Wojciech Hyk

Chronoamperometry of uncharged species under the conditions of deficiency of supporting electrolyte: experiment versus theory

Abstract Chronoamperometry of ferrocene in acetonitrile containing a small amount of supporting electrolyte was studied to verify the theoretical model developed by Hyk et al. (W. Hyk, M. Palys and Z. Stojek, J. Electroanal. Chem., 415 (1996) 13). Some minor extensions were made in this model to adjust it better to the case studied. The experimental chronoamperograms were found to agree well with the theoretical predictions. The biggest deviations were observed for small support ratios. These deviations can be explained by the uncontrolled presence of ionic impurities in the solvent. The impurities influence significantly the conductivity of the solution for support ratios (ratio of concentration of supporting electrolyte and ferrocene) smaller than 0.00075, and cannot be included explicitly in the theoretical model. The influence of the geometry of the electrode and the effect of the double layer capacitance are discussed. A possibility of evaluating the total ion level and the formal potential of the system investigated, by fitting an experimental curve to the theoretical one, is also presented.

Migrational chronoamperometry of uncharged substrates. influence of electron transfer rate

Abstract An improved numerical way of solving the second Ficks law for the purpose of modelling chronoamperometry of uncharged species in a solution containing an insufficient amount of supporting electrolyte is presented. The simulation of the mixed diffusional and migrational transport is based on the Crank-Nicolson method with an exponentially expanded space grid. The simulation scheme includes the electron transfer rate and assumes the use of hemispherical microelectrodes. The electroneutrality condition was not explicitly imposed to the calculation model but deviations from this principle were controlled and found to be negligible for the transport. The scheme that has been worked out allows the computation of changes in the concentration profiles and the electrical variables (current, true electrode potential, resistance and ohmic drop) as functions of time and support ratio, c supp. el / c substrate . The results obtained demonstrate that the true potential of the working electrode in a solution with deficit of supporting electrolyte differs significantly from the applied potential and changes in time. This makes chronoamperometry resemble voltammetry. The standard heterogeneous rate constant of the electron transfer strongly influences the way the current changes in time and therefore can be determined from chronoamperometric experiments.

Quantifying Uncertainty of Determination by Standard Additions and Serial Dilutions Methods Taking into Account Standard Uncertainties in Both Axes

The analytical expressions for the calculation of the standard uncertainty of the predictor variable either extrapolated or interpolated from a calibration line that takes into account uncertainties in both axes have been derived and successfully verified using the Monte Carlo modeling. These expressions are essential additions to the process of the analyte quantification realized with either the method of standard additions (SAM) or the method of serial dilutions (MSD). The latter one has been proposed as an alternative approach to the SAM procedure. In the MSD approach instead of the sequence of standard additions, the sequence of solvent additions to the spiked sample is performed. The comparison of the calculation results based on the expressions derived to their equivalents obtained from the Monte Carlo simulation, applied to real experimental data sets, confirmed that these expressions are valid in real analytical practice. The estimation of the standard uncertainty of the analyte concentration, quantified via either SAM or MSD or simply a calibration curve, is of great importance for the construction of the uncertainty budget of an analytical procedure. The correct estimation of the standard uncertainty of the analyte concentration is a key issue in the quality assurance in the instrumental analysis.

Steady-state operation of porous photoelectrochemical cells under the conditions of mixed diffusional and migrational mass transport

The model of mixed diffusional-migrational transport in photoelectrochemical cells, including a mesoporous semiconductor that operates under steady-state conditions, is developed. The dye-sensitized nanocrystalline TiO 2 solar cell (DSSC) served as a reference system. The model takes into account the presence of the bulk solution layer in contact with the mesoporous semiconductor anode, illumination from either side of the cell, and the attenuation of light due to its absorption by sensitizing molecules. The expressions derived allow one to calculate concentration profiles of each species present in the electrolyte solution, electrostatic potential profiles, and current density-concentration overpotential curves for any level of ionic support and various schemes of the electron exchange reaction. Limitations due to the mass transport arise when DSSC operates under significant light intensities, e.g., at solar illumination levels. The calculation scheme allowing determination of the limiting current density is also included in the model. The impact of changes in various parameters on the steady-state transport-limited current density of dye-sensitized semiconductor-based photoelectrochemical cells has been evaluated for the three classes of redox systems, O - /R - , O + /R 0 , and O 0 /R - . The analytical expressions derived have been implemented for the I - 3 /I - redox mediator, the most widely used in dye-sensitized solar cells. In the calculations the following parameters were varied: the supporting electrolyte concentration, initial concentrations of the redox species, their diffusion coefficients, the porous layer absorbance, the bulk liquid layer thickness, and the photoanode porosity. The effect of migration is altered by these parameters only for the redox mediators involving both charged forms. For the redox couples involving one uncharged species, only varying content of supporting ions affects the migrational transport. The present study also provides definitive answers to certain questions related to the role of migration in the transport-limited operation of the cell, including under what conditions the migrational effects become negligible and how we might take advantage of migration to improve the cell performance.

Migrational chronoamperometry for various reaction stoichiometries and a variety of types of supporting electrolytes

Abstract The theoretical current-time relations are presented for chronoamperometry and the following experimental conditions: (1) the electrode process is of the type vSSZS⇔vPPZP+ne−, where one of the stoichiometric numbers (vS or vP) is an integer greater than or equal to 1; (2) the working electrodes are hemispherical microelectrodes; (3) a wide range of support ratios (the ratio of the bulk concentrations of supporting electrolyte and substrate) is employed; (4) various charges are assigned to the supporting ions. In some cases the influence of stoichiometry of the electrode reaction considered on the I–t dependences and the height of the steady-state current was significant. The influence of charges of supporting ions was clearly seen on the chronoamperometric curves for the most resistive system, i.e., uncharged substrate and small support ratio. Since the time interval used in this paper was appropriately large to reach steady-state, the theoretical results obtained are also an extension to those given by Myland and Oldham [J. Electroanal. Chem., 347 (1993) 49].

Highly efficient and selective leaching of silver from electronic scrap in the base-activated persulfate – ammonia system

A system composed of persulfate salt and ammonia in highly alkaline aqueous solution is developed and examined for leaching metallic silver from elements of the electronic waste materials (e-scrap). Strong base activates persulfate ions providing in situ generation of highly reactive oxygen molecules. The oxidized metal forms then well soluble complex ions with ammonia ligands. The kinetic studies of the leaching process were performed for pure metallic silver. They revealed that the efficiency of the process is affected by the type of the persulfate salt. By employing potassium persulfate one obtains significantly (more than 50% for silver plates and more than 100% for silver powder) increased efficiency of silver dissolution compared to the solution composed of either sodium or ammonium persulfates. In the range of persulfate concentrations between 0.02 and 0.23mol/L the apparent reaction order with respect to the persulfate concentration was similar for all persulfate salts and was estimated to be around 0.5. The room temperature (22±2°C) seems to be an optimal temperature for the leaching process. An increase in the temperature resulted in the significant drop in the silver dissolution rate due to the decreased solubility of oxygen. Based on these results a possible mechanism of dissolving silver is discussed and the optimal composition of the leaching solution is formulated. The obtained formulation of the leaching solution was applied for the extraction of silver coatings of Cu-based e-waste scrap and the obtained results revealed an important effect of copper in the mechanism of the leaching process. The regression analysis of the leaching curve indicated that each gram of base-activated potassium persulfate under the specified conditions may leach almost 100mg of silver coatings in a form of well soluble diamminesilver (I) complex. The silver complex can be relatively easy reduced to metallic silver. The method developed is relatively cheap, low toxic and does not produce harmful by-products.