Gravimetric Analysis
Determination of Phosphorus Content of
Fertilizer: Skill Building Lab
 2013, Sharmaine S. Cady
East Stroudsburg University
Skills to build:
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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 watersoluble 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
Fertilizers
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
N
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
P
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
K
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
S
Essential component of two amino acids
Essential component of Vitamin B1 and
enzymes
Ca
Component of cell membranes
Necessary for plant growth and cell division
Necessary for pollen development
Prevents leaf fall
Mg
Essential component of chlorophyll
Activates enzymes
Other micronutrients (Cu, Mn, Mo, Zn, B
among others)
Essential components of metabolic enzymes
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Fertilizers
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)  HPO42-(aq) + H2O (l)
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Fertilizers
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
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Fertilizers
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 500-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 in Solution Stoichiometry). Add this volume to the beaker. 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, turn on the vacuum and seal the filter paper over the holes
before you filter the solution. To completely transfer the product, rinse the
beaker with 15-mL of distilled water and pour over the product in the funnel.
Repeat the rinse one more time. 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 (see online example in Mass Stoichiometry).
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Fertilizers
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.
References
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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