Reference voltammetry

Electrochemical Detectors Another common group of HPLC detectors are those based on electrochemical measurements such as amperometry, voltammetry, coulometry, and conductivity. Figure 12.29b, for example, shows an amperometric flow cell. Effluent from the column passes over the working electrode, which is held at a potential favorable for oxidizing or reducing the analytes. The potential is held constant relative to a downstream reference electrode, and the current flowing between the working and auxiliary electrodes is measured. Detection limits for amperometric electrochemical detection are 10 pg-1 ng of injected analyte. 

Active electrochemical techniques are not confined to pulse and linear sweep waveforms, which are considered large ampHtude methods. A-C voltammetry, considered a small ampHtude method because an alternating voltage <10 mV is appHed to actively couple through the double-layer capacitance, can also be used (15). An excellent source of additional information concerning active electroanalytical techniques can be found in References 16—18. Reference 18, although directed toward clinical chemistry and medicine, also contains an excellent review of electroanalytical techniques (see also... 

For PPV-imine and PPV-ether the oxidation potential, measured by cyclic voltammetry using Ag/AgCl as a reference are ,M.=0.8 eV and 0.92 eV, respectively. By adopting the values 4.6 eV and 4.8 eV for the work functions of a Ag/AgCl and an 1TO electrode, respectively, one arrives at zero field injection barriers of 0.4 and 0.55 eV. These values represent lower bounds because cyclic voltammetry is carried out in polar solvents in which the stabilization cncigy of radical ions exceeds that in a polymer film, where only electronic polarization takes place. E x values for LPPP and PPPV are not available but in theory they should exceed those of PPV-imine and PPV-ether. 

Electrical methods of analysis (apart from electrogravimetry referred to above) involve the measurement of current, voltage or resistance in relation to the concentration of a certain species in solution. Techniques which can be included under this general heading are (i) voltammetry (measurement of current at a micro-electrode at a specified voltage) (ii) coulometry (measurement of current and time needed to complete an electrochemical reaction or to generate sufficient material to react completely with a specified reagent) (iii) potentiometry (measurement of the potential of an electrode in equilibrium with an ion to be determined) (iv) conductimetry (measurement of the electrical conductivity of a solution). 

From the nature of the process described above it has been referred to as stripping polarography , but the term anodic stripping voltammetry is preferred. It is also possible to reverse the polarity of the two electrodes of the cell, thus leading to the technique of cathodic stripping voltammetry. 

In view of the limitations referred to above, and particularly the influence of electrode characteristics upon the peaks in the voltammogram, some care must be exercised in setting up an apparatus for stripping voltammetry. The optimum conditions require ... 

FIGURE 2-4 Cyclic voltammetry of C60 and C70 in an acetonitrile-toluene solution. (Reproduced with permission from reference 2.)... 

For more detailed information on the theory of cyclic voltammetry, and the interpretation of cyclic voltammograms, see references (1,7,19,20). 

FIGURE 8. Voltammetries in DMF-TBAP 0.1m, stationary mercury electrode, sweep rate lOmVs 1, concentration of sulphones 5 x 10 3m (1) in aprotic DMF, (2) DMF with phenol (10 2m) (after Reference 26). 

The term polarography basically refers to a method, where the current flowing across the electrochemical interface is recorded as a function of the applied electrode potential, historically in most cases a mercury electrode is involved. Thus polarography might be called also voltammetry. This sometimes results in confusing terms like e.g. AC voltammetry, which is obviously equivalent to AC polarography (see following entry). (Data obtained with this method are labelled DCP.)... 

Appropriate electroanalytical procedures to verify the one or other case have been given in the references of this section. The main techniques are cyclic voltammetry, chronoamperometry, chronocoulometry, and rotating disk voltammetry. The last one appears to be best suited since constant mass transport in the film is a very important feature as outlined aixive Table 2 gives examples for... 

FIGURE 11. Cyclic voltammetries of 68 at a mercury microelectrode, concentration 2x10 m, electrolyte OMF/Bu NI 0.1m, reference electrode Ag/Agl/I" 0.1m, sweep rate 300mVs ... 

In this section, we will present and discuss cyclic voltammetry and potential-step DBMS data on the electro-oxidation ( stripping ) of pre-adsorbed residues formed upon adsorption of formic acid, formaldehyde, and methanol, and compare these data with the oxidative stripping of a CO adlayer formed upon exposure of a Pt/ Vulcan catalyst to a CO-containing (either CO- or CO/Ar-saturated) electrolyte as reference. We will identify adsorbed species from the ratio of the mass spectrometric and faradaic stripping charge, determine the adsorbate coverage relative to a saturated CO adlayer, and discuss mass spectrometric and faradaic current transients after adsorption at 0.16 V and a subsequent potential step to 0.6 V. 

The voltammograms at the microhole-supported ITIES were analyzed using the Tomes criterion [34], which predicts ii3/4 — iii/4l = 56.4/n mV (where n is the number of electrons transferred and E- i and 1/4 refer to the three-quarter and one-quarter potentials, respectively) for a reversible ET reaction. An attempt was made to use the deviations from the reversible behavior to estimate kinetic parameters using the method previously developed for UMEs [21,27]. However, the shape of measured voltammograms was imperfect, and the slope of the semilogarithmic plot observed was much lower than expected from the theory. It was concluded that voltammetry at micro-ITIES is not suitable for ET kinetic measurements because of insufficient accuracy and repeatability [16]. Those experiments may have been affected by reactions involving the supporting electrolytes, ion transfers, and interfacial precipitation. It is also possible that the data was at variance with the Butler-Volmer model because the overall reaction rate was only weakly potential-dependent [35] and/or limited by the precursor complex formation at the interface [33b]. 

Potential step voltammetry (chronoamperometry) or normal pulse voltammetry (NPV) and potential sweep or cyclic voltammetry (CV) were employed for investigating drugs at the NB/W or DCE/W interface. A thin O-layer cell [15,16,23] was used to realize the partition equilibrium of neutral species (that is, B) at the O/W interface initially at t = 0 within a reasonably short time. All measurements were carried out at 25°C. Experimental details should be consulted in the references cited. 

Principles and Characteristics Voltammetric methods are electrochemical methods which comprise several current-measuring techniques involving reduction or oxidation at a metal-solution interface. Voltammetry consists of applying a variable potential difference between a reference electrode (e.g. Ag/AgCl) and a working electrode at which an electrochemical reaction is induced (Ox + ne ----> Red). Actually, the exper-... 

For the theory of voltammetry/polarography, the reader is referred to refs [473,476]. 

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