# Gravimetric Determination of Phosphorus in Fertilizer Samples

Topics: Solubility, Colloid, Ammonia Pages: 4 (1089 words) Published: August 13, 2012
Gravimetric Determination of Phosphorus in Fertilizer Samples -------------------------------------------------

Results and Discussion

More than 50% of fertilizers are composed of nitrogen, phosphorus and potassium because these three nutrients are essential but are deficient in soils. To ensure good quality of fertilizers, it is significant that the amounts of these nutrients are quantified. In this light, gravimetric principles are of much use. The objective of the experiment is to determine the percentage of phosphorus and diphosphorus pentoxide in fertilizer samples. Gravimetric analysis follows only a few fundamental steps. Here, the quantity of the analyte is determined by separating it physically from the system. To do this, it should be converted to an insoluble substance. In the experiment, the phosphorus of the fertilizer sample was precipitated as magnesium ammonium phosphate hexahydrate. It follows the equation, 5H2O(l) + HPO42-(aq) + NH4+(aq) + Mg2+(aq) + OH-(aq)→

MgNH4PO4∙ 6H2O(s) (1)
The precipitating reagent used is ammonia, NH3. Ammonia is used instead of ammonium chloride because the latter produces Cl- ions that may react with the Mg2+ ions found in the solution, thus,

forming MgCl2(s). This can interfere to the production of the desired precipitate. Forming the precipitate is critical. First and foremost, the precipitate formed should be free from impurities. Also, the size of the particles (making up the precipitate) should be considered. In gravimetric

analysis, relatively few crystals are preferred over many small ones. Experimentally, it is found that particle size is affected by experimental variables such as precipitate solubility, reactant concentrations in the precipitating solution, the rate of addition and mixing of reactants, and the temperature. Mathematically, it is reflected in the Von- Weimarn equation, which states that R=Q-SS...