Gravimetric Determination of Phosphorus in Fertilizer Samples

Date Performed: July 10, 2009

GRAVIMETRIC DETERMINATION OF PHOSPHORUS IN FERTILIZER SAMPLES

RESULTS AND DISCUSSION

In this experiment, we determined the phosphorus content of a fertilizer sample by separating and precipitating a weighable form of phosphorus in which we did gravimetric calculations from. We applied similar principles of gravimetric analysis from experiment 2. The fertilizer samples were rst dissolved in 40 mL of distilled water. Undissolved particles were treated as contaminants and were ltered out. A clear solution was obtained. 45 mL of 10% MgSO4•7H2O solution was added to the ltrate. While constantly stirring, 150 mL of 2.0 M NH3 was slowly added to the solution, forming a white precipitate. It is important to add NH3 slowly while stirring. This is an application of Von Weimarn equation, R= (Q-S)/S, where R is the relative supersaturation, Q is the analyte concentration, and S is the equilibrium solubility. In principle, a low R is desired to avoid colloidal suspension of precipitates. A low R promotes crystal growth than nucleation. A way of obtaining a low value of R was applied in the experiment; constant stirring and slow addition lowers the Q, the concentration. The solution was then set aside for 15 minutes, to give time for digestion process. Digestion is where we leave the precipitate in its mother liquor for certain amount of time to improve the lterability of precipitate. The precipitate was then quantitatively transferred into a pre-weighed lter paper. The precipitate was washed 3 times with distilled water and 2 times with 95% ethanol. Washing with ethanol removes other contaminants that water alone cannot remove, e.g.

organic substances left in the solution. It also removes extra water from the precipitate, and it is easily removed from the precipitate as it is a volatile substance. Gravimetric data were then collected. Complete data can be found in the appendix. A Q-test was performed at 95% con dence level, and it was...