Steady-state voltammetry of strong and weak acids with and without supporting electrolyte (original) (raw)
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Voltammetric analysis of weak acids with microelectrodes
Journal of Electroanalytical Chemistry, 2004
The role of H + on the steady-state voltammetric limiting current (I L ) of weak acids (HA) reduction was studied considering results from either lactic acid or acetic acid solutions at constant pH. For solutions with pH > pK a or pH ≅ pK a the [H + ] effect on the I L vs c HA slope was important even when the H + current contribution was minor.
Analytical Chemistry, 1998
The steady-state voltammetric behavior for reduction of several polyprotic acids and mixtures of strong and weak mono-and polyprotic acids was studied at platinum microelectrodes. The results demonstrated that over the potential range accessible to reduction of acids in water (up to ∼-1 V vs Ag/AgCl) via a preceding chemical reaction (CE mechanism), the reduction of weak polyprotic acids and mixtures of acids can produce either a single well-defined wave or two waves separated to a different extent, depending on the dissociation constant of each acidic form, on the analytical concentration of each acid, and on the mutual ratio of the acids present at equilibrium in the bulk solutions. The overall reduction mechanism for most of the mixtures examined was interpreted on the basis of a series of CE processes associated to the hydrogen evolution. This interpretation was supported by digital simulation procedures. A theoretical relationship for predicting the steady-state limiting current for any mixture of acidic species, whose dissociation steps are fast, was also derived. This equation proved valid for all those acids with equilibrium constants larger than ∼10-6. On the basis of this theoretical relationship, a simple diagnostic criterion to assess whether or not the reduction process of a mixture of acids is under a kinetic control was also established.
Journal of Electroanalytical Chemistry, 2010
The square wave voltammetric response of the reduction of acids of different strength namely, hydrochloric acid, phosphoric acid, lactic acid, acetic acid and sodium dihydrogenophosphate at a platinum microdisk electrode are presented. The voltammograms of HCl and H 2 PO À 4 display a single peak which height increased linearly with concentration. The acids of intermediary strength exhibit voltammograms resulting from the overlapping of two peaks that are assigned to the reduction of hydrogen ions (H + ) and of the undissociated acid form (HA). Peaks separation depends on the acid strength and concentration. A numerical deconvolution of voltammograms was performed based on Gaussian peaks fitting. The deconvoluted peak current data for H + were validated towards calibration curves of HCl. The validation of the current data of HA was performed using buffer solutions. The methodology present herein allows to assess simultaneously the concentration of each acid form from a single square wave voltammogram.
Cathodic reduction of acids in dimethylformamide on platinum
Electrochimica Acta, 2006
A linear correlation was shown to exist between the acidity and the cyclic voltammetric half-potential of the reduction of acids in DMF for carboxylic and N-acids in the pK a range of 6-16. Chlorophenols are reduced at slightly lower potentials giving a separate parallel line. Applying the obtained equation and employing the same method to literature data in DMSO, the pK a values for conjugate aids of DMF and DMSO can be calculated, showing DMSO·H + to be more acidic (pK a = 2.9) than DMF·H + (pK a = 5.7). The analysis of cyclovoltammetric data demonstrated that a CE mechanism operates in the reduction of strong acids, including the conjugate acid of DMF. Weaker acids are reduced by direct discharge or a mixed mechanism.
Journal of The Electrochemical Society
Linear sweep voltammetry data, obtained for hydrogen evolution on a copper electrode were used to estimate the lability of weak monoprotic acids as proton donors. Analysis of differential equations involving diffusion and kinetic terms shows that the total mass transport of proton donors and acceptors does not depend on the kinetics of chemical steps. Their surface concentrations as functions of the electrode potential can be easily obtained by convolution of experimental voltammograms and used in further analysis. The results show that acetic and glycolic acids are sufficiently labile, whereas the dissociation of the carboxylic group in gluconic acid is kinetically inhibited. The zwitterions formed in glycine solutions cannot be regarded as labile proton donors due to the inertness of the protonated amino group. Transforms necessary for constructing linear Tafel plots are discussed.
Voltammetry of surface-bound species: Proton-coupled electrochemical reduction
Journal of …, 2010
We consider the one electron, one proton and also the two electron, two proton reductions of surfacebound species. Two mechanisms of reaction are considered: stepwise and concerted. The voltammetry is modelled under three regimes of proton transport: infinitely fast (fully buffered solution), infinitely slow (infinitely high surface coverage of electrode) and the intermediate case of a finite rate of diffusional mass transport to electrode surface. The types of voltammograms observed in each case are presented and discussed.
Theory of the diffuse layer when a strong acid is reduced without supporting electrolyte
Journal of Electroanalytical Chemistry, 2000
The value of the limiting current for reduction of a strong acid, HA, without supporting electrolyte is known to be twice the current value in a solution containing a supporting electrolyte. This enhancement is ascribed to acceleration of the flux of H + toward the electrode by the electric field. Since the electrical neutrality equates [A − ] with [H + ], the depletion of H + by the electrode reduction decreases also [A − ] to zero. Consequently, no ion may be present near the electrode, indicative of infinite solution resistance. This is inconsistent with the experimental observation that the voltammogram is sigmoid. The inconsistency is discussed in this report by considering the Nernst-Planck equations and Poisson's equation for H + and A − at the hemi-spherical electrode under the steady-state condition. Because of strong non-linearity in the differential equations, numerical solutions are obtained by combining the Newton method and the iterative method for simultaneous equations. They demonstrate the presence of two domains of collapse of the electrical neutrality. One domain close to the electrode has the relation [A − ] \ [H + ] owing to the electroreduction of [H + ], whereas the other domain further from the electrode has the opposite relation owing to the electrostatic effect. The non-zero value of [A − ] is responsible for a finite value of the solution resistance and hence enables the voltammetric measurement to be made without a supporting electrolyte.
Analytica Chimica Acta, 1995
Normal pulse voltammetric waves obtained under conditions of substantial deficiency of supporting electrolyte may have heights identical to those recorded in the presence of an excess of supporting electrolyte. This is due to fast accumulation of ions in the microelectrode neighbourhood. Even such a small amount of electroinactive ions as that which may be adsorbed at the microelectrode surface can significantly speed up the formation of an "excess electrolyte zone", and correspondingly can lower the total resistance between the electrodes. Solutions of oxygen and ferrocene, and undiluted methanol were taken for the normal pulse and chronoamperometric experiments. The discussion of the experimental results was supported by digital simulation.
Journal of Electroanalytical …, 1998
The steady-state voltammetric behaviour for the reduction of mixtures of labile complexes of two divalent cations was studied in aqueous solutions containing different concentrations of supporting electrolyte. Mixtures of lead(II) plus cadmium(II) salts, and mercuric(II) plus lead(II) salts were investigated using a mercury-coated platinum and a naked platinum microelectrode, respectively. For the mixture containing lead plus cadmium salts, in solution, with no deliberately added supporting electrolyte, the steady-state limiting current of cadmium increased much more than expected, as a consequence of the increased rate of mass transport of lead which reduces at less negative potentials. Conversely, the steady-state limiting current of lead was not affected by the presence of the more easily reduced mercury ions. The different behaviour observed for the two mixtures was due to the inhibition of the migratory component in the mass transport of mercury(II), which formed labile complexes with chloride and hydroxide ions to a considerable extent. In solutions containing supporting electrolyte, the overall steady-state limiting currents, measured in the presence of the more negative reducible ions, depended on the supporting electrolyte to metal ion concentration ratio, provided that the complexation of the metal ions was negligible. The experimental data agreed with those obtained from theoretical equations derived from already known treatments, and adapted to the solutions investigated here.