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Testing for anions

This page introduces how to test for common anions such as the carbonate ion (CO₃^2-), halide ions (Cl^-, Br^-, I^-) and sulfate ions (SO₄^2-).

The carbonate ion (CO₃^2-) will react with dilute acids to release carbon dioxide gas. Limewater is used to test for carbon dioxide; it turns milky or chalky when CO₂ gas is bubbled through it. The set-up to test for the presence of the carbonate ion is shown opposite. To test for the presence of carbonate ions approximately 10ml of dilute hydrochloric acid is added to a test tube containing a few granules of the suspected metal carbonate. If carbonate ions are present then CO₂ gas will be released. The carbon dioxide gas will then bubble through the limewater and turn it a milky/chalky colour e.g. if the carbonate is calcium carbonate then the following reaction will occur:

calcium carbonate_(s) + hydrochloric acid_(aq) → calcium chloride_(aq) + carbon dioxide_(g) + water_(l)

CaCO_3(s) + 2HCl_(g) → CaCl_2(aq) + CO_2(g) + H₂O_(l)

If a solution contains sulfate ions and a few drops of barium chloride solution are added then an insoluble white precipitate of barium sulfate will form. However if the solution also contains carbonate ions then a white precipitate of barium carbonate will also form. So it might seem that this is not really a very good test for sulfate ions as it also gives a positive test for carbonate ions, but if a few drops of hydrochloric acid are added then any carbonate ions present will react with the acid to release carbon dioxide gas, as in the above test for carbonate ions. However if no fizzing is seen then only the sulfate ion is present. An outline of the test is shown below:

Testing for halide ions: chloride, bromide and iodide ions.

Halide ions are chloride, bromide and iodide ions. Most compounds containing the chloride, bromide and iodide ions that you will meet are likely to be soluble. However silver chloride, bromide and iodide are insoluble and have different colours and this fact is the basis of the test to detect halide ions. Silver chloride is white, silver bromide is cream and silver iodide is yellow. To test for the presence of chloride (Cl^-), bromide (Br^-) and iodide (I^-) ions in solution simply add a few drops of nitric acid (the acid is needed in case carbonate ions are present) and then add a few drops of the silver nitrate solution. If the halides are present then a solid insoluble precipitate will be produced. This is summarised in the diagram below.

The equations below may help you to understand these reactions in a little more detail. The key points are the state symbols; recall that (aq) means it's a solution while (s) here indicates that it is an insoluble solid.

sodium chloride_(aq) + silver nitrate_(aq) → silver chloride_(s) + sodium nitrate dioxide_(aq)

NaCl_(aq) + AgNO_3(aq) → AgCl_(s) + NaNO_3(aq)

sodium bromide_(aq) + silver nitrate_(aq) → silver bromide_(s) + sodium nitrate dioxide_(aq)

NaCl_(aq) + AgNO_3(aq) → AgBr_(s) + NaNO_3(aq)

sodium iodide_(aq) + silver nitrate_(aq) → silver iodide_(s) + sodium nitrate dioxide_(aq)

NaI_(aq) + AgNO_3(aq) → AgI_(s) + NaNO_3(aq)

Key Points

• Carbonate ions react with dilute acid to release carbon dioxide gas. Carbon dioxide gas can be detected by bubbling it through limewater. Limewater turns milky or chalky when CO ₂ is bubbled through it.
• Sulfate ions form an insoluble colourless (white) precipitate of barium sulfate when a few drops of hydrochloric acid followed by barium chloride solution are added to it. To test an unknown solution for the presence of sulfate ions firstly acidify the unknown solution by adding hydrochloric acid, then add barium chloride solution. If an insoluble white precipitate forms then this will confirm the presence of sulfate ions. There should be no fizzing. If the solution fizzes then carbonate ions are also present. You should never use sulfuric acid to acidify the unknown solution as this contains sulfate ions.
• The silver nitrate test is used to identify chloride, bromide and iodide ions. The unknown sample can be dissolved in nitric acid and then a few drops of silver nitrate are added. A white precipitate will show the presence of chloride ions, a cream precipitate bromide ions and a yellow precipitate will confirm the presence of iodide ions. It is worth noting that silver chloride, silver bromide and silver iodide are all light sensitive chemical and that if left exposed to light then a black precipitate of silver ions will form. So any colour changes after the addition of silver nitrate solution to your test solution should be observed quickly and the solutions disposed of quickly.

Check your understanding - Questions on tests for anions

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