Titration curves voltammetric

In voltammetric titration the reaction is pursued by means of voltammetry interest is sometimes taken in the complete titration curve, but mostly in its part around the equivalence point in order to establish the titration end-point. 

To obtain accurate values for the parameters of complexation it is necessary to follow the titration curve with sufficient precision and to reach the linear part of the titration curve with certainty (as required by the theory outlined in Section 4.1). To satisfy these requirements numerous metal standard additions (at least 10, but preferably between 15 and 20) are performed throughout the titration experiment. After each standard addition and before the voltammetric measurement a period of 15 min for Cd and Cu, and 25-30 min for Pb are allowed to pass to reach the chemical equilibrium (typically, one titration requires about 20 h). 

Several different types of amperometric titration curves are possible. For example, one can titrate a metal ion that shows a voltammetric wave (e.g., Pb " ) with a titrant that causes its precipitation (e.g., Cx20 ). If the potential is held at the plateau of the voltammetric wave, the current will decrease during the titration and remain at the residual current level for/> 1. 

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.

The shape of the amperometric titration curve in this case, where both the titrant and the substance titrated undergo reversible redox reactions, is illustrated in Figure 3.21A. In the case where the substance titrated does not have a reversible voltammetric wave, the titration curve will have the shape illustrated in Figure 3.2IB. Prior to the equivalence point, the applied voltage is too small to cause both oxidation and reduction of the redox couple of the substance titrated. If the titrant has an irreversible wave, the titration curve will look like that in Figure 3.21C. This type of titration is commonly called a dead-stop titration, because the indicator current falls to zero at the equivalence point. 

The key factor in voltammetry (and polarography) is that the applied potential is varied over the course of the measurement. The voltammogram, which is a current-applied potential curve, / = /( ), corresponds to a voltage scan over a range that induces oxidation or reduction of the analytes. This plot allows identification and measurement of the concentration of each species. Several metals can be determined. The limiting currents in the redox processes can be used for quantitative analysis this is the basis of voltammetric analysis [489]. The methods are based on the direct proportionality between the current and the concentration of the electroactive species, and exploit the ease and precision of measuring electric currents. Voltammetry is suitable for concentrations at or above ppm level. The sensitivity is often much higher than can be obtained with classical titrations. The sensitivity of voltammetric... 

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. 

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