Qualitative analysis Flashcards
What do qualitative tests rely upon?
Qualitative tests rely upon simple observations rather than measurements and can be carried out on a test tube scale very quickly.
When testing unknown in organic compounds, what do we carry out the test for first?
Anions
In what order are the tests for anions done?
Carbonate test, sulphate test and then halide tests
Why do we do the carbonate test first?
So it can be eliminated so carbonate compounds won’t give false positives, such as white precipitate
what do carbohydrates react with acid to form?
as salt, a gas and a harmless liquid
What do we use to identify carbonates?
this reaction of dilute nitric acid with an unknown solid or solution to identify carbonates.
if we see bubbles, what could our compound be? (carbonate test)
a carbonate
how can we prove the gas formed as carbon dioxide?
-Bubbled through limewater, it turns cloudy
why is the carbonate test always undertaken first with an unknown compound?
as there is no risk of an incorrect conclusion being made
What is sulphate test based on?
Precipitation formation
What time is sulphate soluble in?
water
What can be used to identify sulphate in unknown solutions?
Barium sulphate as it is very insoluble
What is barium sulphate form when it forms?
white precipitate
What do we normally use as a source of ba2+ ions in the sulfate test?
barium chloride or barium nitrate
What is barium carbonate?
An insoluble white precipitate in water
what can carbonate salts also be formed by?
Transitional metal ions
If you want to do a test for sulphate and a test for halides one why do you need to use barium nitrate?
-barium chloride would also supply a source of chloride ions which will react with the silver nitrate forming a white precipitate and giving a false positive for chloride ions
what is the test for halide ions based on?
Precipitate formation and solubility
How can the halide ion be determined in aqueous solution?
solution silver nitrate followed by the addition of dilute ammonia
why is the halide test left until last in the sequence?
as both silver carbonate and silver sulphate are insoluble white precipitate and give false positives for chloride ions
what is the test for cations? (ammonium ions)
When ammonium (NH4+) containing salts are heated with aqueous hydroxide (OH-) the reaction produces ammonia gas (NH3).
what is the ammonia formed in the test of the ammonium ions?
alkali and a gas
how can we test ammonia in the test for ammonium ions (NH4+)?
1.Aqueous sodium hydroxide is added to the solution we want to test for ammonium ions.
2. To allow the ammonia to be released as a gas we have to heat the mixture (in a water bath or with a Bunsen burner).
3. The mixture does not present gas with bubbles as ammonia is much more soluble in water than carbon dioxide.
4. If ammonium ions are present the gas that is released is alkali and we test for this using moist pH indicator (red) paper (turns blue).
