Qualitative analysis Flashcards
Describe the test for anions
We would first test for carbonate ions:
Add a dilute strong acid (e.g HNO3) to a sample. In the presence of Carbonate CO2 gas bubbles will form (effervescence). Test for CO2 bubbling through limewater. In the presence of CO2 limewater turns cloudy as CaCO3 (s) precipitate forms.
CaCO3 (aq) + HNO3 (aq) → Ca(NO3)2 (aq) + H2CO3 (aq)
After, we would test for sulfate ions:
we add Ba2+ (e.g BaNO3) to a sample. If SO4^2- are present a white precipitate of BaSO4 (s) will form
Ba^2+ (aq) + SO4^2- (aq) —-> BaSO4 (s)
Finally, we would test for halide ions (X-):
We would add AgNO3 (aq)
AgCl will form a white precipice and is soluble to dilute NH3
AgBr will form a cream precipitate and is soluble to concentrated NH3
AgI will form a yellow precipitate and is insoluble in concentrated NH3
(If we are unable to differentiate by colour we can through its solubility to NH3)
Ag^+ (aq) + X^- (aq) —> AgX (s)
Why do we test for anions in a certain order
In order to prevent a false positive result
e.g: If we were to add barium nitrate to a solution containing a carbonate. Barium carbonate would form creating a white precipitate. As a result this would lead to a false positive for sulfate ions. As barium ions in sulphates also form a white precipitate
Why can we not use certain chemicals for the test for anions:
We cannot use HCl or H2SO4 for the carbonate test as it would leave chloride or sulfate ions
We can’t use barium chloride for the sulfate test as it would leave chloride ions
If these chemicals were used then it would create false positives as the ion could react to give an incorrect result e.g (Barium ions in sulphates precipitates with Carbonate ions too)
What must we add to the halide test to prevent a false negative
The Halide tests are carried out after acidifying with Nitric acid.
This is because solutions can absorb Carbon Dioxide to form Carbonate ions.
CO2(g) + H2O(l) → CO32-(aq) + 2H+(aq)
And the Silver ions we use to test for Halides also precipitate with Carbonate ions
We can’t use HCl to acidify due to the chloride ions
2Ag+(aq) + CO32-(aq) → AgCO3(s)
Describe a test for cations
Test for ammonium ions (NH4^+):
Add NaOH (aq) and gently heat
NH4^+ (aq) + OH^- (aq) —> NH3 (g) + H2O (l)
Test the gas formed using damp litmus paper. If the sample contains NH4^+ (so NH3(g) is formed, the litmus paper will turn blue- ammonia is a base
