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Harris Chapter 7

Harris Chapter 7. TITRATIONS Supplements Information In Zumdahl’s Chapter 14 & 15. Supplemental Content. Titration Nomenclature Calculations Precipitation Titration By regions around the Equivalence Point Titration of Mixtures Titration Spreadsheets End-Point Detection.

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Harris Chapter 7

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  1. Harris Chapter 7 TITRATIONS Supplements Information In Zumdahl’s Chapter 14 & 15

  2. Supplemental Content • Titration Nomenclature • Calculations • Precipitation Titration • By regions around the Equivalence Point • Titration of Mixtures • Titration Spreadsheets • End-Point Detection

  3. Analyte: the solution (soln.) being analyzed. Titrant: soln. added Equivalence point: completed reaction. End point: measured or observed indication of change in soln. Indicator: compound responsible for end point indication. Standardization: fix titrant concentration with primary standard. Back Titration: finds XS titrant to subtract. Titration Nomenclature

  4. Calculations • Buret dispenses mL of titrant, so we use mmol because M mol / L = mmol / mL. • Burette is a diminutive of the French buire, a vase for liquors! • [analyte] = (mmol analyte) / (mL initial volume) • mmol analyte = (mmol titrant)  (stoichiometric ratio of analyte to titrant) • mmol titrant = (mL titrant)  [titrant] • In other words, CV = CV over and over.

  5. Precipitation Titration • Simpler than weak acid since no Kw involved • (unless precipitate is an acid or base). • Prior to equivalence, titrant is the limiting reactant & [analyte] = (mmol analyte – scaled mmol titrant) / (mL current solution) • Post equivalence, analyte is the limiting reactant & [titrant] = (mmol titrant – scaled mmol analyte) / ( mL current solution) • In either case andat equivalence: • Ksp = [analyte] [titrant] (assuming 1:1)

  6. Titration of Mixtures • Endpoints are characterized by inflection points where a curvature changes sign. • In mixtures leading to two precipitates, two inflection points are observed. • The lowest Ksp solid finishes its reaction by precipitating first. Thus the first inflection point. • The more soluble solid precipitates last.

  7. Spreadsheet Titration Curves • While pH is normally plotted against Vtitrant , the equations for Vtitrant = f(pH) are easier as seen in Harris 12-9. The same is true if pM vs. VM is sought in precipitation titrations. • nMX(s) = C°M VM – [M+] ( VM + V°M ) • nMX(s) = C°X VX – [M+] ( VX + V°X ) (now equate) • VM = V°X (C°X+[M+]–[X–]) / (C°M–[M+]+[X–])

  8. Precipitate End-Point Detection • Volhard Titration • Back titrate excess Ag+ (from a halide ppt.) with SCN– in presence of Fe3+ • Red FeSCN2+ appears after AgSCN precipitates. • Fajans Titration • Won’t be used in lab., but involves coprecipitate of colored indicator at end-point.

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