Analytical Chem istry - DePauw University

Chapter 9 Titrimetric Methods489Table 9.16Selected Examples of Redox IndicatorsIndicator Color of In ox Color of In redoE In ox /In redindigo tetrasulfate blue colorless 0.36methylene blue blue colorless 0.53diphenylamine violet colorless 0.75diphenylamine sulfonic acid red-violet colorless 0.85tris(2,2´-bipyridine)iron pale blue red 1.120ferroin pale blue red 1.147tris(5-nitro-1,10-phenanthroline)iron pale blue red-violet 1.25A partial list of redox indicators is shown in Table 9.16. Examples of appropriateand inappropriate indicators for the titration of Fe 2+ with Ce 4+are shown in Figure 9.40.Ot h e r Me t h o d s f o r Fi n d i n g t h e En d p o i n tAnother method for locating a redox titration’s end point is a potentiometrictitration in which we monitor the change in potential while addingthe titrant to the titrand. The end point is found by visually examiningthe titration curve. The simplest experimental design for a potentiometrictitration consists of a Pt indicator electrode whose potential is governed bythe titrand’s or titrant’s redox half-reaction, and a reference electrode thathas a fixed potential. A further discussion of potentiometry is found inChapter 11. Other methods for locating the titration’s end point includethermometric titrations and spectrophotometric titrations.1.61.4E (V)1.21.00.80.6ferroindiphenylaminesulfonic acid0 20 40 60 80 100Volume of Ce 4+ (mL)Figure 9.40 Titration curve for the titration of 50.0mL of 0.100 M Fe 2+ with 0.100 M Ce 4+ . The endpoint transitions for the indicators diphenylaminesulfonic acid and ferroin are superimposed on thetitration curve. Because the transition for ferroinis too small to see on the scale of the x-axis—it requiresonly 1–2 drops of titrant—the color changeis expanded to the right.