Qualitative Analysis and Identification of Ions

What are you trying to identify?

An unknown salt can contain a positive ion, or cation, and a negative ion, or anion. Qualitative analysis identifies them from observations: a flame colour, a gas, or a solid forming when a reagent is added. It does not tell you the concentration. In this practical, you build a case for the identity of each ion and then use both ions to name the compound.

OCR gives identifying an unknown compound using cation tests, anion tests and flame tests as an example activity for its Identification of species Practical Activity Group. The activity can be organised in different ways; what matters here is understanding how the tests are carried out and what their observations show.

Keep each test separate

Place small, similar amounts of the unknown solution in separate clean test tubes. Add one test reagent to each portion, first in small amounts where appropriate, and record what you actually see. Use a fresh portion for the next test. Otherwise, a reagent from an earlier test might react in the next one and give a misleading result. Known solutions tested alongside an unknown can help you compare faint colours or precipitates.

A precipitate is an insoluble solid formed in a reaction between solutions. Record its colour and whether it remains or dissolves when more reagent is added. If a gas is produced, identify it with a gas test rather than relying on bubbles alone. Wear eye protection and follow the safety instructions for the reagents and heating equipment being used.

Tests for positive ions

For a metal-ion test, add sodium hydroxide solution drop by drop to a fresh portion of the unknown. Observe any precipitate, then add more sodium hydroxide and check whether it dissolves in excess. Repeat with ammonia solution on another fresh portion. The pattern across the two reagents is more informative than a single observation. For example, a precipitate associated with copper(II) ions dissolves in excess ammonia solution, whereas one associated with aluminium ions dissolves in excess sodium hydroxide solution. If an observation is faint or uncertain, compare it directly with a known sample rather than guessing.

The ammonium ion, $\mathrm{NH_4^+}$, needs a different test. Add sodium hydroxide solution to a fresh portion and warm it gently, for example in a hot-water bath. If ammonia gas is released, damp red litmus paper held at the mouth of the tube turns blue. Keep the paper out of the liquid: the result should be a test of the gas.

A flame test offers another way to investigate some metal ions. One school method uses a wooden splint prepared by soaking it in water and then in a salt solution; the splint is held in a blue Bunsen flame and the flame colour is observed. Common reference colours include lithium crimson, sodium yellow, potassium lilac, calcium orange-red and copper blue-green. Test known salts for comparison. A strong colour from contamination or a mixture can mask another colour, so use the flame result alongside other evidence rather than treating it as a complete identification.

Tests for negative ions

Different anions need different fresh portions of the unknown:

The acid chosen matters. Do not acidify the halide test with hydrochloric acid: it would add chloride ions of its own before the silver nitrate was added.

Turn observations into an identification

Suppose separate portions of an unknown give a white precipitate with acidified barium chloride, and the cation tests give a precipitate that dissolves in excess ammonia solution. The first result supports sulfate; the second supports copper(II). Together they identify the compound as copper(II) sulfate, $\mathrm{CuSO_4}$. In a practical record, keep the two observations beside the proposed ions so that someone else can follow your reasoning. A negative result is worth recording too: it helps rule out alternatives.