Core Practical 7: Analysis of Inorganic and Organic Unknowns
Test Tube Reactions
- Simple test tube reactions can be done to identify the following ions:
- Ammonium ions (NH4+)
- Halide ions (X-) - covered in 2.3.4 Halide Ion Reactions
- Carbonate ions (CO32-) and hydrogencarbonate ions (HCO3-)
- Sulfate ions (SO42-)
- If the sample to be tested is a solid, then it must be dissolved in deionised water and made into an aqueous solution
Testing for Ammonium Ions
- About 10 drops of a solution containing ammonium ions, such as ammonium chloride, should be added to a clean test tube
- About 10 drops of sodium hydroxide should be added using a pipette
Overall equation: NH4Cl (aq) + NaOH (aq) → NH3 (g) + H2O (l) + NaCl (aq)
Ionic equation: NH4+ (aq) + OH- (aq) → NH3 (g) + H2O (l)
- The test tube should be swirled carefully to ensure that it is mixed well
- The test tube of the solution should then be placed in a beaker of water, and the beaker of water should be placed above a Bunsen burner, so that it can become a water bath
- As the solution is heated gently, fumes will be produced
- A pair of tongs should be used to hold a damp piece of red litmus paper near the mouth of the test tube, to test the fumes
- The red litmus paper will change colour and become blue in the presence of ammonia gas
Damp red litmus paper turning blue in the presence of ammonia gas
Testing for Carbonate Ions
- 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
Overall equation: 2HCl (aq) + Na2CO3 (aq) → 2NaCl (aq) + CO2 (g) + H2O (l)
Ionic equation: 2H+ (aq) + CO32- (aq) → CO2 (g) + H2O (l)
- 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 which contains a small amount of limewater (calcium hydroxide solution)
- Carbonate ions will react with hydrogen ions from the acid to produce carbon dioxide gas
- Carbon dioxide gas will turn the limewater milky
When carbon dioxide gas is bubbled into limewater it will turn cloudy as calcium carbonate is produced
- A similar reaction is seen with sodium hydrogen carbonate, but the equations are:
Overall equation: HCl (aq) + NaHCO3 (aq) → NaCl (aq) + CO2 (g) + H2O (l)
Ionic equation: H+ (aq) + HCO3- (aq) → CO2 (g) + H2O (l)
Testing for Sulfate Ions
- 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) + SO42- (aq) → BaSO4 (s)
A white precipitate of barium sulfate is a positive result for the presence of sulfate ions
Examiner Tip
HCl is added first to remove any carbonates which may be present and would also produce a precipitate and interfere with the results.
