Dilute sodium hydroxide solution is used in tests for some metal ions and is a test to identify ammonium ions in compounds.
Testing for metals
Dilute sodium hydroxide solution reacts with some metal ions in solution, forming metal hydroxides. Some of these metal hydroxides are insoluble, so they appear as precipitates. For example, copper sulfate solution reacts with a few drops of sodium hydroxide solution:
Copper sulfate + sodium hydroxide → sodium sulfate + copper hydroxide
CuSO4(aq) + 2NaOH(aq) → Na2SO4(aq) + Cu(OH)2(s)
Copper hydroxide forms a blue precipitate.
The table shows the coloured precipitates formed by five common metal ions in this test.
| Metal ion | Precipitate colour |
| Aluminium, Al3+ | White |
| Calcium, Ca2+ | White |
| Copper, Cu2+ | Blue |
| Iron(II), Fe2+ | Green |
| Iron(III), Fe3+ | Brown |
Distinguishing between aluminium ions and calcium ions
A few drops of dilute sodium hydroxide solution react to form a white precipitate with aluminium ions and with calcium ions. However, if excess sodium hydroxide solution is added:
- the aluminium hydroxide precipitate reacts to form a colourless solution
- the calcium hydroxide precipitate is unchanged
Question
A green precipitate forms when dilute sodium hydroxide solution is added to a solution containing a metal ion. Identify the metal ion present.
answer
Iron(II) ions, Fe2+, are present.
Testing for ammonium ions
Ammonium ions, NH4+, react with hydroxide ions to form ammonia and water:
NH4+(aq) + OH–(aq) → NH3(g) + H2O(l)
This gives a test for ammonium ions:
- add dilute sodium hydroxide solution to the sample
- warm the mixture
Ammonia gas is given off if ammonium ions are present.
Testing for ammonia
Ammonia turns damp red litmus paper blue.
Question
Explain what the test for ammonia tells you about the properties of ammonia.
answer
Key fact
Ions of group 1 metals (Li+, Na+ and K+) form soluble hydroxides. Therefore, they are identified using flame tests and not by adding sodium hydroxide solution.
