qualitative analysis Flashcards
Test for carbonates:
Add dilute nitric acid and bubble the gas through lime water, fizzing happens, positive test turns the lime water cloudy
Na2CO3 + 2HNO3 -> 2NaNO3 + H2O + CO2
CO2 + Ca(OH)2 -> CaCO3 + H2O
Test for sulphates:
add Barium nitrate If halide involved, If not add nitric acid (to remove impurities) then barium chloride, Positive test forms a white precipitate of barium sulphate
Ba+2 + SO4-2 ->BaSO4
Test for halide ions:
add Dilute nitric acid (to remove impurities) then silver nitrate, Positive result for chloride forms white precipitate which is soluble in dilute ammonia,
bromide - forms a cream precipitate which is soluble in concentrated ammonia
iodide - forms a cream yellow precipitate which are not soluble in ammonia
Ag+(aq) + Cl-(aq) -> AgCl(s)
Test for ammonium ions:
heat with sodium hydroxide, positive result turns Damp red litmus paper blue as it’s an alkali
HN4+ + OH- -> NH3 + H2O
To test for an unknown solution what must the order be and why
carbonate sulphate then halide test
- because Barium ions used in the sulphate test could form BaCO3 or BaSO4 precipitate So we should do the carbonate test first to eliminate CO2 to confirm the precipitate is BaSO4
- And silver ions could form Ag2CO3 or Ag2SO4 or AgCl So it is important to rule out the first 2
Why do you add nitric acid Instead of hydrochloric acid Or sulfuric acid to test for a mixture
The SO4 ions in sulfuric acid would give a false positive for the sulphate test
The Cl- Ions in HCl would give a false positive For the halide test
How to test for a mixture of ions
- Do the carbonate test, if you see bubbles that means carbonate is present so continue adding dilute nitric acid until bubbles stop to get rid of all carbonate ions
- Add excess Barium nitrate To the leftover mixture, barium sulphate precipitate will form If sulphate is present and can be filtered out
- To the leftover solution add silver nitrate any precipitate formed must be a halide ion add NH3 to confirm which halide
