3.1.4 - qualitative analysis

• Anions are negative ions
• Cations are positive ions
• To test for carbonate ions:

  add strong hydrochloric acid

  collect the gas produced

  pass through limewater
• An equation for the carbonate ion test

  CO3 + H+ ————> H2O + CO2
• To test for sulfate ions:

  add dilute hydrochloric acid and barium chloride to the sample
• if sulfate ions are present, you observe a white precipitate of barium sulfate
• An equation for the sulfate ion test is:

  Ba + SO4 ———> BaSO4
• To test for halide ions you use silver nitrate
• You use silver nitrate for halide ions instead of HCl because it removes carbonate ions.

  Adding HCl would add Cl- ions giving it a false positive result
• To test for a halide

  dissolve sample in water

  add aqueous silver nitrate

  record colour change

  if it is difficult to distinguish the colour add aqueous ammonia (dilute first then concentrated)

  note the solubility of precipitate
• If chloride ions are present, a white precipitate is formed. And it is soluble in dilute aqueous ammonia.
• The equation for the chloride test is:

  Ag + Cl ——> AgCl
• If bromide ions are present, a cream precipitate is formed. It is only soluble in concentrated aqueous ammonia.
• The equation for the bromide test is:

  Ag + Br ——> AgBr
• The equation for the iodide test is:

  Ag + I ——> AgI
• The order of testing for anions should be

  1. carbonate
  2. sulfate
  3. halide

  this is because barium ions form an insoluble precipitate of barium carbonate

  and silver ions form insoluble precipitate of silver sulfate
• To test for ammonium ions you

  add sodium hydroxide to the sample and gently warm it

  test the gas produced with red litmus paper
• If ammonium ions are present the red litmus paper turns blue and ammonia has a distinct pungent smell
• The equation for the ammonium ions test is:

  NH4+ + OH- ——> NH3 + H2O