Absorbance-time curves

Using the reaction spectrum at selected wavelengths absorbance-time curves are plotted first. 

Next absorbance diagrams are plotted in various combinations. A typical graph demonstrates curves, which means the reaction contains more than one linear independent step of reaction. 

Furthermore absorbance difference diagrams can be plotted as demonstrated in Fig. 5.7. Whereas the additional information of this type of diagram is small, absorbance difference quotient diagrams can be used to decide whether the number of linear independent reactions amounts to 2 or more as given in Fig. 5.11. 

Taking the data as used for Fig. 4.18 the following linear relationship is obtained demonstrating that the reaction is represented by two linear independent steps. Therefore any of the mechanisms given in Table 2.5 can be used. On the other hand chemical information requires a consecutive reac- 

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Figure 4-12 shows a stopped-flow study of the pyridine-catalyzed hydrolysis of acetic anhydride. The absorbance-time curve reveals tbe formation and decay of the reactive intermediate acetylpyridinium ion. 

Fig. 5 Absorbance time curves for the spectroelectrochemical generation of pery-lene radical anion at 570 nm in dimethylformamide (a), and with the addition of phenol (b). The time to open circuit (dashed line) is 400 ms. (Ahlberg et al., 1979)...

Performance of the experiment, usually termed chronoabsorptometry, includes the registration of the absorbance-time curves for DPA in the absence and in the presence of pyridine from which data for the normalized absorbance-time dependence is obtained. The rate constant can be estimated by fitting these data to the working curve. In the present case [349] the rate constant for the nucleophilic attack of pyridine on DPA" was calculated to be 1.86 x 10" s ... 

Calculate absorbance-time curves for a gold film OTE at which a reduction product is produced with 8r = 10, 10, and 10 cm Let Dq = 1 X 10 cm /s, and Cq = 1 mM. Draw graphs for times ranging from 1 to 100 ms. Conunent on the magnitudes of the absorbances and their experimental implications. 

In consequence without further knowledge such types of consecutive and parallel reactions cannot be distinguished. To evaluate the rate laws derived in Chapters 2 and 3, the concentrations of the reactants have to be known. For this reason the next step is to get the concentrations dependent on time out of the reaction spectrum or the absorbance-time curves. For further details see Sections 5.1 and 5.3.3. 

In Fig. 5.16 the absorbance difference diagram is given as an example for a substituted anthraquinone. Fig. 5.17 demonstrates the absorbance-time curves. All wavelengths have a linear relationship. In Table 5.1 the result of an evaluation according to eq. (5.93) is given for the same compounds. The 1-chloro-substituted compounds demonstrated a comparable small differential yield in neutral methanol. 

Standard deviations are small. The constants obtained are used to recalculate absorbance-time curves for both series of measurement by numerical integration. The curves obtained are plotted in Figs. 5.22 and... 

Unfortunately, even when capillary slit cells are used in the experiment, the spectra of the different species are still superimposed. Therefore, it is necessary to separate or to deconvolute the superimposed spectra in order to obtain information about the reaction kinetics of individual species. In the literature, techniques have been proposed for the deconvolution of the superimposed spectra [68, 69]. Data processing and deconvolution may be achieved with spreadsheet software on a suitable computer system. As soon as the time dependence of the concentration of each component is known, the absorbance-time curves and the charge-time curves calculated from the current passing the electrode can be used to determine rate or equilibrium constants for the chemical system under smdy. 

Fig. 10,3 - Absorbance-time curve for the formation of a coloured species under conditions of diffusion control.

Fig. 10.4 - Experimental absorbance-time curve for the radical cation of DPA formed by oxidation of diphenylanthrancene (DPA), in acetonitrile/water (10moldm ). Data taken from H. N. Blount T. Kuwana, J. Electroanal. Chem., 27, (1970), 464.

Fig. 10.6 — A comparison of absorbance-time curves for a potential step followed by open circuit relaxation for the ece and disp 1 mechanisms. Reproduced with permission from A. Bewick, J. M. Mellor, B. S. Pons, Hlectrochem. Acta, 23, (1978), 77.

As a further practical tip, for an OTE based on a thin conductive optically transparent layer, the resistance of the working electrode can be reduced if an additional thicker metal layer is coated on the optically transparent film in the region not exposed to the light beam under the insulation layer. If the insulation is prepared via lamination, a thin metal foil can be simply inserted instead of the deposited metal layer. This new cell design (Fig. II.6.5) enables cyclic voltam-mograms as well as current time curves in the case of a potential step experiment (chronoamperograms) simultaneously with a series of time-resolved spectra to be recorded. This experimental approach allows absorbance-potential or absorbance-time curves to be recorded to analyse the reaction kinetics of the generation of intermediates and the final product up to a time scale of approximately 0.1 s life-time. 

Figure 2 shows the time-based absorbance of doped PBT-ITO at constant wavelength upon reduction in ascorbic acid solutions of various concentration. When the concentration of ascorbic acid solution is above 20 ppm, the absorbance rose sharply and a maximim absorbance was attained almost immediately. For ascorbic acid solutions of lower concentration, the absorbance-time curve rose in a more gradual manner, suggesting that the time-based spectrophotometric responses of doped PBT coated on ITO electrode could be used, in principle, as a measurement of ascorbic acid concentration. 

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