Quantitative Determination of Phosphorus in Plant Food Using Household Chemicals A Simple but Accurate Variation in Which Introductory Students Can Skip the Time-Consuming Ignition and Ice Bath Precipitation of the Classic Method Sally ~olomon,'Alan ~ee,'and Donald Bates Drexel University, Philadelphia, PA 19104 A quantitative analysis for the determination of phosphorus in plant foods is described. All the reagents used are readily available in drugstores, supermarkets, or variety stores and thus require no special waste-disposal methods. The procedures can bedone with very little equipment in any building that has running water. Like animals, plants require essential nutrients (1, 2) supplied by fertilizers in a form that can be utilized by the plant. The three important nutrients most likely to be lacking in soil are primary nutrients: nitrogen, phosphorus,and potassium. Plant food labels display three numbers that indicate their percentages:
Composition of Plant Foods Brand Name PLANTABBS AfricanVioletPlant Food
11-15-20
RAP-ID-GRO Plant Food SternsMiracle-Gro for Roses
Sterns Miracle-Gro Plant Food Super K-GRO Plant Food PETERS Soluble Plant Fwd
nitrogen (N)
(N)-(p)-(K)
15-30-15
phasphorus (as P?O.) . . - " potassium (as KzO)
For example, a Common a l l - ~ u r ~ owater-soluble se plant food product containing 15%N
The MgNH4P04.6H20 does not form in acidic solution because the concentration of phosphate is reduced by conversion to acid phosphate.
30%Pz05 15%&O
is labeled 15-30-15 to indicate 15%N
13%P
12.5%K
The phosphorous content is quoted as Pz05content even though there is no Pz05in the plant food. Likewise the plant foods contain no KzO. Formulations of plant foods that were used as samples in this experiment are given in the table.
Principles of the Analysis Details of gravimetric schemes once included in analytical chemistry texts ( 3 , 4 ) are covered only briefly in most modern editions.
The hydroxide needed for the neutralization must be provided by a weak base, such as ammonia, because strong bases like NaOH would cause the precipitation of Mg(OHI2 and other undesirable compounds. An excess of ammonium ion (derived from the ammonium ohosohates in the plant food) encourages the formation o ~ M ~ N E ~ 6Hz0 ~Po~ through the common ion effect. Although the solubility of magnesium ammonium phosphate increases with increasing temperature, the procedure described here has been tested a t room temperature with good results. Once filtered, the precipitate must never be warmed because the hexahydrate begins to lose water between 40 OC and 60 'C to form the monohydrate. (It loses all water of hydration a t 100 T.)
.
A Change from the Classic Method
The Precipitate
In classic schemes the precipitate would be converted by ignition to the pyrophosphate MgzP~07and weighed.
The gravimetric determination of phosphorus is based on the precipitation of magnesium ammonium phosphate hexahydrate from a solution that contains acid phosphate ions, ammonium ions, and magnesium ions. The precipitate forms upon slow neutralization with ammonia of a n acidic solution of a phosphate-containing sample.
Fortunately, the MgNH4P04.6Hz0 is sufficiently stable a t room temperature to be dried and weighed, thus avoiding the ignition and the difficulties associated with it. Experimental Equipment
'A~thorlo whom correspondence s h o o ~ be adaressed. 2S~pponea oy an Academy of App ied Sclence REAP grant.
410
Journal of Chemical Education
Beakers, Erlenmeyer flasks, or jars can be used to dissolve samples and prepare solutions. One container must be large enough (1L) to provide room for swirling or stirring 500 or 600 mL of a slurry mixture.
Other items needed are a 100-mm (top diameter) funnel and a spatula or dull knife. Filtering may done with fluted paper basket coffee filters (generic brands are fine). Papers of medium filtering speed, such as Whatman No. 40, can also be used. Cone filter papers (such as the Melitta brand) may collapse under the weight of the wet precipitate. Any weighing device that can measure to the nearest 0.1 g can be used.
carefully scraped offthe filter paper, then weighed. It is not possible to find the weight of the precipitate by subtracting the weight of the dry filter paper. The filter paper used to collect the precipitate will absorb the water that is not entirely removed as the sample dries. Calculations and Write-ups
From the weight of the MgNH4P04.6H20 and the percentwe of ~hosnhorusit contains (12.6%). the nercentaae phos~horo"s inthe plant food sample can be caiculated-
Chemicals
The chemicals needed rubbing alcohol magnesium sulfate (Epsom salts) ammonia
%P=
are all available as household products. The MgS04 7H20solution used for precipitation is prepared by dissolving 10 g of Epsom saltdl00 mL water. Each analysis requires about 150 mL of M e solution. Water can be poured from the tap.3
.
Procedure Dissolving the Sample
A sample of plant food that weighs a little more than 10 g is weighed to the nearest 0.1 g. The plant food is then dissolved in 125-140 mL of tap water. Although the plant foods are all advertised as water-soluble, they may contain a small amount of insoluble residue. This should be removed by filtering the mixture through a single coffeebasket filter. Transfer the solution to a container that holds at least 1L. About 150 mL of a solution of MgS04 7Hz0 is added to the sample solution. The volume of magnesium sulfate solution recommended is 5 mL1100 mg PzOS,.which provides a stoichiometric excess of roughly 50% for plant food samples containing 30% P205.
.
The Precipitation
At this point most of the sample solutions will be too acidic for any precipitate to form. Approximately 200 mL of NHs(aq) is added gradually while swirling or stirring. A white precipitate then forms. The suspension is allowed to stand at room temperature for 15 min. before filtering. The additional amount of precipitate that would form durine " a normal waitine oeriod (several hours) with cooling in an ice bath is not enough to influence the results of this analvsis in which the final result will contain iust three si&ficant figures.
-.
Handling the Precipitate
The precipitate is collwted using doubled fluted basket coffee filters in a 100-mm funnel. Two 50-mL portions of rubbine flask to " alcohol are added to the orecioitation . gather any remaining material. Then they are poured over . soon as the alcohol the solid in the funnel to aid d ~ n aAs has filtered through, the fi1ter"pa;er with precipitate can be carefullv lifted out of the funnel. Then soread it out, and place it on"a flat surface, such as a paperplate 1ined.with a oaoer towel. for faster drvine. 'TGe precipitate should de lift to dry overnight. Before s that the weiehine it. be sure that there are no l u m ~ and solii is powdery with a texture similar to Glc. The solid is
.
3 ~ 1 1water used in this project came from the supply sewing thecity
of Philadelohia.
4The elgmental analysis was carried out by Micro-Analysis, Wilmington, DE 19808. 'The dye structures are nol available.
grams MgNH4P04.6Hz0x 12.6% grams of sample
To comnare their result with the nlant food label, students will need to convert %P to %p20; using 0.437, the weight fraction of phosphorus in P205(62.0 g PI142 g P2O.d.
Laboratory write-ups for students with specific instructions and an instructors guide are available from S. Solomon. If possible, send a self-addressed envelope. If you want the material on disk, send a disk (IBM or Macintosh) with a self-addressedmailing envelope. Postage is not required. Errors
Due to the common oractice of overformulation in which the quantity promisedi~lower than the actual content, the Dercentaae (aqP A ) found msv be too hieh - of nhosnhorus . of phosphoby as much as l%.'~orexample, the rus (as Pz06)in 15-30-15 Plant Food is guaranteed to be 30% (at least) but is likely to be between 30 and 31% (5). Moreover, variations in the blending process can introduce an error estimated to be about 0.5%. Students should evaluate their results with this in mind and retain no more than three significant figures. The experimental procedure was developed by Drexel University students, who were instructed to weigh to the nearest 0.1 g and to use tap water, household ammonia, and the other materials that were sunolied. Thev tested various brands of filter paper, ammonia,.and typesof plant food. The procedure was then tested on another class of 24 students all of whom analyzed the same lot of Miracle-Gro Rose Food (18-24-163. The averaee result obtained bv all 24 students who carried out theUexperimentwas 24.4% Pz05with an average deviation of 1.3%. (The standard error of the mean was 0.33%.) The large amount of potassium present in plant foods could cause the formation of mixed crystals of MgNH4P04.6H20 and MgKPO,. 6Hz0 which crystallize in similar geometric forms (6). The %P present in pure MgNH4P04.6Hz0 is 12.6%, compared to only 11.8%in pure MgKP04. 6Hz0. According to the elemental analysis4 for phosphorous in which the white powdery solid was found to contain 12.4%P, the precipitate formed in this procedure is likely to be mostly ammonium. The amount of the MgKF'04 .6Hz0 that could form can be reduced by reprecipitation: Dissolve the initial precipitate in HC1, and then neutralize once more (3).Cations and dyess present in plant foods that could interfere by forming insoluble precipitate are present in amounts too small to make a difference. Acknowledgment
We wish to acknowledge the Academy ofApplied Science whose apprentice program sypported Alan Lee, a high Volume 70 Number 5 May 1993
411
school student. We would also like to thank Anthony Wambsgans who us to test this experiment in of his laboratory courses, and Chinhyu Hur who helped us coordinate the testing. We are very grateful to all the Drexel University students-nonmajors and engineering and science students--who carried out the ex~edments and helped us to refine our instructions, Michael Dobres "Om the department provided advice On plant nutrients.
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Journal of Chemical Education
Literature Cited L s e J i 8 b w F;Ross, C.PlantPhysialogy; Wsdswo*h: Belmont, CA, 11W5. Z.S~~,,K cherni8*fwfie C O ~ , ~ ~ ; . U I , ~ B X 197,;cbpter9. ~ : B ~ ~ ~ , s fie,, w,; naeniseh,E.;hwyer, D.~ ~ ~ ~ ~4th ed.; i wile,. ~ N~~~~~~ ~ i ~ 1958: P 370. 4. Fishel; R.Quontitatiue ChemizdAmlys% WB. Saundera: Philadelphia, 1961.
5. Myate eommunicaticn Y'th technical experts st S t e m Miraele.Gm, Port Washington, NY
& ,tz.
J.: Sehenk, G. Quontitolive Analytiml Chemishy; Allm and Barn: Boston, 1974; P 53.
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