Module 3.1.4 - Qualitative analysis
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What does CaSH stand for?
Carbonate Sulfate Halide
What is the test for carbonates?
Add dilute acid (HNO3) to the unknown solution - positive: effervescence as CO2 gas is formed
What is the ionic equation for positive carbonate test?
CO3 2- (aq) + 2 H+ (aq) -> H2O (l) + CO2 (g)
How do you test for sulfate?
Add dilute HNO3 and then barium nitrate Ba(NO3)2 - positive: white precipitate forms
What is the ionic equation for positive sulfate test?
Ba2+ (aq) + SO4 2- (aq) -> BaSO4 (s)
What is the test for halide ions?
• Add aqueous silver nitrate solution (AgNO3) to sample of aqueous halide ion until precipitate forms • Record colour of precipitate • Test solubility of precipitate with dilute and concentrated ammonia solution
What is the ionic equation for positive halide test?
Where X is halide: Ag+ (aq) + X- (aq) -> AgX (s)
What is the test for ammonium ions?
Add warm dilute aqueous sodium hydroxide to solution - Positive: Ammonia gas produced when solution is warmed, detected with damp red litmus paper turning blue
What is the ionic equation for positive ammonium test?
NH4+ (aq) + OH- (aq) -> NH3 (g) + H2O(l)
Why should carbonates be tested first?
Only carbonate produce CO2(g) with an acid, no bubbling no carbonate
Why should sulfates be tested second?
• Barium carbonate insoluble in water and forms a white precipitate also
Why should halides be tested third?
Silver carbonate and silver sulfate (Ag2CO3 and Ag2SO4) are insoluble and will form precipitates
What should you do to ensure all tests are completed before moving on?
Carbonate: Add acid until no more effervescence Sulfate: Add excess of Ba(NO3)2 and filter solution to remove precipitate Halide: Carry halide test out on filtrate then use ammonia solution to confirm halide identity