Precipitation

• Precipitation methods


  Titration methods involving the precipitation of a silver salt
• Precipitation titrations are useful in the determination of halides such as chlorides, bromides and iodides
• Precipitation titrations involve the use of a precipitating agent such as silver nitrate, and are therefore also known as argentimetric titrations
• Ways to determine the end point in precipitation methods

  • Cessation of precipitation or the appearance of turbidity
  • Use of indicators
  • Instrumental methods as potentiometric
• Indicators used in official volumetric precipitation assays

  • Ferric Ammonium sulfate TS
  • Potassium chromate TS
  • Adsorption indicators: Dichlorofluorescein (DCF), Eosin Y TS, Tetrabromophenolphthalein ethyl ester (TEE)
• Ferric Ammonium sulfate TS


  8% in water, used when titrating with standard ammonium thiocyanate in the presence of silver nitrate. Forms a red color of the ferric thiocyanate complex when the silver thiocyanate has been completely precipitated.
• Potassium chromate TS


  10% in water, forms a red precipitate of silver chromate which is seen against the background of white silver chloride
• Adsorption indicators


  Organic compounds that tend to be adsorbed onto the surface of the solid in a precipitation titration, resulting in a color change and transfer of color from the solution to the solid (or the reverse)
• Standard solutions used in official assays by precipitation methods

  • 0.1 N silver nitrate
  • 0.1 N ammonium thiocyanate
• Types of volumetric precipitation methods

  • Mohr
  • Volhard
  • Fajans
• Mohr method


  Sodium chromate can serve as an indicator, reacting with silver ion to form a brick-red silver chromate (Ag2CrO4) precipitate in the equivalence-point region
• Volhard method


  Silver ions are titrated with a standard solution of thiocyanate ion, with iron (III) serving as the indicator to form a red complex with the first slight excess of thiocyanate ion
• Volhard method

  1. End point is detected by adding iron (III) as a ferric alum, which forms a soluble red complex with the first excess of titrant
• Fajans method


  Uses adsorption indicators, which are organic compounds that tend to be adsorbed onto the surface of the solid in a precipitation titration, resulting in a color change and transfer of color
• Argentometry


  A type of titration involving silver ion (Ag)