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Determination of Phosphorus Content of Fertilizer: Skill Building Lab © 2011, Sharmaine S. Cady East Stroudsburg University

Gravimetric Analysis

Skills to build:

   

Using vacuum filtration Using a digital balance Using mass stoichiometry Doing a gravimetric analysis

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






Mg Other micronutrients (Cu, Mn, Mo, Zn, B among others)



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...
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