Analytical Chem istry - DePauw University

Chapter 9 Titrimetric Methods471If at least one species in a complexation titration absorbs electromagneticradiation, we can identify the end point by monitoring the titrand’sabsorbance at a carefully selected wavelength. For example, we can identifythe end point for a titration of Cu 2+ with EDTA, in the presence of NH 3 bymonitoring the titrand’s absorbance at a wavelength of 745 nm, where theCu(NH 3 ) 4 2+ complex absorbs strongly. At the beginning of the titrationthe absorbance is at a maximum. As we add EDTA, however, the reaction2+ 4− 2−Cu(NH ) ( aq) + Y ( aq) → CuY ( aq) + 4NH( aq)3 4decreases the concentration of Cu(NH 3 ) 4 2+ and decreases the absorbanceuntil we reach the equivalence point. After the equivalence point the absorbanceremains essentially unchanged. The resulting spectrophotometrictitration curve is shown in Figure 9.31a. Note that the titration curve’sy-axis is not the actual absorbance, A, but a corrected absorbance, A corrA A V + VEDTA= ×corrVwhere V EDTA and V Cu are, respectively, the volumes of EDTA and Cu. Correctingthe absorbance for the titrand’s dilution ensures that the spectrophotometrictitration curve consists of linear segments that we can extrapolateto find the end point. Other common spectrophotometric titration curvesare shown in Figures 9.31b-f.CuCu3(a)(b)A corrA corrVolume of TitrantVolume of Titrant(c)(d)A corrA corrVolume of TitrantVolume of Titrant(e)A corrVolume of Titrant(f)A corrVolume of TitrantFigure 9.31 Examples of spectrophotometric titrationcurves: (a) only the titrand absorbs; (b) only the titrantabsorbs; (c) only the product of the titration reactionabsorbs; (d) both the titrand and the titrant absorb;(e) both the titration reaction’s product and the titrantabsorb; (f) only the indicator absorbs. The red arrowsindicate the end points for each titration curve.