Electronic dilution curves

Figure 2.18. Gradient dilution is based on selecting readouts at the tail of the dispersed sample zone (right-hand side of left curve), where elements of fluid can be located within which the sample material has been diluted by the disperson process. The selection may be mechanical, that is, the zone is selected via the delay time, captured by means of a second valve, and then reinjected into a second FIA system. Electronic dilution is achieved by means of a microprocessor, by selecting via the delay time a detector readout from which a calibration curve is constructed (middle curves). The peaks, recorded with four different concentrations of analyte (labeled 25%, 50%, 75%, and 100%) were sliced at different delay limes yielding readouts from which calibration curves of different slopes were recorded (right curves).

Construction of a calibration curve using electronic dilution. (For color version of this figure, the reader is referred to the online version of this book.)... 

Examination of the behaviour of a dilute solution of the substrate at a small electrode is a preliminary step towards electrochemical transformation of an organic compound. The electrode potential is swept in a linear fashion and the current recorded. This experiment shows the potential range where the substrate is electroactive and information about the mechanism of the electrochemical process can be deduced from the shape of the voltammetric response curve [44]. Substrate concentrations of the order of 10 molar are used with electrodes of area 0.2 cm or less and a supporting electrolyte concentration around 0.1 molar. As the electrode potential is swept through the electroactive region, a current response of the order of microamperes is seen. The response rises and eventually reaches a maximum value. At such low substrate concentration, the rate of the surface electron transfer process eventually becomes limited by the rate of diffusion of substrate towards the electrode. The counter electrode is placed in the same reaction vessel. At these low concentrations, products formed at the counter electrode do not interfere with the working electrode process. The potential of the working electrode is controlled relative to a reference electrode. For most work, even in aprotic solvents, the reference electrode is the aqueous saturated calomel electrode. Quoted reaction potentials then include the liquid junction potential. A reference electrode, which uses the same solvent as the main electrochemical cell, is used when mechanistic conclusions are to be drawn from the experimental results. 

Surface oxide formation undoubtedly is involved in the Fe(II)-dichromate titration curves, which Smith and Brandt found to be different when the direction of titration was reversed (Figure 15-2, right). Kolthoff and Tanaka found that the rate of oxidation with dichromate was slow, whereas the rate of reduction with Fe(II) was fast. Ross and Shain found the same sort of behavior and noted also that the rates of oxidation and reduction decreased in more dilute solutions. The oxidized surface in a dichromate solution may be largely covered with adsorbed dichromate, as chromium surfaces have been shown to be in some experiments with radio-chromium, so that it is relatively ineffective as an electron-transfer surface for the Fe(III)-Fe(II) system. 

Once we have verified that the polyynes in dilute solution are stable to air oxidation, we studied the effect of a more powerful oxidizing agent ozone. Figure 18.12(A) (bottom curve) shows the electronic absorption spectrum of the polyynes solution in DHNP. I he addition of O3 to this solution causes an increase in the intensity of the bands at 220 and 232 nm (Figure 18.12A, top curve) this increase in the absorption intensity is due to ozone dissolved in DHNP which has an absorption maximum just in this wavelength range. However, a reduction in intensity can be observed in Figure 18.12(A) for the other bands at 257, 245, 282, 303, and 322 nm as a consequence of the ozone treatment. This fact can be better appreciated by the spectrum difference... 

All solutions were relatively dilute such that the OD at 290nm was << 0.1. The fluorescence Intensity was sufficiently low that all decay curves were obtained at the Center for Fast Kinetics Research at the University of Texas. This system uses Standard photon-counting electronics but the excitation source is a synch-pumped cavity-dumped dye laser pumped by a NdiYAG laser... 

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