The Damp Fertiliser Store
A farm store holds 40 kg bags of ammonium nitrate fertiliser stacked against a north-facing concrete wall. Over winter the wall becomes damp and the bags nearest it clump into hard lumps. A worker notices a faint smell and places a pH indicator strip in a sample of the run-off water collected from the floor; the strip turns red (pH about 4). The store also contains bags of calcium oxide (quicklime) left over from whitewashing. The worker wants to wash the clumped fertiliser down the drain with water and sweep the quicklime into the same drain to 'neutralise any acid'. The store manager asks you to explain what has happened and to judge whether the worker's plan is safe and chemically sound.
1. Explain why the fertiliser clumped and why the run-off is acidic, using ideas about solubility and ions.
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Ammonium nitrate is very soluble in water. Damp from the wall dissolves the solid, and when the water later evaporates, crystals re-form and bind the granules into lumps. The dissolved salt contains ammonium ions (NH4+) and nitrate ions (NO3-). Ammonium ions are weakly acidic in water: NH4+ + H2O ⇌ NH3 + H3O+. The faint smell is ammonia released by this equilibrium. Because the run-off has a pH near 4, it contains an excess of H3O+ ions from this hydrolysis. The clumping is therefore a physical consequence of dissolving and recrystallising, while the acidity is a chemical consequence of the ammonium ion behaving as a weak acid.
2. Judge the worker's plan to wash the fertiliser into the drain and to add quicklime to the same drain.
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The plan should be rejected. First, ammonium nitrate is a powerful oxidising agent and a nutrient pollutant; discharging 40 kg of dissolved nitrate into a drain can contaminate groundwater and cause eutrophication in streams. Second, adding calcium oxide to water is strongly exothermic: CaO + H2O → Ca(OH)2 releases a great deal of heat and produces a strongly alkaline slurry. Mixing this with ammonium salts can release ammonia gas and, with heat, is hazardous. Third, the quicklime would neutralise the acid only if well mixed and controlled, which a drain cannot guarantee. The correct action is to keep the fertiliser dry, sweep up the lumps and return them to a sealed container, and dispose of contaminated run-off according to local regulations, not down a drain.
3. Suggest how the store could be modified to prevent the problem recurring, and state one measurement that would confirm the fix.
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Raise the bags on pallets so they are not in contact with the concrete, ventilate the store, and repair the damp wall or install a vapour barrier. Keep oxidising fertilisers away from quicklime and other incompatible materials. To confirm the fix, place a sample of the store's atmosphere or a swab of the wall in distilled water and measure the pH; a reading near 7 instead of 4 would show that acidic run-off is no longer forming. Regular weighing of a test bag could also reveal moisture uptake before clumping begins.