A technician in a hospital laboratory analyses a sample of a patient's urine. The technician suspects the sample contains ammonium ions, iron(II) ions and chloride ions. Describe the tests the technician should carry out to confirm the presence of each of these three ions, and state the expected result for each test.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Qualitative analysis of ions can be used to identify the cations and anions present in an unknown sample. Different reagents are used to produce characteristic observations that confirm the identity of specific ions.
Model answer (5 marks)
1. Add a few drops of sodium hydroxide to the urine and warm gently. Ammonium ions give off ammonia gas, which turns damp red litmus paper blue – confirming NH4+.
2. Add sodium hydroxide to the urine. Fe2+ gives a green precipitate of Fe(OH)2 – confirming Fe2+.
3. Acidify the urine with dilute nitric acid and add silver nitrate. A white precipitate of AgCl is formed – confirming Cl–.
2. Add sodium hydroxide to the urine. Fe2+ gives a green precipitate of Fe(OH)2 – confirming Fe2+.
3. Acidify the urine with dilute nitric acid and add silver nitrate. A white precipitate of AgCl is formed – confirming Cl–.
Examiner tips
- Use the exact wording of the reactions (e.g. ‘ammonia gas turns damp red litmus blue’). Include the reagent and the colour/precipitate expected. Keep each point short and to the point. Show the sequence of steps for each ion. Mention the specific ion in the answer. Use UK spelling (e.g. ‘litmus’).
Common mistakes
- Mixing up the order of reagents (e.g. adding AgNO3 before acidifying). Confusing the colour of the litmus change or the colour of the Fe(OH)2 precipitate. Forgetting to specify that the AgCl precipitate is white.
Mark scheme (5 marks)
- Add sodium hydroxide solution to the sample and warm gently — ammonia gas is produced, which turns damp red litmus paper blue, confirming ammonium ions
- Add sodium hydroxide solution to the sample — a green precipitate forms, confirming iron(II) ions
- Acidify the sample with dilute nitric acid and add silver nitrate solution — a white precipitate forms, confirming chloride ions
Key terms in this question
ammonium ions · iron(II) ions · chloride ions
Related
- All Edexcel A-Level Chemistry (9CH0) revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
More Qualitative analysis of ions questions
- A geologist discovers a mineral sample and suspects it contains iron(III) ions, …
- A water quality technician tests a sample of water taken from a borehole. The te…
- A forensic scientist analyses a white powder found at a crime scene. The scienti…
- A student carries out a series of tests to identify the ions present in an unkno…