Study of the Mechanism of Electrode Processes on a Solid Electrode Involving Complex Ions
Abstract Generated abstract
This paper develops relations for analyzing irreversible discharge and ionization processes of complex ions at solid electrodes, motivated by the growing use of graphite and other solid electrodes in polarography and electroplating. Starting from Brainina’s equation for metal electro-dissolution and incorporating Gerischer’s treatment of exchange currents for complex-forming reactions, the authors derive expressions linking peak potentials to ligand concentration and potential sweep rate, allowing estimation of transfer coefficients and the composition of electroactive complexes. The relations are applied to the mercury, thiocyanate system in potassium thiocyanate media, yielding transfer coefficients and ligand numbers consistent with anodic formation of a trithiocyanate mercury complex followed by equilibrium conversion to tetrathiocyanate, and cathodic discharge involving a monothiocyanate mercury species.
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Study of the Mechanism of Electrode Processes on a Solid Electrode Involving Complex Ions
N. A. Kolpakova, A. I. Kartushinskaya
(Presented by the scientific seminar of the CTF)
Recently, solid electrodes made of graphite and other materials have found ever wider application in the practice of polarographic analysis and electroplating.
Therefore, the study of the mechanism of processes occurring during the discharge—ionization of ions on their surface is of great importance for theoretical and applied electrochemistry.
The theory for studying the mechanism of discharge—ionization of complex ions on amalgam and stationary mercury electrodes was developed most fully by A. G. Stromberg.
The purpose of the present work was to derive the basic relations for studying the mechanism of irreversible processes of discharge of complex ions on a solid electrode.
As the basic equation describing the irreversible process of electro-dissolution of a metal from the surface of a solid electrode in a non-complexing medium, the equation of H. Z. Brainina [1] was adopted; it was derived taking into account the change in the activity of the solid phase during electro-dissolution of the deposit,
\[ \varphi_n = \varphi_1 + \frac{RT}{\beta nF}\ln \frac{\beta \omega}{\gamma S k S_2 a_\infty RT}\, l^{\frac{\beta nF}{RT}(\varphi_1-\varphi_0)} \tag{1} \]
Equation (1), for \(\varphi_1=\varphi_p\), is easily reduced to the form
\[ \varphi_n=\varphi_p-\frac{RT}{\beta nF}\ln \frac{i_0RT}{ZFQ\beta\omega} \tag{2} \]
On the basis of Gerischer’s theory [2], the equation for the exchange current with the participation of complexes in the electrochemical reaction is written as:
\[ i_0=nFSK_s C_R^0 C_x^a \exp\left[\frac{\beta nF}{RT}(\varphi_p-\varphi^0)\right]. \tag{3} \]
Taking into account that \(C_R^0=\dfrac{aT}{f}=a_\infty\gamma Q/f\) and assuming \(f\) of the solid phase \(=1\), we substitute equation 3 into 2. Then
\[ \varphi_n=\varphi^0-\frac{2.3RT}{\beta nF}\lg\frac{K_{s_2}SRTa_\infty\gamma}{\beta} +\frac{2.3RT}{\beta nF}\lg w- \frac{2.3RT}{\beta nF}q_2\lg C_x . \tag{4} \]
It follows from equation 4 that the peak potential of electro-dissolution of the metal depends on the \(\lg\) concentration of the addend. From the slope of the straight line in the coordinates \(\varphi_n-\lg C_x\), the composition of the ionizing complexes can be determined. The value of the electron-transfer coefficient in the anodic process, \(\beta\), which is necessary for determining \(q_2\), can also be determined on the basis of equation 4 from the dependence \(\varphi_{na}-\lg w\).
A method for determining the composition of the complexes participating in the cathodic process was proposed in [3]. Proceeding from an analysis of the Matsuda and Ayabe equation [4], below we give the principal relations obtained in [3]:
\[ \varphi_n=\varphi_0+\frac{2.3RT}{\alpha nF} \left(-1.14+\lg K_{s_1}-\frac{1}{2}\lg D,\ \alpha,\ nw\right) -(p-q_1)\lg C_x . \tag{5} \]
Knowing the composition \(p\) of the complexes predominating in solution, from the dependence \(\varphi_{nk}-\lg C_x\) one can determine the composition \(q_1\) of the discharging complexes.
The expressions obtained were used in studying the system \(\mathrm{Hg}^{2+}\)—KSCN. Figure 1 presents the dependences of \(\varphi_{nk}\) and \(\varphi_{na}\) on the rate of change of potential. From the slopes of straight lines 1 and 2, the values of the transfer coefficients of the cathodic and anodic processes were found.
Fig. 1. Dependence of \(\varphi_{nk}\) and \(\varphi_{na}\) of mercury in a KSCN background on the logarithm of the rate of change of potential. Experimental conditions: \(\varphi_\mathrm{e}=-1.0\) V relative to the saturated calomel electrode, \(C_{\mathrm{Hg}}=1\cdot10^{-6}\) g-ion/l–\(2\cdot10^{-5}\) g-ion/l, \(\tau=1\) min.
Fig. 2. Dependence of \(\varphi_{na}\), \(\varphi_p\), \(\varphi_{nk}\) of mercury on the logarithm of the concentration of rhodanide ions. Experimental conditions: \(\varphi_\mathrm{e}=1.0\) V relative to sat. cal. el.; \(\tau=1\) min; \(W=0.0053\) V/sec; background KSCN + KNO\(_3\) \((I=2)\).
- \(\varphi_{na}-\lg C_{\mathrm{lig.}},\ C_{\mathrm{Hg}}=1\cdot10^{-6}\) g-ion/l.
- \(\varphi_p-\lg C_{\mathrm{lig.}},\ C_{\mathrm{Hg}}=1\cdot10^{-6}\) g-ion/l.
- \(\varphi_{nk}-\lg C_{\mathrm{lig.}},\ C_{\mathrm{Hg}}=2\cdot10^{-5}\) g-ion/l.
\[ \alpha=0.232,\qquad \beta=0.82. \]
From the tangents of the angles of inclination of straight lines 1, 2, 3 (Fig. 2), the values
\[ q_2=3,\qquad p-q_1=3,\qquad p=4 \]
were found.
In accordance with the data obtained, the mechanism may be represented as follows:
a) of the anodic process
\[ \mathrm{Hg}^{0} + 3\mathrm{SCN}^{-} - 2e \rightarrow \mathrm{Hg}(\mathrm{SCN})_{3}^{-} \quad \text{electrode reaction} \]
\[ \mathrm{Hg}(\mathrm{SCN})_{3}^{-} + \mathrm{SCN}^{-} \rightleftarrows \mathrm{Hg}(\mathrm{SCN})_{4}^{2-} \quad \text{equilibrium reaction;} \]
b) of the cathodic process
\[ \mathrm{Hg}(\mathrm{SCN})_{4}^{2-} \rightleftarrows \mathrm{Hg}(\mathrm{SCN})^{+} + 3\mathrm{SCN}^{-} \quad \text{equilibrium reaction,} \]
\[ \mathrm{Hg}(\mathrm{SCN})^{+} \rightarrow \mathrm{Hg}^{0} + \mathrm{SCN}^{-} - 2e \quad \text{electrode reaction.} \]
REFERENCES
- Kh. Z. Brainina. Elektrokhimiya, vol. 2, no. 9, 1966.
- H. Gerischer. Z. phys. Chem., 202, 292, 302, 1953.
- A. G. Stromberg, L. N. Popova. Izv. TPI, in press.
- H. Matsuda, V. Auabe. Z. Elektrochem., 59, 494, 1955.