L3 - Anion Qualitative Tests

objectives

• conduct qualitative investigations – using flame tests, precipitation and complexation reactions as appropriate – to test for the presence in aqueous solution of the following ions:
• anions: chloride (Cl– ), bromide (Br– ), iodide (I – ), hydroxide (OH– ), acetate (CH3COO– ), carbonate (CO3 2– ), sulfate (SO4 2– ), phosphate (PO4 3– )

Testing for Carbonates

• We should remember from Module 7 that Carbonates react with Acids to form a Salt, Water and bubbles of Carbon Dioxide gas.

Carbonate + Acid --> Salt + H₂O + CO₂

We can bubble this gas through Limewater to make the limewater go cloudy

Testing for Hydroxide Ions

The common test for hydroxide ions is that when we dip red litmus paper into a solution of hydroxide ions it turns the paper BLUE as it is a Base.

We can also Add Ammonium ions and heat them. This then produces Ammonia Gas which also turns red litmus paper blue.

Testing with Barium Nitrate

We can use Barium Nitrate to test for the presence of both Phosphate and Sulphate ions.

In both cases - the Barium Phosphate and Barium Sulphate form White Precipitates.

To distinguish between them we place them in acidic solutions.

In Low pH solutions Barium Phosphate does not form as it only forms in alkaline solutions

Barium Sulphate is insoluble in all pH and thus is always a white precipitate at all pH

Testing with Silver Nitrate

Halide Ions

• On addition of a few drops of Silver Nitrate in Nitric Acid (Acidified Silver Nitrate) the Halide ions form Different Coloured Precipitates
• The ionic equations can be shown as:

Ag⁺_(aq) + Cl^-_(aq) → AgCl_(s)

Ag⁺_(aq) + Br^-_(aq) → AgBr_(s)

Ag⁺_(aq) + I^-_(aq) → AgI_(s)

• Silver Chloride is a White Precipitate
• Silver Bromide is a Cream Precipitate
• Silver Iodide is a Yellow Precipitate

From the picture below you should be able to discern the difference between the 3 PPT's however it is difficult to distinguish between the Cl- and Br-

To discern the difference we then use Ammonia - a few drops of dilute Ammonia result in the complexation of silver and this causes the Precipitates to redissolve. However - only the chloride completely redissolves, the bromide partially dissolves ad the iodide never dissolves. See below:

Ethanoate ions

When Concentrated Silver Nitrate is added to solutions containing Ethanoate ions (CH₃COO^-) a White Precipitate will form.

A note on Silver Nitrate - As Silver forms many Precipitates it is always used last when testing for ions.