = 0.01\ \gamma\); curve 2 — \(C_{\mathrm{Hg}} = 0.005\ \gamma\); curve 3 — \(C_{\mathrm{Hg}} = 0.0045\ \gamma\).
Thus, on the basis of the material presented above, it may be considered that the formation of a new phase occurs in the form of nuclei, and not in the form of monolayers.
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