Tests for anions in solution

Anions are negative ions.

The anions you need to be able to identify are:

• carbonate, CO32–

• hydroxide, OH–

• chloride, Cl–

• iodide, I–

• sulfate, SO42–

• nitrate, NO3–

Carbonate

Most metal carbonates, including all coloured carbonates, are insoluble.

If your unknown solution is coloured, it will not contain carbonate ions.

Pour a little of the unknown solution into a clean test tube.

Add a little dilute acid. If the solution contains carbonate ions you will see bubbles of a colourless, odourless gas.

Confirm the evolution of a gas by adding a larger volume of acid.

2H+(aq) + CO32–(aq) → CO2(g) + H2O(l)

The identity of the gas can be confirmed with limewater if required.

First, pour a little limewater into a test tube, then put a little fresh test solution into a second test tube.

Squeeze as much air as possible out of a dropper and keep it squeezed.

Position the tip of the dropper a few cm above the surface of the liquid.

Add the acid, and release the bulb of the dropper to suck some of the gas into the dropper.

Put the dropper tip into the limewater and squeeze the bulb to expel the gas.

Suck limewater into the dropper and expel it out again.

The white precipitate confirms the gas as carbon dioxide.

Hydroxide

Most metal hydroxides, including all coloured hydroxides, are insoluble.

If your unknown solution is coloured, it will not contain hydroxide ions.

Put a little of the test solution into a clean test tube.

Add a little dilute acid. If it fizzes, it is a carbonate, not a hydroxide.

Add a few drops of silver nitrate solution. If hydroxide is present, a mud-brown precipitate of silver oxide forms. (Silver hydroxide which forms first decomposes to form the oxide.)

2Ag+(aq) + 2OH–(aq) → Ag2O(s) + H2O(l)

Chloride

Pour a small amount of the solution to be tested into a clean test tube.

Add a little nitric acid.

If fizzing occurs, the unknown is NOT a chloride.

(This step is not necessary if you have already eliminated the presence of carbonate.)

Add a few drops of silver nitrate solution, AgNO3.

If chloride is present a white precipitate will form.

Ag+(aq) + Cl–(aq) → AgCl(s)

To confirm that the white precipitate is silver chloride, rather than silver bromide or silver iodide, add 1 mL of ammonia solution (which contains NH3(aq)).

Silver chloride redissolves in ammonia solution to form a colourless solution.

AgCl(s) + 2NH3(aq) → [Ag(NH3)2]+(aq) + Cl–(aq)

Iodide

Pour a small amount of the solution to be tested into a clean test tube.

Add a little nitric acid.

If fizzing occurs the unknown is NOT an iodide.

This step is not necessary if you have already eliminated the presence of carbonate ions.

Add a few drops of silver nitrate solution, AgNO3.

If iodide is present a pale yellow precipitate will form.

Ag+(aq) + I–(aq) → AgI(s)

Silver chloride forms a white precipitate.

Silver iodide forms a pale yellow precipitate.

To confirm that the pale yellow precipitate is silver iodide, rather than silver chloride or silver bromide, add 5 mL of ammonia solution (which contains NH3(aq)).

Silver iodide will not redissolve in ammonia solution.

Alternative test for iodide

Pour a little of the test solution into a clean test tube.

Add one or two drops of lead nitrate solution.

If iodide is present a bright yellow precipitate will form.

Pb2+(aq) + 2I–(aq) → PbI2(s)

Note: lead chromate is also bright yellow, however solutions containing chromate ions will be yellow, not colourless.

You should only use this test on colourless solutions.

Sulfate

Pour a small amount of the test solution into a clean test tube.

Add 1 mL of either dilute nitric acid or dilute hydrochloric acid.

If fizzing occurs the unknown is NOT a sulfate.

Add a few drops of barium chloride or barium nitrate solution. If sulfate ions are present a white precipitate will form.

Ba2+(aq) + SO42–(aq) → BaSO4(s)

Without acid, this test will also detect the presence of SO3

2– ions, though they are outside the range of this course.

Some metal sulfates form coloured solutions.

When barium chloride is added the solution turns cloudy.

The formation of a precipitate with Ba2+(aq) and acid is sufficient to confirm sulfate.

NitrateFor this course, you can assume that a solution that does not fizz with dilute acid, form a precipitate with silver nitrate solution, or form a precipitate with barium chloride solution will contain the nitrate ion, NO3

–.