GRAVIMETRIC DETERMINATION OF PHOSPHORUS
IN PLANT FOOD
DISCUSSION
Gravimetric analyses may be difficult and time consuming but are inherently quite
accurate. The accuracy of an analysis is often directly proportional to the time expended
in carrying it out. The ultimate use of the analytical result governs how much time and
effort the analytical chemist should expend in obtaining it. For example, before building a
mill to process gold ore, an accurate analysis of the ore is required. Mills are very
expensive to build and operate, and economic factors determine whether of not
construction of the mill is worthwhile. Because of the value of gold, the difference of
only a few hundredths of a percent of gold in an ore may be the governing factor as to
whether of not to construct a mill. While, on the other hand, the analysis of an
inexpensive commodity chemical such as a plant food requires much less accuracy; the
economic consequences of giving the consumer an extra 0.2% of an active ingredient are
usually small even for a large volume of product. Time is too valuable, whether it be the
students’ or scientists’, to be wasted in the pursuit of the ultimate in accuracy when such
is not needed.
Consumer chemicals are subject to quality control by the manufacturer and by various
consumer protection agencies. Consumer chemicals are usually analyzed both
qualitatively to determine what substances they contain and quantitatively to determine
how much of these substances are present. For example, plant foods are analyzed this
way.
Plant foods contain three essential nutrients that are likely to be lacking in soils. These
are soluble compounds of nitrogen, phosphorus and potassium. The labels on the plant
food usually have a set of numbers such as 15-30-15. These numbers mean that the plant
food is guaranteed to contain at least 15% nitrogen, 30% phosphorus (expressed as P2O5)
and 15% potassium (expressed as K2O). The rest of the product is other anions or cations
necessary to balance charge in the chemical compounds, dyes to provide a pleasing color,
and fillers.
In this experiment we will illustrate one of the quality control analyses for plant food
by gravimetric determination of its phosphorus content. Phosphorus will be determined
by precipitation of the sparingly soluble salt magnesium ammonium phosphate
hexahydrate according to the reaction:
5H2O(l) + HPO4 2-
(aq) + NH4 + (aq) + Mg
2+ (aq) + OH
- (aq) MgNH4PO4 6H2O (s)
EXAMPLE 1:
What is the minimum phosphorus percentage of a plant food whose P2O5 percentage is
guaranteed to be 15%?
Solution: Assuming 100 g of plant food, we would have 15 g of P2O5. Using this
quantity we can calculate the amount of P in the sample:
(15 g P2O5)(1 mol P2O5/141.9 g P2O5)(2 mol P/1 mol P2O5)(30.97 g P/1 mol P) = 6.5 g P
%P = 6.5 g P/100 g sample x 100 = 6.5%
EXAMPLE 2:
If a 10.00g sample of soluble plant food yields 10.22g of MgNH4PO4 6H2O, what are
the percentages of P and P2O5 in this sample?
Solution: First, solve for the grams of P in the sample.
(10.22g MgNH4PO4 6H2O)(1 mol MgNH4PO4 6H2O/245.3g MgNH4PO4 6H2O) X
(1 mol P/1 mol MgNH4PO4 6H2O)(30.97g P/1 mol P = 1.290g P
%P = (1.290g P/10.00g Sample) X 100 = 12.90% P
Similarly, solve for grams of P2O5.
(10.22g MgNH4PO4 6H2O)(1 MOL MgNH4PO4 H2O/245.3g MgNH4PO4 6H2O) X
(1 mol P/1 mol MgNH4PO4 6H2O)(1mol P2O5/2 mol P)(141.9g P2O5/ 1 mol P2O5) =
2.956g P2O5
%P2O5 = (2.956g / 10.00g) = 29.56% P2O5
PROCEDURE
Weigh to the nearest 0.01g 1.5 to 2.0grams of your assigned brand of fertilizer onto
weighing paper. Transfer the sample quantitatively to a 250 mL beaker and record the
sample weight. Add 35 to 40 mL of distilled water and stir the mixture with a glass
stirring rod to dissolve the sample. Although plant foods are all advertised to be water-
soluble, they may contain a small amount of insoluble residue. To the filtrate add about
45 mL of a 10% MgSO4 7H2O solution. Then add approximately 150 mL of 2 M NH3(aq)
slowly while stirring. A white precipitate of MgNH4PO4 6H2O will form. Allow the
mixture to sit at room temperature for 15 minutes to complete the precipitation. Collect
the precipitate on a preweighed piece of filter paper.
Transfer the precipitate to the filter paper with the aid of a rubber policeman. Wash the
precipitate with two or three 5mL portions of distilled water. Do this by adding each
portion to the beaker in which you did the precipitation to transfer any remaining
precipitate; then pour over the solid on the funnel. Finally, pour two 10 mL portions of
75% isopropyl alcohol through the filter paper. Remove the filter paper, place it on a
numbered watch glass, and store it in your drawer until the next lab period to dry.
Repeat the above procedure with two more samples. In the next period, when the
MgNH4PO4 6H2O is dry, weigh it and calculate the percentages of P and P2O5 in your
original samples.
Date ______________ Section _________ Name ____________________
Data and Calculations Sheet: GRAVIMETRIC DETERMINATION OF PHOSPHORUS IN PLANT FOOD
Data:
Trial 1 Trial 2 Trial3
Weight of sample __________ __________ __________
Weight of filter paper
And MgNH4PO4 6H2O __________ __________ __________
Weight of filter paper __________ __________ __________
Calculations: (You MUST show your work on a separate sheet)
Weight of MgNH4PO4 6H2O__________ __________ __________
Weight of P in original
Sample
__________ __________ __________
Percentage P in
original sample __________ __________ __________
Average percent phosphorus __________
Average percent P2O5 __________
Guaranteed minimum percent P2O5
from label __________
Post Lab Questions:
1.MgNH4PO4 6H2O has a solubility of 0.023g/100mL in water. Suppose a 5.02g
sample were washed with 20 mL of water. What fraction of the MgNH4PO4 6H2O
would dissolve?
2.MgNH4PO4 6H2O loses H2O stepwise as it is heated. Between 40 and 60C the
monohydrate is formed and above 100C the anhydrous material is formed. What are
the phosphorus percentages of the monohydrate and of the anhydrous material?
3. Ignition of MgNH4PO4 6H2O produces NH3, H2O and magnesium pyrophosphate,
Mg2P2O7. Complete and balance the equation for this reaction. If 5.00g of
MgNH4PO4 6H2O are ignited, how many grams of Mg2P2O7 would be formed?
4. How closely did your result for the % P (as P2O5) agree with the minimum percentage claimed on the label? If it did not agree propose an explanation for
this result.
Date ___________ Section _________ Name________________________
ADVANCED STUDY ASSIGNMENT: Gravimetric Determination of Phosphorus
in Plant Food.
1. Why would only three significant figures be required for the analysis of a
consumer chemical such as P2O5 in plant food?
2. The label on a plant food reads 23-19-17. What does this mean?
3. What is the minimum percentage of phosphorus in the plant food in question 2?
4. What is the minimum percentage of potassium in the plant food in question 3?
5. If an experiment’s results are 12.1g, 12.4g and 12.6g, find the mean, the average
deviation from the mean and the standard deviation from the mean.