Chemistry Investigation: Volumetric and Colorimetric Analysis of the copper content of coins.
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Contents: Introduction
Aim
Summary
A &rief history of the penny
'nderlying Chemistry
4
(issolving 'nreactive )etals
4
Colorimetric Analysis
6
Volumetric Analysis
!0
%rocedures
!"
%roducing %roducing a Copper *itrate solution from a ! penny coin Colorimetric Analysis
!
Volumetric Analysis
!+
(iscussion
!"
"0
Conclusion
"0
$valuation
"!
Colorimetric Analysis
"!
Volumetric Analysis
""
Appendices
""
'ncertainties
"
%reparation of sodium thiosulphate solution
"4
Calcu alcula lattion ion of of the the re,u e,uired ired volu volum me of of -I -I sol solu utio tion eferences
"4 "/
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Introduction Aim: he aim of this investigation investigation was to compare compare the techni,ues of colorimetric colorimetric and volumetric analysis for the determination of the copper content of pre !11" one penny coins.
Summary Colorimetric Analysis: he average percentage percentage of copper &y colorimetric analysis analysis was found to &e !0."2!./3
Volumetric Analysis: he average percentage percentage of copper &y volumetric analysis analysis was found to &e 14.#2!."3 14.#2!."3 herefore herefore &ased on royal mint specications5 specications5 that copper coins coins are 1#3 copper5 "./3 inc and 0./3 tin. 7royalmint.com8 he volumetric method was &oth &oth more accurate and precise.
A &rief history of the penny he '- has used pennies as as a denomination of currency currency since #+/A( #+/A( which were initially produced as an alloy of silver and copper. copper. he weight and composition of these pennies changed throughout the dar9 ages and medieval period. ith the rule of the Stuart (ynasty a true copper farthing was introduced which would &e more e,uivalent to a modern penny. penny. ;owever when the ;ouse of ;anover too9 over rule of $ngland5 $ ngland5 the silver penny was phased out gradually &eing replaced &y copper. copper. A the &eginning of the "0th century the composition was standardised at 1/./3 copper5 3 tin and !./3 inc. his composition remained until the Second orld ar ar when the tin composition was decreased and copper was increased due to a national tin shortage. he old composition was restored at the conclusion of the war. war. he minting of pennies was ceased during the !1/0s5 &ut when it resumed in !16" the composition used during the Second orld ar ar 71#3 Cu5 0./3 Sn5 "./3
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'nderlying Chemistry (issolving an unreactive metal hough nitric acid generally &ehaves li9e other dilute acids at lower concentrations 70.!= !mol.l=!8 it can e>hi&it ,ualities not shown &y other acids as its concentration is increased. hese are thought to &e due to the increased concentration of the nitrate ion. hen using a very dilute acid such as nitric acid 70.! mol.l=!85 the primary reaction ta9ing place when the acid is added to a metal is:
METAL + ACID→ SALT + HYDROGEN
−¿ ( aq ) → Mg ( N O ) + H ( g ) +¿ N O ¿ ¿ Mg ( s )+ 2 H 3 2
2
3
here the hydrogen ions are reduced &y the metals5 releasing hydrogen gas. ;owever this reaction will only occur with particularly reactive metals 7i.e. a&ove hydrogen in the electrochemical series8 such as magnesium and inc. his is &ecause the overall change in free energy in a reaction must always &e negative. he free energy in a reaction can &e dened &y
∆G
=−nF E
energy for the reaction
n
0
∆G
where
is the change in free
F
is the num&er of electrons used in the redo> step5
is faraday?s constant 7the charge carried &y one mole of electrons5 1.6/>!04C8 and
E
0
is the overall reduction potential for the e,uation.
∆G
@or the e>ample of copper5 the overall for
−¿ → H +¿+ 2 e ¿
is positive5
2
E
¿
0
= 0.00 V
2 H
−¿ → Cu 2 +¿+ 2 e
Cu
¿
¿
0
E = 0.34 V 2+¿+ H 2
+¿ →Cu ¿ is the dierence &etween ¿ Cu+ 2 H E = 0.00−0.34 =−0.34 V
So the overall reduction potential for the redo> that of the two relevant half e,uations:
0
4 ∆ G =−2∗9.65∗ 10 ∗−0.3 4 3 ∆ G= 65.62∗10 J
so the action is thermodynamically unfeasi&le. hereas the ∆ G value for magnesium is negative5 indicating the reaction is thermodynamically feasi&le.
−¿ → H +¿+ 2 e ¿
2
E
¿
0
= 0.00 V
2 H
−¿ → Mg 2+ ¿+ 2 e ¿
¿
E
0
=−2.37 V
Mg
2+¿+ H 2
+¿ → Mg ¿ is the dierence &etween ¿ Mg + 2 H E = 0.00 −(−2.37 )= 2.37 V
So the overall reduction potential for the redo> that of the two relevant half e,uations:
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4
∆ G=−2∗9.65∗10
∗2.37
3
∆ G =−457.4∗10 J
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If the concentration of nitric acid is increased up to appro>imately !=" mol.l=! then a dierent e,uation is produced. 2 +¿( aq )+ 2 NO ( g ) + 4 H 2 O ( l )
+¿ → 3 Cu¿ −¿ ( aq ) + 8 H ¿ ¿ 3 Cu ( s ) + 2 N O 3
In this reaction the *itric B>ide species is evolved. In addition this reaction is not limited to those a&ove hydrogen on the electrochemical series hen concentrated nitric acid is used however5 a dierent reaction is again produced: 2 +¿( aq )+ 2 NO ( g ) + 2 H 2 O ( l )
+¿ → Cu¿ −¿ ( aq ) + 4 H ¿ ¿ Cu ( s ) + 2 N O 3
his was the reaction used to dissolve the coins for the further e>periments. It had to &e performed in a fume cup&oard as large volumes of the highly to>ic nitrogen dio>ide species were evolved. 7arvie5 eid D o&ertson5 !1#68
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;ere are two e>amples of Conc. *itric acid dissolving copper coins5 producing &rown nitrogen dio>ide. he green &ea9er on the left has nearly nished reacting as the volume of gas produced has decreased su&stantially. he &ea9er on the right however has only Eust started reacting5 shown &y the large volume of &rown gas in the &ea9er.Colorimetric
Analysis
Colorimetric determination relies on the su&stance under test a&sor&ing certain wavelengths of light while transmitting others. Solutions of copper 7II8 ions transmit primarily in the cyan region of the $) spectrum as they have a pale &lueFcyan colour. his corresponds to them a&sor&ing strongly in the red region. his was also conrmed &y e>periment using the dierent lters in the colorimeter.
Hello
ed
ree
Cyan
)agent
Glue
7&log.asmart&ear.com 8 @rom the colour wheel a&ove5 you can see that if a su&stance a&sor&s primarily in one region5 the region opposite will &e the colour the solution appears. @or e>ample a solution that appears yellow will do so &ecause it a&sor&s strongly in the &lue region of the spectrum leaving only yellow regions to pass through.
Colour a&sor&ed
Colour o&served
Glue
Hellow
reen
)agenta
ed
Cyan
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hen the copper " ions are dissolved in water the water molecules form ligands around the copper ions. A ligand is a molecule or negative ion with at least one lone pair of electrons that is attracted to an ion 7e>amples &eing water5 ammonia5 $(A and o>alate ions8. 7chemwi9i.ucdavis.edu8
6 water ligands formed an octahedral arrangement around a central atom or ion. The δ- oxygen ion with its lone pairs is attracted towards the central ion or atom
All electrons e>ist in energy levels. hese can &e further divided into su&shells which are further separated into or&itals. In free transition metal atoms and ions5 the ions have a d su&shell which is su÷d into / degenerate or&itals 7i.e. they all have the same energy8.
7home.freeu9.net8
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hen in an octahedral comple> KCu7;"B86L"5 the water ligands will approach the copper ion along the >5 y and a>es and repel the electrons in the or&itals oriented along these a>es 7d>"=y" and d"8 and so have a higher energy. his leads to a loss of degeneracy in the d or&itals with the d>y5 d> and dy or&itals having a lower energy. (ue to this loss of degeneracy5 when light passes though the solution5 light of wavelengths a&le to e>cite electrons causing them to Eump from the lower d or&itals to higher d or&itals will &e a&sor&ed causing the solution to appear coloured. his appearence of colour is due to the energy of photons a&sor&ed corresponding to the wavelength &y the e,uation
Lhc where E is the energy of the photon5 L is avagadro?s constant λ 76.0"M!0 "85 h is %lanc9?s constant 76.6M!0=485 c is the speed of light 7M!0+8 and λ is the wavelength of light a&sor&ed E =
$nergy
large energy dierence small dierence in energy
degenerate d or&itals in JfreeJ ion d or&itals split &y a Jwea9 eldJ ligand such as I= d or&itals split &y a Jstrong eldJ ligand such as C*=
(iagram from 7i&& D ;awley5 "0!08 he strength of these ligands is determined from the spectrochemical series: C*=N*;N;"BNB;=N@=NCl=NGr=NI=
$ach of these copper chloride solutions has increasing volumes of a,ueous ammonia solution 7left to right8. he change in the colour of light transmitted is &ecause the sie of the d=d transition increases due to ammonia &eing higher in the spectrochemical series than water. 7dw&.unl.edu8 $.g for copper
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o analyse compounds &y colorimetry5 one must use a colorimeter. hey operate &y using a light source to shine a &eam of light through a sample contained in a cuvette. he light transmitted through the cuvette then hits a light sensor 7li9ely a photoresistor8 which measures the light transmitted. Gefore running the sensor on the desired sample5 a reference sample must also &e tested to set a &aseline a&sor&ance for the cuvette. his reference cuvette is commonly lled with deionised water.
Simplied Colorimeter diagram (docbrown.info o determine the a&sor&ance of the sample5 the colorimeter calculates a value &ased on the &eer=lam&ert law. Gy this law5 the a&sor&ance of a solution can &e e>pressed as
A =− ln and
I 0
() I I 0
where
ln
represents the natural log functionO
I
are the intensities of transmitted and incident r adiation respectively where
intensity is measured in power per unit area 7cm="8 Gecause A ∝ Cnc it is possi&le to create a standard curve &y plotting 9nown values for concentration 7&y ma9ing standard solutions of Cu7*B8" 8 against a&sor&ance and comparing the curve with a&sor&ance values from the prepared samples from the penny coins which will contain Cu" ions from their reaction with nitric acid 7pg/8
A&s.
KCu"L
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Volumetric Analysis o analyse the copper content of copper nitrate solutions produced from coins &y Volumetric Analysis5 the solution must rst &e converted into a dierent set of compounds that will react with an indicator to produce a colour change. o do this any e>cess nitric acid rst had to &e neutralised. he reason for this is &ecause a side reaction occurs &etween e>cess nitric acid and the sodium thiosulphate used in the titration 7this would decrease the yield of
−¿¿ I
ions and move the reactionPs end
point8.
Na2 S2 O3+ 2 HN O 3 → 2 NaN O 3 + S O2+ S + H 2 O
o deal with e>cess acid5 it can &e reacted with a metal car&onate5 such as calcium or sodium producing the following reaction:
HN O 3 + Na 2 C O3 → 2 NaN O 3 + C O2 + H 2 O o determine the concentration of Cu7II8 ions &y titration an iodide salt such as potassium iodide can &e reacted with the copper ions where copper 7II8 ions are reduced to copper 7I8 &y iodides as it o>idises to elemental iodine however this is unusual as the reduction ¿ → 2 I ¿ I 2 2 e
−¿
potential for
+¿
−¿ +
0
7$ Q0./4 V8 is higher than that of
−¿ → Cu¿ ¿ 2+¿+ e Cu
¿
7 $0Q0.!# V8. ;owever5 &ecause copper iodide produced in the reaction is very insolu&le creating very low concentrations of copper iodide and &ecause reduction potentials are measured at !mol.l=! this value is inaccurate and in reality is closer to 0.++V causing the reaction go as o&served.
N O 3
¿ −¿ ¿¿ ¿ ¿ ¿ +¿ ¿ 2 2 Cu
¿
¿
Sodium thiosulphate can &e reacted with iodine produced in the a&ove reaction:
−¿+ Na S O ¿ I + 2 Na S O → 2 I 2
2
2
2
4
6
3
So the overall e,uation for the reaction can &e thought of as:
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N O 3
¿ −¿ ¿¿ ¿ ¿ ¿ 2 +¿ ¿ 2 Cu
¿
his gives a ratio &etween the reaction components of: 2−¿ 2 +¿ : I 2 : S 2 O 3
¿
¿
Cu 2:1:2
his ratio allows the num&er of moles of copper ions to &e determined from the num&er of moles of thiosulphate ions directly as the ratio &etween the two is !:!.
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I 2 8 solutions have a very strong &rown colour while At high concentrations5 iodine 7 at lower concentrations such as near the end point the colour is a far more pale yellow ma9ing it diRcult to determine the end point. o ma9e this end point easier to detect5 a starch indicator is used as it creates a colour change &y creating a &lue coloured comple>
−¿¿
with
ions through adsorption with
I 3
−¿ ¿ −¿ → I ¿ I + I 3
−¿¿ I 3
ions &eing formed &y the reaction
. his reaction is highly reversi&le at low concentrations however at higher
2
concentrations the &onding &etween starch and iodine ions &ecomes much stronger and slows down the adsorption of iodine ma9ing end points more diRcult to spot. his is a common occurrence if starch is added early on in the titration when iodine concentrations are still high. 7titrations.info8 o remove the e>cess nitric left over from the initial reaction with the coin5 it can &e neutralised with an a,ueous solution of calcium car&onate5 following the reaction &elow 2 H
NO 3+ CaC O 3 →Ca ( N O 3 )2+ H 2 O + C O 2
%rimary standards are used in analytical chemistry to compare and test other chemicals as their properties are very well understood and are sta&le. %roducing a very accurate solution of a primary standard simply re,uires weighing out a mass and diluting to an appropriate volume. o meet the criteria of &eing a primary standard the compound must meet a set of strict re,uirements: Sta&le in air over long periods ;ighly pure and cheap *o water of hydration which could change with atmospheric conditions such as humidity and temperature (issolves readily in a chosen solvent to produce a sta&le solution A large molecular mass • • •
• •
Sodium thiosulphate5 while satisfying many of these criteria does not ,ualify as a primary standard as it e>ists as a hydrated salt which can change in water content with changing conditions. $>amples of true primary standards include: Anhydrous sodium car&onate5 silver nitrate and potassium hydrogen phthalate. ;ow a solution of sodium thiosulphate could &e standardised is detailed in the evaluation. 7csudh.edu8
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%rocedures %roducing a Copper *itrate solution from a ! penny coin he two procedures used for determining the percentage of copper in coins re,uired that a solution of Copper *itrate was produced. his same procedure was used in &oth methods. $,uipment: • • • •
!00cm glass &ea9ers 7pyre> glass8 @ume cup&oard Galance 7accurate to d.p. or more8 "/0cm volumetric as9
eagents: • • •
Concentrated nitric acid5 "/cm per coin %re=!11" ! pence copper coins (eionised water as re,uired
)ethod: Appro>imately "/cm of conc nitric acid was poured into a !00cm glass &ea9er using the &ea9erPs mar9ed scale. he &ea9er was then placed into the fume cup&oard as to>ic nitrogen dio>ide would &e evolved during the reaction. A coin was weighed on a tared &alance and this mass was noted down. his coin was then dropped carefully into the &ea9er then the &ea9er was pushed further into the fume cup&oard to minimise the ris9 of nitrogen dio>ide escaping. he coinFacid solution was then left until the coin had completely reacted5 indicated &y no more nitrogen dio>ide &eing produced and all eervescence ceasing in the &ea9er. he dar9 greenF&lue li,uid remaining was then diluted with deionised water so the energy released &y any e>cess acid &eing diluted could &e done under controlled conditions in a reinforced &ea9er. his partially diluted solution was then poured into a "/0cm volumetric as9 and further diluted up to the &eginning of the nec9 of the as95 stoppered5 inverted then left to settle5 then lled up to the graduation mar9 and nally stoppered5 inverted and settled again.
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Colorimetric Analysis !8 A&sorption Spectrum o determine the lter most suited to the e>periment an a&sorption spectrum was created using a stoc9 solution of copper nitrate 7apro> ! mol.l =!8 o create the curve a colorimeter was used5 a sample of copper 7II8 nitrate and deionised water were poured into " identical cuvettes. he following procedure was repeated for each lter: he reference 7deionised water8 cuvette was inserted into the colorimeter and the reference point was set to ero. he reference cuvette was removed and replaced with the test cuvette 7copper 7II8 nitrate8 and a reading was ta9en.
@ilter wavelength 7nm8 440 4#0
A&sor&ance 7A8 un ! un " =0.0! 0
0.0" 0.04
410
0.06
/"0 /+0 /10
0." 0./4 !.0!
0.06 0.01 0./
6+0
o top of scale
0.1! o top of scale
!.6 !.4 !." ! Absorbance (A)
0.+
A&sor&ance
0.6
A&sor&ance run "
0.4
$>ponential 7A&sor&ance run "8
0." 0 =0."400 /00 600 #00 Wavelength (nm)
Conclusion his graph shows that with the lters that were availa&le5 the lter that would give the greatest a&sor&ance would &e lter allowing 6+0nm 7red8 light to pass through. So this was the lter used for all su&se,uent tests.
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"8 Cali&ration curve o allow the results of the e>periments done with coins to &e analysed a series of cali&ration curves were created. he procedure for these was as follows: $,uipment: !00cm glass &ea9ers 7pyre> glass8 Galance 7accurate to d.p.8 • • •
/0cm volumetric as9s
Colorimeter with 6+0nm lter " )atching Cuvettes eighing &oats eagents: Solid Copper *itrate 7up to 40g per graph created8 • • •
•
(eionised water as re,uired )ethod: )asses of appro>imately !."g5 ".4g5 .6g5 4.+g5 6.0g5 #."g and +.4g solid copper nitrate were accurately weighed out on a tared &alance in weighing &oats. $ach of these masses were poured from the weighing &oats used to weigh them out into a &ea9er and the weighing &oat was rinsed over the &ea9er with deionised water to collect all the copper nitrate which was then partially diluted to ease pouring into volumetric as9s. he concentrated solution of copper nitrate was then poured into a /0cm volumetric as9. A cuvette was used lled with deionised water. he cuvette with deionised water was then inserted into the colorimeter and then a ero value for a&sor&ance was set. he cuvette was then removed. A matching cuvette was then lled with the copper nitrate5 inserted into the colorimeter and an a&sor&ance value was ta9en. he preparation of the cuvette and measurement of a&sor&ance was repeated times for each value of concentration5 and the whole procedure repeated at each concentration. he whole process was duplicated to produce a second cali&ration curve. *ote: care was ta9en to hold the cuvettes &y the opa,ue sides and insert with the clear sides along the light path. •
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Cali&ration graph ! Sample calculation 7formula mass for Cu7*B 8".;"0Q"4!.6g8: ! 1.210 = =0.005 !l n= F! 241.6 C =
1000 n
V
=
eighed mass of copper nitrate 7g8 !."!0 ".1/ .6!# 4.+!/ 6.006 #."6/ +.44!
1000 ∗0.005 50
−1
=0.1 !l"l
Calculated concentrati on 7mol.l=!8 0.!00 0.!1+ 0."11 0.11 0.41# 0.60! 0.611
A&sor&ance 7A8 un ! *o data 0.66 0.11 !."1 !./+ !.+6 o top of scale
un "
un
0./ 0.66 0.1# !."! !./! !.+! o top of scale
Average
0."1 0.6" 0.14 !." !./ !.+" o top of scale
0." 0.6/ 0.1# !."4 !./4 !.+ o top of scale
Cali&ration graph " eighed mass of copper nitrate 7g8 !."0 ".10 ./## 4.+6 6.066 #."4" +.44+
Calculated concentrati on 7mol.l=!8 0.!00 0.!1+ 0."16 0.400 0./0" 0.600 0.611
A&sor&ance 7A8 un ! 0./ 0.6# 0.1# !.0# !.6 !.10 o top of scale
un "
un
0./ 0.61 !.! !.!0 !.6/ !.1# o top of o top of scale
Average 0. 0.61 !.00 !."6 !.6/ !.1+ scale
0.4 0.6+ !.0 !.!4 !.64 !.1/ o top of scale
Cali&ration graphs
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8 Colorimetric determination of copper content of coins $,uipment: • • •
!00cm glass &ea9ers 7pyre> glass8 Colorimeter with 6+0nm lter " )atching Cuvettes
eagents: • •
%re=!11" ! pence coins (eionised water as re,uired
)ethod: hree !p coins were weighed separately on a tared &alance. hen each coin was placed in a separate &ea9er containing appro>imately "/cm of concentrated nitric acid and allowed to react completely 7when no more *B" fumes were given o8. he solution was then diluted with deionised water and transferred into a "/0cm volumetric as9 with washings. he as9 was then made up to the "/0cm mar9 then stoppered and inverted 7this procedure is analogous with that on pg!"8 A reference value was set &y inserting a cuvette lled with deionied water into the colorimeter and setting it as the reference value. A small volume of coinFcopper nitrate solution was then poured into a matching cuvette. he cuvette was placed into the colorimeter and its a&sor&ance was ta9en. cuvettes were prepared for each solution. he previous procedure was completed for each coin. he a&sor&ance results were then inserted into the e,uations from the cali&ration curves to produce concentration values as seen on the ta&le.
esults: aw (ata: Coin no.
Coin mass 7g8
! "
./44 .6"0 ./!0
Coin no.
un ! 0.1! 0.+0 0.#"
A&sor&ance 7A8 un un Averag " e 0.10 0.1" 0.1! 0.#1 0.#1 0.#1 0.#" 0.#" 0.#"
'sing Cali&ration Curve !
'sing Cali&ration Curve "
Concentrati on 7mol.l=!8
)as s 7g8
3 Coppe r Conte nt
Concentrati on 7mol.l=!8
)as s 7g8
3 Coppe r Conte nt
!
0."1
4./+
!"1
0."+
4.4"
!"/
"
0.""
./0
16./
0.""
.4!
14.!
0."0
.0+
+#.1
0.!1
.04
+6./
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o calculate the 3 of copper5 the following se,uence of calculations was used: !. Concentration was calculated using the average a&sor&ance and the line of &est t e,uation from the cali&ration curve:
Cncen#$a#%n=
A&s$&ance − ' %n#e$ce(# G$a)%en#
". *um&er of moles was calculated using
n=
V ∗C 1000
where
n
is the
num&er of moles in a solution5 V is the volume of the solution and the concentration of the solution. . hen the mass is calculated using ! = F!∗n where ! mass of the solution component5 n
compound and
is
is the
is the relative formula mass of the
is the num&er of moles in a solution.
4. @inally percentage is calculated &y percentage5 ! mass of the coin.
F!
C
=
100 !
!c%n
is the mass of copper in the coin and
where
!c%n
is the is the initial
$.g: coin !5 using cali&ration graph one:
C = n=
0.91−0.0442 2.9998
0.29∗250 1000
−1
= 0.29 !l"l
=0.073 !l
!=0.073∗63.5= 4.58 g
=
Cu
4.58 3.544
∗100 =129
o nd the mean value for this method5 all of the nal percentages were used giving !0."2!.63
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Volumetric Analysis $,uipment: • • • • • •
!00cm glass &ea9ers 7pyre> glass8 "/cm &ul& pipette dropper "/0cm volumetric as9 /0cm &urette !0cm measuring cylinder
eagents: • • • • •
•
(eionised water as re,uired !mol.l=! calcium car&onate solution /0cm per coin (ilute 7!mol.l=!8 ethanoic acid 7"/cm8 !mol.l=! potassium iodide solution5 /0cm per coin Sodium hiosulphate solution5 0.""mol.l=! as re,uired 7see appendi> " for preparation8 0."3 starch solution5 !0cm per coin
)ethod: hree !p coins were weighed separately on a tared &alance. hen each coin was placed in a separate &ea9er containing appro>imately "/cm of concentrated nitric acid and allowed to react completely 7when no more *B" fumes were given o8. he solution was then diluted with deionised water and transferred into a "/0cm volumetric as9 with washings. he as9 was then made up to the "/0cm mar9 then stoppered and inverted 7this procedure is analogous with that on pg!"8.@or each solution "/cm was pipetted into a conical as9 this was repeated a further ties to give 4 as9s of Copper7II8*itrate solution. A ! mol.l =! solution of calcium car&onate was carefully added to each conical as9 until a small mass of precipitate was formed and remained in the as9 indicating any e>cess nitric acid had &een neutralised. A volume of dilute ethanoic acid was then added to the conical as9 until any remaining precipitate had dissolved. !0cm of ! mol.l= ! potassium iodide measured in a !0cm measuring cylinder was added to the solution which produced a colour change from transparent &lue to cloudy &rown. A fresh solution of sodium thiosulphate was then produced as the titre. he & urette was prepared for titration &y rinsing out with the prepared thiosulphate solution then lling with the same solution and noting the start point. he titration was then carried out &y adding thiosulphate until the solution &ecomes very pale then !cm of starch indicator was added producing a purpleF&lue colour at the point of contact and turning the solution a pale pin9 colour. Small volumes of thiosulphate should continue to &e added followed &y drops of starch indicator until no more purple colour is p roduced. he volume of thiosulphate re,uired to complete the reaction is then noted from the end point on the &urette. he dierence &etween start and end volumes is the itre. he titration solution in conical as9 and titration was then repeated as necessary to produce concordant results. $ach set of results was repeated for each coin.
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esults: Coin *o.
un no
!
!
"
" 4 !
$nd Vol 7cm8 "4.0
"4.0 0.0
4+.# "4.# nFa 4+.+
"4.+
" 4 ! " 4
Start Vol 7cm8 0.0
0.0 "4.1 0.0 ".0 0.0 ".4
Coi n no.
)ass of coin 7g8
! "
.6!! ./1+ ./+4
*um&er of moles of copper in "/cm 0.00/4 0.00// 0.00/"
itre 7cm8 ".0= N"4.0 "4.# "4.#
Average itre 7cm8 "4.#
".0= N"/.0 "4.1 "/.0
"4.1/
""=N" "". ".4 "./
".4/
*um&er of moles of copper in "/0cm
)ass of copper in coin 7g8
3 coppe r in coin
0.0/4 0.0// 0.0/"
.4 .41 .0
1/.0 16.1 1".!
"4.1 41.1 nFa ".0 4/. ".4 46.1
Sample Calculation 7coin !8: As the ratio of hiosulphate ions to Copper ions is !:!5 n thiosulphateQncopper
n=
VC 1000
=
24.7∗0.22 1000
=0.0054 !l
this value for n applies to the copper in the "/cm pipette5 n for the "/0cm volumetric as9 is 0.0/4mol as its volume is !0 times that of the pipette.
!=n∗ FM =0.054 ∗63.5= 3.43 g Cu=
!Cu ! c%n
∗100=
3.43 3.611
=95.0
Conclusion he average percentage of copper in a !p coin is 14.#2!.!3
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(iscussion Conclusion )ethod
Average percentage copper content of a coin
Colorimetric Analysis
!0."2!.63
Volumetric Analysis
14.#2!.!3
hen compared with the royal mint value for the percentage of copper in ! penny coins of 1#3 it can &e seen that volumetric analysis is &oth a more accurate and precise method as it is closer to the e>pected value and the uncertainty in this value is also lower.
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$valuation T(issolvingP the Coins Gecause this procedure is common to &oth techni,ues the errors involved can &e disregarded for the purpose of comparison. he errors that could contri&ute however which are not covered &y the measurement uncertainties are as follows: he coin did not entirely react with the nitric acid5 however this was avoided &y adding e>cess nitric acid to the &ea9er. Any splashes from the &ea9er which could result in lost copper5 though none were o&served. ;ad any of these other errors occurred they would li9ely have caused a decrease in the copper in the solution5 causing a decrease in the concentration and a lower result overall for the percentage of copper in the coin.
Colorimetric Analysis !8 he errors involved in determining the ma>imum a&sor&ance come only from the colorimeter scale as all other factors were 9ept constant. he dierence &etween results was far larger than the inaccuracy in the scale on the colorimeter giving my greater condence that the red lter was the correct one to use. "8 he percentage errors in the mass of copper nitrate salt weighed out decreases as the mass of salt increases. )a9ing it unusual that the lines in the cali&ration curve &egin to drift apart5 however this can &e e>plained &y assumptions ta9en &y the graphing software treating all points e,ually. he lines are however very close together allowing me to &e condent they are accurate. 8 hough the procedure uncertainty for this was 2!.635 the random uncertainty for the results was 2#."3 suggesting that other errors were present causing the result to err signicantly a&ove the e>pected value. he possi&le reasons for this include: he cuvettes &eing inserted incorrectly or selecting a non=identical set of cuvettes 7&oth of which I too9 care not to do85 any contact with greasy ngerprints against the clear sides of the cuvette 7which was possi&le while removing the cuvettes from the pac9aging8 this error would cause an increase in the read a&sor&ance value and lead to an increased value for concentration and nal percentage of copper in the coin. his causes me to &elieve that it could at least in part &e responsi&le for the overall result &eing higher than e>pected. o remove this error in a future e>periment more care could &e ta9en when handling the cuvettes. he other primary factor that could possi&ly inuence the result would &e interactions &y the inc and tin ions in the solution. hese ions have either full or empty outer d or&itals ma9ing d=d transitions unli9ely to &e the cause for the colour eect. ;owever permanganate ions are a&le to produce a strong purple colour despite having an empty d or&ital. he presence of inc and tin ions may cause the result for a&sor&ance &ecome higher and in turn lead to an increased result for copper content in the coin ma9ing it pro&a&le that it plays a part in producing results that do not conform to the stated gures. o determine of this uncertainty in future I would create the cali&ration curves with a mi>ture similar to that of a coinPs composition 71#3 copper5 "./3 inc5 0./3 tin8.
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Volumetric Analysis he procedure uncertainty for this procedure was 2!.!3 while the random uncertainty was 2!.43 &ased on the results. he e>pected value 71#38 however does not fall within the range generated &y either uncertainty from the average result. his suggests that many other factors were at play that could &e not controlled. his could include side reactions in the conical as9 prior to titration 7such as e>cess acid reacting with iodine8 as well as not correctly identifying the end point d ue to a colour change that was hard to determine. hen determining the end point it was li9ely that I pre=empted this5 causing a l ower volume of thiosulphate to &e recorded and therefore a lower percentage copper calculated. Bther eects that could inuence the result were impurities in the reagents used. @or e>ample the sodium thiosulphate crystals may have &een impure causing the e>pected concentration to &e incorrect. his would have decreased the num&er of moles in solution. his would mean that the calculated value should have &een lowered further still. As sodium thiosulphate is not a primary standard due its varia&le water content5 this could have either inceased or decreased the concentration of the solution &ased on am&ient conditions. It would have &een possi&le to verify the concentration of the solution using the primary standard %otassium Iodate. his was not done during my e>periment due to time constraints and &ecause the thiosulphate solutions were made up fresh on the day of use from the crystals to minimise any errors that may occur d uring storage. he titrations were all carried out in a single afternoon5 and so the am&ient conditions were constant. ;owever this error cannot &e ,uantied. hen adding the %otassium iodide used to create the colour5 the reaction relies on an e>cess volume of -I solution &eing added to ensure all the copper was reacted. he actual volume added was not important and so a measuring cylinder could &e used. his solution was added using a !0cm measuring cylinder. he minimum volume for this was calculated5 in retrospect5 to &e !!cm on average. So it is very li9ely that an inade,uate volume of potassium iodide was added to solution leading to a li9ely source of error. herefore the mass of copper would &e less than was present. he maEor errors in this procedure were in determining the end point and in using insuRcient -I solution5 with the use of insuRcient -I having the most mar9ed eect on the results5 resulting in a calculated value of copper content &eing lower than was actually present.
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Appendices: Appendi> !: 'ncertainties he procedure uncertainties were derived from the valued stated on the e,uipment and the specications stated for class G scientic glassware 7Eaytecglass.co.u98
Item
oleranc e
d.p. eighing &alance
20.00! g
/0cm volumetric as9 7class G8
20.!"c m
"d.p. colorimeter
20.0!A
"/0cm volumetric as9 7class G8
20.cm
"/cm pipette
20.06c m
/0cm &urette
20.!cm
'sing the values from the ta&le a&ove:
'ncertainty in colorimetric analysis: 'ncertainty in weighing
0.001
C%n s#a$#%ng !asses 'ncertainty in /0cm volumetric as9 'ncertainty in a&sor&ance value Bverall uncertainty 'ncertainty in 3 copper content
0.12 50
∗100 =a*e$age 0.03
∗100=0.24
0.01
=1 + 1.3 + 1.4 A&s$&ances !."#35 !./#35 !.6#3 average !./03 !./03 of average copper content Q !.63
'ncertainty in volumetric analysis: 'ncertainty in weighing
0.001
C%n s#a$#%ng !asses 'ncertainty in "/0cm volumetric as9 'ncertainty in pipette
0.3 250
'ncertainty in &urette readings Bverall uncertainty 'ncertainty in 3 copper content
∗100=0.12
0.06 25
∗100 =a*e$age 0.03
= 0.24 0.2
∗100=0.8 + 0.8 + 0.9 #%#$e *lu!es !.!135 !.!135 !."13 average !.""3 !.""3 of average copper content Q !.!3
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he random uncertainty in the results was calculated using the formula
,nce$#a%n#' =
∆ Values 5 this gave random uncertainties of #3 for colorimetric analysis n *alues
and !.63 for volumetric analysis. e.g. for volumetric analysis
,nc=
96.9 −92.1 3
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Appendi> ": %reparation of sodium thiosulphate solution $,uipment: • • •
!00cm glass &ea9er 7pyre> glass8 Galance 7accurate to d.p. or more8 "/0cm volumetric as9
eagents: • •
Crystals of Sodium thiosulphate (eionised water as re,uired
)ethod: A mass of sodium thiosulphate was weighed in a weighing &oat on a tared &alance and recorded. his was then transferred to a &ea9er and the weighing &oat washed with deionised water to provide washings. he crystals were then partially diluted and transferred to the "/0cm volumetric as9 then made up to the "/0cm mar95 stoppered then inverted. he mass of sodium thiosulphate used was: !.6/6g he formula mass stated on this container of sodium thiosulphate crystals was: "4+gFmol o calculate the concentration5 I rst fount the num&er of moles:
! 13.656 = n= F! 248 n =0.055 !l 1000 n 1000 ∗0.055 = C = 250 V −1 C =0.22 !l"l
Appendi> : Calculation of the re,uired volume of -I solution 7retrospectively8 o predict the average num&er of moles of copper in each conical as9 I used the following calculation: )ass of copper is 1#3 the mass of each coin 7average mass .6g8 )ass copper: .41g *um&er of moles of copperQ.41F6./ Q0.0//moles *um&er of moles of copper per conical as9Q0.00//moles
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