Module 3.1.4- Qualitative analysis

• Test for anions- carbonates


  add dilute nitric acid

  if there are bubbles, the unknown compound can be a carbonate

  bubble gas through lime water -> white cloudy precipitate

  if CO3 2- ions are present
• Test for anions- sulfate


  add dilute hydrochloric acid

  and then add barium chloride solution

  if a white precipitate forms, sulfate ions are present.

  
• Test for anions- halides


  add aqueous silver nitrate to aqueous solution of an halide

  chloride ions - white precipitate

  bromide ions - cream precipitate

  iodide ions - yellow precipitate
• Further test to distinguish the difference between precipitates in the test for halides

  Using aqueous ammonia

  AgCl is soluble in dilute ammonia

  AgBr is soluble in concentrated ammonia

  AgI is insoluble in both

  AgBr is insoluble in dilute ammonia
• Sequence of tests to carry out for anions

  Carbonate ions, Sulfate ions, Halide ions

  CaSH
• Why must the carbonate ions test be carried out first

  In the carbonate test, you are adding dilute acid and are looking for effervescence

  neither sulfate or halide ions produce bubbles with dilute acid
• Why must the sulfate ions test be carried out second

  If you carry out a sulfate test on a carbonate, you will get a white precipitate too (because of BaCO3) so it is important to do the carbonate ion test first.
• Why must the halide ions test be carried out third

  Silver carbonate and silver sulfate are both insoluble in water and will form precipitates in this test so it is best to rule these out by carrying this test out last
• Testing a mixture of ions (on the same solution)

  Carbonate- add dilute nitric acid until bubbling stops so all carbonate ions have been removed

  Sulfate- add excess Ba(NO3)2 and any sulfate ions will precipitate out as barium sulfate. filter this off

  Halide- add aqueous silver nitrate, and then aqueous ammonia to confirm (normal test)
• What must you ensure when testing a mixture of ions on the same solution (consecutively)

  When testing for carbonate, use only HNO3, and not H2SO4 or HCl since they will show a false positive in the other tests

  When testing for sulfate, don't use barium chloride as this will be present in the halides test
• Tests for cations- NH4+


  aqueous sodium hydroxide is added

  ammonia gas is produced

  mixture is warmed and ammonia gas is released

  you can smell ammonia or use moist pH indicator paper. ammonia is alkaline so it will turn the paper blue