Background, absorption electrolyte

Typically, a potential-difference spectrum of the electrode-electrolyte interface presents negative, positive, and bipolar bands and smooth background absorption, which are due to aU species in the path of IR radiation that are affected by the electrode potential O igs. 3.40, 3.43a, 4.47a, 4.50a, 7.45, and 7.47). In addition to the absorption bands that reflect change in the population in the HL of reagents/products for the reaction under study, these may include the bands due to (1) the electrolyte species in the diffuse layer, (2) the electrolyte species in the HL, (3) the reagent/product species whose absorption (parameters of the elementary oscillators) is modulated by potential and coadsorption of electrolyte species, (4) delocalized and localized charge carries, and (5) optical effects of various nature. Consider these effects in more detail. 

In determining the values of Ka use is made of the pronounced shift of the UV-vis absorption spectrum of 2.4 upon coordination to the catalytically active ions as is illustrated in Figure 2.4 ". The occurrence of an isosbestic point can be regarded as an indication that there are only two species in solution that contribute to the absorption spectrum free and coordinated dienophile. The exact method of determination of the equilibrium constants is described extensively in reference 75 and is summarised in the experimental section. Since equilibrium constants and rate constants depend on the ionic strength, from this point onward, all measurements have been performed at constant ionic strength of 2.00 M usir potassium nitrate as background electrolyte . 

Figure 16. Fluorescence detected X-ray absorption spectra for a 4-nm iron film in emersion cell. Spectra are for (A) dry film (B) cathodi-cally protected film (C) passivated film (D) background electrolyte. (From Ref. 74, with permission.)...

Water is so transparent that ultraviolet detectors can operate at wavelengths as short as 185 nm. where most solutes have strong absorption. To take advantage of short-wavelength ultraviolet detection, background electrolyte must have very low absorption. Borate buffers are com-... 

Buffers can be used to control pH, provided it is determined that the buffer components do not interact, e.g., with the metal ion. However, buffers often absorb in the UV. In any event, a blank consisting of background electrolyte plus buffer should be used in the reference cell. Sodium or potassium nitrate usually can t be used as background electrolyte, because of the absorption band of nitrate centered at 300 nm. 

For capillary zone electrophoresis the electrical and thermal detection modes have insufficient sensitivity. This is because in capillary zone electrophoresis there is a relatively large background of supporting electrolyte (buffer) upon which a low concentration of sample ion is superimposed. Detecting the exceedingly small changes in electrical properties or temperature associated with sample zones is difficult. Thus UV absorption and fluorescence detection have been of greatest use in capillary zone electrophoresis. 

Fig. 11-33. Characteristic UV absorption spectra of NaOBS solutions recorded in HCl04/NaC104 background electrolyte [38]...

The bands discussed above are again present, although very weak. A distinction between absorption caused by the aniline in solution and the aniline in the polymer is impossible. With the electrolyte solution containing the monomer as a background, a different result is observed. 

The last step in processing the experimental PM IRRAS spectra involves the subtraction of the background. This background arises from the slowly varying broad-band absorbance of infrared radiation by the aqueous electrolyte. In order to remove the background, a procedure similar to that published by Earner et al. [66] was developed by Zamlynny [36]. The baseline is created from the experimental data points using spline interpolation. Successful interpolation requires knowledge of the exact positions of the absorption bands and a little experience. 

Popular techniques for the analysis of small ions include ion-exchange chromatography and flame atomic absorption spectrometry but CE is becoming more popular for such applications due to its simplicity and speed of method development and analysis, elimination of the need for specialized columns, high resolving power, and simple sample treatment steps (typically, the sample just needs to be diluted in the background electrolyte and injected onto the capillary). Commercial ion analysis... 

As discussed in Section 3.7, a smooth background that increases at lower wavenumbers is characteristic of free-carrier absorption, also termed Drude absorption (DA) (Figs. 3.44 and 3.47). The potential dependence of DA for the silicon-fluoride electrolyte interface is shown in Fig. 7.31. Free-carrier absorption is negligible at negative potentials, at which the silicon electrode is depleted. Since DA obeys the Schottky-Mott law (inset in Fig. 7.31), the flat-band potential was found to be - -0.4 V. Weak absorption by flee carriers... 

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