Lab 3: Gravimetric Measurement
of Phosphorus in Plant Food*
* Adapted from "Laboratory
Experiments for Chemistry: The Central Science", J. H. Nelson and K.
C. Kemp, 7th ed., Prentice Hall, 1997.
July 10, 2001
Purpose. The goal of this experiment will be to introduce you to gravimetric methods of analysis, one of the "classical" methods of chemical analysis. In these procedures, an analyte species is selectively converted into a new compound, the mass of which is easily measured. The mass of the product can then be used to calculate the amount of analyte species that was initially in the sample.
Introduction. In this experiment, you will determine the level of phosphorus (as wt% P2O5) in a sample of commercial plant food. This is accomplished by the precipitation of dissolved phosphorus in the form Mg(NH4)PO4. 6H2O. By measuring the amount of precipitate that forms, you can calculate the percent of phosphorus in the fertilizer sample.
Procedure: Perform the following steps on three separate samples; these can be done simultaneously to save time - just be sure to clearly label each sample so that they will not be confused during the experiment.
1) Measure approximately 3.0 - 3.5 grams (to the nearest 0.0001 g) and transfer to a 250 mL beaker. Add 35 to 40 mL of distilled water and stir with a glass stirring rod.
2) If the resulting solution from step 1 is cloudy or has undissolved material, filter the solution as shown in lab.
3) Add approximately 5 g of epsom salts (MgSO4.7H2O); stir and dissolve. Then gradually add 150 mL of ~4% ammonia to the solution while slowly stirring. In your notebook record any changes in the appearance of the solution. Allow the solution to sit at room temperature for about 15 minutes to allow the precipitate to fully develop.
4) Weigh a piece of filter paper and then filter the precipitate; use a rubber policeman to help completely transfer the solid into the filter paper. Wash the precipitate with two portions of 5 mL deionized water; first use the water to rinse the beaker, then pour the rinse over the precipitate.
5) Rinse the precipitate with two 10 mL portions of 75% isopropyl alcohol. After the rinsings are complete, place the filter paper on a labeled watch glass (your name and the sample number) and place in drawer to dry for one week
6) When dry, weigh each sample and determine
the amount of precipitate collected.
Data Analysis & Report. Calculate
the mass of P2O5 in your sample. What weight percent
does this correspond to?