The Influence of tert-Butanol on a Two-step Zn 2 + Ion Electroreduction in Concentrated NaClO 4 Solutions

The two-step reduction of Zn ions at the dropping mercury electrode in 2, 3, and 4 mol dm NaClO4 with the addition of tert-butanol, using the impedance method in wide potential and frequency ranges was examined. An inhibiting influence of tert-butanol is shown in the used experimental systems, which increases with the rise of the base electrolytes concentration. This effect caused by NaClO4 concentration change is probably linked with the electrodes surface and the Zn aquaion composition change. (doi: 10.5562/cca1726)


INTRODUCTION
Adsorption at the electrode/electrolyte solution interface plays an important role in the study of electrode kinetics.The presence of adsorbed nonelectroactive species can have a drastic influence on the rate of an electrode reaction, either in an accelerating or a inhibiting sense.The organic adsorbates inhibiting effect on various inorganic cations electroreduction is generally known. 1,2uasi-reversible electrode processes are the most suitable for the studies on the influence of organic compounds because they enable standard rate constant measurements in a wide range of quantities.The Zn 2+ ion is such a model reactant.Electroreduction of Zn 2+ ions on the mercury electrode is a typical example of a reaction controlled both by diffusion and charge transfer.In the literature several papers discuss [3][4][5][6][7][8] the possibility of a mechanism with two single electron transfers.
The Zn 2+ ion electroreduction at a mercury electrode in NaClO 4 as the base electrolyte is an example of an electrode reaction which is inhibited by tert-butanol (TB). 9The base electrolyte concentration influences the degree of Zn 2+ ion aquacomplexes and electrode surface hydration and hence on the depolarizer electroreduction kinetics.The influence of NaClO 4 concentration on Zn 2+ electroreduction in presence of tert-butanol was investigated recently but it concerned diluted solutions. 9The present paper is a continuation of this problem referring to 2, 3, and 4 mol dm -3 NaClO 4 .The results obtained are a compilation of tert-butanol adsorption and Zn 2+ ions kinetic electroreduction parameters in the presence of tert-butanol.In the supporting electrolytes concentrated solutions the Zn 2+ ions undergo a two step reduction where the inhibition effect of Zn 2+ ion reduction is the strongest in 4 mol dm -3 NaClO 4 .The results obtained from the kinetic studies of this process permitt a qualitative description of the adsorption-desorption balance of tert-butanol in Zn 2+ ions potential reduction range in NaClO 4 , so in an area distant from maximum adsorption potential.

EXPERIMENTAL
The experiments were performed in a three-electrode cell with a dropping mercury electrode made by MTM Poland as a working electrode, Ag/AgCl as a reference electrode and a platinum spiral as an auxiliary electrode.The reference electrode was fitted with a Luggin capillary probe.The capillary was filled with the cell solution.The impedance measurements were carried out with a 9121FR Analyzer and 9131 electrochemical interface (Atlas-Sollich, Gdańsk, Poland).The complex impedance data were collected at 36 frequencies ranging from 15 to 50000 Hz within the Faradaic potential region with 10 mV intervals.The ohmic resistance of the electrolyte solution was obtained at a potential outside the Faradaic region.

Polarographic measurements and voltammetric
experiments were performed employing the Autolab frequency response analyzer (Eco Chemie, Netherlands).
Chemicals of analytical grade were used from Fluka.Optimal accuracy was achieved by maintaining the Zn 2+ concentration around 0.005 mol dm -3 .Solutions of tert-butanol were prepared to cover the range from 0.01 to 0.5 mol dm -3 in NaClO 4 solutions.The hydrolysis of Zn 2+ was suppressed by maintaining the solutions at pH = 3.
The solutions were deaerated using nitrogen.This gas was passed over the solution during the measurements.Water and mercury were distilled twice.Measurements were carried out at 298.0 ± 0.1 K.

Double Layer Analysis
The double layers parameter calculations for the adsorption were based on the data from differential capacitypotential curves obtained experimentally for several concentrations of tert-butanol in NaClO 4 solutions of the following concentrations: 2 mol dm -3 , 3 mol dm -3 and 4 mol dm -3 . 10he curves ( ) C f E = obtained in NaClO 4 solutions with the addition of increasing tert-butanol amounts are characterized by a significant decrease in differential capacity as compared with the basic electrolyte, typical for inhibitors.The potential range in which this decrease occurs extends from -0.3 V to -1.1 V.The shape of the respective differential capacity curves obtained in the used concentrations of NaClO 4 is similar to the mentioned above potential range.The differences concern only the desorption peaks heights, which decrease in the following order: 4 mol dm -3 > 3 mol dm -3 > 2 mol dm -3 NaClO 4 .
In order to calculate the potential profile in the diffuse double layer it is necessary to estimate the charge density, σ i , of specifically adsorbed where c is the bulk concentration of NaClO 4 .Table 1 collects the electrode charge densities σ m and the potentials of the outer Helmholtz plane Φ 2 pertaining to the Zn 2+ ions electroreduction potentials for various NaClO 4 and tert-butanol concentrations.Electrode surface charge, σ m , takes on higher values with the increase of tert-butanol concentration.This is consistent with the change of zero charge potential in the presence of tert-butanol.The Φ 2 values however slightly depend on tert-butanol concentration like in the presence of tetramethylthiourea. 14  (10 / Cm ) m σ − potentials of the outher Helmholtz plane (Φ 2 / V) and potentials in the reaction plane (Φ r / V) as a function of tert-butanol and NaClO4 concentration at potentials: -0.96 V in 2 mol dm -3 NaClO 4 , -0.94 V in 3 mol dm -3 NaClO 4 and -0.93 V in 4 mol dm -3 NaClO 4 above values of potentials are close to from the electrode than the outer Helmholtz plane. 15ndreu et al. 7 proved that the reaction plane is 0.28 nm further from the electrode, which corresponds to the diameter of one water molecule

H O . d
The potential in the reaction plane Φ r can be calculated 8 by where κ is the double-layer thickness parameter expressed in cm -1 and 298 K is equal to where c is the bulk z:z electrolyte concentration in mol dm -3 .The calculated Φ r values are collected in Table 1.
They practically do not depend on tert-butanol concentration but with the increase of NaClO 4 shift in the direction of less negative potentials.
Figure 1 shows the relative surface excess of tertbutanol plotted at constant NaClO 4 concentration versus Φ r the potential.In the range of Zn 2+ reduction potentials the relative surface excess, Γ', of tert-butanol distinctly increases with the increasing tert-butanol and NaClO 4 concentration.Such Γ' value changes with NaClO 4 concentration increase clearly point (as in the case of tetramethylthiourea 14 ) to a facilitated tertbutanol adsorption on a less hydrated electrode surface in the presence of higher quantities of 4 ClO − ions.In all studied systems two areas of change for the Γ' value in the Φ r function can be distinguished.The first area in which the obtained dependence ' ( ) Φ has a nearly linear character apply to concentrations from 0.01 mol dm -3 to 0.1 mol dm -3 tert-butanol.The second area of higher tert-butanol concentrations from 0.2 mol dm -3 to 0.5 mol dm -3 the ' ( ) Φ dependencies loose their linear character and the observed Γ' shifts more clearly depend on the Φ r potential as well as on the tertbutanol concentration.

Polarographic and Voltammetric Measurements
The approximate diffusion coefficients of Zn 2+ in the examined solution were calculated using the Ilkovič equation for a diffusion-controlled limiting current.The polarographic wave of Zn 2+ in 0.1 mol dm -3 KNO 3 with a value of D ox = 6.9×10 -6 cm 2 s -1 for the Zn 2+ diffusion coefficient was used as a standard. 17The D ox values in the 2, 3, and 4 mol dm -3 NaClO 4 change with an increase of tert-butanol concentration ranging from 6.06×10 -6 cm 2 s -1 to 5.75×10 -6 cm 2 s -1 , from 5.72×10 -6 cm 2 s -1 to 4.9×10 -6 cm 2 s -1 and from 4.68×10 -6 cm 2 s -1 to 3.93×10 -6 cm 2 s -1 .The decrease of D ox values for Zn 2+ ions in the absence of tert-butanol, with NaClO 4 concentration increase results from much stronger solu-tion viscosity.The addition of the highest tert-butanol concentration generaly decreases the D ox values.
The reversible potentials of the half wave 1/ 2 r E of the Zn 2+ ions reduction were estimated from the cyclic voltammetric curves 17 with the reproducibility ±0.002 V, using the sweep rates of 0.005 to 0.1 V s -1 where E pc and E pa are cathode and anode peak potentials respectively.
In the studied NaClO 4 concentrations the increase in tert-butanol concentration does not cause significant 1/ 2 r E value changes.These small 1/ 2 r E changes in the studied electrolytes can be linked to the liable adsorption balance of tert-butanol in the potential range of Zn 2+ ions reduction. 10In this potential range tert-butanol desorption peaks start to form.
Figure 2 shows voltammetric curves of Zn 2+ electroreduction in 2, 3, and 4 mol dm -3 NaClO 4 with and without the addition of tert-butanol.A distinct decrease of the anodic peak current, the strongest in 4 mol dm -3 NaClO 4 should be noticed.In the case of Zn 2+ ions reduction acceleration by tetramethylthiourea 14 an increase of anodic and cathodic peak currents takes place.
From the results presented in Table 2 it can be stated that the base electrolyte concentration increase was accompanied by the decrease of ΔE values for Zn 2+ ions electroreduction attesting an acceleration of the electroreduction process.This tendency continued in c ≤ 0.05 mol dm -3 tert-butanol solutions.The introduction of higher amounts of tert-butanol caused a strong inhibition of Zn 2+ ions in 4 mol dm -3 NaClO 4 which is confirmed by the highest ΔE values.The observed effects correspond with the Γ' values for tert-butanol (Figure 1).

The Rate of Electroreduction
The complex impedance data were collected at 36 frequencies.Figure 3 shows some examples of impedance diagrams for the electroreduction of Zn 2+ ions in 3 mol dm -3 NaClO 4 with the addition of tert-butanol.The Z' and Z'' measurements accuracy was about 2 %.Just as in the diluted NaClO 4 solutions 9 the increased values of the charge-transfer resistance in the presence of tertbutanol demonstrate unequivocally the inhibiting influence of tert-butanol.Even a 0.09 mol dm -3 tert-butanol concentration causes a distinct increase of charge-transfer resistance, the most distinct in 4 mol dm -3 NaClO 4 .
The charge-transfer resistance values were used to determine the apparent rate constants k f values. 7The details are described elsewhere. 19,20The k f values obtained for the solutions of a constant NaClO 4 concentration and varying concentrations of tert-butanol indicate an inhibiting tert-butanol influence on the Zn 2+ electroreduction in the studied systems.Based on the command of the k f and Φ r values true rate constants were appointed based on the dependence ( ) ( )exp ( ) Figure 4 shows the potential dependence of the true rate constants k f of Zn 2+ ion electroreduction calculated at OHP + 0.28 nm, obtained at various tert-butanol concentrations.
In some cases the dependence ln t f k vs. Φ r seems to be linear.Then to characterize the regression quality a coefficient of determination, R 2 was used.In this case, the R 2 coefficient of determination is a statistical measure of how well the regression line approximates the real data points.An R 2 of 1.0 indicates that the regression line perfectly fits with the data making it easy to evaluate the quality of the fit.Table 3 shows the values of R 2 for the cases when experimental points were adjusted by linear function.It can be seen, that the use of higher tert-butanol concentration: c ≥ 0.4 mol dm -3 in the case of 2 mol dm -3 NaClO 4 , c ≥ 0.3 mol dm -3 in the case of 3 mol dm -3 NaClO 4 and c ≥ 0.2 mol dm -3 in the case of 4 mol dm -3 NaClO 4 results in rectilinear dependencies ln ( ).
In this case it was nearly impossible to determine a constant rate of individual transfer stages consecutive electrons during Zn 2+ ions electroreduction on a mercury electrode.This effect could probably be explained by the Zn 2+ ions electroreduction irreversibility in the presence of a substantial amount of tert-butanol.The strong irreversibility of the studied process did not allow two separate electron transfer stages.
For lower tert-butanol concentrations the relations ln ( ) Φ are not rectilinear, and the slope of the curves changes with the potential and the tert-butanol concentrations.This nonlinear dependence indicates a two-step character of Zn 2+ ions electroreduction mechanism in the studied solutions.This multistep mechanism is determined by the following properties: 1.A two one-electron transfer processes (mechanism EE), 21 2. A slow transfer of zinc cation through the double layer with two distinct steps to a site at the interface where a metal ion reactant is adsorbed (mechanism IA), 21,22 3. A chemical step followed by electron transfer (mechanism CE).The chemical step is assumed to involve loss or exchange of a ligand, which could be a solvent molecule. 23On the basis of the kinetic analysis we cannot detect the presence of chemical reactions 24 4.An ion transfer step followed by an electron transfer (mechanism IE).As the dependence on Figure 4 shows the introduction of the lowest tert-butanol concentrations to the studied systems caused a Zn 2+ electroreduction rate decrease.This tert-butanol inhibiting effect is the clearest at less negative potentials and increases with the rise of tert-butanol concentration.
It is assumed that for the studied systems which obtained non-linear dependencies ln ( ) Φ the charge transfer takes place via two-consecutive one electron transfer steps. 7,8The first electron transfer is rate determining 1 ( ) k k = at the most negative potentials.At less negative potentials, the overall rate is determined by both steps simultaneously where K 1 is the formal equilibrium constant of the first stage.The balance constants for each of the electron transfer stages were connected with a Nernstian-like potential dependence. 7By using the above relationship and adopting the experimental data ln ( ),  ing accelerating effect of the second electrons transfer in concentrated NaClO 4 solutions in the presence of small tert-butanol concentrations is linked with the Zn + aquaion structure.In 3 and 4 mol dm -3 NaClO 4 solutions the Zn + ion is less hydrated.The electrodes surface is also less hydrated for example for a 0.1 mol dm -3 tert-butanol concentration at 0 f E potential the Γ' values are 1.1×10 -6 , 1.8×10 -6 and 1.7×10 -6 mol m -2 w 2, 3, and 4 mol dm -3 NaClO 4 , respectively.
-For tert-butanol maximum concentration only the t s k values (Table 4) could have been determined.At the highest tert-butanol concentration used 0.5 mol dm -3 the highest inhibiting effect was observed in a 4 mol dm -3 NaClO 4 .The t s k value for this concentration decreases over 100 times in relation to the t s k value determined in the absence oftert-butanol , in a 3 mol dm -3 NaClO 4 over 90 times, and in a 2 mol dm -3 about 30 times.In reference to the results obtained from diluted NaClO 4 solutions 9 it can be said that the tert-butanol inhibiting effect increases in the following order:0.1 mol dm -3 < 0.5 mol dm -3 < 1.0 mol dm -3 < 2.0 mol dm -3 < 3.0 mol dm -3 < 4.0 mol dm -3 NaClO 4 .

CONCLUSIONS
From the presented results it can be stated that: 1. Slight 1/ 2 r E value shifts in all the studied systems 2 mol dm -3 , 3 mol dm -3 and 4 mol dm -3 NaClO 4 may be the result of a weak tert-butanol adsorption in the Zn 2+ ion reduction potential region.2. Determination of constant rate stages of Zn 2+ ions electroreduction was possible in solutions with lower tert-butanol concentrations for which ' ( ) Φ dependencies had a generally linear character whereas the ln ( ) Φ dependence was non-linear.In higher tert-butanol concentrations responsible for the strong Zn 2+ ions reduction irreversibility process ln ( ) dependencies were linear and did not permit the determination of constant rate stages.3. The analysis of the true rate constant t s k depending on tert-butanol and the base electrolytes concentration as well as the work 9 results allows to conclude that the strongest inhibiting effect at the highest tert-butanol concentration occurs in a 4 mol dm -3 NaClO 4 .The tert-butanol inhibiting effect increases in the order: 0.1 mol dm -3 < 0.5 mol dm -3 < 1.0 mol dm -3 < 2.0 mol dm -3 < 3.0 mol dm -3 < 4.0 mol dm -3 NaClO 4 .4. The rectilinear and parallel dependences

4 ClO−
ions as a function of the charge density on the electrode, σ m .Data of the specifically adsorbed charge σ i of 4 ClO − were taken from the Parsons and Payne study. 11It was assumed, as in Reference 12, that the amount of specific 4 ClO − adsorption is the same for HClO 4 and NaClO 4 at the same concentration and charge density.The potentials of the outer Helmholtz plane (OHP), Φ 2 , were derived in the usual manner assuming the validity of the Gouy-Chapman-Stern theory:

Figure 3 .
Figure 3. Impedance diagrams measured at 0 f E for the electro- reduction of Zn 2+ in 3 mol dm -3 NaClO 4 and in the presence of tert-butanol.

8 Figures 5 Figure 5 .
Figure 5.The true standard rate constants t s k for the Zn 2+ reduction vs. the relative surface excess of tert-butanol in 2, 3, and 4 mol dm -3 NaClO 4 .The dashed lines denote t s k values in

Figure 6 .
Figure 6.The individual true standard rate constants 1 t s k for the Zn 2+ reduction vs. the relative surface excess of tert-butanol in 2, 3, and 4 mol dm -3 NaClO 4 .The dashed lines denote 1 t s k f Γ suggest that the used NaClO 4 concentra- tions do not influence the first electron transfer mechanism.From the 2 it follows that the Zn + aquaions and the hydration degree composition has a clear influence on the second electrode transfer mechanism.

Figure 7 .
Figure 7.The individual true standard rate constants 2 t s k for the Zn 2+ reduction vs. the relative surface excess of tert-butanol in 2, 3, and 4 mol dm -3 NaClO 4 .The dashed lines denote 2 t s k

Table 2 .
The ΔE value of the potential differences of the anode and cathode peak in 2, 3, and 4 mol dm -3 NaClO 4 in the presence of different amounts of tert-butanol c TB / mol dm-3

Table 3 .
The coefficient of determination R 2 values which characterize the linear regression quality in cases where the

Table 4 .
The true standard rate constant t s k of Zn 2+ ion elec- troreduction extrapolated to the 0 f E for various concentrations of NaClO 4 and tert-butanol c TB / mol dm -3