What are the principles behind gravimetric analysis of phosphorus in fertilizer samples? What is the significance of digestion in a gravimetric analysis?

Corrie writes ... Gravimetric analysis relies on the formation, isolation and thorough drying of an insoluble solid (usually as a precipitate) from a solution. The accurate mass of the solid enables the amount of a particular element present in it to be calculated. In the gravimetric determination of phosphorus, the phosphate present in the fertilizer is usually converted into an insoluble magnesium ammonium phosphate or ammonium molybdophosphate. Digestion of precipitates, i,e, allowing them to stand in contact with the mother liquor for several hours, allows the smaller crystals to re-dissolve and then re-precipitate out on the surface of larger crystals. This reduces co-precipitation (incorporation of other substances into crystals of the required precipitate), and allows easier filtration of the precipitate as it is composed of larger particles of solid. Risk assessment Before attempting any practical work based on the advice and suggestions on this website, you must do the ...  more