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Unformatted text preview: ‘ K
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il'l"'. Gravimetric ‘ Determination of
Phosphorus in
Plant Food To illustrate an application of gravimetric analysis to a consumer product. Apparatus
balance funnel support
beakers (6), any combination, ring stand 250 mL or larger stirring rods (3) with
filter paper (Whatman No. 40) rubber policeman
funnels (3)
Chemicals :9
75% aqueous isopropyl alcohol" 2 M NH3(aq)
10% aqueous MgSO4  7 H20:E plant food Analytical chemistry is concerned with determining how much of one or
more constituents is present in a particular sample of material. Two common
quantitative methods used in analytical chemistry are gravimetric and
volumetric analysis. Gravimetric analysis derives its name from the fact that
the constituent being determined is isolated in some weighable form. Volu
metric analysis, on the other hand, derives its name from the fact that the
amount of a constituent being determined involves measuring the volume of
a reagent. Volumetric analyses are generally less time consuming and less ac
curate than gravimetric analyses. Gravimetric analyses may be difficult and time consuming, but they are in—
herently quite accurate. The accuracy of an analysis is often directly propor
tional to the time expended in carrying it out. The ultimate use of the
analytical result governs how much time and effOri‘ 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 or not con
struction of the mill is worthwhile. Because of the value of gold, the difference
of only a few hundredths of apercent of gold in an ore may be the governing
factor as to whether or not to construct a mill. 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. "Epsom salts, household ammonia (nonsudsy), and rubbing alcohol, respectively, may be
substituted for these reagents. 88 Experiment 9 o Gravimetric Determination of Phosphorus in Plant Food Consumer chemicals are subject to quality control by the manufacturer
and by various consumer protection agencies. Consumer chemicals are usu
ally 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 153015.
These numbers mean that the plant food is guaranteed to contain at least
15% nitrogen, 30% phosphorus (expressed as P205), and 15% potassium (ex—
pressed as K20). 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. EXAMPLE 9.1 What is the minimum percentage of phosphorus in a plant food whose P205 per
centage is guaranteed to be 15%? SOLUTION: Assuming 100 g of plant food, we would have 15 g of P205. Using
this quantity, we can calculate the amount of P in the sample: 1molP205)( 21110112 )(30.97gP)_65 P
1:11.9ng05 11110119205 1molP ' g (15 s P205)(
Thus, g P 6.5 g
%P = X 100 — X 100 — 6.5%
g sample 100 g In this experiment we will illustrate one of the quality—control analyses for
plant food by gravimetric determination of its phosphorus content. Phos
phorus will be determined by precipitation of the sparingly soluble salt mag
nesium ammonium phosphate hexahydrate according to the reaction 5HZO(I) + HPOf’Qaq) + NH4+(aq) + Mg2+(aq) + OH—(aq)
—> MgNH4PO4  6H20(s) EXAMPLE 9.2 If a 10.00g sample of soluble plant food yields 10.22 g of MgNH4PO4  6H20,
what are the percentages of P and P205 in this sample? SOLUTION: First, solving for the grams of P in the sample
1 mol MgNH4PO4  6H20 )
245.4 g MgNH4PO4  6HZO 30.97 P
X 2 1.290 g P
1 mol MgNH4PO4  6H20 1 mol P (10.22 g MgNH4PO4 6HZO)( Thus, g P 1.290 g P
%P = w X 100 I —~ X 100 = 12.90%P
g sample 10.00 g sample C
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. Laboratory Experiments
Similarly, solving for grams of P205: 1 mol ' (10.22 g MgNH4PO4  6H20)( ) .4 g ‘ 1 l 1 1P 0 141.9 P O
x( m P m0 2 5X g 2 5) — 2955ng05
1 mol ' 2 11101 P 1 mol P205
2.955 g and 0/0 P205 = 10.00 g x 100 e 29.55 513205
Weigh by difference to the nearest hundredth gram 1.5 to 2.5 g of your un l PROCEDURE
known sample, using weighing paper. Transfer the sample quantitatively ' to a 250mL beaker and record the sample mass. Add 35 to 40 mL of dis tilled 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. If your sample does not completely dissolve, remove the insoluble material by filtration. To the fil trate add about 45 mL of a 10% MgSO4  7HZO solution. Then add approx imately 150 mL of 2 M NH3(aa) slowly while stirring. A white precipitate of MgNH4PO4  61120 will form. Allow the mixture to sit at room tempera ture for 15 minutes to complete the precipitation. Collect the precipitate
on a preweighed piece of filter paper (Figure 9.1). Fold and crease lightly. Seal the moistened edge of the ﬁlter
Tear off comer paper against the
unequally. funnel, making sure
Open out to form ' that the paper over
a cone with one piece ‘ the bottom portion of paper against one
side and three pieces
of paper against the other side of the funnel. is set firmiy against
the tunnel to prevent
air from being sucked
down the side of the paper. Pour down a glass rod
to aid in transfer. The ﬁltrate should run down
the walls of the beaker. The
weight of the water column
hastens ﬁltration. ‘
Use a rubber policeman to transfer the last traces of precipitate from the beaker. A FIGURE 9.] Filter paper use. (Filtration of MgNH4PO4 ' 6H20 is slow. Time may be saved byﬁlteriag by suction with a
Biichnerﬁmnel.) 89 _. 90 Experiment 9 0 Gravim'etric Determination of Phosphorus in Plant Food Instructor demonstration Obtain a filter paper (three of these will be needed) and weigh it accurate
ly. (Be certain that you weigh the paper after it has been folded and torn, not
before.) Fold the paper as illustrated in Figure 9.1 and fit it into a glass fun
nel. Be certain that you open the filter paper in the funnel so that one side has
three pieces and one side has one piece of paper against the funnel—not two
pieces on each side. Why? Your instructor will also demonstrate this for you.
Wet the paper with distilled water to hold it in place in the funnel. Com—
pletely and quantitatively transfer the precipitate and all of the solution from
the beaker onto the filter, using a rubber policeman (your laboratory instruc
tor will show you how to use a rubber policeman). Wash the precipitate with
two or three 5—mL 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 to dry in your locker
until the next period. Repeat the above procedure with two more samples. In the next period,
when the MgNH4PO4  6H20 is thoroughly dry, weigh the filter papers plus
MgNH4PO4 ' 6HZO. Record the mass and calculate the percentages of phos—
phorus and P205 in your original samples. Standard Deviation As a means of estimating the precision of your results, it is desirable to cal
culate the standard deviation. Before we illustrate how to do this, however, we will define some of the terms above as well as some additional ones that
are necessary. Accuracy: measure of how closely individual measurements agree with
the correct (true) value. Precision: the closeness of agreement among several measurements of the
same quantity; the reproducibility of a measurement. Error: difference between the true result and the determined result. Determinate errors: errors in method or performance that can be discov
ered and eliminated. Indeterminate errors: random errors that are incapable of discovery but
which can be treated by statistics. Mean: arithmetic mean or average (,u), where sum of results
number of results For example, if an experirnent's results are 1, 3, and 5, then _1+3+5_ 3 3 )1. Median: the midpoint of the results for an odd number of results and the
average of the two middle results for an even number of results (In). For example, if an experiment’s results are 1, 3, and 5, then m : 3. If results
are 1.0, 3.0, 4.0, and 5.0, then 3.0+.
=_40=35 m 2 e.
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uni UUUUUUUU ll‘llllllﬂllll The scatter about the mean or median—that is, the deviations from the
mean or median—are measures of precision. Thus the smaller the devia
tions, the more reproducible or precise the measurements. EXAMPLE 9.3 If an experiment’s results are 1.0, 2.0, 3.0, and 4.0, calculate the mean, the devia tions from the mean, the average deviation from the mean, and the relative aver
age deviation from the mean. SOLUTION: The mean is calculated as follows:
1.0 + 2.0 + 3.0 + 4.0 _ 10.0 ,u, — 4 , 4 2.5
The deviations from the mean are
‘25 s 1.0] 2 1.5
12.5 — 2.0! = 0.5
I25  3.0I = 0.5
[2.5 — 4.0; = 1.5
The symbol I I means absolute value, so all differences are positive. The aver  age deviation from the mean is therefore 1. + . +0. +1.
5 054 5 5:10 The relative average deviation from the mean is calculated by dividing the aver
age deviation from the mean by the mean. Thus, Relative deviation 2 = 0.40
This can be expreSSed as 40%, 400 parts/ thousand (ppt), or 40,000 parts/million
(ppm). Note, if the mean were larger, say 100 instead of 2.5, and the average
deviation still 1.0, the relative deviation becomes 1.0/100 or 1.0% (10ppt). In this case the precision is better because the relative deviation is smaller. Standard deviation (3) is related to statistics and is a better measure of pre—
cision and is calculated using the formula \/ sum of the squares of the deviations from the mean
S = number of observations  1 Eva — #2 N—l where s = standard deviation from the mean, X, = members of the set, p, = mean, and N = number of members in the set of data. The symbol 2,
means to sum over the members. EXAMPLE 9.4 An experiment's results are 1, 3, and 5. Calculate the mean, the deviations from
the mean, the standard deviation, and the relative standard deviation for the data. SOLUTION: The mean is as follows: 1+3+5
=—=3 ”‘ 3 Laboratory Experiments 91 92 Experiment 9 0 Gravimetric Determination of Phosphorus in Plant Food The deviations from the mean are I X1 — id = deviation I1—M=2 133I=0 5—m22
f+02+ﬁ 3r1
M+0+4 2
f'
_ E = 4:2 s=——— The results of this experiment would be reported as 3 :t 2. The relative standard deviation is g 2 0.7, or 70% Calculate the standard deviation of your data and report the results on your report sheet.
The standard deviation may be used to determine whether a result should be retained or discarded. As a rule of thumb, you should discard any result
that is more than two standard deviations from the mean. For example, if
you had a result of 49.65% and you had determined that your percentage of
phosphorus was 49.25 :l: 0.09%, this result (49.65%) should be discarded.
This is because 5 = 0.09 and [49.25  49.65I = 0.40, which is greater than
2 X 0.09. This result is more than two standard deviations from the mean. PRE LAB Before beginning this experiment in the laboratory, you should be able to an—
QUESTIONS swer the following questions; 1. Which method of analysis generally is the faster method, gravimetric
or volumetric? ' 2. Why would only three significant figures be required for the analysis of
a consumer chemical such as P205 in plant food? The label on a plant food reads 231917. What does this mean? 4. What is the minimum percentage of phosphorus in the plant food in
question 3? 5. What is the minimum percentage of potassium in the plant food in
question 3? What is the percent sulfate in CuSO4  5H20?
Does standard deviation give a measure of accuracy or precision? If an experiment’s results are 12.1, 12.4, and 12.6, find the mean, the av
erage deviation from the mean, the standard deviation from the mean,
and the relative average deviation from the mean. 9. What is meant by the term indeterminate errors? 10. Differentiate between qualitative and quantitative analysis. 11. What is meant by the term accuracy? “any:cannnnnnnnnnnnnnnn. ...
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 Fall '11
 TRICCA
 Standard Deviation, Mean, Plant Food

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