Self-Enhancement of Voltammetric Waves of Weak Acids in the Absence of Supporting Electrolyte

Abstract

Weak acids such as acetic, ascorbic, and salicylic acids are easily reduced at platinum microelectrodes in the absence of the supporting electrolyte. The current is mass transport controlled, and the reduction of proton is preceded by dissociation of the acid in the reaction layer. Since these acids are only slightly dissociated, transport should not be enhanced by migration and the heights of their voltammetric waves should be nearly independent of supporting electrolyte concentration. However, transport-limited currents diminish by 50% when a small amount of a supporting electrolyte is added to the solution. Thus the wave height in the absence of electrolyte exceeds that with electrolyte present by a factor of 2, as expected for oneelectron reduction of a singly charged reactant. This change in the wave height is connected with the small amount of hydrogen ion in the solution arising from the dissociation of the acid. Since the equilibrium concentration of hydrogen ion, which is the only cation available in the solution, is much lower than that of undissociated acid, only a small ratio of the concentration of the supporting electrolyte to that of the acid is needed to eliminate this effect. Dihydrogen phosphoric acid, which is negatively charged, behaves differently. Taking into account this outcome and the limited dissociation of weak acids, the diffusion coefficients of acetic, ascorbic, salicylic, and dhydrogen phosphoric acids were determined at 20 OC. They are 0.97 X 5.6 X 10-6, 7.7 X loa, and 6.4 X loa cm2 s-1, respectively.It has beendemonstrated recently that the heights of steadystate voltammetric waves of strong and weak acids at platinum microelectrodes depend linearly on the total or formal acid concentration over a very wide range of concentration.' Apparently this dependence can be used for analytical purposes. Examples in which measuring the voltammetric waves could be competitive with use of the glass electrode include measurements in media containing charged colloidal particles (where standard pH electrodes cannot be used2 ), probing proton concentration in a small region (e.g., near a large electrode), or any situation where one cannot wait for the potential of a pH electrode to stabilize. Reduction of hydrogen ion also has the potential to be an excellent test system for electrochemical examination of microelectrodes, due to the large and well-known diffusion coefficient value of the hydrogen ion. Perchloric acid is superior as a test system since both hydrogen ion and perchlorate anion are easily rinsed from most surfaces, and carry-over of perchlorate would not interfere in most measurements.The height of the wave for reduction of hydrogen ion in solutions of strong acid was found to depend on the concentration of the supporting electrolyte' as predicted by the theory for transport assisted by m i g r a t i~n .~-~ For uncharged substrates the theory predicts no dependence on the electrolyte concentration. For weak acids of the type HA, which are only slightly dissociated, it could be expect...

Electrocatalytic Hydrogen Evolution under Densely Buffered Neutral pH Conditions

“…1a indicates either successful buffering action of phosphate ions to release hydronium ions at close proximity of the surface or occurrence of direct reduction of weak acid, as previously proposed. [37][38][39][40] Taking various rate determining steps with different reactants, the hydrogen evolution reaction (HER) kinetics can be either zero-order or first-order in weak acid activities (the details of discussion are described in the supporting information). Due to the rapid buffering action and rapid kinetics of HER on Pt, we failed to experimentally differentiate the true reactant (hydronium ion vs. weak acid) on the Pt surfaces.…”

“…Alternatively, some claim that the weak acid itself 3 acts as the reactant under such conditions, although clear evidences of such "direct weak acid reduction " have not yet been documented. [37][38][39][40] The authors of such claims have shown that electric currents increase with the solute concentration up to 0.5 mol L −1 . Although higher concentrations were not investigated, 40 the performances under such conditions are speculated to be much higher.…”

Role of the reversible electrochemical deprotonation of phosphate species in anaerobic biocorrosion of steels

“…The electrochemical reduction of phosphates and other weak acids on various electrodes has been the subject of several investigations [29,[34][35][36][37][38][39][40][41][42][43][44][45]. The most widely accepted mechanism for the reduction of weak acids is of the CE (chemical-electrochemical) type, where the dissociation of the acid takes place before the electrochemical reduction of free protons [32][33][34]:…”

“…This model permitted the development of electrochemical techniques for the measurement of total acid and free proton concentration in solutions of strong and weak acids [35][36][37]. An alternative mechanism has been proposed by other authors who claim that the hydrogen atoms of undissociated weak acids (HA) could be directly reduced without a dissociation step [29,[38][39][40]. Stojek et al [38] studied the influence of supporting electrolyte in the reduction of polyprotic acids on platinum electrodes.…”

“…An alternative mechanism has been proposed by other authors who claim that the hydrogen atoms of undissociated weak acids (HA) could be directly reduced without a dissociation step [29,[38][39][40]. Stojek et al [38] studied the influence of supporting electrolyte in the reduction of polyprotic acids on platinum electrodes. They proposed that, after the electrochemical reduction of the undissociated acid in the presence of a supporting electrolyte, the conjugate base of the acid (A À ) reacted with water or with protons (H + ) in order to re-establish the acid-base equilibrium in the solution.…”