Question

What is the form in which phosphorus exists in plant food and what ion could you use to precipitate the phosphorus.

Answer 1

Generally as phosphate. You can ppt PO4^3- from an ammoniacal solution with Mg^2+ as MgNH4PO4. This is then fired in a furnace to form Mg2P2O7, magnesium pyrohosphate. That is the final weighing form. This is a typical gravimetric analysis.