CHAPTER II. POTENTIOMETRY AND REDOX TITRATIONS I ...

The larger the difference in standard potential between titrant and analyte, thesharper the break in titration curve at equivalent point. A redox titration is usuallyfeasible if the difference between analyte and titrant is ≥ 0.2 V. If the difference informal potentials is ≥ 0.4 V, then a redox indicator usually gives a satisfactory endpoint. Otherwise, the end point should be detected potentiometrically.iii/. Gran PlotThe Gran plot uses data from well before the equivalent point (V e ) to locate V e .Potentiometric data close to V e are the least accurate because electrodes are slow toequilibrate with species in solution when one member of a redox couple is nearlyused up.e.g., For the oxidation of Fe 2+ to Fe 3+ , the potential prior to V e isE = [E°’ − 0.0592⋅log([Fe 2+ ]/[Fe 3+ ])] − E ref (1)If volume of analyte is V 0 and volume of titrant is V, and if the reaction goes “tocompletion” with each addition of titrant, then[Fe 2+ ]/[Fe 3+ ] = (V e − V)/V (2)Substituting (2) into (1) and rearranging,V⋅10 −nE/0.0592 = −(V − V e )⋅10 −n(E ref − E°’)/0.0592A graph of V⋅10 −nE/0.0592 versus V should be a straight line whose x-intercept is V e .FIGURE 2-20. “Harris” Fig. 16-4 (p. 356).iv/. Adjustment of Analyte Oxidation StateSometimes it is necessary to adjust oxidation state of analyte before it can betitrated. Preadjustment must be quantitative and the excess preadjustment reagentmust be removed or destroyed.a) Preoxidation1. Peroxydisulfate (Persulfate)S 2 O 2− 8 is a strong oxidant that requires Ag + as a catalyst.S 2 O 2− 8 + Ag + → SO 2− 4 + SO − 4 + Ag 2+8