7/20/2010
Iodometric Determination of
Copper
I US O
In
One C
Centt C
Coins
i M
Made
d B
Before
f
and
d Aft
After 1982
XX YY
Pennies are an interesting
commodity They are everywhere,
commodity.
everywhere
an always present means of
currency. The penny is ingrained in
our society for many different
social and economic reasons.
1
7/20/2010
Penny Composition
• 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
2
7/20/2010
Wet Chemical Analysis
• M
More accurate
t and
d precise
i th
than iinstrumental
t
t l
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
• U
Used
d widely
id l to
t quantitatively
tit ti l 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"
3
7/20/2010
Iodometry
• Ch
Chosen for
f its
it straightforward
t i htf
d procedure
d
• 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 titration
is a well documented method of determining
the amount of an ion in a solution.
Volumetric Determination
• A primary standard
– selected as appropriate to the ion being measured
– dissolved into an aqueous solution
– used in a titration to standardize a secondary standard
solution of sodium thiosulfate
• The analyte is reacted with iodide to oxidize ion to
iodine
• The iodine is then titrated with thiosulfate with a
starch indicator to determine the endpoint
• With the known amount of thiosulfate used to titrate
the freed iodine, we can determine how much iodide
reacted with our analyte and then determine how
much analyte is present
4
7/20/2010
Iodometry
• The iodine (triiodide) – iodide redox system
system,
I3 + 2 e ֖ 3 I• The iodide ion is a strong reducing agent that many oxidizing
agents can react with completely
• The amount of iodine liberated in the reaction between iodide
ion and an oxidizing agent is a measure of the quantity of
oxidizing agent originally present in the solution
q
to
• The amount of standard sodium thiosulfate solution required
titrate the liberated iodine is then equivalent to the amount of
oxidizing agent
• Iodometric methods can be used for the quantitative
determination of strong oxidizing agents such as potassium
dichromate, permanganate, hydrogen peroxide, oxygen, and in
my case, the cupric ion.
Sodium Thiosulfate
• M
Made
d ffrom dissolving
di
l i sodium
di
thi
thiosulfate
lf t
pentahydrate salt
• Solution is standardized against a primary
standard
• Iodine oxidizes thiosulfate to the
tetrathionate ion:
I2 + 2 S2O32- ֖ 2 I- + S4O62-
5
7/20/2010
Primary Standard
• IIodine
di iis rarely
l used
d as a primary
i
standard
t d d
for thiosulfate because it presents a problem
in weighing and maintaining its solution
concentration
• Pure copper in the form of copper wire is
used as the primary standard for sodium
thiosulfate when it is to be used for the
determination of copper as is the case with
this experiment
Secondary Standard
• Upon addition of excess iodide to a solution of
copper(II), a precipitate of CuI is formed along with
I2. The liberated iodine is then titrated with standard
sodium thiosulfate
2 Cu2+ + 5 I- ֖ 2 CuI(s) + I3I3- + 2 S2O32- ֖ 3 I- + S4O62• Formation of the precipitate and the addition of
excess iodide force the equilibrium to the right in a
rapid quantitative reaction
• The net stoichiometry of the reaction is 1:1 since 2
moles of copper indirectly requires 2 moles of
thiosulfate
6
7/20/2010
Indicator
• The endpoint in a titration of iodine with thiosulfate is
signaled by the color change of the starch indicator.
• The active ingredient in starch is a helical polymer of
α-D-gluocse called β-amylose, which forms a deep
blue-black complex with molecular iodine
• For use in this indirect method, the indicator is
point when virtually
y all of the iodine has
added at a p
been reduced to iodide ion, this helps the the
disappearance of color to be rapid and sudden
• The starch indicator solution must be freshly
prepared since it will decompose and its sensitivity
is decreased
Preparation of Copper
• Complex ions are ions formed by the bonding of a metal atom
or ion to two or more ligands by coordinate covalent bonds
• A ligand is a negative ion or neutral molecule attached to the
central metal ion in a complex ion
• In this experiment, copper and copper-clad pennies are
dissolved 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 NO3-(aq)
• Once the pennies have been dissolved, the copper is free to
react with iodide ions
7
7/20/2010
Experimental Procedure
Standardizing Thiosulfate
• Measure out ~1 L of de-ionized water
• Heat to boiling, boil for at least five minutes
• Add 25 g of Na2S2O3·5H2O to 1-L volumetric
flask, 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
• Store in darkness when not in use to help
prevent decomposition of the thiosulfate, which
is catalyzed by light.
8
7/20/2010
Standardizing Thiosulfate
•
•
•
•
•
•
•
•
Three pieces of clean copper wire were weighed using about
0.20 to 0.25 g each and placed in seperate 250-mL Erlenmeyer
flasks
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 added
and the solution boiled for a few more minutes to remove more
nitrogen oxides
Add 5 mL of urea solution (1 g in 20 mL of water) with
continued boiling for another minute.
g removes oxides of nitrogen
g which result from the
Boiling
following 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
plate and cooled
• The excess acid is neutralized with 4 M
aqueous ammonia added dropwise in
the hood until a blue-green precipitate of
copper(II) hydroxide barely form
• To dissolve the precipitate and bring the
solution
l ti tto a pH
Hb
beneficial
fi i l tto th
the tit
titration
ti
reactions, 5 mL of glacial acetic acid was
added to each flask.
9
7/20/2010
Analysis of Pennies
•
•
•
•
•
•
•
•
•
For the pennies, six pennies were selected, three from pre1982 minting and three from post-1982
They were cleaned with soap and water, dried, and then
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 flasks
were placed on a hot plate in the hood to dissolve the pennies
After three hours of heating and additional 6 M nitric acid and
15.9 M nitric acid added in small amounts and de-ionized water
to keep the solution level constant, the pennies were dissolved
The flasks are removed from the hot plate and allowed to cool
in an ice bath
Then ~10 mL of 9 M sulfuric acid is slowly added to each flask
by pouring down the side of the flask
The solution is reheated until white fumes of sulfur trioxide
appeared
Throughout the heating process, red fumes of nitrogen oxides
could been seen over each of the solutions
Analysis of Pennies
• The flasks are then removed again and cooled in an ice
bath
• 4 M aqueous ammonia is added dropwise right until the
point where the copper hydroxide precipitate has formed
• To dissolve the precipitate and bring the pH to a level
appropriate for the titration system, 5 mL of glacial acetic
acid is added
• The pre-1982 penny solutions has to be diluted to get a
proper amount of copper concentration that will be
appropriate 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 mL
Erlenmeyer flask which is then ready for titration
preparation
10
7/20/2010
Iodometric Titration
•
•
•
•
At this point, the samples were prepared and titrated one at a
time.
Potassium iodide is weighed and then added
The flask is covered with a watch glass and allowed to stand for
~2 minutes
The solution is then titrated with thiosulfate solution until the
brownish color of iodine is almost gone indicated by a light tan
or 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
11
7/20/2010
Iodometric Titration
• The flask is swirled for about 15 seconds
• Then the titration is completed by adding thiosulfate
dropwise
• At the end point, the bluish-gray color of the solution
disappears and the precipitate appears whitish, or slightly
gray
• The second and third samples were treated in the same
manner and titrated with thiosulfate
Data and Analysis
12
7/20/2010
Standardization of Thiosulfate
Solution
Sodium thiosulfate pentahydrate
FM
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
Solution
Trial 1
Trial 2
Trial 3
M
Mass
off copper wire
i (g)
( )
0 2160
0.2160
0 2438
0.2438
0 2388
0.2388
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 0.1013 M 0.1017 M
solution
0.1016 M
The average molarity was determined to be 0.1015 ± 0.0002 M.
13
7/20/2010
Pre-1982 Pennies
Year of Mint
1977
1978
1979
M
Mass
off penny (g)
( )
3 0717
3.0717
3 0876
3.0876
3 1191
3.1191
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%
0 17%
0.17%
0 17%
0.17%
Propagation of Error from Molarity 0.17%
0 17%
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 Pennies
Year of Mint
1983
1985
1986
M
Mass
off penny (g)
( )
2 5147
2.5147
2 5488
2.5488
2 5236
2.5236
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%
Propagation of Error from Molarity 0.004%
0 004%
Calc
0 005%
0.005%
0 005%
0.005%
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.
14
7/20/2010
Confidence Interval t-test
• The 95% confidence interval range with two degrees of
pre-1982 p
pennies was found to be
freedom for p
– 96.95 to 93.20 %wt copper
– The true value given at the Department of Treasury website is
• (1962–1982) 95% copper, 5% zinc (brass)
– which falls within the CI range
• The 95% confidence interval range with two degrees of
freedom for post-1982 pennies was found to be
– 2.863
2 863 to 2.165
2 165 %wt copper
– 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 I2
is slowly released into the solution by the CuI[I2] complex
– This "after-bluing" can be avoided by adding potassium
thiocyanate just before the end point is reached
– The thiocyanate ion, SCN-, replaces the complexed I2 from
CuI[I2], releasing the I2 to solution where its reaction with
thiosulfate is rapid.
• Iodine may be lost by evaporation from the solution
– This is minimized by having a large excess of iodide in order to
keep the iodine in the tri-iodide ion state
– Titrations involving iodine should be made in cold solutions in
order to minimize loss through evaporation.
15
7/20/2010
Sources of Error
• In acidic solutions, the iodide ion can oxidize
from the atmosphere
– Therefore, prompt titration of the liberated iodine is
necessary in order to prevent oxidation.
• If the starch solution is not made fresh daily or
improperly prepared, the indicator will not
behave properly at the endpoint resulting in a
sluggish or improper endpoint
endpoint.
Sources of Error
• The addition of concentrated H2SO4 and
continued heating until white sulfur trioxide
fumes appear will ensure that all the HNO3
has been removed
– NO3- will oxidize I- and would will seriously
interfere with the procedure.
• The presence of the concentrated sulfuric acid aids
in the exclusion of the remaining nitrogen oxides
from the hot solution.
• The red oxides first boil off, then fumes of sulfur
trioxide beginning to escape.
16
7/20/2010
Sources of Error
• If excess ammonia is added, copper(II)
hydroxide redissolves
redissolves, forming the deep blue
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
iodide ions from being complete
• 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
dehydrating agent
• Combined with the heating and evaporation of water in the sample,
this resulted in a supersaturated aqueous ionic solution which forced
some 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 to
precipitate out in well formed crystals
de
• These two precipitates were easily redissolved by adding more deionized water and slightly mixing
Copper
Sulfate
Crystal
17
7/20/2010
Sources of Error
• Dealing with concentrated acids and evaporation of the
little amount of water in these acids can cause
"b
"bumping"
i " or littl
little explosions
l i
off steam
t
in
i the
th
superheated liquid
• Using tongs will keep hands far enough from the mouth
of the flask should the solution suddenly "bump" and
cause acid burns from the flung drops of concentrated
acid
• Bumping can also cause small amounts of the analyte to
fly from the flask
flask, so preventing bumping by swirling the
flask occasionally will ensure that none of the analyte is
lost.
Elimination of Error
• This experimental method has built in
corrections for each of the errors
mentioned previously that can occur.
• These systematic errors have been easily
corrected by the refinement of the process
• The replication of the experiment is
ensured
db
by th
the elimination
li i ti off th
these
systematic errors in a concise and
repeatable manner
18
7/20/2010
Conclusion
• Iodometric determination of copper gives a reliable
measurement of copper in a sample
• The amount of error is low because of the correction
of systematic error, and the lowered possibility of
introduction of random error through the
straightforward procedure
• The indirect titration of the iodine with thiosulfate is
simple, easy to control, accurate and precise
• The given value of copper content of a penny fell
within the measured 95% confidence interval with
three trials on each type of penny
• With more trials, the confidence interval could be
honed down to find an even more precise measured
value.
19