PRECIPITATION TITRIMETRY

• Precipitation Titrimetry


  A form of volumetric analysis wherein a sample in solution is made to react with a standard solution of precipitating reagent (for the target analyte) until the endpoint is reached
• Precipitimetry


  Another name for precipitation titrimetry
• Saturation method


  Another name for precipitation titrimetry
• Precipitation titrations are based on a reaction that yields an ionic product that has limited solubility
• Precipitation titrimetry is one of the oldest analytical techniques
• Precipitation Titrimetry


  • Applicable to pharmacy practice: Group 1 ions (silver, mercury)
  • Precipitating agent for silver: HCl
  • Precipitating agent for halides (Cl, Br): AgNO3
• Limitations of Precipitation Titrimetry


  • Slow rate of precipitation for most analytes limits the number of precipitating agents that can be used
  • Limited number of ions that can be analyzed using precipitation methods
  • End point may be difficult to detect
• Precipitation titration analysis are commonly applied for Ag+, Hg+, halide (Br-, Cl-), and theophylline containing compounds
• KSCN (Potassium Thiocyanate)


  Available as a primary standard but has hygroscopic properties; it will absorb moisture in air and may have inaccurate weight
• Standardizing KSCN


  Best standardized using std. AgNO3 solution to determine the concentration
• KSCN may require restandardization due to its indefinite stability
• Types of Precipitation Indicators

  • First Type: Forms a colored compound with the excess titrant (e.g. ferric alum [FeNH4(SO4)2] and chromate indicators [CrO4-2])
  • Adsorption Indicators: Color dyes; becomes adsorbed on the precipitate at the equivalence point; color changes when the indicator has been absorbed (e.g. fluorescein, dichlorofluorescein [DCF], eosin)
• Different Adsorption Indicators

  • Fluorescein (Cl- with Ag+, pH 7-8)
  • Dichlorofluorescein (Cl- with Ag+, pH 4)
  • Bromocresol green (SCN- with Ag+, pH 4-5)
  • Eosin (Br-, I-, SCN- with Ag+, pH 2)
  • Methyl violet (Ag+ with Cl-, acid solution)
  • Rhodamine 6 G (Ag+ with Br1-, HNO3 (</= 0.3 M))
  • Thorin (SO4-2 with Ba2+, pH 1.5 – 3.5)
  • Bromophenol blue (Hg2+ with Cl-, 0.1 M solution)
  • Orthochrome T (Pb2+ with CrO4-2, Neutral, 0.02 M solution)
• Mohr Method


  Involves formation of a colored secondary precipitate, carried out at pH 7-10, uses a K2CrO4 as an indicator and a standard AgNO3 as a titrant (argentometric method)
• Fajan's Method


  Involves formation of a colored adsorption product, uses fluorescein as the common indicator and a std. solution of AgNO3 as the titrant (argentometric method)
• Direct Thiocyanate Method


  The titration involves the use of standard SCN solution and as a result leads to the formation of a colored thiocyanate complex (standard solution), used for mercury analysis or mercury-type titration
• Volhard's Method


  The "residual method" of precipitimetry, uses ferric alum as an indicator and involves the formation of a colored complex ion
• Precipitation Titration Methods


  • Mohr
  • Fajan
  • Direct Thiocyanate
  • Volhard
• Analytes in Precipitation Titrimetry

  • Halides
  • Halide-like anions (SCN-1, CN-1, CNO-1)
  • Mercaptans
  • Fatty acids
  • Several divalent inorganic anions