S V Ivanov, P A Manorik - Cathodic reduction of monoglycinate complexes of copper(ii) on a solid electrode - страница 1
CATHODIC REDUCTION OF MONOGLYCINATE COMPLEXES OF COPPER(II) ON A SOLID ELECTRODE
S. V. Ivanov, P. A, Manorik, and I. V. Trosyuk
UDC 541.138 3
The mechanism was studied and kinetic parameters were determined of discharge on a solid electrode of monoglycinate complexes of copper (II). It was established that discharge occurs in stages. The stage of addition of the first electron is reversible; the stage of addition of the second electron is irreversible. It is preceded by a chemical reaction of cleavage of the glycine molecule, due to which the copper(I) aqua ion participates in the electrochemical process.
This paper is devoted to the study of the kinetics and mechanism of discharge of copper(II) complexes with glycine (Gly) of composition Cu(H2O)4Gly+. The conditions of formation of such complexes in the selected pH range of the solution and component concentrations, the experimental method, and apparatus are described in .
In solutions, containing monoglycinate complexes of copper(II), the voltamograms are characterized by cathodic peaks, the number and the position on the potential axis of which depend on the composition and pH of the solution and also the potential scanning rate. Cathodic voltamograms at different potential-scanning rates in a solution of CiuGly = 1:3 at copper concentration of 1.6-10'2 mole// and a solution pH 5 are presented in Fig. 1. A limiting current is observed in the region of potentials of 0.15-0.05 V under these conditions, while a current maximum is observed at a potential of -0.2 to -0.25 V. The electrode process in the region of potentials, preceding the limiting current, does not depend on the potential-scanning rate, which is indicated by the agreement of initial segments of voltamograms. At the same time, the size of the limiting-current segment is shortened with an increase in potential-scanning rate, and if it is greater than 1 V/sec a limiting current is not observed. The second branch of the voltamogram is shifted to more negative potential values in proportion to a decrease in the potential-scanning rate, and the potential of its maximum is shifted very insignificantly in the same direction. A lower value of the limiting current and of the peak current (Fig. la) correspond to a lower potential-scanning rate.
An increase in the glycine concentration at a constant copper concentration (and a fixed pH) causes a shift of the voltamogram to the region of negative potentials, and the shift is greater, the higher the solution pH.
The solution pH has the greatest effect on the form of the voltamogram. Dependences for solutions of Cu:Gly = 1:2 at a copper concentration of 1.6-10'2 mole// at pH 1-5 are presented in Fig. 2. One broad current maximum, dose to the limiting current, is observed on the voltamogram in strongly acidic solutions (at pH 1-2). The Cu(H2O)62+ copper aqua ion predominates in solution under these conditions [2-4]. An additional current maximum is observed in the region of potentials of -0.8 V upon an increase in the solution pH to 3. According to [1,3,5], discharge of the diglycinate complex of copper Cu(H2O)2(Gly)2 occurs under these conditions. Upon further alkalization of the solution the form of voltamograms becomes even more complicated: a third current maximum is observed in the region of potentials of 0.150.05 V. The values of current maxima uniformly decrease at potentials of 0.15 and -0.2 V and an increase in solution pH, while they change  at a potential of -0.8 V in the range of pH 5-7. This is associated with the fact that under these conditions virtually all copper ions are bonded into the Cu(H2O)2(Gly)2 complex, and no redistribution of copper ions between copper complexes with glycine of various compositions occurs. Potentials of the indicated cathodic peaks of voltamograms are shifted insignificantly in the direction of negative values with an increase in solution pH. As was already noted, an increase in solution pH in copper glycinate systems leads to the formation of Cu(H2O)4Gly+ and Cu(H2O)2(Gly)2 complexes, the equilibrium concentration of which are determined by the solution pH and the concentration of components. The diffusion coefficient of metal complexes is always lower than of their aqua ions; therefore, in the examined case upon diffusion control of the process the limiting current and (or) peak current should decrease with an increase in solution pH.
Fig. 1 Fig. 2
Fig. 1. a) Voltamograms of the copper electrode, recorded in a solution of Cu:Gly = 1:3 at a potential-scanning rate, V/sec: 1) 0.01; 2) 0.02; 3) 0.05; 4) 0.1; b) dependence of currents of cathodic peaks of voltamograms on potential-scanning rate: 1) for peak I; 2) for peak II.
Fig. 2. Voltamograms of the copper electrode, recorded in a solution of Cu:Gly = 1:2 at a potential-scanning rate of 0.01 V/sec at pH: 1) 1; 2) 2; 3) 3; 4) 4; 5) 5.
Processes were identified, characterized by cathadic peaks on voltamograms in the region of low polarizations (Fig. 1). According to , joint discharge of Cu(H2O)62* and the copper monoglycinate complex Cu(H2O)4Gly+, present in solution, is observed under such conditions. Therefore, it is logical to propose that each of the observed current peaks is due to discharge of one of the present ions. However, calculation with the Delahey equation  for peak currents showed that the concentration of Cu(H2O)62+ copper aqua ions, present under these conditions in solution, makes it possible to provide a current, approximately 60 times slower than that observed experimentally. The concentration of the Cu(H2O)4Gly* complex exceeds the concentration of the Cu(H2O)62+ aqua ions by more than two orders of magnitude; therefore, it should be considered that both peaks (Fig. 1) in the region of low polarizations are due to discharge of the copper monoglycinate complex.
During the study of kinetics and mechanism of electrode processes of polyvalent metals, including divalent metals, splitting of cathodic or anodic peaks into two peaks can be achieved by using for this purpose fast or slow potential-scanning rates . According to , splitting of the cathodic peak is theoretically possible upon a difference in potentials of cathodic and anodic peaks of less than -0.06 V. This splitting was observed  during discharge of ammoniacal complexes of copper. The process of electrolytic reduction of copper(II) ions to monovalent copper occurs at potentials of the first peak, and reduction of copper(I) ions to the metal occurs at potentials of the second peak.
Potentiostatic discharge at potentials of the first peak leads to the formation of a dark loose deposit; the increase in electrode weight indicates an apparent increase in yield of copper as a function of current, reaching 240%. This can be explained if it is proposed that the final cathodic reduction product is CAO (theoretical yield as a function of current amounts to 225% in this case). This conclusion agrees well with results of , according to which the formation of a second type of electrode, containing a covering Cuf) layer, is possible in copper(II) glycine solutions at a solution pH, exceeding 4. Electrolysis at potentials, corresponding to the second current peak, leads to the formation of metallic copper on the electrode. On the whole the stepwise discharge of copper (II) ions with formation of products of intermediate reduction, containing copper(I) compounds, is well described in the framework of the model of a bifunctional electrochemical system .
Dependences of potential Ep' and current of the first peak Ip1 on the potential-scanning rate were studied. Dependences of the peak currents orr the scanning rate in Ip'-V1/2 coordinates fit a line (Fig. la), i.e., values of
Fig. 3. Dependence of parameter Ip/V on the potential-scanning rate in a solution of Cu:Gly = 1:3 at a copper concentration of 1.6-10'2 mole// and pH 5: 1) for peak I; 2) for peak II.
Fig. 4. Determination of orders of the cathodic process: 1) with respect to glycine ions at pH 5; 2, 3) with respect to hydrogen ions for the system Cu:Gly = 1:3 at a copper concentration of 1.6-10 mole//.
Ip'/V172 remain constant in the whole studied range of potential-scanning rates (Fig. 3). The peak potential E ' practically does not change with a decrease in the potential-scanning rate. An increase in the potential-scanning rate leads to an increase in the difference AE = E'p/2 - E'p (Table 1). The set of obtained data on the basis of diagnostic Nickelson-Shein criteria  makes it possible to conclude that the first stage of the electrode process of discharge of the Cu(H2O)4Gly complex occurs reversibly by the following scheme:
Calculation of the number of electrons n, transferred in the first stage of the electrode process was carried out with the equation :
Results of calculations are presented in Table 1 and show that n - 1. The diffusion coefficient of the Cu(H20)4Gly complex was determined upon the use of high potential-scanning rates, i.e., under conditions of independence of diffusion of electrochemically active species by the Rcndls-Shevchik equation [111
/„ = 0.447pi/2
where !„ is the peak current, A; S is the electrode surface, cm2; C°Cu(AGiy- is the concentration of the
Cu(H20)4Gly+ complex in solution, mole/cm3. According to calculation, the diffusion coefficient of the monoglycinate complex of copper(II) is equal to 7.3-10"* cm2/sec, which is close to diffusion coefficients of the diglycinate complex of copper(II) Cu(H2O)2(Gly)2  and the copper aqua ion Cu(H2O)62+ 
' Further analysis of the electrochemical process (1) shows that it is not completely reversible. The difference in potentials of cathodic and anodic peaks for the reversible process should be equal to 2.2RT/nF, which amounts to 0.057 V ; in our case it reaches significantly larger values. A correlation between function i J, associated with the difference in potentials of cathodic and anodic peaks AEp, and the standard rate constant of the electrode reaction K, was established in :
With the use of data of  on values of AEp and л the standard rate constants of electrode reaction (1) amounted to K, = 1.55-10"3 cm/sec. Dependences of the current of the second peak (Fig. la) on the potential scanning rate V were studied. The peak current Ip" depends linearly on Vl/2, and parameter Ip"/V1/2 decreases uniformly with an increase in potential-scanning rate (Fig. 3). At the same time, the peak potential is shifted to the region of positive potentials (Table 2) with an increase in scanning rate. On the basis of the obtained diagnostic criteria  the process of transfer of the second electron can be classified as irreversible and preceded by a chemical stage. This process can most probably be presented by the following scheme:
GiGIy^OT - Gly. Cu~ - e-~Cu.
The value of ana, the product of electrochemical transfer coefficients and the number of electrons, transferred at the stage, determining the rate of the electrode process, was determined with the Matsuda-Ayabc equation :
-E"2 - - 1.851
Values of a with consideration that n = 1 are presented in Table 2. The mechanism of the cathodic process was investigated by methods of stationary polarization curves in a potentiostatic regime. Dependences of the logarithm of exchange current densities i0 on pH of the investigated solution and glycine concentrations are presented hi Fig. 4. With consideration of the fact that glycine at pH 4-8 is present in solution in zwitter-ion and deprotonated forms, equilibrium concentrations of the corresponding forms of glycine, calculated with the use of known pK values , were considered during the construction of Ig i0-lg [Gly] dependences. As is seen from Fig. 4, the exchange current density depends on the solution acidity and the glycine concentration. Slopes of the indicated dependence are: dig i0/31g [Gly] = -0.47 and dig, i)/31g [H+] = -0.81.
The order of the electrode reaction with respect to glycine ions was determined with the Fetter equation :
Kinetic Parameters of the Electrolytic Reduction Process (6)
where Р0Л. is the order of reaction in glycine ions; vatf- is the stoichiometric coefficient, with which glycine enters into the overall electrode reaction; z is the number of electrons, participating in the elementary stage of the reaction; n is the total number of electrons, participating in the electrode reaction.
With consideration that z = n = 1 and vatf = -1, the value of Рд determined with Eq. (8), amounted to -0.77. Thus, the order of the electrode process in glycine ions is equal to -1, which indicates the presence of a chemical reaction, preceding the discharge stage.
Because the value of ph+ is not known, the order of the electrode process in hydrogen ions was determined with the enuatinn f14l-
where E,, is the stationary electrode potential.
The value of 3Eo/dlg [H*] was determined experimentally from the dependence of the stationary potential on pH of the investigated solution (Fig. 4). The slope of the dEA/dlg [H+] line amounted to 0.125. The value of P0fC, calculated from these data, amounted to -0.18; consequently, the order of the electrode reaction in hydrogen ions is equal to zero.
The set of obtained experimental data indicates that the copper(I)-containing intermediate reduction product of the monoglycinate complex of copper(II) is the Cu(H2O)2+ aqua ion. Consequently, the overall process of cathodic reduction of the copper(II) monoglycinate complex can be presented by Eqs. (1), (5), (6).
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