Qualitative analysis

• Identifying positive ions:

  • transitions metal ions
  • ammonium ions, NH4+
• Transition metal ions:

  • aqueous sodium hydroxide produces ppts with aqueous transition metal ions. these reactions can be used for identifying transition metal ions in unknown solution
• Ammonium, NH4+:

  • when heated with hydroxide ions, NH4+ reacts to produce ammonia gas, NH3
  • NH4+ (aq) + OH- (aq) = NH3 (g) + H2O (l)
  • to test for the ammonium ions, aqueous sodium hydroxide is heated gently with the solution being analysed.
  • if ammonia is evolved, damp red pH indicator paper will turn blue, confirming the presence of NH4+ ions
• Identifying anions:

  • tests can be used to identify anions in solutions of unknown compounds
  • where the tests are being carried out in the same solution it is important to carry them out in the following order:
• • (1) carbonate, CO3-
  • add dilute nitric acid, HNO3 (aq)
  • effervescence as carbon dioxide is evolved
  • CO32- (aq) + 2H+ (aq) = CO2 (g) + H2O (l)
• • (2) sulfate SO42-
  • add Ba2+ (aq) ions
  • white ppt of BaSO4 s formed:
  • Ba2+ (aq) + SO42- (aq) = BaSO4 (s)
• • (3) halide ions
  • add Ag+ (aq) ions
  • white ppt of AgCl if Cl- ions present, soluble in dilute NH3 (aq)
  • Ag+ (aq) + Cl- (aq) - AgCl(s)
  • cream ppt of AgBr if Br- ions present, soluble in conc NH3 (aq)
  • yellow ppt of AgI if I- ions present, soluble in conc NH3 (aq)