Qualitative Analysis (OCR A Level Chemistry A) : Revision Note

Halide Ions

• Halide ions can be identified in an unknown solution by dissolving the solution in nitric acid and then adding silver nitrate solution dropwise

• The nitric acid is to prevent any false positive results from carbonate ions precipitating out with silver ions

• The halide ions will react with the silver nitrate solution as follows:

Ag⁺ (aq) + X^- (aq) → AgX (s)

(ionic equation)

• Where X^- is the halide ion

• The state symbols are key in this equation

• If the unknown solution contains halide ions, a precipitate of the silver halide will be formed (AgX)

  

A silver halide precipitate is formed upon addition of silver nitrate solution to halide ion solution

• Silver chloride (AgCl) is a white precipitate

• Silver bromide (AgBr) is a cream precipitate

• Silver iodide (AgI) is a yellow precipitate

  

The silver halide precipitates are dense and characteristically coloured

Adding ammonia

• Because the white, cream and yellow precipitates could look very similar in colour, ammonia is often used as a follow up test to determine which halide ion is present

• Dilute followed by concentrated ammonia is added to the silver halide solution to identify the halide ion

• If the precipitate dissolves in dilute ammonia the unknown halide is chloride

• If the precipitate does not dissolve in dilute, but does dissolve in concentrated ammonia the unknown halide is bromide

• If the precipitate does not dissolve in dilute or concentrated ammonia, then the unknown halide is iodide

  

Silver chloride and silver bromide precipitates dissolve on addition of ammonia solution whereas silver iodide is insoluble in ammonia

Reaction of Halide Ions with Silver Nitrate & Ammonia Solutions

Qualitative Analysis of Ions

Test for Carbonates

• A small amount (around 1 cm³) of dilute hydrochloric acid should be added to a test tube using a pipette

• An equal amount of sodium carbonate solution should then be added to the test tube using a clean pipette

• As soon as the sodium carbonate solution is added, a bung with a delivery tube should be attached to the test tube

  • The delivery tube should transfer the gas which is formed into a different test tube that contains a small amount of limewater (calcium hydroxide solution)

• If a carbonate compound is present then effervescence should be seen and the gas produced is CO₂ which forms a white precipitate of calcium carbonate when bubbled through limewater:

CO₃^2- (aq) + 2H⁺ (aq) → CO₂ (g) + H₂O (l)

CO₂ (g) + Ca(OH)₂ (aq) → CaCO₃ (s) + H₂O (l)

Limewater turns milky in the presence of CO₂ caused by formation of insoluble calcium carbonate

Test for Sulfates

• Acidify the sample with dilute hydrochloric acid and then add a few drops of aqueous barium chloride

• If a sulfate is present then a white precipitate of barium sulfate is formed:

Ba²⁺ (aq) + SO₄^2- (aq) → BaSO₄ (s)

• The test can also be carried out with barium nitrate solution

  

A white precipitate of barium sulfate is a positive result for the presence of sulfate ions

Testing for ammonium ions

• We can test for the presence of ammonium ions, NH₄⁺, by reacting with warm aqueous sodium hydroxide forming ammonia gas

NH₄⁺ (aq) + OH^- (aq) → NH₃ (g) + H₂O (l)

• Ammonia gas is produced. You are unlikely to see gas bubbles as ammonia is very soluble in water

• Ammonia gas can be identified by its pungent smell or by turning red litmus paper blue