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Determination of Phosphorus Content of Fertilizer

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Fertilizer Scientists estimate that the earth’s soil contains less than twenty percent of the necessary organic nutrients needed to meet our current food production. Carbon, hydrogen, and oxygen used to synthesize compounds needed for structural integrity are readily available from air and water. Nitrogen, phosphorus, and potassium, the building blocks for healthy plants, are usually not present in sufficient amounts in the soil to support plant growth. These are known as macronutrients. In addition, micronutrients, such as calcium, sulfur, and magnesium, are also necessary, but in smaller amounts.Table 1 shows the role of these nutrients in plant growth. Commercial fertilizers are inorganic or organic compounds that replenish the soil with additional nutrients to increase growth and keep plants healthy.

In 2001, 53 million tons of fertilizer was used by the agricultural industry in the United States. Nitrogen fertilizers use ammonia as the building block for the production of water-soluble nitrogen-containing compounds, such as ammonium nitrate. Phosphorus is mined from natural geological deposits and mixed with sulfuric acid to make water-soluble phosphate compounds. Potassium is supplied as potash (potassium chloride) from evaporation of sea water. In the United States, the Association of American Plant Food Control Officials (AAPFCO) is concerned with consumer protection. AAPFCO works with industry to promote the safe and effective application of fertilizers in an effort to protect soil and water resources. It also oversees standard methods for fertilizer analysis to ensure that fertilizer labels are accurate and consistent with nutrient content.

Table 1. Role of Nutrients in Plants Nutrient Role Essential component of amino acids used to make proteins Component of DNA and RNA Necessary for chlorophyll production Produces the greatest yield response in crops Essential component of enzymes used in photosynthesis Essential component of nucleic acids (DNA and RNA) Stimulates root growth Essential for seed germination Aids in efficient use of water Ensures carbon assimilation Aids in transporting and storing sugars and proteins Activates enzymes Essential for water regulation and uptake Ensures resistance to frost, drought, and disease Essential component of two amino acids Essential component of Vitamin B1 and enzymes Component of cell membranes Necessary for plant growth and cell division Necessary for pollen development Prevents leaf fall Essential component of chlorophyll Activates enzymes Essential components of metabolic enzymes

The label on a fertilizer bag or container contains three numbers. These numbers give the percentage of nitrogen, phosphorus, and potassium, respectively, available to plants based upon chemical analysis. Note that the phosphorus analysis is reported as percent by mass P2O5 and potassium as percent by mass K2O. Originally, fertilizers were heated in air to convert the phosphate and potash into these compounds, which were then used to report the nutrient content. Although analysis techniques have changed, the label definition has not. In this experiment, a sample of fertilizer is analyzed for phosphorus using gravimetric analysis to determine if its label percentage is accurate. Gravimetric Analysis Gravimetric analysis is a technique that involves the determination of the mass of a chemical species of known composition that can be related to the analyte by mass stoichiometry. You should review mass stoichiometry relationships before starting the experiment.

The “Stoichiometry” link listed for this experiment provides some examples of using mass stoichiometry. The most common gravimetric technique involves precipitation. In this experiment, the water-soluble phosphate in fertilizer is precipitated from solution with magnesium sulfate. The precipitate is magnesium ammonium phosphate hexahydrate. The chemical reaction equation may be written as PO43-(aq) + Mg2+(aq) + NH3 (aq) + 7 H2O (l) → MgNH4PO4·6H2O(s) + OH-(aq) The sulfate ion is a spectator ion and is not included in the chemical reaction equation. Based on the mass of fertilizer dissolved and the fertilizer analysis, you will need to calculate the volume of MgSO4 solution needed to form your precipitate. See the online theory section for an example calculation. The solution is made basic with aqueous ammonia in order to precipitate the entiremass of phosphate ion. If the solution is not sufficiently basic, the following reaction occurs, and some of the phosphate is not precipitated,which produces inaccurate results: PO43-(aq) + H3O+ (aq)  H2PO42-(aq) + H2O (l) The precipitate must be dried at room temperature to prevent the waters of hydration from being lost. Once the mass of MgNH4PO4·6H2O(s) is obtained, the stoichiometric ratios between P, P2O5, and the product will yield the amount of P2O5 in the fertilizer.

3D Molecules Go to the web site and click on the diphosphorus pentoxide link. Use the radio buttons to show different views and properties of each
molecule. Answer the questions based on the manipulation of the molecules.

Experimental Methods and Materials

Safety considerations Wear suitable protective clothing, gloves, and eye/face protection! You should read the online MSDS for: Ammonia Magnesium Ammonium Phosphate Magnesium Sulfate

Preparation of fertilizer sample Weigh between 3 and 5 g of fertilizer. Record the brand and type of fertilizer and the percentage analysis. Place in a 1000-mL beaker. Add 20 mL of distilled water, a magnetic stir bar and stir to dissolve the fertilizer. If it does not completely dissolve, add another 20 mL of distilled water and stir. Continue until all the fertilizer is dissolved or no more solid appears to dissolve. Do not exceed 100 mL of distilled water. If water-insoluble components remain in the beaker, vacuum filter and save the filtrate (liquid).

Precipitation of phosphate Determine the volume of 0.400 M MgSO4 needed to precipitate the phosphate based on the label analysis and your mass of fertilizer (see online example). Add this volume to the beaker. Calibrate the pH meter. Place the pH probe into the beaker and record the pH. Add 6 M aqueous ammonia (NH3) to the beaker in 5-mL increments until the pH is at least 9.0. Stand the mixture in an ice bath for 30 minutes. Place a piece of filter paper into the top of a Buchner funnel. Weigh the top of the Buchner funnel with the filter paper. Use vacuum filtration to isolate the product (see diagram at right). First, using distilled water, seal the filter paper over the holes before you filter the solution. Rinse with 2 15-mL portions of distilled water. Pull a vacuum for 5 minutes to dry the product as much as possible.

Product purification The product contains excess water which must be allowed to evaporate. Remove the top of the Buchner funnel and cover with a Kimwipe. Give to instructor to dry until the next lab period. Reweigh the Buchner funnel top. Determine the mass of P2O5 in your fertilizer sample and its mass percentage.

Laboratory Report Answers to the following questions should appear in the conclusion section of your report: 1. How does your percentage P2O5 compare with the given value on the label? Explain any differences. 2. Use Excel to calculate the average percentage P2O5 for the class and the standard deviation. A large standard deviation indicates that the data points are far from the average. Does the standard deviation indicate that the class as a whole had more or less success in obtaining precise results? See the online instructions for using Excel.


European Manufacturers Fertilizer Association Home Page.http://www.efma.org/(accessed August 2005) The Fertilizer Institute Home Page.http://www.tfi.org/ (accessed August 2005) Wink, D. J.;Gislason, S. F.; Kuehn, J. E.Working with Chemistry, 2e; W. H. Freeman & Company: New York, 2000; pp C-11-C-12.

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