Analysis of Phosphorous in Fertilizer

Objectives
To determine the percentage of phosphorous (as P2O5) in fertilizer.
To become familiar with gravimetric analysis.

Discussion
[This laboratory exercise was adapted from "Quantitative Determination of Phosphorous in Plant Food Using Household Chemicals", by Sally Solomon, Alan Lee, and Donald Bates, Journal of Chemical Education, p.410., May 1993]

Plants, like animals, require essential nutrients to live. The three main nutrients contained in plant foods are nitrogen, phosphorous (as P2O5), and potassium (as K2O). For example, a common all-purpose plant food labeled 15-30-15 would contain 15% nitrogen, 30% P2O5 , and 15% K2O. However, the percentage of the elements present in this fertilizer would be 15% nitrogen, 13% phosphorous, and 12.5% potassium.

The gravimetric analysis of phosphorous in this experiment is based on the precipitation of phosphorous as MgNH4PO4 . 6H2O from a solution that contains the monohydrogen phosphate ion (HPO42- ), ammonium ions, and magnesium ions. The balanced reaction is:
5 H2O + HPO42- + NH41+ + Mg2+ + OH1- =====> MgNH4PO4 . 6H2O

The precipitate forms only upon the slow neutralization of the solution with ammonia water. In an acid solution the phosphate ions react with hydronium ions to form monohydrogen phosphate ions. The net effect of this is to reduce the concentration of the phosphate ions (PO43- ) and prevent the formation of MgNH4PO4 . 6H2O.
PO43- + H3O1+ =====> HPO42- + H2O

The hydroxide ions necessary for the neutralization must be supplied by a weak base such as ammonium hydroxide. A strong base, like NaOH, would cause the precipitation of Mg(OH)2 instead of the double salt magnesium ammonium phosphate hexahydrate.

Procedure
1. Mass out to the nearest milligram approximately 10 grams of plant food. Dissolve in about 150 mL of tap water. Plant foods generally contain some insoluble material even though they are advertised as "water soluble". If your plant food contains some insoluble material remove it by filtration.

2. Transfer the dissolved plant food to a 1 L Erlenmeyer flask and add 0.40 M MgSO4 . 7H2O . The amount of magnesium sulfate added should be about 5 mL/100 mG of P2O5 that is believed to be in your sample. Your instructor will tell you the approximate amount of P2O5 in your plant food.

3. Add a few drops of phenolphthalein to the solution and then slowly add 1M aqueous NH3 (NH4OH) with constant swirling or stirring until a white precipitate forms or until the phenolphthalein endpoint is reached. (If no precipitate forms contact your instructor.) Let the solution stand for at least 15 minutes to insure that precipitation is complete.

4. Collect the precipitate by vacuum filtration. Any precipitate left behind in the flask can be rinsed on to the filter paper by adding small amounts (50 mL) of 70% isopropanol (2-propanol) to the flask. Spread out your filter paper to dry overnight.

5. Carefully scrape the precipitate from the filter paper and collect on a watch glass. Mass the precipitate to the nearest milligram.

Questions
1. From the data obtained determine the percentage of phosphorous (as P2O5 ) in the plant food.

2. A large amount of potassium in plant foods could cause the formation of MgKPO4. 6H2O along with MgNH4PO4 . 6H2O. What would be the effect on the calculated percentage of P2O5 in question 1 if MgKPO4. 6H2O was mixed in with MgNH4PO4 . 6H2O in the precipitate? That is, would the percentage of P2O5 calculated be higher or lower that if the precipitate contained only MgNH4PO4 . 6H2O?

3. How would the calculated percentage of P2O5 be affected if the precipitate was not thoroughly dry?

4. Determine the average and standard deviation from the class results.

5. From the class values listed on the chalkboard for the percentage of P2O5 in the plant food determine which ones would be considered outliers at the 96% confidence level and rejected. After rejecting these values calculate the class average for the percentage of P2O5 in the plant food and the standard deviation.

6. After step 3 the normal procedure is to let the solution stand for several hours instead of 15 minutes. What percentage error (if any) results in letting it stand for only 15 minutes instead of several hours?

7. Summarize your results in a nice neat table.


Teacher Notes
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