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Iodometric Determination of Copper

I US O C t C i M d B f d Aft 1982In US One Cent Coins Made Before and After 1982

XX YY

Pennies are an interesting commodity They are everywherecommodity. They are everywhere, an always present means of currency. The penny is ingrained in our society for many different social and economic reasons.

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Penny CompositionPenny 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

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Wet Chemical AnalysisM t d i th i t t l• More accurate and precise than instrumental 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 - IodineU d id l t tit ti l• Used widely to quantitatively 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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IodometryCh f it t i htf d d• Chosen for its straightforward procedure

• 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• 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 aThe 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

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Iodometry• The iodine (triiodide) iodide redox system• The iodine (triiodide) – iodide redox 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

• The amount of standard sodium thiosulfate solution required to qtitrate 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 ThiosulfateM d f di l i di thi lf t• Made from dissolving sodium thiosulfate pentahydrate salt

• Solution is standardized against a primary standard

• Iodine oxidizes thiosulfate to the tetrathionate ion:

I2 + 2 S2O32- 2 I- + S4O6

2-

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Primary StandardI di i l d i t d d• Iodine is rarely used as a primary standard 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 sodiumused 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 StandardUpon addition of excess iodide to a solution of• 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) + I3-

I3- + 2 S2O32- 3 I- + S4O6

2-

• Formation of the precipitate and the addition ofFormation 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

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IndicatorThe endpoint in a titration of iodine with thiosulfate is• 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 added at a point when virtually all of the iodine has p ybeen 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• 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

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Experimental Procedure

Standardizing Thiosulfate• Measure out ~1 L of de-ionized water• Heat to boiling, boil for at least five minutesHeat 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• Store in darkness when not in use to help

prevent decomposition of the thiosulfate, which is catalyzed by light.

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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.

• Boiling removes oxides of nitrogen which result from the g gfollowing reactions:

3 Cu(s) + 8 HNO3 3 Cu(NO3)2 + 2 NO + 4 H2O2 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 cooledplate 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 l ti t H b fi i l t th tit tisolution to a pH beneficial to the titration

reactions, 5 mL of glacial acetic acid was added 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• They were cleaned with soap and water, dried, and thenThey 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• 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

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Iodometric Titration• At this point, the samples were prepared and titrated one at a

time. • Potassium iodide is weighed and then addedPotassium 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

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Iodometric Titration• The flask is swirled for about 15 seconds• Then the titration is completed by adding thiosulfate

dropwisedropwise• 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

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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 f i ( ) 0 2160 0 2438 0 2388Mass of copper wire (g) 0.2160 0.2438 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 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

M f ( ) 3 0717 3 0876 3 1191Mass of penny (g) 3.0717 3.0876 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%

Propagation of Error from Molarity 0 17% 0 17% 0 17%Propagation of Error from Molarity Calc

0.17% 0.17% 0.17%

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

M f ( ) 2 5147 2 5488 2 5236Mass of penny (g) 2.5147 2.5488 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% 0 005% 0 005%Propagation of Error from Molarity Calc

0.004% 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.

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Confidence Interval t-test• The 95% confidence interval range with two degrees of

freedom for pre-1982 pennies was found to be p 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 to 2 165 %wt copper2.863 to 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 I2is 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• 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.

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Sources of Error• In acidic solutions, the iodide ion can oxidize

from the atmospherefrom 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 endpointsluggish or improper endpoint.

Sources of Error

• The addition of concentrated H2SO4 and continued heating until white sulfur trioxidecontinued heating until white sulfur trioxide fumes appear will ensure that all the HNO3has been removed– NO3

- will oxidize I- and would will seriously interfere with the procedure.

• The presence of the concentrated sulfuric acid aidsThe 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.

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Sources of Error

• If excess ammonia is added, copper(II) hydroxide redissolves forming the deep bluehydroxide 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 completeiodide 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 agentdehydrating 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

• These two precipitates were easily redissolved by adding more de-These two precipitates were easily redissolved by adding more deionized water and slightly mixing

Copper Sulfate Crystal

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Sources of Error• Dealing with concentrated acids and evaporation of the

little amount of water in these acids can cause "b i " littl l i f t i th"bumping" or little explosions of steam in the 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 so preventing bumping by swirling thefly from the 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 errorscorrections 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 d b th li i ti f thensured by the elimination of these

systematic errors in a concise and repeatable manner

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Conclusion• Iodometric determination of copper gives a reliable• 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 precisesimple, 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.