Identifying anions
Qualitative analysis is the identification of the ions present in a substance. Common anion tests:
- Carbonate, CO32−: add dilute acid — CO2 fizzes off and turns limewater milky.
- Chloride, Cl−: add dilute HNO3 then AgNO3 — a white precipitate of AgCl forms.
- Sulphate, SO42−: add dilute HCl then BaCl2 — a white precipitate of BaSO4 forms.
Identifying cations with NaOH and NH3
Adding sodium hydroxide or aqueous ammonia to a solution of the salt gives a precipitate whose colour and behaviour identify the cation:
| Cation | With a little NaOH(aq) | In excess NaOH(aq) |
|---|---|---|
| Cu2+ | Blue precipitate | Insoluble |
| Fe2+ | Green precipitate | Insoluble |
| Fe3+ | Brown precipitate | Insoluble |
| Al3+ | White precipitate | Dissolves |
| Zn2+ | White precipitate | Dissolves |
Key idea / 要点
Al3+ and Zn2+ both give a white precipitate that dissolves in excess NaOH. In excess ammonia, only the Zn(OH)2 dissolves, so ammonia distinguishes them.
Worked example
A salt solution gives a white precipitate with a little NaOH that dissolves in excess NaOH, but the precipitate stays undissolved in excess ammonia. Which cation is present?
Answer: Al3+. It gives a white precipitate soluble in excess NaOH but insoluble in excess ammonia (Zn2+ would dissolve in excess ammonia).
A full analysis usually combines several tests. First the effect of heat and the appearance of the solid are noted, then anion tests are carried out on a solution, and finally NaOH and ammonia are used to pin down the cation. Because a few ions behave alike, confirmatory tests are essential — for example, adding acid before AgNO3 or BaCl2 removes carbonate ions that would otherwise give misleading white precipitates.
Remember
- Cl− ⇒ white AgCl with AgNO3; SO42− ⇒ white BaSO4 with BaCl2.
- Cu2+ blue, Fe2+ green, Fe3+ brown precipitate with NaOH.
- Al3+ and Zn2+ dissolve in excess NaOH; ammonia tells them apart.