Analytical Chem istry - DePauw University

512 Analytical Chemistry 2.0continuously monitoring a property of the titrand’s solution—absorbance,potential, and temperature are typical examples—that changes as the titrationprogresses. In either case, an accurate result requires that the end pointclosely match the equivalence point. Knowing the shape of a titration curveis critical to evaluating the feasibility of a titrimetric method.Many titrations are direct, in which the analyte participates in the titrationas the titrand or the titrant. Other titration strategies may be usedwhen a direct reaction between the analyte and titrant is not feasible. Ina back titration a reagent is added in excess to a solution containing theanalyte. When the reaction between the reagent and the analyte is complete,the amount of excess reagent is determined by a titration. In a displacementtitration the analyte displaces a reagent, usually from a complex, and theamount of displaced reagent is determined by an appropriate titration.Titrimetric methods have been developed using acid–base, complexation,redox, and precipitation reactions. Acid–base titrations use a strongacid or a strong base as a titrant. The most common titrant for a complexationtitration is EDTA. Because of their stability against air oxidation,most redox titrations use an oxidizing agent as a titrant. Titrations withreducing agents also are possible. Precipitation titrations often involve Ag +as either the analyte or titrant.9HProblemsAnswers, but not worked solutions, tomost end-of-chapter problems are availablehere.Some of the problems that follow require one ormore equilibrium constants or standard state potentials.For your convenience, here are hyperlinksto the appendices containing these constantsAppendix 10: Solubility ProductsAppendix 11: Acid Dissociation ConstantsAppendix 12: Metal-Ligand Formation ConstantsAppendix 13: Standard State Reduction Potentials1. Calculate or sketch titration curves for the following acid–base titrations.a. 25.0 mL of 0.100 M NaOH with 0.0500 M HClb. 50.0 mL of 0.0500 M HCOOH with 0.100 M NaOHc. 50.0 mL of 0.100 M NH 3 with 0.100 M HCld. 50.0 mL of 0.0500 M ethylenediamine with 0.100 M HCle. 50.0 mL of 0.0400 M citric acid with 0.120 M NaOHf. 50.0 mL of 0.0400 M H 3 PO 4 with 0.120 M NaOH2. Locate the equivalence point for each titration curve in problem 1.What is the stoichiometric relationship between the moles of acid andthe moles of base at each of these equivalence points?3. Suggest an appropriate visual indicator for each of the titrations inproblem 1.4. In sketching the titration curve for a weak acid we approximate the pHat 10% of the equivalence point volume as pK a – 1, and the pH at 90%of the equivalence point volume as pK a + 1. Show that these assumptionsare reasonable.