Testing a Fertiliser Sample in the School Laboratory
A Form 5 student is given a white crystalline sample labelled only 'fertiliser salt, possibly ammonium sulphate or ammonium chloride'. She has dilute sodium hydroxide, dilute nitric acid, silver nitrate solution, barium chloride solution, a Bunsen burner, test tubes and red litmus paper. The school has no fume cupboard and the laboratory window opens onto a corridor. She heats a portion with sodium hydroxide and a pungent gas is produced that turns damp red litmus paper blue. She then adds nitric acid and silver nitrate to a fresh solution of the sample and gets a white precipitate. Her partner says 'that proves it is ammonium chloride, let's write it up'.
1. Evaluate the evidence. Does the silver nitrate result prove the presence of chloride? Explain what further test or control is needed.
Read the worked answer
The first test is sound. Warming the sample with sodium hydroxide releases a gas that turns damp red litmus blue. That identifies ammonia, and therefore the ammonium ion (NH4+) in the sample. The equation is: NH4+ + OH- -> NH3 + H2O. This confirms the sample is an ammonium salt but does not identify the anion.
The silver nitrate result is not conclusive as described. Adding silver nitrate to a solution acidified with nitric acid gives a white precipitate for chloride (AgCl). However, the student must check two things:
- Was the acid added first, and was it nitric acid? Acidifying with dilute nitric acid before adding silver nitrate prevents false positives from carbonate, which would also give a white precipitate (Ag2CO3) that dissolves in acid. If the acid was added after the silver nitrate, or if the order is unclear, the result is unreliable.
- A control is needed. The student should run the same test on distilled water and on a known ammonium sulphate solution. If ammonium sulphate also gives a white precipitate under her procedure, the test is not specific.
Distinguishing sulphate: Barium chloride solution acidified with dilute hydrochloric acid gives a white precipitate of barium sulphate with sulphate ions. If the sample gives no precipitate with barium chloride, sulphate is absent and chloride becomes the likely anion.
Verdict: The ammonium part is proven; the chloride part is probable but not proven until the acidification order is confirmed, a control is run, and a barium chloride test is negative.
2. The partner wants to repeat the ammonia test by heating a larger sample in an open test tube near the corridor window. Give a risk assessment and a safer procedure.
Read the worked answer
Hazards identified:
- Ammonia gas is produced. It is irritant to the eyes, throat and respiratory tract, and at higher concentration can cause coughing and breathing difficulty.
- Heating a larger sample in an open tube increases the amount of gas released and the risk of the tube cracking or spitting hot liquid.
- The corridor window is not a fume cupboard; opening it does not guarantee that fumes are carried away from people passing by.
- Sodium hydroxide is corrosive to skin and eyes.
Risk rating: Moderate to high for an open, unsupervised larger-scale test in a room without extraction, especially with other students nearby.
Safer procedure:
- Use a small sample (about a spatula tip, no more than 0.5 g) in a test tube.
- Add only 2-3 cm3 of dilute sodium hydroxide, not concentrated.
- Hold the tube with a test tube holder and heat gently, moving the tube in the flame, pointing the mouth away from everyone.
- Hold damp red litmus paper at the tube mouth with tongs; do not put your face near the mouth.
- If the school lacks a fume cupboard, perform the test in a well-ventilated area with the demonstrator or teacher present, or use a gas syringe or inverted funnel arrangement to contain the gas.
- Wear goggles and keep the acid and silver nitrate away from the ammonia test area.
Recording: Note the observation (pungent gas turning damp red litmus blue) and the conclusion (ammonium ion present), and note the limitation that the test does not identify the anion.