Analytical Chem istry - DePauw University

Chapter 9 Titrimetric Methods4151412end pointpH of 11.610pH8642pH at V eq = 7.000 10 20 30 40 50V NaOH (mL)V eq = 25.0 mLend pointpH of 6.8Figure 9.1 Typical acid–base titration curve showing howthe titrand’s pH changes with the addition of titrant. Thetitrand is a 25.0 mL solution of 0.100 M HCl and the titrantis 0.100 M NaOH. The titration curve is the solid blue line,and the equivalence point volume (25.0 mL) and pH (7.00)are shown by the dashed red lines. The green dots show twoend points. The end point at a pH of 6.8 has a small titrationerror, and the end point at a pH of 11.6 has a largertitration error.The titration curve in Figure 9.1 is not unique to an acid–base titration.Any titration curve that follows the change in concentration of a species inthe titration reaction (plotted logarithmically) as a function of the titrant’svolume has the same general sigmoidal shape. Several additional examplesare shown in Figure 9.2.The titrand’s or the titrant’s concentration is not the only property wecan use when recording a titration curve. Other parameters, such as thetemperature or absorbance of the titrand’s solution, may provide a usefulend point signal. Many acid–base titration reactions, for example, areexothermic. As the titrant and titrand react the temperature of the titrand’ssolution steadily increases. Once we reach the equivalence point, furtheradditions of titrant do not produce as exothermic a response. Figure 9.3shows a typical thermometric titration curve with the intersection ofthe two linear segments indicating the equivalence point.(a)15(b)1.6(c) 101.48101.26pCd50E (V)1.040.80.620 10 20 30 40 50 0 10 20 30 40 50 0 10 20 30 40 50V EDTA (mL) V Ce 4+ (mL) V AgNO3 (mL)Figure 9.2 Additional examples of titration curves. (a) Complexation titration of 25.0 mL of 1.0 mM Cd 2+ with 1.0 mMEDTA at a pH of 10. The y-axis displays the titrand’s equilibrium concentration as pCd. (b) Redox titration of 25.0 mLof 0.050 M Fe 2+ with 0.050 M Ce 4+ in 1 M HClO 4 . The y-axis displays the titration mixture’s electrochemical potential,E, which, through the Nernst equation is a logarithmic function of concentrations. (c) Precipitation titration of 25.0 mLof 0.10 M NaCl with 0.10 M AgNO 3 . The y-axis displays the titrant’s equilibrium concentration as pAg.pAg