Chemistry C3 - Identifying Ions

When you have an ionic compound, you have to test and identify two different ions - the cation (the atom that loses electrons, so positively charged) and the anion (the atom that gains electrons, so negatively charged).

There are two types of analysis:

• qualitative - what is present
• quantitative - how much is present

There are several reasons why you would need to identify ions:

• to determine alcohol levels in blood
• to check for toxic substances in food
• for water companies to check the purity of water
• for companies to check the purity of their products (eg copper pipes)

Calcium (Ca 2+)

• add sodium hydroxide solution drop by drop
• white cloudy suspension, insoluble in excess

Copper (Cu 2+)

• add sodium hydroxide solution drop by drop
• blue gelatinous precipitate, insoluble in excess

Iron II (Fe 2+)

• add sodium hydroxide solution drop by drop
• moss-green gelatinous precipitate, insoluble in excess

to be continued...

Iron III (Fe 3+)

• add sodium hydroxide solution drop by drop
• rust-red gelatinous precipitate, insoluble in excess

Aluminium (Al 3+)

• add sodium hydroxide solution drop by drop
• white gelatinous precipitate, dissolves to colourless solution in excess

Ammonium (NH4 4+)

• add sodium hydroxide, warm gently and test gas given off with damp UI paper
• no precipitate, UI paper turns blue, smell of ammonia

Hydroxide (OH-)

• add some ammonium chloride, warm, and test gas given off with damp UI paper
• UI paper turns blue - ammonia given off

Carbonate (CO3 2-)

• add hydrochloric acid and bubble gas given off through limewater
• limewater turns cloudy - carbon dioxide given off

Sulphite (SO3 2-)

• add hydrochloric acid, warm gently and test gas given off with potassium dichomate solution on a piece of filter paper
• potassium dichromate solution turns from orange to green

to be continued...

Chloride (Cl )

• add nitric acid, then silver nitrate drop by drop
• white precipitate

Bromide (Br -)

• add nitric acid, then silver nitrate drop by drop
• cream precipitate

Iodide (I -)

• add nitric acid, then silver nitrate drop by drop
• yellow precipitate - turns grey in the sunlight

Sulphate (SO4 2)

• nitric acid then barium chloride
• white gelatinous precipitate