Differential Alternative Pulses Voltammetry Application in Organic and Inorganic Analysis
Authors: Zlatev Roumen , Stoytcheva Margarita 
Published: May 2020

The simultaneous voltammetric determination of species with close peak potentials presents some difficulties, because of the overlapping of their redox peaks at conventional solid electrodes. Therefore, two approaches were proposed to overcome this drawback: (i) chemometric resolution of the overlapped peaks by semi-derivative or multivariate analysis and (ii) development of chemically modified electrodes with better electrochemical performances, permitting peaks separation. However, the mathematical treatment habitually results in a precision decrease, while the electrode modification often requires the use of expensive materials and involves sophisticated techniques. Hence, another strategy for the direct simultaneous species determination is suggested in this work. It involves the application of Differential Alternative Pulses Voltammetry (DAPV) [1], which provides the great resolution of the second order voltammetric techniques, as Radio-Frequency Polarography, Second Harmonic AC Voltammetry, and Differential Faradaic Rectification Polarography (DFRP), in combination with the high sensitivity, precision, and instrumental and operational simplicity of the most common voltammetric technique, the differential pulse voltammetry (DPV).
The second order voltammetric techniques including DAPV are based on the nonlinearity of the electrochemical system I/E characteristic yielding voltammograms shaped as a derivative of voltammetric peak resulting from small sinusoidal or rectangular bipolar deviations of the DC potential. Two single pulses of small amplitude, one cathodic and one anodic are successively applied in DAPV for every value of the scanned DC potential with delay time between the pulses equal to the delay time between the scan step and the first one (Fig. 1). The sum of the two pulse faradaic current components with opposite polarities is registered vs. the scanned potential as DAPV voltammogram. The experimentally determined half-width of the DAPV peaks (37 mV at 2 electrons reduction) is 1.22 times smaller than that of the most common voltammetric technique DPV (45.3 mV) at the same conditions and equals that obtained by the application of another second order technique, the DFRP. Thus, the DAPV voltammogram’s shape as second derivative of voltammetric wave combined with the small half-widths of the anodic and the cathodic peaks allows simultaneous quantification of species having very small peak potential difference.
Taking advantage from the great resolution power of DAPV, it was applied for the simultaneous determination of a range of organic (catechol and hydroquinone [2]; o-nitrophenol, m-nitrophenol and p-nitrophenol [3]) and inorganic species (Cd2+/In3+; Tl+/Pb2+ [1, 4]) with close peak potential difference, as shown in Figs. 2-5.
The proposed approach offers a very simple alternative route for the simultaneous determination of a broad range of species and overcomes the disadvantages of the currently applied strategies involving electrode modification or mathematical data processing.

References

1. Zlatev, M. Stoytcheva, B. Valdez, J.P. Magnin, P. Ozil. Electrochem. Communications, 2006, 8, 1699-1706.
2. Ayala, M. Stoytcheva, R. Zlatev, Z. Velkova, V. Gochev, B. Valdez, G. Montero. Electroanalysis, 2018, 30, 1913-1917.
3. Stoytcheva, R. Zlatev, Z. Velkova, V. Gochev, A. Ayala, G. Montero, B. Valdez. Electroanalysis, 2019, 31, 652-660.
4. Zlatev, M. Stoytcheva, B. Valdez. Electroanalysis, 2018, 30, 1902-1905.

Figures captions

Fig. 1. DAPV potential-time waveform. M1-M4 are the current measuring intervals; DT is the wait time for the pulse capacitive current decay.
Fig. 2. HQ (30 mmol L-1) and CC (10 mmol L-1) determination by DAPV. PBS 0.1 mol L-1, pH 7; Spectroscopic graphite electrode.
Fig. 3. Nitrophenol (NP) isomers simultaneous determination by DAPV (m-NP 125 mmol L-1, p-NP 125 mmol L-1, and increasing o-NP concentrations: 25, 50, 75, 100 and 125 mmol L-1). PBS 0.1 mol L-1; pH 6.5; glassy carbon electrode modified by a nanocomposite of graphite nanopowder and Au nanoparticles.
Fig. 4. Cd2+ and In3+ (E1/2 difference 45 mV) simultaneous determination by DAPV, 0.1 mol L-1 HCl; Hanging Mercury Drop Electrode.
Fig. 5. Tl+ and Pb2+ (E1/2 difference 50 mV) simultaneous determination at 12:1 concentration ratio by DAPV and DPV. 0.1 mol L-1 HCl; Hanging Mercury Drop Electrode.

Figure 1