Voltammetric indicator electrodes

Let us introduce into the titrant one Pt indicator electrode vs. an SCE and maintain in the electric circuit a low constant current + /, as indicated by the broken horizontal line in Fig. 3.71. For this line we shall consider the successive points of its intersection with the voltammetric curves during titration and observe the following phenomena as expressed in the corresponding electrode potentials. Immediately from the beginning of the titration E remains high (nearly 1.44 V), but falls sharply just before the equivalence point (E = 1.107 V), and soon approaches a low E value (below 0.77 V) (see Fig. 3.72, cathodic curve +1). 

Again for the titration of Ce(IV) with Fe(II) we shall now consider constant-potential amperometry at one Pt indicator electrode and do so on the basis of the voltammetric curves in Fig. 3.71. One can make a choice from three potentials eu e2 and e3, where the curves are virtually horizontal. Fig. 3.74 shows the current changes concerned during titration at e1 there is no deflection at all as it concerns Fe(III) and Fe(II) only at e2 and e3 there is a deflection at A = 1 but only to an extent determined by the ratio of the it values of the Ce and Fe redox couples. The establishment of the deflection point is easiest at e2 as it simply agrees with the intersection with the zero-current abscissa as being the equivalence point in fact, no deflection is needed in order to determine this intersection point, but if there is a deflection, the amperometric method is not useful compared with the non-faradaic potentiometric titration unless the concentration of analyte is too low. 

Voltammetry is a term used to include all the methods that measure current-potential curves (voltammograms) at small indicator electrodes other than the DME [6], There are various types of voltammetric indicator electrodes, but disk electrodes, as in Fig. 5.17, are popular. The materials used for disk electrodes are platinum, gold, graphite, glassy carbon (GC), boron-doped diamond8, carbon paste, etc. and they can be modified in various ways. For electrode materials other than mercury, the potential windows are much wider on the positive side than for mercury. However, electrodes of stationary mercury-drop, mercury-film, and mercury-pool are also... 

The boron-doped diamond film electrode is the newest of the voltammetric indicator electrodes. It is promising because of its high chemical stability, low background currents, and wide potential windows (high overpoten-... 

Fig. 5.17 Examples of voltammetric indicator electrodes, (a) Disk electrode (GC, platinum, gold, etc.) (b) carbon paste electrode (c) simple hanging mercury-drop electrode.

The conventional voltammetric indicator electrodes are 0.5-5 mm in diameter. However, ultramicroelectrodes (UME) [8] that have dimensions of 1-20 pm, are also used as indicator electrodes. The tiny electrodes have some definite advantages over conventional ones ... 

In contrast, channel electrode voltammetric studies at a platinum electrode on unbuffered maleic acid solutions of this concentration indicate that there is little dissociation of HA" on the timescale of the channel electrode experiment [263]. Thus, the calcite "sees HA" and H+, but little A2" and,... 

A titration in which measurement of the current flowing at a voltammetric indicator electrode is used for detection of the equivalence point is termed an amperometric titration. The current measured is almost always a limiting current which is proportional to concentration, and can be due to the substance titrated, to the titrant itself, to a product of the reaction, or to any two of these—depending on the potential of the electrode and the electrochemical characteristics of the chemical substances involved. The titration curve is a plot of the limiting current, corrected for dilution by the reagent and, if necessary, for any residual current, as a function of the volume of titrant. Ideally, the titration curve consists of two linear segments which intersect at the equivalence point. 

Figure 3.21. Titration curves for amperometric titrations with two polarized or indicator electrodes. A Both the titrant and the substance titrated have reversible voltammetric curves. B The substance titrated displays irreversibility, and the titrant reversibility. C The substance titrated displays reversibility, and the titrant irreversibility.

Reference electrodes are necessary to control the potential of a working electrode (e.g., during voltammetric measurements) or to measure the potential of an indicator electrode in potentiomet-ric measurements, since the Galvani potential... 

Voltammetric, respectively, amperometric sensors using the concentration-dependent current which is generated at the indicator electrode by the electrochemical transformation of the analyte itself or of an electrochemical active product after an appropriate reaction chain. The working electrode can be an inert metal as well as a chemically modified electrochemical active surface. 

After all the above considerations and experimental data have been taken into account one arrives at the following general conclusions regarding the electroactivity of organic substances at non-mercury voltammetric indicator electrodes ... 

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