3.1.4. Qualitative analysis Flashcards
What are anions called?
Negative ions
Test for carbonate ions, CO32-
Add strong acid to sample
Collect the gas produced
Pass the gas through limewater
What are the observations for a positive test of carbonate ions?
Fizzing
Limewater turns cloudy
Equation for carbonate ion test
CO32- (aq) + 2H+ (aq) -> CO2 (g) + H2O (aq)
Test for sulfate ions, SO42-
Add dilute hydrochloric acid and barium chloride to the sample
What are the observations for a positive test of sulfate ions?
White precipitate of barium sulphate is produced (BaSO4)
Equation for sulfate ion test
SO42- (aq) + Ba2+ (aq) -> BaSO4 (s)
What is used to test for halide ions?
Dissolve sample in water
Add aqueous silver nitrate
Record colour of precipitate formed
Add dilute, then concentrated aqueous ammonia
Record solubility of precipitate
Why is HNO3 added to test for halide ions and not HCl
To remove carbonate ions
Adding HCl would give a false positive result for chloride ions
Colour and solubility of chloride ions
White precipitate, dissolves in dilute ammonia
Colour and solubility of bromide ions
Cream precipitate, dissolves in concentrated ammonia
Colour and solubility of iodide ions
Yellow precipitate, insoluble in ammonia
Which order should tests for carbonate, sulfate and halide ions be carried out and why?
- Carbonate test
- Sulfate test
- Halide test
Barium ions in sulfate test form insoluble precipitate BaCO3 - which would give a false negative in the carbonate fest
Silver ions in halide test form insoluble precipitate Ag2SO4 - which would give a false positive in the sulfate test.
Test for ammonium ions NH4+
Heat sample with NaOH.
What is the observation for a positive test for ammonium ions
Using a PH indicator - colour change to blue/purple
Equation for ammonium ion test
NH4+ + OH- -> NH3 + H2O
