9.3C - Sodium hydroxide tests for cations
Use sodium hydroxide solution to distinguish the six cations in this topic. You will interpret precipitate colours, test a white precipitate with excess reagent, and identify ammonium ions by the ammonia released on warming.
How sodium hydroxide tests work
A cation is a positive ion. In this lesson the required cations are:
| Required cation | Formula |
|---|---|
| aluminium ion | |
| calcium ion | |
| copper ion | |
| iron(II) ion | |
| iron(III) ion | |
| ammonium ion |
Sodium hydroxide solution contains hydroxide ions, . For most of the metal cations in this lesson, the hydroxide ions form an insoluble metal hydroxide. The solid that appears in the liquid is called a precipitate.
Precipitate
A precipitate is an insoluble solid formed in a solution during a chemical reaction.
For metal cations, the usual method is:
1. Put a small volume of the sample solution in a test tube.
2. Add sodium hydroxide solution drop by drop.
3. Observe the precipitate colour.
4. If a white precipitate forms, add excess sodium hydroxide solution and observe whether it dissolves.
If the starting sample is a soluble solid, dissolve a small amount in distilled or deionised water first. Use a separate portion of solution for each test if more than one test is being carried out.
Sodium hydroxide solution is alkaline and can damage eyes or skin. Use small volumes, avoid splashes, and warm mixtures gently only when the ammonium test requires it.
Coloured precipitates
For copper and iron ions, the precipitate colour is the key observation.
| Ion in the sample solution | Observation when sodium hydroxide solution is added | Identification |
|---|---|---|
| copper ion, | blue precipitate forms | copper ion present |
| iron(II) ion, | green precipitate forms | iron(II) ion present |
| iron(III) ion, | brown / red-brown precipitate forms | iron(III) ion present |
Record whether a solid forms, as well as its colour. A coloured solution alone is not evidence of a precipitate.
Iron(II) and iron(III) are easy to confuse because they are both iron ions. The Roman numeral is part of the ion's identity:
| Ion | Charge | Observation with sodium hydroxide solution |
|---|---|---|
| iron(II), | 2+ | green precipitate |
| iron(III), | 3+ | brown / red-brown precipitate |
Fresh iron(II) hydroxide is green, but it can darken or become brownish if it is oxidised in air. In an exam answer, identify a fresh green precipitate as iron(II) and a brown or red-brown precipitate as iron(III).
Distinguishing white precipitates
Among the cations in this lesson, aluminium ions and calcium ions both give a white precipitate when sodium hydroxide solution is first added. That first result is not unique, so the test must continue.
Continue this lesson.
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