PRECIPITATION

• What is precipitation titrimetry based on?

  Reactions yielding ionic compounds of limited solubility
• What is the general reaction format in precipitation titrimetry?

  Analyte + Titrant → Precipitate
• What assumption is made about the precipitate in precipitation titrimetry?

  The precipitate is virtually insoluble
• When did precipitation titrimetry originate?

  Mid-1800s
• Why are there few precipitating agents used in titrimetry?

  Due to the slow rate of formation of precipitates
• What is the most widely used precipitating reagent?

  Silver nitrate
• What types of ions can silver nitrate determine?

  Halides and halide-like anions
• What are titrimetric methods based on silver nitrate called?

  Argentometric methods
• What is the purity grade of silver nitrate available?

  Primary standard-grade purity
• How can less pure silver nitrate be standardized?

  With primary-standard potassium chloride
• What precaution must be taken when preparing silver nitrate?

  Prevent skin contact to avoid staining
• What is the reaction involved in the titration of a halide ion with silver nitrate?

  AgNO3 + NaX → AgX(s) + NaNO3
• What does a titration curve for this method plot?

  pAg versus the volume of silver nitrate added
• What are the regions of a precipitation titration curve?

  • Preequivalence point: AgX(s) + NaNO3 + NaX
  • Equivalence point: AgX(s) + NaNO3
  • Postequivalence point: AgX(s) + NaNO3 + AgNO3
• What effect do high concentrations of analyte and titrant have on the titration curve?

  They produce larger and sharper end point breaks
• What does a very insoluble product lead to in a titration reaction?

  A very complete titration reaction
• What are the three types of end points in argentometric titrations?

  • Potentiometric: measures potential between electrodes
  • Amperometric: plots current generated in solution
  • Chemical: color change or turbidity appearance
• How does sodium chromate serve as an indicator in the Mohr Method?

  It forms a brick-red silver chromate precipitate
• Why does AgCl precipitate before silver chromate in the Mohr Method?

  AgCl is much less soluble than silver chromate
• What is the Ksp of AgCl?

  $1.820 \times 10^{-10}$
• What is the Ksp of silver chromate?

  $1.20 \times 10^{-12}$
• How can the concentration of CrO4$^{2-}$ be computed at the equivalence point?


  From the Ksp of silver chromate
• What are the three chemical indicators used for argentometric methods?

  • Chromate ion
  • Adsorption indicator
  • Iron(III) ion
• What concentration of CrO4$^{2-}$ imparts an intense yellow color?


  $6.6 \times 10^{-3} \text{ M}$
• What pH is best for the Mohr Method?

  About 8
• What is an adsorption indicator?

  An organic compound adsorbed onto solid surface
• What is a typical adsorption indicator used for chloride ion titration?

  Fluorescein
• What happens to the indicator after the equivalence point in the Fajans Method?

  Indicator is attracted to the positive surface
• What must be true for the indicator ion in the Fajans Method?

  It must not displace the primary adsorbed ion
• What factors affect the adsorption end point?

  Indicator concentration and precipitate surface area
• Why must the pH be high enough in the Fajans Method?

  To prevent conversion of In to HIn
• What is the direct method in the Volhard Method?

  Silver ions are titrated with thiocyanate ion
• What role does iron (III) serve in the Volhard Method?

  It acts as an indicator
• What happens when there is a slight excess of thiocyanate ion in the Volhard Method?

  The solution turns red
• Why must the titration in the Volhard Method be acidic?

  To prevent precipitation of Iron (III)
• What is the indirect method in the Volhard Method?

  Excess silver nitrate is added, then back titrated
• Why is silver chloride filtered off before back titration?

  AgCl is more soluble than AgSCN
• What can happen if AgCl is not removed before back titration?

  It can cause overconsumption of thiocyanate
• What is an advantage of the Volhard method over other methods?

  It requires a strong acidic environment