experiment 6 - gravimetric determination of iron

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


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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.


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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.


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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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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.


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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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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.).


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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.


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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.


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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.


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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

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