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Iodometric Determination ofCopper
n ne en ons a e eore an er
XX YY
Pennies are an interesting
. ,an always present means of
currency. The penny is ingrained in
our society for many different
social and economic reasons.
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Pennies made from 1962 to 1982 consist of 95%
copper and 5% zinc. These copper rich pennies are
currently worth more as a metal source than their
face value.
Pennies minted after 1982 to the present day are
only made up of 2.5% copper. This copper is plated
onto a zinc core that makes up 97.5% of the penny.
The determination of copper in both pennies made
before and after 1982 will be determined using the
same method.
Introduction to Iodometric
Determination
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Wet Chemical Analysis
ore accurate an prec se t an nstrumenta
methods.
Systematic errors have been detected in the
past and can be accounted.
More involved process
Help to learn new techniques
Reinforce the techniques learned so far
Process selection - Iodine
se w e y to quant tat ve y measure
chemicals reactions
Many analytical procedures are based on the
release or uptake
An analyte that is an oxidizing agent is
added to excess iodide to produce iodine The iodine produced is determined by
titration with sodium thiosulfate
This method is called "iodometry"
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Iodometry
osen or ts stra g t orwar proce ure
The reaction is rapid and quantitative
The starch used is a clear and determinant
indicator
The reagents involved are all easily obtained
and in good supply.
Volumetric determination with a back titrationis a well documented method of determining
the amount of an ion in a solution.
Volumetric Determination
selected as appropriate to the ion being measured
dissolved into an aqueous solution
used in a titration to standardize a secondary standardsolution of sodium thiosulfate
The analyte is reacted with iodide to oxidize ion toiodine
The iodine is then titrated with thiosulfate with astarch indicator to determine the endpoint
With the known amount of thiosulfate used to titratethe freed iodine, we can determine how much iodidereacted with our analyte and then determine howmuch analyte is present
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Iodometry ,
I3- + 2 e- 3 I-
The iodide ion is a strong reducing agent that many oxidizingagents can react with completely
The amount of iodine liberated in the reaction between iodideion and an oxidizing agent is a measure of the quantity ofoxidizing agent originally present in the solution
The amount of standard sodium thiosulfate solution re uired totitrate the liberated iodine is then equivalent to the amount ofoxidizing agent
Iodometric methods can be used for the quantitativedetermination of strong oxidizing agents such as potassiumdichromate, permanganate, hydrogen peroxide, oxygen, and inmy case, the cupric ion.
Sodium Thiosulfate
a e rom sso v ng so um t osu ate
pentahydrate salt
Solution is standardized against a primary
standard
Iodine oxidizes thiosulfate to the
tetrathionate ion:I2 + 2 S2O3
2- 2 I- + S4O62-
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Primary Standard
o ne s rare y use as a pr mary stan ar
for thiosulfate because it presents a problem
in weighing and maintaining its solution
concentration
Pure copper in the form of copper wire is
thiosulfate when it is to be used for the
determination of copper as is the case with
this experiment
Secondary Standard
copper(II), a precipitate of CuI is formed along withI2. The liberated iodine is then titrated with standardsodium thiosulfate
2 Cu2+ + 5 I- 2 CuI(s) + I3-
I3- + 2 S2O3
2- 3 I- + S4O62-
excess iodide force the equilibrium to the right in arapid quantitative reaction
The net stoichiometry of the reaction is 1:1 since 2moles of copper indirectly requires 2 moles ofthiosulfate
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Indicator
signaled by the color change of the starch indicator.
The active ingredient in starch is a helical polymer of-D-gluocse called -amylose, which forms a deepblue-black complex with molecular iodine
For use in this indirect method, the indicator isadded at a oint when virtuall all of the iodine hasbeen reduced to iodide ion, this helps the thedisappearance of color to be rapid and sudden
The starch indicator solution must be freshlyprepared since it will decompose and its sensitivityis decreased
Preparation of Copper
or ion to two or more ligands by coordinate covalent bonds
A ligand is a negative ion or neutral molecule attached to thecentral metal ion in a complex ion
In this experiment, copper and copper-clad pennies aredissolved in a concentrated aqueous solution of nitric acid,HNO3
In aqueous solution, most of the first-row transition metals form
octahedral complex ions with water as their ligands:Cu(s) + 4 HNO3(aq) + 4 H2O(l)
Cu(H2O)62+(aq) + 2 NO2(g) + 2 NO
3-(aq)
Once the pennies have been dissolved, the copper is free toreact with iodide ions
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Experimental Procedure
Standardizing Thiosulfate Measure out ~1 L of de-ionized water
Heat to boilin boil for at least five minutes
Add 25 g of Na2S2O35H2O to 1-L volumetricflask, and mix with hot boiled water
Add ~0.1 g of Na2CO3 as a preservative
Fill to the mark and mix
Transfer the solution to a 1-L plastic bottle
prevent decomposition of the thiosulfate, whichis catalyzed by light.
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Standardizing Thiosulfate
Three pieces of clean copper wire were weighed using about0.20 to 0.25 g each and placed in seperate 250-mL Erlenmeyerflasks
About 5 mL of 6 M nitric acid is added to each flask
Dissolution is promoted by warming the solution on a hot plate
De-ionized water is added to keep the volume constant.
After the copper has dissolved, ~25 mL of water was addedand the solution boiled for a few more minutes to remove morenitrogen oxides
Add 5 mL of urea solution (1 g in 20 mL of water) withcontinued boiling for another minute.
Boilin removes oxides of nitro en which result from thefollowing reactions:
3 Cu(s) + 8 HNO3 3 Cu(NO3)2 + 2 NO + 4 H2O
2 NO + O2 2 NO2 and the oxides are decomposed by urea:
2 HNO2 + (NH2)2CO CO2 + 2 N2 + 3 H2O
Standardizing Thiosulfate
The solution is removed from the hot
The excess acid is neutralized with 4 Maqueous ammonia added dropwise inthe hood until a blue-green precipitate ofcopper(II) hydroxide barely form
To dissolve the precipitate and bring the
so u on o a p ene c a o e ra onreactions, 5 mL of glacial acetic acid wasadded to each flask.
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Analysis of Pennies
For the pennies, six pennies were selected, three from pre-1982 minting and three from post-1982
, ,weighed on an analytical balance
Each of the pennies are placed into a 250-mL Erlenmeyer flask
To each flask, ~10 mL of 6 M nitric acid is added and the flaskswere placed on a hot plate in the hood to dissolve the pennies
After three hours of heating and additional 6 M nitric acid and15.9 M nitric acid added in small amounts and de-ionized waterto keep the solution level constant, the pennies were dissolved
The flasks are removed from the hot plate and allowed to coolin an ice bath
Then ~10 mL of 9 M sulfuric acid is slowly added to each flaskby pouring down the side of the flask
The solution is reheated until white fumes of sulfur trioxideappeared
Throughout the heating process, red fumes of nitrogen oxidescould been seen over each of the solutions
Analysis of Pennies The flasks are then removed again and cooled in an ice
bath
point where the copper hydroxide precipitate has formed
To dissolve the precipitate and bring the pH to a levelappropriate for the titration system, 5 mL of glacial aceticacid is added
The pre-1982 penny solutions has to be diluted to get aproper amount of copper concentration that will beappropriate for the framework of the experiment
The copper solution is quantitatively transferred to a 250
mL volumetric flask and diluted to the mark After dilution, 25 mL of this solution is transferred with a
transfer pipet quantitatively to another 250 mLErlenmeyer flask which is then ready for titrationpreparation
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Iodometric Titration
At this point, the samples were prepared and titrated one at atime.
The flask is covered with a watch glass and allowed to stand for~2 minutes
The solution is then titrated with thiosulfate solution until thebrownish color of iodine is almost gone indicated by a light tanor creamed coffee color
Iodometric Titration Enough iodine has then been reacted to add the 5mL of
starch solution
And 2 g of sodium thiocyanate is added to displace any
adsorbed iodine from the precipitate
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Iodometric Titration
The flask is swirled for about 15 seconds
Then the titration is completed by adding thiosulfate
At the end point, the bluish-gray color of the solutiondisappears and the precipitate appears whitish, or slightlygray
The second and third samples were treated in the samemanner and titrated with thiosulfate
Data and Analysis
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Standardization of Thiosulfate
SolutionSodium thiosulfate pentahydrateFM
248.18 g/mol
Copper molar mass 63.546 g/mol
Mass of thiosulfate used (g) 25.1074
Volume of solution (L) 1.00
Calculated Molarity of thiosulfate
solution
0.101 M
Standardization of Thiosulfate
SolutionTrial 1 Trial 2 Trial 3
ass o copper w re g . . .
Mass of KI added (g) 3.0024 3.0155 3.0012
Mass of KSCN added (g) 1.9816 2.0383 2.0165
Volume of thiosulfate titrated
mL
33.54 37.72 37.00
Measured Molarity of thiosulfate
solution
0.1013 M 0.1017 M 0.1016 M
The average molarity was determined to be 0.1015 0.0002 M.
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Pre-1982 Pennies
Year of Mint 1977 1978 1979
ass o penny g . . .
Volume of thiosulfate titrated (mL) 45.27 45.85 45.59
Measured mass of copper (g) 2.921 2.958 2.963
Measured % of copper in penny 95.10% 95.82% 94.31%
Calc
. . .
The average %wt of copper in pre-1982 pennies is 95.08 0.10
with a standard deviation of 0.7530 from the three trials.
Post-1982 PenniesYear of Mint 1983 1985 1986
ass o penny g . . .
Volume of thiosulfate titrated (mL) 9.27 9.89 10.40
Measured mass of copper (g) 0.05981 0.06382 0.06711
Measured % of copper in penny 2.379% 2.504% 2.659%
Calc
. . .
The average %wt of copper in post-1982 pennies is 2.514 0.003
with a standard deviation of 0.1405 from the three trials.
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Confidence Interval t-test
The 95% confidence interval range with two degrees offreedom for pre-1982 pennies was found to be 96.95 to 93.20 %wt copper
The true value given at the Department of Treasury website is
(19621982) 95% copper, 5% zinc (brass)
which falls within the CI range
The 95% confidence interval range with two degrees offreedom for post-1982 pennies was found to be . .
The true value given at the Department of Treasury website is
(1982 present) 97.5% zinc core, 2.5% copper plating
which falls within the CI range
Sources of Error Copper(I) iodide forms a weak complex with molecular
iodine which slows down its reaction with thiosulfate. As a consequence, once the starch indicator has turned from
blue to colorless, the blue color returns after a few seconds as I2is slowly released into the solution by the CuI[I2] complex
This "after-bluing" can be avoided by adding potassiumthiocyanate just before the end point is reached
The thiocyanate ion, SCN-, replaces the complexed I2 fromCuI[I2], releasing the I2 to solution where its reaction withthiosulfate is rapid.
This is minimized by having a large excess of iodide in order tokeep the iodine in the tri-iodide ion state
Titrations involving iodine should be made in cold solutions inorder to minimize loss through evaporation.
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Sources of Error
In acidic solutions, the iodide ion can oxidize
Therefore, prompt titration of the liberated iodine isnecessary in order to prevent oxidation.
If the starch solution is not made fresh daily orimproperly prepared, the indicator will notbehave properly at the endpoint resulting in a
.
Sources of Error
The addition of concentrated H2SO4 and
fumes appear will ensure that all the HNO3has been removed
NO3- will oxidize I- and would will seriously
interfere with the procedure. The resence of the concentrated sulfuric acid aids
in the exclusion of the remaining nitrogen oxidesfrom the hot solution.
The red oxides first boil off, then fumes of sulfurtrioxide beginning to escape.
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Sources of Error
If excess ammonia is added, copper(II)
,
copper(II) ammonia complex, Cu(NH3)4
If the excess of ammonia is large, some
ammonium acetate will be produced later which
may keep the reaction between copper(II) and
The excess ammonia can be removed by
boiling, and the precipitate will re-form
Sources of Error Concentrated sulfuric acid has a very strong affinity for water
With nitric acid itself, sulfuric acid acts as both an acid and a
Combined with the heating and evaporation of water in the sample,this resulted in a supersaturated aqueous ionic solution which forcedsome precipitates out at certain stages in the experiment
Before the sulfuric acid was added to the pre-1982 penny solutions,the concentration caused copper nitrate to precipitate out
After the sulfuric acid was added, it caused copper sulfate toprecipitate out in well formed crystals
These two reci itates were easil redissolved b addin more de-
ionized water and slightly mixing
Copper
Sulfate
Crystal
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Sources of Error
Dealing with concentrated acids and evaporation of thelittle amount of water in these acids can cause
ump ng or e exp os ons o s eam n esuperheated liquid
Using tongs will keep hands far enough from the mouthof the flask should the solution suddenly "bump" andcause acid burns from the flung drops of concentratedacid
Bumping can also cause small amounts of the analyte to,
flask occasionally will ensure that none of the analyte islost.
Elimination of Error
This experimental method has built in
mentioned previously that can occur.
These systematic errors have been easilycorrected by the refinement of the process
The replication of the experiment is
ensure y e e m na on o esesystematic errors in a concise andrepeatable manner
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Conclusion
measurement of copper in a sample
The amount of error is low because of the correctionof systematic error, and the lowered possibility ofintroduction of random error through thestraightforward procedure
The indirect titration of the iodine with thiosulfate issim le eas to control accurate and recise
The given value of copper content of a penny fell
within the measured 95% confidence interval withthree trials on each type of penny
With more trials, the confidence interval could behoned down to find an even more precise measuredvalue.