Low Frequency Square‐Wave Voltammetry of Weak Acids at Platinum Microelectrodes

Abstract

The reduction process of monoprotic weak acids (lactic and acetic acid, and dihydrogen phosphate ion) and mixtures of a strong and a weak acid (perchloric and one of the above weak acids) have been investigated by square-wave voltammetry (SWV) at a platinum microelectrode 12.5 mm radius. The SW measurements have been performed at low frequencies (i.e., 3 Hz), where the heterogeneous electron transfer process, involving hydrogen evolution (2H þ þ 2e ¼ H 2 ), is essentially reversible, and non-planar diffusion conditions prevail in the mass transport. Under these conditions, the forward and the reverse current-potential profiles are sigmoidal, whereas the net current-potential profile is peak shaped. The influence of dissociation constant and analytical concentration of the weak acid on the features of the square-wave voltammograms has been examined in detail. In general, both these characteristics of the weak acids affect the square-wave parameters peak potential (E p ), width at half height (W 1/2 ) and net peak current (i p ) in a complex manner. Their dependence on concentration is due to the 2 : 1 stoichiometry of the heterogeneous electron transfer, while the dissociation constant contribution arises because of the CE mechanism involved in the overall electrode process. The kinetic of the homogeneous reaction has been investigated by the use of the forward current plateau (achieved by the end of the sweep) along with a theoretical equation. Mixtures of a strong and a weak acid can provide one or two waves, depending on the acid strength and on the analytical concentration of each acid. Peak potential differences of about 80 mV are needed to discern two peaks. Calibration plots, examined for all the weak acids, have shown that linearity may exist only over relatively small concentration ranges.

Simultaneous evaluation of the dissociated and undissociated acid concentrations by square wave voltammetry using microelectrodes

“…30 mV for a 10-fold increase of acid concentration, while W 1/2 remained approximately unchanged up to 20 mM, these observations are in agreement with reported results from HClO 4 [22]. On the other side, the voltammograms of H 2 PO À 4 shifted towards negative potentials and the peaks became wider as the acid concentration increased, which is also in agreement with the reported results on weak acids [4]. For H 3 PO 4 the I p vs. C follows a non-linear trend ( Fig.…”

“…For lower acid concentrations (<40 mM for HL and <10 mM for HAc) square wave voltammograms display a unique asymmetric peak. In these concentration ranges the increase of concentration leads to the peak enlargement [4] and to a marked decrease of the peak symmetry. For the higher acid concentrations (P40 mM for HL and P10 mM for HAc) a shoulder starts to be noticeable, at the right or at the left side of the main peak of HL and HAc, respectively.…”

“…Moreover, the preceding reaction was found to be very fast for acids whose dissociation constants are larger than 1 Â 10 À5 [2] and therefore current is diffusion controlled. Microelectrode studies on the reduction of acids were reported in different experimental conditions, such as: in single acid solutions [3][4][5][6], in the presence of low concentrations of supporting electrolyte [7][8][9][10][11][12], in buffer solutions [6,13,14], in mixtures of weak and strong acids [6,15,16] and in complex media [4,[17][18][19][20][21]. Linear sweep voltammetry (LSV) [8] and square wave voltammetry (SWV) [22] under quasi-steady-state conditions of strong acids provided current data that follow a linear relation with the acid concentration and therefore can be used for analytical purposes.…”

Voltammetric Determination of the Adsorption Kinetics of Acetic Acid on Activated Carbon

“…11 The voltammetric behaviors of various acids at platinum microelectrodes under steady state conditions have been extensively investigated in the past two decades. [12][13][14][15][16][17][18][19][20] Strong acid, which is completely dissociated, has been found to prove a well-defined, transport-controlled wave. Reduction current is proportional to acid concentration over a wide range as well as to hydrogen ion transport rate.…”

“…The potential and current of a reduction peak were found to depend on both the analytical concentration (C HA ) and the dissociation constant (K a ) of the acid. 15,16 At low weak acid concentrations, hydrogen ion plays an important function in mass transport with a current that is not linear with C HA . At high concentrations, weak acid is only slightly dissociated, and the current is linearly dependent on the acid concentration.…”