Test-tube Reactions to Identify Anions
This lesson covers:
- Identifying carbonate ions
- Identifying sulfate ions
- Identifying halide ions
- Eliminating interfering ions
Identifying carbonate ions
To confirm the presence of carbonate (CO32-) ions:
- Add dilute nitric acid (HNO3) dropwise to a sample containing an unknown ion.
- Observe carefully for effervescence caused by bubbles of carbon dioxide (CO2) gas being evolved:
CO32-(aq) + 2H+(aq) ➔ CO2(g) + H2O(l)
Confirm the presence of CO2 by passing bubbles of the gas through limewater (calcium hydroxide solution), in a connecting test tube. If CO2 is present, the limewater will turn cloudy due to the formation of a white calcium carbonate precipitate, confirming the original presence of carbonate ions in the sample:
CO2(g) + Ca(OH)2(aq) ➔ CaCO3(s) + H2O(l)
Identifying sulfate ions
To confirm the presence of sulfate (SO42-) ions:
- Add dilute nitric acid (HNO3) dropwise to a sample containing an unknown ion.
- Add a solution containing aqueous Ba2+ ions, such as barium chloride (BaCl2) or barium nitrate (Ba(NO3)2), dropwise to the sample.
- Observe carefully for the formation of a white precipitate of barium sulfate (BaSO4):
Ba2+(aq) + SO42-(aq) ➔ BaSO4(s)
Identifying halide ions
To distinguish between chloride, bromide, and iodide ions:
1. Add dilute nitric acid (HNO3) dropwise to a sample containing an unknown ion.
2. Add silver nitrate solution (AgNO3) dropwise.
3. Observe the colour of any precipitate formed and compare to the known precipitate colours of silver halides:
| Halide ion | Precipitate | Colour | Ionic equation |
|---|---|---|---|
| Cl- | AgCl | White | Ag+(aq) + Cl-(aq) ➔ AgCl(s) |
| Br- | AgBr | Cream | Ag+(aq) + Br-(aq) ➔ AgBr(s) |
| I- | AgI | Yellow | Ag+(aq) + I-(aq) ➔ AgI(s) |
4. Take separate portions of the silver halide precipitate and add to test tubes containing dilute ammonia solution (NH3) and concentrated ammonia solution (NH3).
5. Observe solubility of precipitate in each ammonia solution. Compare observations to the known solubilities of silver halides in dilute and concentrated NH3 summarised below:
| Halide ion | Precipitate | Soluble in dilute NH3(aq)? | Soluble in concentrated NH3(aq)? |
|---|---|---|---|
| Cl- | AgCl | Yes | Yes |
| Br- | AgBr | No | Yes |
| I- | AgI | No | No |
6. The solubility tests in ammonia solutions can definitively confirm the identity of the halide ion originally present by distinguishing between the silver halides.
Importance of eliminating interfering ions
It is vital to be aware of potential false positives in negative ion tests and take measures to avoid them:
- Carbonate (CO3)2- ions interfere with the sulfate test because carbonates precipitate as white barium carbonate, similar to the expected barium sulfate. Acidifying the sample first with nitric acid eliminates CO32- ions as CO2.
- Sulfate (SO4)2- ions interfere in halide tests since sulfates precipitate as white silver sulfate, resembling positive silver halide precipitates. Adding aqueous Ba2+ ions and filtering out any precipitate eliminates SO42- ions as BaSO4.
- Hydrochloric acid introduces additional chloride (Cl-) ions that can lead to inaccurate halide test results even if chlorides were not originally present. Acidifying the sample with nitric acid eliminates the risk of a false positive result.
To avoid issues with similar reactions from interfering ions, it is important to test for negative ions in the correct order:
- Test for carbonate ions
- Test for sulfates
- Test for halides