Analytical Chem istry - DePauw University

508 Analytical Chemistry 2.0Table 9.19 Representative Examples of PrecipitationTitrationsTitrand Titrant a End Point b3–AsO 4 AgNO 3 , KSCN VolhardBr – AgNO 3Mohr or FajansAgNO 3 , KSCN VolhardCl – AgNO 3Mohr or FajansAgNO 3 , KSCN Volhard*2–CO 3 AgNO 3 , KSCN Volhard*2–C 2 O 4 AgNO 3 , KSCN Volhard*2–CrO 4 AgNO 3 , KSCN Volhard*I – AgNO 3FajansAgNO 3 , KSCN Volhard3–PO 4 AgNO 3 , KSCN Volhard*S 2– AgNO 3 , KSCN Volhard*SCN – AgNO 3 , KSCN Volhard*a When two reagents are listed, the analysis is by a back titration. The first reagentis added in excess and the second reagent used to back titrate the excess.b For those Volhard methods identified with an asterisk ( *) the precipitated silversalt must be removed before carrying out the back titration.9E.3 Quantitative ApplicationsAlthough precipitation titrimetry is rarely listed as a standard method ofanalysis, it may still be useful as a secondary analytical method for verifyingother analytical methods. Most precipitation titrations use Ag + as eitherthe titrand or the titration. A titration in which Ag + is the titrant is calledan argentometric titration. Table 9.19 provides a list of several typicalprecipitation titrations.Qu a n t i t a t i ve Ca l c u l at i o n sThe quantitative relationship between the titrand and the titrant is determinedby the stoichiometry of the titration reaction. If you are unsure ofthe balanced reaction, you can deduce the stoichiometry from the precipitate’sformula. For example, in forming a precipitate of Ag 2 CrO 4 , eachmole of CrO 4 2– reacts with two moles of Ag + .Example 9.14A mixture containing only KCl and NaBr is analyzed by the Mohr method.A 0.3172-g sample is dissolved in 50 mL of water and titrated to theAg 2 CrO 4 end point, requiring 36.85 mL of 0.1120 M AgNO 3 . A blanktitration requires 0.71 mL of titrant to reach the same end point. Reportthe %w/w KCl in the sample.