Titration curves amperometric

Fig. 3.79. Dead-stop end-point titration, i.e. measuring the current across two Pt-IE s with constant potential difference AE (differential amperometric titration), curves being obtained from Fig. 3.78.

Figure 17.1 Typical Amperometric Titration Curves (a) A is Reduced R is not (b) R is Reduced A is not (c) Both R A are Reduced (d) End-point Indicated by Zero-current A = Analyte R = Reagent.

Discuss the four typical amperometric titration curves obtained in amperometric method of analysis and examine them critically with appropriate examples. 

Figure 3.43 Titration of Cl- with Ag+. (CF + Ag+ =5= AgCl(s)). (A) Volt-ammograms at Ag electrode for 0, 50, 100, 150, and 200% of the equivalence point during the titration. (B) Amperometric titration curve for applied potential at Ej and E2. (C) Amperometric titration curve for two Ag electrodes. AE indicated by the shaded areas in A.

The appearance of a cathodic limiting current after the equivalence point reflects the reduction of excess Ag+ titrant. Figure 3.43B shows resulting amperometric titration curves for two values of applied potential. Their shapes are determined by the behavior of the limiting current of the voltammogram at the particular potential during the titration. 

Figure 4.5 Amperometric titration curves for the titration of Pb(II) ions by Ct2Oj ions at Edme = —0.8 V versus SCE (curve a) and at Edme = 0.0 V versus SCE (curve b).

Figure 4.6 Dual-polarized electrode amperometric titration curves. Both curves result from the application of a 0.25-V potential across two identical platinum electrodes that are immersed in the titration solution.

Figure 6.16. Bi-amperometric titration curves, (a) Both electrode reactions reversible, e.g. Fe + against Ce4+. (b) Only titrant reaction is reversible, c.g. AsOJ against 12. (c) Only substance titrated is reversible, e.g. 12 against Na2S20j...

Fig. 13 Amperometric titration curves (A) analyte is reduced, reagent is not (B) reagent is reduced, analyte is not (C) both reagent and analyte are reduced.

Fig. 14 Amperometric titration curves with twin polarized electrodes (A) both reactants behave reversibly at the electrode (B) only reagent behaves reversibly (C) only analyse titrated behaves reversibly.

Amperometric titration curves typically take one of the forms shown in Figure 23-14. The curve in part a represents a titration in which the analyte reacts at the electrode while the titrant does not. Figure 23-14b is typical of a titration in which the reagent reacts at the electrode and the analyte does not. Figure 23-14c corresponds to a titration in which both the analyte and the titrant react at the working electrode. 

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. 

Figure 11.5.5 One-electrode amperometric titration curve (dilution neglected) for titration of Fe " with Ce " with the platinum indicator electrode held at E in Figure 11.5.4.

The shape of the two-electrode amperometric titration curve depends strongly on the reversibility of the electrode reactions of the titrant and titrate systems. For example, if the titrate involves reversible electrode reactions (e.g., and the titrant elec-... 

The amperometric titration curves for coulometric titrations have somewhat different shapes than the ones for the manual titrations described, because usually one form of the couple exists in large excess. For example, in the titration of Fe " with electrogenerated Ce, Ce " will be present from the start at a concentration that is large compared to that of the Fe. Titrations for multicomponent systems can be treated in a similar manner. In all cases the curves can be derived by consideration of the i-E curves that arise during the titration. 

Based on the curves in Figure 11.10.1, how could one determine a mixture of Br2 and I2 by titration with Sn using one-electrode amperometry Sketch the current-potential curves that would be obtained for various stages of the titration and the amperometric titration curves that would result from the method you propose. Sketch the titration curve of a mixture of Br2 and I2 by titration with Sn using two-electrode amperometry with an impressed voltage of 100 mV. 

Consider carrying out a one-electrode amperometric titration for the system Fe -Ce as shown in Figure 11.5.1 at several widely different potentials and sketch the amperometric titration curves that result. Consider, in each case, situations in which (a) the mass-transfer coefficients for all species... 

Fiqure 3.19. Current-versus-voltage curves and amperometric titration curve for the titration of Pir with Na SOi solution. A Successive current-versus-voltage curves for the reduction of ion at a mercury electrode, made after increments of SO were added. B The resulting amperometric titration curve for currents O o, j l, i a.. .) measured at an applied potential of — 1 V versus SCE. 

One advantage of amperometric titrations is that the substance titrated does not have to be electroactive if an appropriate titrant with electrolytic properties is used. For example, sulfate ion can be determined by titration with Pb +. In this case, an essentially constant residual current flows until there is excess titrant in the test solution. After the endpoint a linearly increasing current appears which is proportional to the concentration of the excess titrant. The amperometric titration curve will have a shape the reverse of that shown in Figure 3.19B —/-shaped, or reverse L-shaped. ... 

In general, the best way to predict the shape of amperometric titration curves is to look at or construct the current-versus-voltage curves of the test solution during the course of the electrolysis. 

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. 

Figure 2 Amperometric titration curves for systems where (A) oniy anaiyte is reduced (B) oniy the reagent is reduced and (C) both anaiyte and reagent are reduced.

Figure 4 Amperometric titration curves for redox reactions on dual polarizable electrodes (A) reversible half-reactions for titrant and sample (B) reversible half-reaction for sample only and (C) reversible half-reaction for titrant only, e.p., equivalent point (Reprinted with permission from Peters GD, Hayes JM, and Hieftje GM (1974) Chemical Separations and Measurements Brooks/Cole, a division of Thomson Learning www.thomson-rights.com. fax 800 730-2215.)...

Figure 4 Amperometric titration curve showing diffusion current versus titrant volume for a reducible analyte and a titrant that is not reducible.

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