Inverse voltammetry

More recent research provides reversible oxidation-reduction potential data (17). These allow the derivation of better stmcture-activity relationships in both photographic sensitization and other systems where electron-transfer sensitizers are important (see Dyes, sensitizing). Data for an extensive series of cyanine dyes are pubflshed, as obtained by second harmonic a-c voltammetry (17). A recent "quantitative stmcture-activity relationship" (QSAR) (34) shows that Brooker deviations for the heterocycHc nuclei (discussed above) can provide estimates of the oxidation potentials within 0.05 V. An oxidation potential plus a dye s absorption energy provide reduction potential estimates. Different regression equations were used for dyes with one-, three-, five-methine carbons in the chromophore. Also noted in Ref. 34 are previous correlations relating Brooker deviations for many heterocycHc nuclei to the piC (for protonation/decolorization) for carbocyanine dyes the piC is thus inversely related to oxidation potential values. 

N. A. Tananaev, simultaneously with F. Feigl, developed the spot analysis. Known Tserevitinov method for determining labile hydrogen atoms in organic compounds (1902-1907 should be noted (the method was later improved by A.P Terent ev). In the USSR, there were powerful schools in liquid-liquid extraction and inverse voltammetry. 

As a result a new approaehes in DCP-ai e atomie-emission speetrometry were applied for Ca, Mg, Cu, Zn, Fe and P determination in blood semm and Ca, Mg, Cu, Zn, Fe, P, Mn, Pb, Cd, Sn, Sr et al. - in human and animals hair with relative standai d deviation (RSD) about 10-20 %. The aeeuraey eontrol has been realized by a eomparison of data produeed with the results of independent methods (atomie-absorption speetrometry and inverse voltammetry). 

Figure 15.7 Logarithm of the kinetic current for the ORR in oxygen-saturated liquid electrolytes versus inverse diameter for Pt particles supported on Vulcan XC-72 (1) 0.9 V vs. RHE at 60 °C [Gasteiger et al., 2005] (2) 0.85 V vs. RHE at room temperature [MaiUard et al., 2002] (3)0.85 V vs. SHE at room temperature [Guerin etal., 2004]. For curves 1 and 2, measurements were performed with the thin-layer RDE in 0.1 M HCIO4 for curve 3, they were performed with stationary voltammetry in 0.5 M H2SO4. (Curves have been replotted from MaiUard et al. [2002] Gasteiger et al. [2005], Copyright 2002 and 2005, with permission from Elsevier and from Guerin et al. [2004], Copyright 2004 American Chemical Society.)...

Monien et al. [515] have compared results obtained in the determination of molybdenum in seawater by three methods based on inverse voltammetry, atomic absorption spectrometry, and X-ray fluorescence spectroscopy. Only the inverse voltammetric method can be applied without prior concentration of molybdenum in the sample, and a sample volume of only 10 ml is adequate. Results of determinations by all three methods on water samples from the Baltic Sea are reported, indicating their relative advantages with respect to reliability. 

These expressions are designed for cyclic voltammetry. The expressions appropriate for potential step chronoamperometry or impedance measurements, for example, are obtained by replacing IZT/Fv by the measurement time, tm, and the inverse of the pulsation, 1/co, respectively. Thus, fast and slow become Af and Ah I and -C 1, respectively. The outcome of the kinetic competition between electron transfer and diffusion is treated in detail in Section 1.4.3 for the case of cyclic voltammetry, including its convolutive version and a brief comparison with other electrochemical techniques. 

This is a case where another electrochemical technique, double potential step chronoamperometry, is more convenient than cyclic voltammetry in the sense that conditions may be defined in which the anodic response is only a function of the rate of the follow-up reaction, with no interference from the electron transfer step. The procedure to be followed is summarized in Figure 2.7. The inversion potential is chosen (Figure 2.7a) well beyond the cyclic voltammetric reduction peak so as to ensure that the condition (Ca) c=0 = 0 is fulfilled whatever the slowness of the electron transfer step. Similarly, the final potential (which is the same as the initial potential) is selected so as to ensure that Cb)x=0 = 0 at the end of the second potential step whatever the rate of electron transfer. The chronoamperometric response is recorded (Figure 2.7b). Figure 2.7c shows the variation of the ratio of the anodic-to-cathodic current for 2tR and tR, recast as Rdps, with the dimensionless parameter, 2, measuring the competition between diffusion and follow-up reaction (see Section 6.2.3) ... 

The previous chapters dealt with ISE systems at zero current, i.e. at equilibrium or steady-state. The properties of the interface between two immiscible electrolyte solutions (ITIES), described in sections 2.4 and 2.5, will now be used to describe a dynamic method based on the passage of electrical current across ITIES. Voltammetry at ITIES (for a survey see [3, 8, 9, 10, 11, 12,18]) is an inverse analogue of potentiometry with liquid-membrane ISEs and thus forms a suitable conclusion to this book. 

Heeb, R. Inverse Polarographie und Voltammetrie. Weinheim/Bergstrafie Verlag Chemie 1969. 

If the potential is inverted at a given value (inversion or final potential) until the initial potential is reached again, the two above techniques are denoted Cyclic Staircase Voltammetry (CSCV) and Cyclic Voltammetry (CV), respectively (see Scheme 5.3). The potential waveform in CV can be written as a continuous function of time... 

In the case of an irreversible reaction of the type 0 + we - R, linear sweep and cyclic voltammetry lead to the same voltammetric profile, since no inverse peak appears on inversing the scan direction. 

Fig. 9.9. Cyclic voltammetry in the investigation of systems of more than one component, showing the importance of the inversion potential in the identification of the peaks on the inverse scan.

Potential or current step transients seem to be more appropriate for kinetic studies since the initial and boundary conditions of the experiment are better defined unlike linear scan or cyclic voltammetry where time and potential are convoluted. The time resolution of the EQCM is limited in this case by the measurement of the resonant frequency. There are different methods to measure the crystal resonance frequency. In the simplest approach, the Miller oscillator or similar circuit tuned to one of the crystal resonance frequencies may be used and the frequency can be measured directly with a frequency meter [18]. This simple experimental device can be easily built, but has a poor resolution which is inversely proportional to the measurement time for instance for an accuracy of 1 Hz, a gate time of 1 second is needed, and for 0.1 Hz the measurement lasts as long as 10 seconds minimum to achieve the same accuracy. An advantage of the Miller oscillator is that the crystal electrode is grounded and can be used as the working electrode with a hard ground potentiostat with no conflict between the high ac circuit and the dc electrochemical circuit. 

Fourier transform voltammetry — Analysis of any AC or transient response using (fast) Fourier transformation (FFT) and inverse (fast) Fourier transformation (IFFT) to convert time domain data to the frequency domain data and then (often) back to time domain data but separated into DC and individual frequency components [i-ii]. See also - Fourier transformation, AC voltamme-... 

Inverse Polarographie und Voltammetrie. — Neuere Verfahren zur Spuren-... 

As the main aim of trace analysis is usually determination of the mass (expressed as the number of moles) of a given component in a studied sample, molar concentration is generally not used. Some exceptions are electrochemical methods, where the analytical signal (e.g., current intensity) is a direct function of molar concentration [9]. Therefore, in voltamperometric techniques the detection limits are usually given in molar concentration units (Table 1.2). Thus, for nickel (molar mass M = 58.7 g/mol) the detection limit in inverse voltammetry is approximately 6 X 10 mol/L, and is expressed as a mass fraction, 3.5 x 10 g/dm, or as a percentage, 0.35 x 10 %. In spectrophotometry, when concentrations are given in molar units then molar absorptivities are also used. For example, molar absorptivity e = 5xl0 " L/mol cm corresponds (for molar mass M = 58.7 g/mol) to molar absorptivity a = 5 x 10 /(58.7 x 10 )mL/g cm (i.e., 0.85 mL/g cm). 

Neeb, R. Kiehnast, I.Z. Effect of salts on the anodic peak height in inverse voltammetry. Anal. Chem. 1967,241, 142-155. 

Yet when applied to current reversal techniques, such as double-step chronampero-metry of cyclic voltammetry, these methods require that an appreciable current be observed during the backward perturbation, that is, for t > 0, in potentiostatic methods or after the potential scan inversion in cyclic voltammetry. This requires that the characteristic time 0 of the method is adjusted to match the half-life ti/2 of the electrogenerated intermediate. Today, owing to the recent development of ultramicroelectrodes, 0 can be routinely varied from a few seconds to a few nanoseconds [102]. Yet with basic standard electrochemical equipment, 0 is usually restricted from the second to the low millisecond range. Thus for experimental situations involving faster chemical reactions, current rever-... 

Preconcentration in a slightly different way is described by Eisner and Mark (40) who equilibrated small areas of cation exchange membranes with sample solutions and then used the membrane as a source of ions for deposition in an anodic stripping voltammetry system. The concentration of the ion in the membrane is linearly related to its concentration in the bulk sample solution. The pre-equilibrated membrane was also analyzed by neutron activation thus extending the range of ions for which the technique is useful. Data are quoted for Ag+, Cu2+, Zn2+, Co2+ and In3+, all of which show favourable distribution for the membrane phase. Equilibration times are inversely proportional to concentration ranging from several minutes at lO"1 M to one day or more at 10 6 M. The method affords a convenient separation from nonionic and anionic species which interfere with the measurement technique. 

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