Practical work 5: identifying mineral fertilizers

To identify a fertilizer you first judge its appearance and solubility in water (white crystals or grey powder, well or poorly soluble), then run ion tests. Ammonium ion — ammonia is released when warmed with alkali (moist red litmus turns blue); chloride — white AgCl with AgNO₃; sulfate — white BaSO₄ with BaCl₂; phosphate — yellow Ag₃PO₄ with AgNO₃; nitrate — brown NO₂ with copper and concentrated H₂SO₄ (this test is very dangerous, teacher only). Potassium ion — a violet flame seen through blue glass, also shown only by the teacher. For example, to tell apart three tubes of KNO₃, NH₄NO₃ and superphosphate: superphosphate is grey, poorly soluble and gives a yellow precipitate with AgNO₃; NH₄NO₃ gives ammonia with alkali and KNO₃ does not. The plan is made so that the easiest test comes first and each test removes one substance. The practical work is done under the teacher’s supervision, in goggles and gloves, in small amounts; fertilizers are not tasted or smelled, and hands are washed.