Qualitative Analysis

• Testing for Negative ions (anions)

  1. Add any dilute acid and observe effervescence

  2. Bubble gas through limewater to test for CO2 – will turn limewater cloudy
• Carbonate


  Fizzing due to CO2 would be observed if a carbonate was present
• Testing for presence of a sulfate

  1. Acidified BaCl2 solution is used as a reagent

  2. If barium chloride is added to a solution that contains sulfate ions a white precipitate forms
• Sulfuric acid cannot be used to acidify the mixture because it contains sulfate ions which would form a precipitate
• Testing for halide ions with silver nitrate

  1. The test solution is made acidic with nitric acid

  2. Silver nitrate solution is added dropwise
• Fluorides


  Produce no precipitate
• Chlorides


  Produce a white precipitate
• Bromides


  Produce a cream precipitate
• Iodides


  Produce a pale yellow precipitate
• Treating silver halide precipitates with ammonia solution


  1. Silver chloride dissolves in dilute ammonia to form a colourless complex ion

  2. Silver bromide dissolves in concentrated ammonia to form a colourless complex ion

  3. Silver iodide does not react with ammonia
• Hydrochloric acid cannot be used to acidify the mixture because it contains chloride ions which would form a precipitate
• The sequence of tests required is carbonate, sulfate then halide
• Testing for positive ions (cations)


  Test for ammonium ion NH4+ by the reaction with warm NaOH(aq), forming NH3 gas
• Ammonia gas


  Can be identified by its pungent smell or by turning red litmus paper blue
• Testing for Negative ions (anions)

  1. Add any dilute acid and observe effervescence

  2. Bubble gas through limewater to test for CO2 – will turn limewater cloudy
• Carbonate


  Fizzing due to CO2 would be observed if present
• Testing for presence of a sulfate

  1. Acidified BaCl2 solution is used as a reagent

  2. If barium chloride is added to a solution that contains sulphate ions a white precipitate forms
• The acid is needed to react with carbonate impurities that are often found in salts which would form a white barium carbonate precipitate and so give a false result
• Sulfuric acid cannot be used to acidify the mixture because it contains sulphate ions which would form a precipitate
• Testing for halide ions with silver nitrate

  1. The test solution is made acidic with nitric acid, and then silver nitrate solution is added dropwise

  2. Fluorides produce no precipitate

  3. Chlorides produce a white precipitate

  4. Bromides produce a cream precipitate

  5. Iodides produce a pale yellow precipitate
• The role of nitric acid is to react with any carbonates present to prevent formation of the precipitate Ag2CO3
• Hydrochloric acid cannot be used to acidify the mixture because it contains chloride ions which would form a precipitate
• The sequence of tests required is carbonate, sulfate then halide. (This will prevent false results of as both BaCO3 and Ag2SO4 are insoluble.)
• Testing for positive ions (cations)


  Test for ammonium ion NH4+ by reacting with warm NaOH(aq) forming NH3 gas
• Ammonia gas


  Can be identified by its pungent smell or by turning red litmus paper blue
• Precipitation Reactions with sodium hydroxide and ammonia

  1. The bases OH- and ammonia when in limited amounts form the same hydroxide precipitates

  2. [Cu(H2O)6]2+ + 2OH- → Cu(H2O)4(OH)2 (s) + 2H2O

  3. [Mn(H2O)6]2+ + 2OH- → Mn(H2O)4(OH)2 (s) + 2H2O

  4. [Fe(H2O)6]3+ + 3OH- → Fe(H2O)3(OH)3 (s) + 3H2O
• Precipitation reactions


  • Cu2+ + 2OH- → Cu(OH)2 (s)
  • Mn2+ + 2OH- → Mn(OH)2 (s)
  • Fe2+ + 2OH- → Fe(OH)2 (s)
  • Fe3+ + 3OH- → Fe(OH)3 (s)
• Excess OH-


  With excess NaOH, the Cr hydroxide dissolves. Cr becomes [Cr(OH)6]3- (aq) - green solution
• Amphoteric


  • The Cr hydroxide can react with alkali to give a solution and react with acid to form the aqueous salt
• Reaction with excess NH3

  1. [Mn(H2O)6]2+ + 2NH3 → Mn(H2O)4(OH)2 (s) + 2NH4+

  2. [Fe(H2O)6]3+ + 3NH3 → Fe(H2O)3(OH)3 (s) + 3NH4+
• Ligand exchange reactions


  • With excess NH3, ligand exchange occurs with Cu and Cr, and their hydroxide precipitates dissolve in excess ammonia
  • Cr becomes [Cr(NH3)6]3+ - purple solution
  • Cu becomes [Cu(NH3)4(H2O)2]2+ - deep blue solution