Qualitative Analysis (OCR AS Chemistry)

• 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)

• Silver chloride (AgCl) is a white precipitate
• Silver bromide (AgBr) is a cream precipitate
• Silver iodide (AgI) is a yellow precipitate

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

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:

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

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