Photometric titration curves

The shape of a photometric titration curve will be dependent upon the optical properties of the reactant, titrant, and products of the reaction at the wavelength used. Some typical titration plots are given in Fig. 17.23. 

For rigorous calculations of equilibrium concentrations of the various species in solution, values of the constants for all the equilibria involved are needed use of a computer may be indicated. For example, SeUers, Eller, and Caruso developed a theoretical model that assumes the existence of a wide variety of species such as can be found in acetonitrile solution. Eleven equations in eleven unknowns are solved with the aid of a computer to give information about conductance, potentiometric, and photometric titration curves in nonaqueous media. 

Possible shapes of photometric titration curves are shown in Fig. 15. Curve A is typical of the titration where the titrant alone absorbs and where the absorbance readings are taken at the wavelength of the titrant. Curve B is characteristic of systems where the product of the reaction absorbs, and curve C is typical for reactions where the analyte is converted to a non-absorbing product. When a colored analyte is converted to a colorless product by a colored titrant, curves similar to D are obtained. 

A photometric titration curve is a plot of absorbance (corrected for volume change) as a function of titrant volume. If conditions are chosen properly, the curve consists of two straight-line regions with different slopes, one occurring prior to the equivalence point of the titration and the other located well beyond the equivalence-point region the end point is taken as the intersection of extrapolated linear portions of the two lines. 

Figure 26-1 2 Typical photometric titration curves. Molar absorptivities of the analyte titrated, the product, and the titrant are e, Sp, and e, respectively.

Figure 26-13 Photometric titration curve at 745 nm for 100 mL of a solution that was 2.0 X 10 M in and Cu. (Reprinted with permission from A, L. Underwood, AnaZ. Chem., 1954,26, 1322. Copyright 1954 the American Chemical Society.)...

Sketch a photometric titration curve for the titration of Sn with MnO. What color radiation should be used for this titration Explain. 

Iron(ni) reacts with thiocyanate ion to form the red complex, Fe(SCN) +. Sketch a photometric titration curve for Fe(III) with thiocyanate ion when a photometer with a green filter is used to collect data. Why is a green filter used ... 

FIGURE 14-13 Photometric titration curves (a) tola hardness of water, (b) determination of sulfate. In (a), total water hardness is obtained by titration with 0.10 M EDTA at 610 nm lor 100 ml of a solution that contained 2.82 mmol/L total hardness. Eriochrome Black T was the indicator, fn (b). 10.0 ml of a solution containing sulfale was titrated with 0.050 M BaCI using Thorin as an indicator and a wavelength of 523 nm. The response shown is proportional to transmittance. (From A. L, Underwood. Ana/. Chom.. 1954, 26. 1322. Figure t, p. 1323. Copyright 1954 American Chemical Society.)... 

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