I. Introduction

The evolution of scientific knowledge owes its pace to the intelligence of mankind that

paved techniques to be passed to one generation after another, from the discovery of the nature

of atom, to Mendeleev’s creation of the periodic table, leading to T.W. Richards and his

students’ determination of the atomic masses of certain elements. The nature of Analytical

Chemistry is the identification of composition of materials, either through quantitative analysis

or qualitative analysis. The latter involves a method called Gravimetric Analysis that plays

significant role in the determination of the amount of species in a material through the

conversion of that species to a product that can be isolated completely and weighed (Gammon et.

al, 2009). Most traditional gravimetric methods require the knowledge of stoichiometric

reactions, solubility rules and the calculation of mass of substance.  

Gravimetry is comprised of sub-procedures such as precipitating the sample, filtering the

solution, washing the precipitate free of contaminants, igniting the precipitate and finally

weighing the precipitate and determining its mass by difference. Precipitation is a process in

which the sample is reacted with another sample to form an insoluble product which is called the

precipitate while the manner of separating the precipitate from the mother liquor is filtration. It is

necessary to assure that the precipitate is free from impurities within, large enough to filter and

negligibly soluble. Washing of precipitate with liquid removes all soluble impurities sticking

with the precipitates (Hage et. al, 2011). After separation, the substance must undergo ignition

before weighing by heating up the precipitate to drive off excess solvent and volatile electrolytes

but it is subjected to change the chemical composition of the precipitate.

An advantage of gravimetric analysis is that identifying the mass of a substance is one of

the most accurate measurements that can be made with errors of less than 0.2% (Hage et. al,

2011). This method of analysis has a real life applications such as the determination of chemicals

in contaminated water, amount of fat a food may contain, chemical analysis of ores and other

industrial materials, in the calibration of instruments, and in the elemental analysis of inorganic

compounds and measurement of the essential elements in plant foods. Although the process is

time-consuming and tedious, the method guarantees an accurate result.

In this experiment, the Gravimetric Determination of Iron, the purpose is to define the

principles and standard techniques involved in precipitation and gravimetric analysis. It aims to

obtain the percent composition of the analyte, which is Iron, in an unknown sample using

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gravimetric data. Gravimetric factor is defined to be the algebraic expression that converts grams

of a compound into grams of a single element. It is the ratio of the formula weight of the

substance being sought to that of the substance weighed. The formula for gravimetric factor is:

m(FW of Substance Sought )n(FW of Substance Weighed)

,where m and n are molar masses.

On the other hand, the formula for the percentage of an element in a sample is given as:

%substance sought=weight of substance sought (g)

weight of sample (g)x100 %

where,

weight of substance sought (g )=weight of precipitate (g ) xFW of substancesought ( g

mol )FW of substance weighed ( g

mol )x

mol substance soughtmol substance weighed

The experiment is designed to measure whether the techniques in precipitation and

gravimetric analysis were properly performed by the analysts in order to yield an accurate

outcome. Thus, in this experiment, one will analyze the amount of an iron in a given sample by

precipitating, from basic solution, the hydrated Iron Oxide. The reaction is immediately followed

by a dehydrated reaction to produce the solid Fe2O3. The gelatinous hydrous oxide can block

impurities. Therefore, the initial precipitate is dissolved in acid and re-precipitated. Because the

concentration of impurities is lower during the second precipitation, occlusion is diminished

(Harris, 2003).

II. Methodology

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Constant Mass of the Crucible

In the beginning of the process, two crucibles were labeled as crucible A and

crucible B. Both crucibles were weighed so that the initial mass can be bases of comparison to

the mass of the crucibles when heated. The crucible A should be heated in a way of bringing

the flame in and out every two seconds. When the crucible glowed orange, the heating

continued for 10 more minutes while the ceramic supports on the clay triangle also glowed.

After heating, the set-up was cooled until the crucible was at room temperature. The crucible

was placed in a dessicator which was then brought to the balance room. The crucible was taken

out of the dessicator and was weighed in an analytical balance. After recording the mass, the

crucible was placed back in the dessicator to keep the crucible dry. If the crucible were not

placed in the dessicator it is subjected to moisture and, therefore, can make the mass of the

crucible heavier since what was weighed was the mass of the crucible plus moisture. The steps

done with crucible A were repeated with crucible B. Alternately, while crucible B was being

cooled down, the 2nd trial for crucible A was being performed. The process was continued until

the 2nd trial for crucible B was performed. Successive weighing must agree within 0.3 mg.

Gravimetric Determination of Iron

Two trials were performed in this part of experiment. In the first trial, the obtained

mass of the sample, named sample A, was 0.546g and then placed in a 400-mL beaker. For the

second trial, sample B with a mass of 0.606g was also placed in a 400-mL beaker. Both

samples were added with 15-mL water and 10-mL of 3M HCl. Both samples were not filtered

and, therefore, was added with 5-mL of 6M HNO3 to the solution and boiled for a few minutes

until the solution becomes clear yellow. Both samples were diluted to 200-mL distilled water

and were added with 3M of NH3. The solutions were constantly stirred until it was basic. The

basicity was determined through the use of litmus paper. The precipitate was then digested by

boiling for 5 minutes. Since both samples did not boil, the solutions were heated vigorously for

25 minutes and then allowed the precipitate to settle.

In filtering both solutions, an ashless filter paper was used to each in order to

avoid contamination of the sample. Note that sample A and sample B were not filtered

simultaneously due to unavailability of equipments. The ashless filter paper was wetted to

make it stick to the funnel. The supernatant liquid of both solutions were decanted not higher

than 1 cm from the top of the funnel. All solid from the beaker were quantitatively transferred

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to the filter paper through the use of rubber policeman and hot 1% (w/w) NH4NO3. The filter

paper that contained the precipitate was drained thoroughly until it was ready to lift out of the

funnel. The filter paper was folded in a manner in which it must be flatten, the edges were

folded then the top and finally placed inside the crucible with point pushed against bottom. The

crucibles were properly labeled with sample A and sample B.

The crucibles that each contained the samples were left in a container for two

nights. The experiment was then continued, starting with the process of ignition. Each

uncovered crucible was subjected to heating with full heat coming out from the burner to

completely burn the filter paper. Through the use of crucible tongs, the cover was placed on the

crucible, which contained the precipitate formed in sample A, every time the filter paper inside

the crucible was caught with flame. However, the cover was also removed every after a few

seconds to have access to air to avoid turning the carbon into graphite or to avoid reduction of

iron. Moreover, the crucible was also moved every once in a while so that the heat was not

concentrated on one side of the base. By the time, the filter paper was charred, the position of

the crucible was moved in a manner that the flame evenly touches the surface of its base. It was

subjected to full heat for about 15 minutes to ensure complete ignition of the Iron Oxide. When

ignition was completed, the crucible was cooled in air until it reached room temperature level

before it was transferred to the dessicator. The crucible and the lid were individually weighed.

The sum of the recorded mass of crucible and lid was then subtracted to the constant mass of

the heated crucible A without the precipitate and the difference was the mass of Iron Oxide.

The process was repeated for the second trial of crucible A, first trial of crucible B and second

trial of crucible B with both trials in crucible B containing the precipitate were each subtracted

to the constant mass of the heated crucible B without the precipitate. After the weight percent

of iron in each sample was obtained, the average and average deviation were then calculated.

III. Results and Discussion

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The initial part of the experiment focused on the weighing of two crucibles until constant

mass was achieved. When the difference of the two mass

of each crucible from the first trial and the second trial

does not lie within the range of 0.3 mg, the procedure

must then be repeated until successive weighings become

constant in order to draw out all the volatiles and moisture

of the sample.

CRUCIBLE A CRUCIBLE B

INITIAL MASS 39.423 g 38.298 g

1ST HEATING 39.424 g 38.299 g

2ND HEATING 39.424 g 38.299 g

Figure 6.1 Mass of two crucibles at initial condition and when heated.

On the second-half of the experiment which focuses on the Gravimetric determination of

Iron from the sample, the principle of gravimetric analysis in which one measures the mass of a

material formed in the reaction of the analyte with

the reagent was employed. General procedures

included in gravimetric analysis are precipitation,

filtration, washing, and ignition.

When the sample was added with H2O, the product

of the reaction was then added with HCl. Nitric acid

plays an important role when added to the solution

to convert Fe2+ to Fe3+. The solution was heated,

diluted and added with 3M of ammonia until the

solution was basic, which was determined through

the use of litmus paper. Prolonged digestion was

then performed on the solution when the precipitate is left to rest in contact with the hot or cold

solution for a period of time. In this process, coagulation for solids is formed. There are factors

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The solution of the unknown sample, added with H2O, HCl, HNO3, diluted, added with 3M NH3

then finally digested, was being cooled down.

Each of the crucibles was heated through the use of Bunsen Burner.

needed to be considered while performing the precipitation method. First, precipitates must be

unbounded from soluble impurities and it must insoluble in solution. Precipitates must be

readily separated from the solution by filtration.

By the time the precipitate has settled, the solution was filtered with the use of ashless

filter paper. Using an ashless filter paper, one can avoid improper measurement and get an

accurate mass of the unknown sample without contaminating the sample because it will not

leave ashes behind after combustion.  Size and diameter of filter paper should be taken

recognition and must be in accord to the bulk of the precipitates. Moreover, pores of filter must

be smaller than the size of the particle of the precipitates. Filtration of a gelatinous precipitate

done while the solution is still hot because it helps coagulation which means, solvent can pass

smoothly when filtrated.

In washing the precipitate, the properties of

ideal washing liquids should be taken consideration of

in which there is no solvent action on precipitates but

must removes all foreign impurities. It should not

form any volatile product with precipitates, should

easily volatile on ignition, should have no dispersive

action on the precipitates and should not interfere with

precipitates (Gammon et. al, 2009). The precipitate was then subjected to ignition through the

use of Bunsen burner. Ignition of the precipitate was a time-consuming process since the filter

paper in the crucible must be completely dry for faster burning. After ignition, the crucibles

were then placed in a dessicator. The dessicator contains

silica gel beads that remove water from the precipitate.

The crucibles should immediately be placed inside the

dessicator when it reached room temperature. If it were not

placed in the dessicator, it is subjected to moisture since

the material used for crucible is porous.

MASS OF CRUCIBLE MASS OF LID MASS OF IRON OXIDE

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The precipitate obtained from the filtered solution being ignited through Bunsen burner.

The ignited precipitate left black stains on the inside of the crucible lid.

Trial 1 27.989 g 11.611 g 0.171 g

Trial 2 27.989 g 11.608 g 0.173 g

Fig.6.2 Mass of Iron Oxide from the difference of the mass of crucible A and its lid.

MASS OF CRUCIBLE MASS OF LID MASS OF IRON OXIDE

Trial 1 28.318 g 10.521 g 0.54 g

Trial 2 28.318 g 10.522 g 0.541 g

Fig.6.3 Mass of Iron Oxide from the difference of the mass of crucible B and its lid.

To compute for the percentage of Fe, the formula below was used,

g Fe2O3×2 Fe

Fe2O3

× 100

g of sample=% Fe

The following was the computation for the percentage of Fe in the sample in crucible A,

g Fe2O3×2 Fe

Fe2O3

× 100

g of sample=% Fe

0.173 g Fe2O3×2(55.85) Fe

(159.7)Fe2O3

× 100

0.546 g of sample=22.16 % Fe

The following was the computation for the percentage of Fe in the sample in crucible B,

0.541g Fe2 O3×2(55.85) Fe

(159.7)Fe2O3

×100

0.606 g of sample=62.37% Fe

The following presents the computation for the expected amount of Fe2O3 that would

precipitate when 0.6094g of FeCl3 is used:

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Using gravimetric factor,

0.6094 g Fe Cl2×159.69 g Fe2O3

(2)162.2 g Fe2 Cl3

=0.2999 g Fe2O3 is obtained . 

Using stoichiometry for elaboration and details,

 

0.6094 g FeCl3 ×1 mol Fe2 Cl3

162.2 g Fe2 Cl3

x1mol Fe 2 O32mol Fe2Cl3

x159.69 g Fe2O3

1mol Fe2Cl3

=0.2999 g Fe2O3 is obtained .

Thus, the mass of Fe2 O3obtained is 0.2999 g.

In calculating the percent Fe in the iron (III) oxide, the following calculation was done,

0.2999 g Fe2O3×55.85 g Fe

159.69 g Fe2 O3

×4 Fe

2Fe2O3

0.2999 g Fe2 O3

x 100 % = 69.95% of Fe in Iron(III) Oxide.With the equation of percent relative error,

x−μμ

x100 % and a mean of,

x=22.16 %+62.37 %

2 = 42.265%

the percent relative error, using the expected result, may be calculated as,

¿42.265 %−69.95 %∨ ¿69.95 %

¿ x 100%= 39.58%

In this result, it is proper to say that there is a random error. One error is that the crucibles

used in the experiment were mixed with other crucibles and there is uncertainty whether the

crucible obtained for ignition is still the same crucible. This can be a factor because every

crucible used doesn’t have an equal weight. Another error is that, not all of the ashless paper is

incinerated. There is a few amount of paper when we weighed the sample leaving it to lower or

higher percentage from the theoretical value.

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IV. Conclusion

Through gravimetric analysis we were able to determine the percent composition of Fe in

the unknown sample. One of the most important sources of error in this experiment would have

to be the nature of the precipitate was in colloidal form. The form of colloidal precipitation is

caused by adding the precipitant in a hasty manner instead of slowly which is the reason why the

solution in the beaker does not clearly distinguish the separation of the solvent to the precipitate.

Moreover, using high concentrations of analyte and precipitant can also lead to form a not-so-

good precipitate. To achieve a good precipitate, it is important to digest the colloidal sample by

heating. Another error accounted was that the crucibles left in the dessicator were switched to

another group, thus, leaving a hunch that the walls of crucibles were contaminated if the person

who held it with bare hands. If fingerprints were left on the walls of the crucible, it could add

more mass to the crucible when weighed in an analytical balance. Furthermore, when crucibles

were not immediately placed in the dessicator after cooling, the moisture absorbed by the

crucible can also add more weight compared to the original. Crucibles are made in materials that

are porous and can, therefore, easily absorb moisture. The ashless filter paper also took some

time to burn completely. However, in the experiment performed the filter paper was not

completely incinerated.

Judging by the computed result, the experiment performed did not yield great accuracy

because of systematic errors. However, if errors were minimized, the experiment could have

proved that Gravimetric Analysis through the determination of Iron in an unknown sample can

really yield accurate result as much as stoichiometric methods can.

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V. References

Ebbing, Darrell D. and Gammon, Steven D. 2009. General Chemistry. New York: Cengage Learning.

Hage, David S. and Carr, James D. 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.

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