experiment 6 - gravimetric determination of iron
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Experiment 6 - Gravimetric Determination of IronTRANSCRIPT
MALAYAN COLLEGES LAGUNA
EXPERIMENT NO. 6
GRAVIMETRIC ANALYSIS OF IRON
Experiment 6: Gravimetric Determination of Iron 1 | P a g e
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I. OBJECTIVES
Upon completion of the experiment, the student should be able to:
Define the principles and proper techniques involved in precipitation and gravimetric
analysis
perform properly the relevant techniques in precipitation and gravimetric analysis; i.e.
washing, ignition and digestion; and,
calculate the amount of iron in an unknown sample using gravimetry.
II. LABORATORY EQUIPMENT / INSTRUMENTS / REAGENTS
Equipment/Apparatus Quantity
Crucible and cover 2 Glass funnel 1 Wire triangle 1 Glass rod 1 Rubber policeman 1 Dessicator 1 250 mL beaker 2 400 mL beaker 2 Tongs/test tube holder 1 Iron ring 1 Clamps 1 Hot hands 1 Heating pad 1 Bunsen burner 1 Analytical balance 1
Chemical/ Reagents/ Materials
3M HCl (corrosive) Litmus paper 6M HNO3 (corrosive, oxidizing agent) 3M NH3 (corrosive) 12M HCl (corrosive, releases fumes) Filter paper (regular & ashless) Unknown sample with iron Distilled water in wash bottle
MALAYAN COLLEGES LAGUNA
EXPERIMENT NO. 6
GRAVIMETRIC ANALYSIS OF IRON
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III. DISCUSSION OF FUNDAMENTALS
Introduction
Analytic chemistry is concerned with the critical measurements of anything that involves
chemicals and substances and such, quantitatively or qualitatively. One of the methods acquainted
with analytical chemistry would be classical methods, which involves the various measures of solid-
state substances. Under this classification would be one of the most important aspects of analytic
chemistry, which is gravimetry, which is the simplified counterpart of stoichiometry. Stoichiometry is
the relation between quantities of substances that take part in a reaction or form a compound
(typically a ratio of whole integers). Using stoichiometric relations is not hard typically if you
understand how it moves around back and forth, but writing it down includes some length of time.
Alas, gravimetry is born. To make up for this kind of lengthy solving, gravimetry only uses a single
ratio that converts one another but still follows Dalton’s law of mass and proportions, which is called
the gravimetric factor. GF is used to convert substances that are specifically related to one another,
which is a principle of mass conservation. Though in a way, gravimetry is limited in such a way that
the experimental process of determining the values should be undergoing a ‘gravimetric analysis’. It
includes preparing the solution, precipitating, digesting, filtering, weighing and drying/ignition. This
experiment will focus on the gravimetric determination of iron, from an unknown compound with
an iron composition. Gravimetric analysis steps will be used, as the unknown compound will be
prepared to precipitate to gelatinous rust ( ). It is ignited to lose the water on it,
forming rust. Continuous constant weighing will in the end get the percentage iron of the sample.
Discussion
Gravimetric analysis is an analytical method which uses mass measurements and knowledge of
reaction stoichiometry to determine the amount of analyte/s in a sample.
Such procedures are exemplified in the gravimetric determination of iron. The analysis is based
on the fact that iron as Fe3+ species forms an insoluble precipitate as iron (III) hydroxide. The
formation of this precipitate is pH dependent (above pH 5), and the precipitates in a mixture include
iron (III) hydroxide, Fe (OH)3 and FeO(OH), usually represented as Fe2O3xH2O. The latter is a
gelatinous precipitate which contains appreciable amount of water that makes quantitative
determination of iron difficult. In such a case, the iron should be completely precipitated in a form
that is useful for gravimetric analysis.
MALAYAN COLLEGES LAGUNA
EXPERIMENT NO. 6
GRAVIMETRIC ANALYSIS OF IRON
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The first part should ensure that iron in a sample mixture is converted to Fe3+ form (Hage and
Carr 2011). The samples can be heated with a mild oxidizing agent such as bromine water or nitric
acid to convert Fe2+ to Fe3+. The pH is controlled by precipitating the iron in ammonia instead of
NaOH. The use of ammonia not only controls the pH of the solution but also prevents
coprecipitation of any other insoluble metallic ions into metal hydroxides. Since ammonia is highly
volatile, it can be easily removed from the final sample such that the sample is free from impurities.
After precipitation, the sample undergoes filtration, dissolution of the precipitate with HCl to
lower the pH, and a second precipitation step by addition of fresh ammonia. This second
precipitation step helps to further lower the amount of metal ions aside from Fe3+ to precipitate.
This final precipitate is then washed several times with ammonium nitrate until the gelatinous iron
(III) hydroxide is formed. Ignition is then done to convert the hydroxide into a well-defined form of
iron, iron (III) oxide, Fe2O3.
Application
Gravimetry has many practical applications, and from what we are discussing in the lecture,
gravimetry involves the conversion of a compound to another compound that is related by their
molecular formulas, i.e. both have arsenic, though one is a sulphate whereas one is a nitrate.
Gravimetric analysis provides information as to the determination of percentages of metallic
substances in a sample, as to which the sample contains any compound that is molecularly related
to the metal in query.
MALAYAN COLLEGES LAGUNA
EXPERIMENT NO. 6
GRAVIMETRIC ANALYSIS OF IRON
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IV. METHODOLOGY
The crucible was heated until it started
to glow orange.
After heating, the set up was let to cool in
the open for 20-30 minutes.
After the crucible has been cooled, it
was then placed in the desiccator and
was weighed in the analytical
balance.
The steps were repeated until
constant weigh was observed.
Figure 1. Constant mass of the crucible
About 0.60 g of the sample
was weighed into a 400-mL
beaker. 15-mL water and 10-
mL of 3M HCl. 5-mL of 6M
HNO3 was added. It was then
boiled until it was clear
yellow.
The sample was diluted to
200-mL with distilled water.
3M ammonia was added
with constant stirring until
the solution was basic. The
precipitate was digested by
boiling.
The filtration set up was
assembled while waiting for
the precipitate to settle. The
supernatant liquid was
decanted through the filter
paper.
The filter paper was allowed
to drain thoroughly.
The filter paper containing
the precipitate was heated
while inside the crucible
previously heated to
constant mass.
When ignition was
completed, the crucible was
let to cool in the open and
was transferred to the
desiccator.
The precipitate and crucible
were weighed and the steps
were repeated until constant
weight was seen.
Weight percent of iron was
calculated in every sample.
Average and average
deviation were also
reported.
Figure 2. Gravimetric determination of iron
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GRAVIMETRIC ANALYSIS OF IRON
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V. DESCRIPTION OF THE APPARATUS / SET - UP
Figure 1. Ignition Set-up
The ignition set-up is done to primarily dry the washed filtrate, which is usually in its gelatinous
form. This is very important to do, especially since we didn’t know how much hydrate molecules
have been stuck up in the precipitate, so in all doing getting the pure precipitate will lead you to
more accurate results as pertained to the gravimetric determination of a certain substance.
MALAYAN COLLEGES LAGUNA
EXPERIMENT NO. 6
GRAVIMETRIC ANALYSIS OF IRON
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VI. DATA SHEET
Table 1. Constant weight of Crucible
Trial Mass (g)
1 42.241
2 42.241
Table 2. Weight of unknown sample
Weight of unknown iron sample (g) 0.598
Table 3. Constant weight of Crucible + Sample
Trial Mass (g)
1 42.4266
2 42.4173
3 42.4171
Ave 42.4172
Weight
Iron 0.1761
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EXPERIMENT NO. 6
GRAVIMETRIC ANALYSIS OF IRON
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Table 4. Percentage iron
Trial % Iron
2 20.58 %
3 20.61 %
Average 20.60%
VII. SAMPLE CALCULATIONS
( (
)
)
VIII. RESULTS AND DISCUSSIONS
The experiment is all about the gravimetric determination of iron. Gravimetric analyses rely on
some final determination of weight as to getting on quantifying an analyte. Since weight can be
measured better and more precisely than any other state, since liquids are always measured by their
volume, and their measuring instruments prove to be quite estimate-friendly, which proves it to be
systematic error related. And another, it is the most accurate to be measured among any other
fundamental properties. With this, gravimetric analysis is one of the most accurate classes of
analytical methods available. This branch of the classical method amongst analytical chemistry is
one of oldest techniques, and proved to be very important. The experimental process may be quite
lengthy and tedious, since samples are extensively treated to remove interfering substances (or the
matrix). As a result, gravimetric methods in experiments are not popularly used amongst
environmental analysis (i.e. petroleum gathering in the depths of the Earth, etc.).
MALAYAN COLLEGES LAGUNA
EXPERIMENT NO. 6
GRAVIMETRIC ANALYSIS OF IRON
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The gravimetric analysis process is subdivided among four different types; physical gravimetry,
electrodeposition, precipitate gravimetric analysis and thermogravimetry. The differences between
these types is on the acquisition of the sample before weighing the analyte, or in simple terms, how
to prepare it. Physical gravimetry is the one that is commonly used in environmental engineering. It
involves the physical separation and classification of the matter that is found in the samples in the
environment, from which the separation and classification would be differed on their particle size
(i.e. total suspended solids or colloids) and its volatility (evaporating readily at normal temperatures
in pressures, for liquids or for those who behave like liquids i.e. Non-Newtonian fluids).
Electrodeposition is the electrochemical reduction of a cation (a metal ion, to be specific) at a
cathode, and the simultaneous deposition of the ions on the cathode itself. Precipiate gravimetric
analysis is pretty self explanatory, as it acquires its analyte from precipitating it from a sample
through a series of chemical reactions. Since this is the easiest methods amongst all four, this
proved to be a great method in the environmental field, specifically pertaining to sulfite (since
sulfite is generally insoluble to any other ion you will mix it with, citing a few exceptions, for most).
Lastly, thermogravimetric analysis samples with hydrates or something that can be evaporated is
heated, and the changes in mass are also recorded. Volatile solids are of major concern to this type
of gravimetric analysis.
In this experiment, we are going to gravimetrically determine the percentage of iron through an
unknown iron sample, which the combined process of thermogravimetric and precipitate
gravimetric analysis.
The experiment started with the determination of the
constant weight of the crucible. The crucible is to be heated
strongly with a Bunsen burner as shown. This process is done
since all solids have a certain affinity to water, even the
containers. If exposed in the air, even under laboratory
conditions, it might pick up water molecules from vapor,
adding weight to the object. Also, water is a viable medium
to grow bacteria, which can also contribute to the mass of
the object. With this, the subject must be either heated or be put in a dessicator. The crucible is
then heated to become orange. To further ensure that the heat is evenly spread among the crucible,
the clay triangle that supports the crucible should be glowing. This can be done in a faster way when
the crucible is placed in the hottest position of the flame, in this flame’s case, the light blue portion
inside. This process is continued for ten minutes, letting the heat in and out to avoid cracking of the
crucible (since it cannot sustain that much heat at a prolonged period of time). After heating, it will
be cooled by putting it in a rubber mat with aluminum sheets, to room temperature. The crucible
Figure 3. Heating set-up.
MALAYAN COLLEGES LAGUNA
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GRAVIMETRIC ANALYSIS OF IRON
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can be cooled in the triangle after heating, however, the heat that the triangle received can be
transferred to the crucible. Moreover, the aluminum in the mat provides conduction of the heat
from the crucible and absorbing it, then the rubber under regulated that conducted heat, making
the cooling process faster. By replacing the crucible to various positions in the mat and touching its
previous position, by the time that all heat has been dissipated from the crucible, it can now be
transferred to a dessicator. Since it is just previously heated, the switching from a hot temperature
to a cooler temperature, the water vapor that is in the air can project into the crucible, and
condensate. This event is what we are trying to avoid, so in this case, the dessicator is used to get rid
of water molecules to take part in the weighing process. After such time that is in the dessicator, the
crucible is now weighed with the cover. This process is repeated until the mass of the crucible is
constant to 0.3 mg.
While the determination of the constant weight of the crucible is in progress, the original
purpose of the experiment can be started. By getting about 0.60 g of the unknown sample in a 400-
mL beaker, the solid is diluted with water and 3M HCl. This should be done under the fume hood
since there might be unnecessary reactions that might bring out fumes that are harmful if released
to the working environment. If the solution contains impurities that cannot be further dissolved,
filter through normal filter paper. After this, put 5mL of nitric acid (HNO3) to the solution and heat it
to a boil, for 5 minutes. By putting the beaker to the rubber mat, regardless of temperature, we
dilute the solution to 200 mL. This step is to be done under no confusion that we are going to dilute
it to a total of 200 mL, not dilute it with 200 mL of distilled water, as this will bring up errors in the
final result. After this, 3M ammonia (NH3) is qualitatively added until such time that the solution is
basic. The basicity of the solution can be determined with the litmus paper. It need not be on a
certain pH; nevertheless its basicity is what is only important. This step will cause the solution
change from yellow to a yellowish white, and there would be coagulation of the precipitate, or the
colloidal form of the precipitate. The precipitate that can be seen is in now in the formula
Fe2O3·xH2O, meaning its hydrate molecules are not specifically known. By continuing the process,
the precipitate that is formed is digested to the solution by letting it boil for another 30 minutes.
This is done primarily because of the missed out ions in the solution, and by digesting it any other
ions that were not precipitated out (hence called the coprecipitates) will now form and join the
precipitate.
MALAYAN COLLEGES LAGUNA
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GRAVIMETRIC ANALYSIS OF IRON
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While the precipitate has not yet precipitated, the
filtration set-up is then assembled. The filter paper used
is a coarse, ashless filter paper (Whatman 41, 110-mm
diameter). This is not the same as the regular filter paper
that is used when filtering impurities. We are using an
ashless filter paper since an ordinary filter paper will
combusted but its remains will not be disintegrated into
the atmosphere, wherein the ashless paper is. The net
effect of this is when you will be using a regular filter
paper, and then the weight of the ignited substance will
account the burnt weight of the filter paper, though not
that accurate since some of its ashes might be disintegrated into the air, or a similar event that
would rise to that. To make the filter paper stick into the
funnel, wet it with distilled water. After the precipitate has
been settled, the filtration process can be started. Regardless
of its temperature, one can already filter even though it just
came off from heating. Also, the heat from the solution helps
since the solution is less dense, and therefore its viscosity (or
the resistance to the relative flow speed) is low, therefore
the solution will be filtered up faster than what it should be
when it is at room temperature. The excess solution (or the
supernatant) is to be decanted first, using the stirring rod.
Be careful not to splash, spill, splatter or pour the liquid 1 cm
higher than the funnel, for the liquid might bring out some of
the precipitate and be left out when spilled or when anything
above happens, making the error bigger. When the
supernatant liquid is decanted to a close, transfer the
precipitate quantitatively using a rubber policeman, fixated
on the stirring rod. When there is still precipitate on the
beaker, pour the supernatant liquid on it all over again,
reform the solids that was all over in the beaker, then re-filter. After the entire solid has been
transferred, wash with pre-prepared hot ammonium chloride (NH4NO3) in the sides of the filter
paper to further center the filtrate for lesser work in the next step.
After all or little of the supernatant liquid is drained from the filter paper, carefully lift the paper
from the funnel and fold it, and putting it to the crucible that has been weighed to constant mass,
as shown. The filter paper must be carefully removed from the funnel where it has been stuck, as to
Figure 5. Decantation.
Figure 4. Filtration set-up.
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where the filter paper must not be torn down during this step.
Position the crucible to the heating process again, as shown in
Figure 2. Heat the set-up to low flame. Do not cover the crucible
as the ignition of the precipitate inside will require a substantial
amount of oxygen, and the water that has not been drained will
evaporate. When the precipitate and crucible are already dry
(by inspection or by checking), turn the burner to full heat to
burn the filter paper. After such time that the filter paper is
burned, the gelatinous red precipitate now turned into a black
powdery substance, which is hence known as rust, or Fe2O3. By
inflaming the crucible for 15 minutes to ensure complete
ignition of the iron oxide, it was then allowed to cool at room
temperature via the same conditions when determining the constant mass of the crucible, then
weighing it. By repeating ignition, cooling and weighing until the successive weighing that comply
agree to 0.3 mg, the weight of the iron oxide can be computed through weighing by difference and
finally getting the percentage iron of the sample by the formula:
(
)
.
IX. SUMMARY AND CONCLUSIONS
By following the steps involving gravimetric analysis, this experiment concluded the concepts
that involves on this classical analysis method. By having an unknown compound with an iron
composition, it was prepared with an acid-base reaction to form a gelatinous precipitate which is
reddish. After digesting it, it will be filtered by a coarse, ashless filter paper, as to avoid any
irrelevant weight being in the final procedure. By continuous ignition and getting a constant weight,
the final form, which is rust, is now used as to get the gravimetric determination of iron, which the
percentage is computed to be 20.60%, which is within the range of how the theoretical weight will
be in this experiment. Errors should be in particular be very much avoided, since every step in the
experiment will provide crucial inconsistency with the final form of the sample when errors are
introduced for they are very sensitive, even the slightest of errors would cause a dramatic change
since the values that we are talking of here is significantly small, and putting a small error to a small
range would by ratio give you a bigger ratio than what you expected.
Figure 6. Precipitate ignition set-up.
MALAYAN COLLEGES LAGUNA
EXPERIMENT NO. 6
GRAVIMETRIC ANALYSIS OF IRON
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X. POST LAB QUESTIONS
1. What is the effect of prolonged digestion?
Answer: Digestion is referred to as the need to dissolve or dissolute a colloid, and
subject it to reform again in a much bigger state than what it was before, an event that we call
as recrystallization. During rest comes the completion of coagulation for solids with colloidal
reduction of the surface and thus there will be less adsorption than it would normally be,
therefore flocculation occurs, swelling the existing crystals. If this process is prolonged, then we
could get all the other ions that we missed out (or the coprecipitates) that is in the solution that
is unprecipitated, so we could get a lower percentage difference.
2. What is the purpose of using an ashless filter paper? Answer: We have to use an ashless filter paper since the weight of an ordinary filter paper will contribute to the overall weight of the dried precipitate, which is undesirable because it will introduce a systematic error in the process. An ashless filter paper burns out to become tiny ashes that deteriorate in contact with air (which is proved by our group as to which we burned an ashless filter paper, that disintegrated in the air leaving no clumps of ashes behind), therefore leaving out only the filtrate alone.
3. Why is filtration of a gelatinous precipitate done while the solution is still hot?
Answer: The filtration of a gelatinous precipitate that is hot will be faster than a cold one,
since the solution is less dense, therefore the solution would have practically less viscosity (even
though it is similar to water but is basic) and there would be greater flow in the filtration
process. Also, heat helps in the coagulation of the substance, which is very much needed for the
colloidal formation of the precipitate.
4. Calculate the expected amount of Fe2O3 (in g) that would be precipitated when 0.6094 g of FeCl3 is used. Calculate the percent Fe in this iron (III) oxide.
Answer: (
)
( (
)
)
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5. Using the expected results from #4, calculate your percent relative error. Are your errors systematic or random? Explain the sources of errors.
Answer:
Any errors that were made would be random, as there are certain environmental
disturbances that cannot be accounted for, and certain estimations to the measurements would
arise to the discrepancy of the theoretical to the experimental values.
XI. REFERENCES
Christian, Gary D. 2004. Analytical chemistry (6th ed.). John Wiley and Sons Inc.
Hage, David S. and James D. Carr. 2011. Analytical chemistry and quantitative analysis. New Jersey:
Pearson Prentice Hall.
Harris, Daniel C. 2003. Quantitative chemical analysis. (6th ed). New York: W. H. Freeman and
Company.
Skoog, Douglas et. al. 2004. Fundamentals of Analytical Chemistry (8th ed.). Singapore: Thomson
Learning.