Testing for negatively charged ions

The formation of different precipitates allows us to identify different negatively charged ions in solution.

Testing for carbonate ions

Carbonate ions, CO32- are detected using a dilute acid. Bubbles are given off when an acid, usually dilute hydrochloric acid, is added to the test compound.

The bubbles are caused by carbon dioxide. Limewater is used to confirm that the gas is carbon dioxide. It turns milky when carbon dioxide is bubbled through it.

Unlike the other tests described on this page, the test for carbonate ions works whatever acid is added.

Testing for sulfate ions

Barium ions react with sulfate ions, SO42- to form insoluble white barium sulfate:

Ba2+(aq) + SO42-(aq) → BaSO4(s)

To test for sulfate ions:

  1. add a few drops of dilute hydrochloric to the sample
  2. add a few drops of dilute barium chloride solution

A white precipitate forms if sulfate ions are present.

A white precipitate of barium sulfate in a test for sulfate ions

Example

Explain why, in a test for sulfate ions, the sample is acidified with hydrochloric acid first.

Carbonate ions also produce a white precipitate with barium chloride solution. The acid reacts with any carbonate ions present. This removes them, so stopping them giving a false positive result.

Question

Explain why, in a test for sulfate ions, dilute sulfuric acid cannot be used to acidify the sample.

Sulfuric acid, H2SO4(aq), contains sulfate ions. These would react with barium ions in the test, giving an incorrect positive result.

Testing for halide ions

Silver ions react with halide ions (Cl-, Br- or I- ions) to form insoluble precipitates. The table shows the colours of these silver halide precipitates.

Halide ionPrecipitate colour
Chloride, Cl-White
Bromide, Br-Cream
Iodide, I-Yellow

To test for halide ions:

  • add a few drops of dilute nitric acid to the sample
  • add a few drops of dilute silver nitrate solution

Observe and record the colour of any precipitate that forms.

Example

Explain why, in a test for halide ions, the sample is acidified with dilute nitric acid first.

Carbonate ions also produce a white precipitate with silver nitrate solution. The acid reacts with any carbonate ions present. This removes them, so stopping them giving an incorrect positive result for chloride ions.

Question

Explain why, in a test for halide ions, dilute hydrochloric acid cannot be used to acidify the sample.

Hydrochloric acid, HCl(aq), contains chloride ions. These would react with silver ions in the test, giving a false positive result.

curriculum-key-fact
It may help to remember the silver halide precipitate colours by the mnemonic 'milk, cream, butter' for 'white, cream, yellow'.