Peak-current method

According to Eq. (27), Stromme et al.125,126 developed systematically the peak-current method to determine the fractal dimension of the electrode surface by using cyclic voltammetry. It must be recalled that this method is valid when the recorded current is limited by diffusion of the electroactive species to and away from the electrode surface. Since the distribution of the reaction sites provides extensive information about the surface geometry, the fractal dimension of the reaction site distribution may agree with the fractal dimension of the electrode surface which is completely electrochemical-active. In addition, it is well known that this method is insensitive to the IR drop in the electrolyte.126... 

Keeping in mind that the dc sputter-deposited Pt films have the completely electrochemical-active surface, the fractal dimensions of the rough film surfaces were calculated from Eq. (27) according to the peak-current method by taking the slopes of the (log 7peak - log v) plots within the scan rate range of v0 to... 

Fractal Dimensions of Self-Affine Fractal Electrodes Determined by the Perimeter-Area Method (2nd Column), the Peak-Current Method (3rd Column), and the Triangulation... 

Cyclic voltammetry provides a simple method for investigating the reversibility of an electrode reaction (table Bl.28.1). The reversibility of a reaction closely depends upon the rate of electron transfer being sufficiently high to maintain the surface concentrations close to those demanded by the electrode potential through the Nemst equation. Therefore, when the scan rate is increased, a reversible reaction may be transfomied to an irreversible one if the rate of electron transfer is slow. For a reversible reaction at a planar electrode, the peak current density, fp, is given by... 

Thus, the limiting current, is a linear function of the concentration of O in bulk solution, and a quantitative analysis is possible using any of the standardization methods discussed in Chapter 5. Equations similar to equation 11.35 can be developed for other forms of voltammetry, in which peak currents are related to the analyte s concentration in bulk solution. 

The concentration of copper in a sample of sea water is determined by anodic stripping voltammetry using the method of standard additions. When a 50.0-mL sample is analyzed, the peak current is 0.886 )J,A. A 5.00-)J,L spike of 10.0-ppm Cu + is added, giving a peak current of 2.52 )J,A. Calculate the parts per million of copper in the sample of sea water. 

The resultant peak current is greater than the dilfusion current recorded with a conventional d.c. polarograph by a factor of ten or even more. The method thus shows enhanced sensitivity and it can be used to make measurements with solutions having concentrations as low as 10 6 — 10 7M and with a resolution of the order of 0.05 V. 

The cyclic voltammograms of ferrlcyanlde (1.0 mM In 1.0 M KCl) In Fig. 2 are Illustrative of the results obtained for scan rates below 100 mV/s. The peak separation is 60 mV and the peak potentials are Independent of scan rate. A plot of peak current versus the square-root of the scan rate yields a straight line with a slope consistent with a seml-lnflnlte linear diffusion controlled electrode reaction. The heterogeneous rate constant for the reduction of ferrlcyanlde was calculated from CV data (scan rate of 20 Vs using the method described by Nicholson (19) with the following parameter values D 7.63 X 10 cm s , D, = 6.32 X 10 cm s, a 0.5, and n =1. The rate constants were found to be... 

As mentioned in the introduction to controlled potential electrolysis (Section 2.3), there are various indirect methods to calculate the number of electrons transferred in a redox process. One method which can be rapidly carried out, but can only be used for electrochemically reversible processes (or for processes not complicated by chemical reactions), compares the cyclic voltammetric response exhibited by a species with its chronoamperometric response obtained under the same experimental conditions.23 This is based on the fact that in cyclic voltammetry the peak current is given by the Randles-Sevcik equation ... 

For the catalytic electrode mechanism, the total surface concentration of R plus O is conserved throughout the voltammetric experiment. As a consequence, the position and width of the net response are constant over entire range of values of the parameter e. Figure 2.35 shows that the net peak current increases without limit with e. This means that the maximal catalytic effect in particular experiment is obtained at lowest frequencies. Figure 2.36 illustrates the effect of the chemical reaction on the shape of the response. For log(e) < -3, the response is identical as for the simple reversible reaction (curves 1 in Fig. 2.36). Due to the effect of the chemical reaction which consumes the O species and produces the R form, the reverse component decreases and the forward component enhances correspondingly (curves 2 in Fig. 2.36). When the response is controlled exclusively by the rate of the chemical reaction, both components of the response are sigmoidal curves separated by 2i sw on the potential axes. As shown by the inset of Fig. 2.36, it is important to note that the net currents are bell-shaped curves for any observed kinetics of the chemical reaction, with readily measurable peak current and potentials, which is of practical importance in electroanalytical methods based on this electrode mecharusm. 

Licklider and co workers experimented with automating the sample introduction step in nanoscale LC-MS. In order to achieve pre-concentration and desalting prior to sample analysis, they created a 2 cm vent after the head of the analytical column. Experimental results demonstrated 50 nanoliter (nL) elution peak volumes while retaining low-to subfemtomole detection levels. Additionally, implementing this pre-concentration technique requires minimal changes in current methods and equipment. 

In this method, the peak current was found to be directly proportional to the drug concentration over the range of 1 -20 mg/dL at a half-wave potential of-0.8 V vj. S.C.E. A derivative polarogram was reported for chlorpromazine. 

Takamura et al. have reported an electrochemical method for the determination of chlorpromazine with an anodically pretreated vitreous carbon electrode [164]. Optimal conditions for the pre-treatment were attained by the anodic oxidation of vitreous carbon electrodes in 0.5 mM phosphate buffer (pH 6.7) at 1.6 V V5. S.C.E. for 2 minutes. This was found to enhance the oxidation peak of the cyclic voltammogram for chlorpromazine by a factor of simeq 30. The peak current at +0.75 V was directly proportional to the concentration of chlorpromazine over the range of 0.2-40 pM and the detection limit was 0.1 pM. 

Takamura et al. have reported a voltammetric method for the determination of chlorpromazine using an anodically oxidized carbon electrode [167]. A vitreous-carbon electrode was maintained at +1.6 V vi. S.C.E. for 2 minutes (in 0.5 M phosphate buffer at pH 6.8). Under these conditions, chlorpromazine gave an oxidation peak current on cyclic voltammograms that varied linearly with concentration over the range of 50 nM to 1 pM. 

Method 1 - Anodic differential pulse voltammetry, involving voltage scanning from +0.3 to +1.0 V and recording the peak current, was found to yield an instrumental response proportional to the concentration of chlorpromazine in the range 9.6 to 340 pM. 

Method 2 -An adsorptive stripping method also involving pulse voltammetry, which yielded a peak current that was proportional to the concentration of chlorpromazine in the range of 83 to 2000 nM. 

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