RP4 - Identifying Anions & Cations

• Inorganic ions: Charged species formed by elements or compounds in aqueous solution (e.g. , , , ).

• Analytical tests allow identification of specific ions via characteristic reactions and their accompanying observations (precipitation, gas evolution, etc.).

• In solution, soluble ionic compounds dissociate into positive ions (cations) and negative ions (anions). One or both of these can be identified through a series of tests.

Step 1: Add dilute nitric acid ()

Step 2: Add silver nitrate solution ()

A silver halide () precipitate forms, and its colour will allow identification of the halide ion that was present in solution.

Ionic Equation:

These precipitates can look very similar, so ammonia solution is added to distinguish between them. The silver chloride precipitate () is soluble in dilute ammonia, and the silver bromide precipitate () is soluble in concentrated ammonia.

Step 3: Add dilute ammonia solution. If the precipitate dissolves, it was .

Step 4: Add concentrated ammonia solution. If the precipitate dissolves, it was . If the precipitate persists it was

Ionic Equation:

Test for Carbonate Ions ()

Reagents: Add dilute or dilute to the solution sample

Observation: Effervescence (bubbling) due to gas.

Ionic equation:

Confirmatory Test: Bubble gas through limewater turns milky (white precipitate of )

Equation:

Test for Sulfate Ions ()

Step 1: Add dilute to the solution sample

Step 2: Add barium chloride solution ()

Observation: White precipitate of barium sulfate () forms

Ionic equation:

The tests for halide and sulfate ions require the addition of a dilute acid before the test reagent was added.

The acid removes any carbonate or hydroxide ions which may give a false positive with the test reagent.

, , and are all white solids. These would form as white precipitates in solution if the interfering and/or ions were not removed first by adding acid

The acid used should have the same anion as the test reagent so as not to add interfering ions e.g. silver nitrate used for halide test, therefore nitric acid used to acidify the solution.

Step 1: Add sodium hydroxide () and warm gently

Step 2: Hold damp red litmus paper over the mouth of the test-tube

Observation: Gas is produced that turns damp red litmus paper blue

Ionic equation:

Test for hydroxide ions ()

Hydroxide ions can be tested for using universal indicator paper or damp red litmus paper both of which would turn blue if present.

Group 2 ions ()

Group 2 ions can be identified through knowledge of the solubility of their sulfate and hydroxide compounds.