Qualitative analysis

• CARBONATE TEST

  Carbonates react with acids to form carbon dioxide gas:

  1. in test tube add dilute nitric acid to the solid or solution to be tested
  2. if you see bubbles it could be a carbonate
  3. to prove it is CO2, bubble the gas through lime water (saturated aqueous calcium hydroxide)
  4. CO2 will react to form a fine white precipitate of calcium carbonate which turns the lime water cloudy
• SULFATE TEST

  • Most sulfates are water soluble but barium sulfate is very insoluble. The formation of a white precipitate of barium sulfate is the basis of the sulfate test:
  • Aqueous barium ions are added to a solution of an unknown compound
  • Usually the Ba2+ ions are added as aqueous barium chloride or barium nitrate
  • If you intend to carry out a halide test after wards, use barium nitrate because introducing chloride ions to the solution will confuse the halide test
• HALIDE TEST

  Aqueous silver ions react with aqueous halide ions to form precipitates of silver halides: $Ag^+$$(aq) +$$X^-(aq) -> AgX(s)$

  1. add aqueous silver nitrate AgNO3 to an aqueous solution of a halide
  2. The silver halide precipitates are different colours - AgCl is white, AgBr is cream, AgI is yellow
  3. Add aqueous ammonia to test the solubility of the precipitate because the precipitate colours are difficult to tell apart - AgCl is soluble in dilute NH3, AgBr is soluble in conc. NH3, AgI is insoluble in conc. NH3
• Carry out the tests for a ions in the correct order to avoid confusing results and incorrect identification:

  1. carbonate CO3 2- test
  2. sulfate SO4 2- test
  3. halides test
• In the carbonate test, you add a dilute acid and are looking for effervescence from CO2 gas. Neither sulfate nor halide ions produce bubbles with dilute acid. The carbonate test can be carried out without the possibility of an incorrect conclusion.
• In the sulfate test, you add a solution containing Ba2+ (aq) ions and are looking for a white precipitate of BaSO4 (s).

  Barium carbonate is white and insoluble in water so if you carry out a sulfate test on a carbonate, you will get a white precipitate too . Therefore it is important to carry out the carbonate test first and only continue to the sulfate test when you know that no carbonate is present
• In the halide test, you add a solution containing Ag+ ions as AgNO3 and are looking for a precipitate.

  Silver carbonate and silver sulfate are both insoluble in water and will form as precipitates in this test so it is important to carry out the halide test last, after carrying out carbonate and sulfate tests to rule out those possibilities
• AMMONIUM ION TEST

  When heated together, aqueous ammonium ions and aqueous hydroxide ions react to form ammonia gas: $NH_4^+$$(aq) +$$OH^-(aq) -> NH_3(g) +$$H_2O(l)$

  1. aqueous sodium hydroxide NaOH is added to a solution of an ammonium ion
  2. ammonia gas is produced . you are unlikely to see gas bubbles as ammonia is very soluble in water
  3. the mixture is warmed and ammonia gas is released
  4. you may be able to smell the ammonia but it is easy to test the gas with moist pH indicator paper. AMMOnia is alkaline and its presence will turn the paper blue