POLAROGRAPHIC STUDY OF MIXED POTENTIAL, MIXED CURRENT, AND CEMENTATION OF METALS ON AMALGAMS

Yu. N. Zhikharev

(Presented by the scientific seminar of the CTF)

Mixed potential and mixed current constitute the theoretical basis for corrosion and cementation of metals [1, 2]. Polarography is a convenient method for studying and interpreting mixed potential and mixed current [3, 4, 5]. In our previous works [6, 7, 8], a number of questions associated with the polarographic study of mixed potential and mixed exchange current were considered.

The aim of the present work is the polarographic interpretation of the mixed exchange current in the nonequilibrium system amalgam of a (more negative) metal—solution of ions of a (more positive) metal, in connection with the theory of cementation of metals on amalgams, as well as the establishment of quantitative relationships between mixed current and mixed potential.

The nonequilibrium system studied was: $\mathrm{Zn(Hg)}$, $\mathrm{Zn(2+)}$, $\mathrm{Bi(3+)}$ in a $1\,M$ $\mathrm{HClO_4}$ solution. The work used an LP-60 recording polarograph and a polarographic cell with an amalgam dropping electrode, described in [10]. The characteristic of the amalgam dropping electrode was $L = 5.22\ \mathrm{mg}^{2/3}\cdot \mathrm{sec}^{-1/6}$. The anode and auxiliary electrode in the lower cell was a saturated mercury-sulfate electrode. The concentration of zinc in the amalgam (in different series of experiments) varied from $3\cdot 10^{-5}$ to $8\cdot 10^{-4}$ g-atom/l and was determined from data for the height of the anodic zinc wave using the Ilkovič equation. The concentrations of $\mathrm{Zn(2+)}$ and $\mathrm{Bi(3+)}$ ions were prepared from prepared standards.

Mixed potential (and current) arise as a result of the simultaneous occurrence, on the amalgam dropping electrode, of anodic and cathodic processes in a thermodynamically nonequilibrium system. Let us consider the physical meaning of the potential on a dropping amalgam electrode in the system under consideration in the absence of current in the circuit (with the circuit open). In [9] it was shown that, for an individual electrochemical system $\mathrm{M(Hg)}, \mathrm{M(2+)}$, the point of intersection of the anodic-cathodic wave on the amalgam dropping electrode with the residual-current line corresponds to the equilibrium potential $\varphi_r$. In the presence in solution of ions of another (more electropositive) metal $\mathrm{M}_1^{z_1+}$, the anodic-cathodic wave of the system $\mathrm{M_2(Hg)}, \mathrm{M_2}^{z_2+}$ rises, and at the point of intersection of the anodic-cathodic wave and the residual current we already have not an equilibrium, but a mixed potential; in this case, an equal current flows through the amalgam surface in two directions (the mixed exchange current) $i_c$, caused by two electrode reactions that compensate each other. With the circuit open $(i=0)$ in the system under consideration

of a mixture of two electrochemical systems, a certain stationary mixed potential is established. As the concentration of ions of the electropositive metal in the solution increases (in our case, bismuth ions), the mixed potential shifts into the positive region, first along the anodic wave of zinc and then along the cathodic wave of bismuth, tending toward the equilibrium potential of bismuth. The values of the mixed potential in the irreversible system under study can be calculated from equations derived on the basis of the theory of slow discharge–ionization [8].

The experimental and calculated data are in sufficiently good agreement. It should be noted that the theoretical quantitative relations obtained can be used directly in amalgam metallurgy, provided that the solution and the amalgam are stirred and that the cementation process has a stationary character. In addition, it is necessary to make sure that both processes are diffusion-controlled, i.e., that the cathodic limiting current of the metal ions \(M_{1}^{z_1+}\) in solution is reached at potentials more negative than the potential at which the anodic limiting current of the electrodissolution of the metal \(M_{2}^{z_2+}\) from the amalgam is reached. In this case, the mixed exchange current directly characterizes the rate of cementation of the metal ions \(M_{1}^{z_1+}\) from solution (if only one metal \(M_{1}^{z_1+}\) is cemented from the solution).

By experimentally measuring the mixed potential (of the stirred amalgam and solution), one can directly calculate the ratio of the mixed current to the limiting diffusion current, \(\frac{i_c}{I_1}\) or \(\frac{i_c}{I_2}\), knowing the half-wave potentials of both metals from polarographic data (\(I_1\) is the limiting current of bismuth, \(I_2\) is the limiting anodic current of zinc). To calculate the absolute value of the cementation rate (mixed exchange current), it is necessary to know, from additional electrochemical measurements, the limiting diffusion currents \(I_1\) and \(I_2\), under stirring conditions approximately identical to those of the polarographic setup for cementation. If \(I_1 \gg I_2\), then the cementation process is limited by the stage of delivery of the metal ions \(M_{1}^{z_1+}\) to the surface of the amalgam. If \(I_1 \gg I_2\), then the cementation process is limited by the stage of delivery of metal atoms from the amalgam to its boundary with the solution.

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