Investigation into the copper content of penny coins

Chemistry Investigation: Volumetric and Colorimetric Analysis of the copper content of coins.

Iain Sword

064312723

Elgin Academy

Iain Sword 604312723 Elgin Academy Page 1

Contents:

Introduction 3

Aim 3

Summary 3

A brief history of the penny 3

Underlying Chemistry 4

Dissolving Unreactive Metals 4

Colorimetric Analysis 6

Volumetric Analysis 10

Procedures 12

Producing a Copper Nitrate solution from a 1 penny coin 12



Discussion 20

Conclusion 20

Evaluation 21



Appendices 22

Uncertainties 23

Preparation of sodium thiosulphate solution 24

Calculation of the required volume of KI solution 24References 25


Introduction

Aim:

The aim of this investigation was to compare the techniques of colorimetric and volumetric analysis for the determination of the copper content of pre 1992 one penny coins.

Summary

Colorimetric Analysis:

The average percentage of copper by colorimetric analysis was found to be 103.2±1.5%

Volumetric Analysis:

The average percentage of copper by volumetric analysis was found to be 94.7±1.2%

Therefore based on royal mint specifications, that copper coins are 97% copper, 2.5% zinc and 0.5% tin.

(royalmint.com)

The volumetric method was both more accurate and precise.

A brief history of the penny

The UK has used pennies as a denomination of currency since 785AD which were initially produced as an alloy of silver and copper. The weight and composition of these pennies changed throughout the dark ages and medieval period. With the rule of the Stuart Dynasty a true copper farthing was introduced which would be more equivalent to a modern penny. However when the House of Hanover took over rule of England, the silver penny was phased out gradually being replaced by copper. A the beginning of the 20th century the composition was standardised at 95.5% copper, 3% tin and 1.5% zinc. This composition remained until the Second World War when the tin composition was decreased and copper was increased due to a national tin shortage. The old composition was restored at the conclusion of the war. The minting of pennies was ceased during the 1950s, but when it resumed in 1962 the composition used during the Second World War (97% Cu, 0.5% Sn, 2.5% Zn) continued to be used to the present day as a decimalised currency since 1970


Underlying Chemistry

Dissolving an unreactive metalThough nitric acid generally behaves like other dilute acids at lower concentrations (0.1-1mol.l-1) it can exhibit qualities not shown by other acids as its concentration is increased. These are thought to be due to the increased concentration of the nitrate ion.When using a very dilute acid such as nitric acid (0.1 mol.l-1), the primary reaction taking place when the acid is added to a metal is: METAL+ACID→SALT +HYDROGEN

Mg ( s )+2H+¿NO 3

−¿ ( aq )→Mg(NO3)2+H

2( g)¿ ¿

Where the hydrogen ions are reduced by the metals, releasing hydrogen gas. However this reaction will only occur with particularly reactive metals (i.e. above hydrogen in the electrochemical series) such as magnesium and zinc. This is because the overall change in free energy in a reaction must always be negative. The free

energy in a reaction can be defined by ∆G=−nF E0 where ∆G is the change in free energy for the

reaction n is the number of electrons used in the redox step, F is faraday's constant (the charge carried by

one mole of electrons, 9.65x104C) and E0 is the overall reduction potential for the equation.

For the example of copper, the overall for ∆G is positive,

2H+¿+2e−¿→H 2¿ ¿ E0=0.00VCu2+¿+2e−¿→Cu¿¿ E0=0.34VSo the overall reduction potential for the redox Cu+2H+¿→Cu2+¿+H 2¿ ¿ is the difference between that of the two

relevant half equations: E0=0.00−0.34=−0.34V∆G=−2∗9.65∗104∗−0.34

∆G=65.62∗103 J so the action is thermodynamically unfeasible. Whereas the ∆G value for magnesium is negative, indicating the reaction is thermodynamically feasible.

2H+¿+2e−¿→H 2¿ ¿ E0=0.00VMg2+¿+2e−¿→Mg¿ ¿ E0=−2.37VSo the overall reduction potential for the redox Mg+2H+¿→Mg2+¿+H 2¿ ¿ is the difference between that of the

two relevant half equations: E0=0.00−(−2.37)=2.37V∆G=−2∗9.65∗104∗2.37

∆G=−457.4∗103 J


If the concentration of nitric acid is increased up to approximately 1-2 mol.l-1 then a different equation is produced.

3Cu (s )+2N O3−¿ (aq) +8H +¿→3Cu

2+¿ ( aq)+ 2NO (g ) +4H2O (l) ¿

¿ ¿

In this reaction the Nitric Oxide species is evolved. In addition this reaction is not limited to those above hydrogen on the electrochemical series

When concentrated nitric acid is used however, a different reaction is again produced:

Cu (s )+2N O3−¿ (aq )+4 H +¿→Cu

2+ ¿( aq)+ 2NO (g )+2H2O (l) ¿

¿¿

This was the reaction used to dissolve the coins for the further experiments. It had to be performed in a fume cupboard as large volumes of the highly toxic nitrogen dioxide species were evolved.

(Garvie, Reid & Robertson, 1976)

Here are two examples of Conc. Nitric acid dissolving copper coins, producing brown nitrogen dioxide. The green beaker on the left has nearly finished reacting as the volume of gas produced has decreased substantially. The beaker on the right however has only just started reacting, shown by the large volume of brown gas in the beaker.


Colorimetric Analysis

Colorimetric determination relies on the substance under test absorbing certain wavelengths of light while transmitting others. Solutions of copper (II) ions transmit primarily in the cyan region of the EM spectrum as they have a pale blue/cyan colour. This corresponds to them absorbing strongly in the red region. This was also confirmed by experiment using the different filters in the colorimeter.

(blog.asmartbear.com)

From the colour wheel above, you can see that if a substance absorbs primarily in one region, the region opposite will be the colour the solution appears. For example a solution that appears yellow will do so because it absorbs strongly in the blue region of the spectrum leaving only yellow regions to pass through.

Colour absorbed Colour observed

Blue Yellow

Green Magenta

Red Cyan


Yellow Red

Magenta

Blue

Cyan

Green

When the copper 2+ 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 (examples being water, ammonia, EDTA and oxalate ions). (chemwiki.ucdavis.edu)

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 exist in energy levels. These can be further divided into subshells which are further separated into orbitals. In free transition metal atoms and ions, the ions have a 3d subshell which is subdivided into 5 degenerate orbitals (i.e. they all have the same energy).

(home.freeuk.net)


degenerate d orbitals in "free" ion

Energy

d orbitals split by a "weak field" ligand such as I-

d orbitals split by a "strong field" ligand such as CN-

small difference in energylarge energy difference

When in an octahedral complex [Cu(H2O)6]2+, the water ligands will approach the copper ion along the x, y and z axes and repel the electrons in the orbitals oriented along these axes (dx

2-y

2 and dz2) and so have a higher

energy. This leads to a loss of degeneracy in the d orbitals with the dxy, dxz and dyz orbitals having a lower energy. Due to this loss of degeneracy, when light passes though the solution, light of wavelengths able to excite electrons causing them to jump from the lower d orbitals to higher d orbitals will be absorbed causing the solution to appear coloured. This appearence of colour is due to the energy of photons absorbed

corresponding to the wavelength by the equation E=Lhcλ

where E is the energy of the photon, L is

avagadro's constant (6.02*1023), h is Planck's constant (6.63*10-34), c is the speed of light (3*108) and λ is the wavelength of light absorbed

Diagram from (Gibb & Hawley, 2010)

The strength of these ligands is determined from the spectrochemical series:

CN->NH3>H2O>OH->F->Cl->Br->I-

Each of these copper chloride solutions has increasing volumes of aqueous ammonia solution (left to right). The change in the colour of light transmitted is because the size of the d-d transition increases due to ammonia being higher in the spectrochemical series than water.(dwb.unl.edu)E.g for copper



Abs.

[Cu2+]

To analyse compounds by colorimetry, one must use a colorimeter. They operate by using a light source to shine a beam of light through a sample contained in a cuvette. The light transmitted through the cuvette then hits a light sensor (likely a photoresistor) which measures the light transmitted. Before running the sensor on the desired sample, a reference sample must also be tested to set a baseline absorbance for the cuvette. This reference cuvette is commonly filled with deionised water.

Simplified Colorimeter diagram(docbrown.info)To determine the absorbance of the sample, the colorimeter calculates a value based on the beer-lambert law.

By this law, the absorbance of a solution can be expressed as A=−ln( II 0 ) where ln represents the natural

log function; I and I 0 are the intensities of transmitted and incident radiation respectively where intensity is

measured in power per unit area (Wcm-2)

Because A∝Conc it is possible to create a standard curve by plotting known values for concentration (by making standard solutions of Cu(NO3)2 ) against absorbance and comparing the curve with absorbance values from the prepared samples from the penny coins which will contain Cu2+ ions from their reaction with nitric acid (pg5)


Volumetric Analysis

To analyse the copper content of copper nitrate solutions produced from coins by Volumetric Analysis, the solution must first be converted into a different set of compounds that will react with an indicator to produce a colour change. To do this any excess nitric acid first had to be neutralised. The reason for this is because a side reaction occurs between excess nitric acid and the sodium thiosulphate used in the titration (this would decrease the yield of I−¿¿ ions and move the reaction’s end point).

Na2S2O3+2HN O3→2NaN O3+S O2+S+H 2O

To deal with excess acid, it can be reacted with a metal carbonate, such as calcium or sodium producing the following reaction:

HNO3+Na2C O3→2NaN O3+CO2+H 2O

To determine the concentration of Cu(II) ions by titration an iodide salt such as potassium iodide can be reacted with the copper ions where copper (II) ions are reduced to copper (I) by iodides as it oxidises to

elemental iodine however this is unusual as the reduction potential for I 2+2e−¿→2 I−¿¿ ¿ (E0=0.54 V) is higher

than that of Cu2+¿+e−¿→Cu+ ¿¿¿¿ ( E0=0.17 V). However, because copper iodide produced in the reaction is very

insoluble creating very low concentrations of copper iodide and because reduction potentials are measured at 1mol.l-1 this value is inaccurate and in reality is closer to 0.88V causing the reaction go as observed.

2Cu2+¿¿ ¿¿¿¿

Sodium thiosulphate can be reacted with iodine produced in the above reaction:

I 2+2Na2S2O3→2 I−¿+Na2 S4O6¿

So the overall equation for the reaction can be thought of as:

2Cu2+¿¿ ¿¿¿

This gives a ratio between the reaction components of:

Cu2+¿ : I2 :S2O32−¿ ¿¿

2 :1:2

This ratio allows the number of moles of copper ions to be determined from the number of moles of thiosulphate ions directly as the ratio between the two is 1:1.


At high concentrations, iodine (I 2) solutions have a very strong brown colour while at lower concentrations

such as near the end point the colour is a far more pale yellow making it difficult to determine the end point. To make this end point easier to detect, a starch indicator is used as it creates a colour change by creating a

blue coloured complex with I 3−¿¿

ions through adsorption with I 3−¿¿

ions being formed by the reaction

I 2+ I−¿→I3

−¿¿ ¿. This reaction is highly reversible at low concentrations however at higher concentrations the

bonding between starch and iodine ions becomes much stronger and slows down the adsorption of iodine making end points more difficult to spot. This is a common occurrence if starch is added early on in the titration when iodine concentrations are still high.

(titrations.info)

To remove the excess nitric left over from the initial reaction with the coin, it can be neutralised with an aqueous solution of calcium carbonate, following the reaction below

2H NO3+CaCO3→Ca (N O3 )2+H 2O+CO2

Primary standards are used in analytical chemistry to compare and test other chemicals as their properties are very well understood and are stable. Producing a very accurate solution of a primary standard simply requires weighing out a mass and diluting to an appropriate volume. To meet the criteria of being a primary standard the compound must meet a set of strict requirements:

Stable in air over long periods Highly pure and cheap No water of hydration which could change with atmospheric conditions such as humidity and

temperature Dissolves readily in a chosen solvent to produce a stable solution A large molecular mass

Sodium thiosulphate, while satisfying many of these criteria does not qualify as a primary standard as it exists as a hydrated salt which can change in water content with changing conditions. Examples of true primary standards include: Anhydrous sodium carbonate, silver nitrate and potassium hydrogen phthalate. How a solution of sodium thiosulphate could be standardised is detailed in the evaluation.

(csudh.edu)


Procedures

Producing a Copper Nitrate solution from a 1 penny coin

The two procedures used for determining the percentage of copper in coins required that a solution of Copper Nitrate was produced. This same procedure was used in both methods.

Equipment:

100cm3 glass beakers (pyrex glass) Fume cupboard Balance (accurate to 3d.p. or more) 250cm3 volumetric flask

Reagents:

Concentrated nitric acid, 25cm3 per coin Pre-1992 1 pence copper coins Deionised water as required

Method:

Approximately 25cm3 of conc nitric acid was poured into a 100cm3 glass beaker using the beaker’s marked scale. The beaker was then placed into the fume cupboard as toxic nitrogen dioxide would be evolved during the reaction. A coin was weighed on a tared balance and this mass was noted down. This coin was then dropped carefully into the beaker then the beaker was pushed further into the fume cupboard to minimise the risk of nitrogen dioxide escaping. The coin/acid solution was then left until the coin had completely reacted, indicated by no more nitrogen dioxide being produced and all effervescence ceasing in the beaker. The dark green/blue liquid remaining was then diluted with deionised water so the energy released by any excess acid being diluted could be done under controlled conditions in a reinforced beaker. This partially diluted solution was then poured into a 250cm3 volumetric flask and further diluted up to the beginning of the neck of the flask, stoppered, inverted then left to settle, then filled up to the graduation mark and finally stoppered, inverted and settled again.



1) Absorption Spectrum

To determine the filter most suited to the experiment an absorption spectrum was created using a stock solution of copper nitrate (aprox 1 mol.l-1)

To create the curve a colorimeter was used, a sample of copper (II) nitrate and deionised water were poured into 2 identical cuvettes. The following procedure was repeated for each filter:

The reference (deionised water) cuvette was inserted into the colorimeter and the reference point was set to zero. The reference cuvette was removed and replaced with the test cuvette (copper (II) nitrate) and a reading was taken.

Conclusion

This graph shows that with the filters that were available, the filter that would give the greatest absorbance would be filter allowing 680nm (red) light to pass through. So this was the filter used for all subsequent tests.


400 450 500 550 600 650 700-0.2

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

AbsorbanceAbsorbance run 2Exponential (Absorbance run 2)

Wavelength (nm)

Abso

rban

ce (A

)

Absorbance (A)Filter wavelength (nm) Run 1 Run 2

440 -0.01 0.02470 0 0.04490 0.06 0.06520 0.23 0.09580 0.54 0.5590 1.01 0.91680 off top of scale off top of scale

2) Calibration curve

To allow the results of the experiments done with coins to be analysed a series of calibration curves were created. The procedure for these was as follows:Equipment:

100cm3 glass beakers (pyrex glass) Balance (accurate to 3d.p.) 50cm3 volumetric flasks Colorimeter with 680nm filter 2 Matching Cuvettes Weighing boats

Reagents: Solid Copper Nitrate (up to 40g per graph created) Deionised water as required

Method:Masses of approximately 1.2g, 2.4g, 3.6g, 4.8g, 6.0g, 7.2g and 8.4g solid copper nitrate were accurately weighed out on a tared balance in weighing boats. Each of these masses were poured from the weighing boats used to weigh them out into a beaker and the weighing boat was rinsed over the beaker with deionised water to collect all the copper nitrate which was then partially diluted to ease pouring into volumetric flasks. The concentrated solution of copper nitrate was then poured into a 50cm3 volumetric flask. A cuvette was used filled with deionised water. The cuvette with deionised water was then inserted into the colorimeter and then a zero value for absorbance was set. The cuvette was then removed. A matching cuvette was then filled with the copper nitrate, inserted into the colorimeter and an absorbance value was taken. The preparation of the cuvette and measurement of absorbance was repeated 3 times for each value of concentration, and the whole procedure repeated at each concentration. The whole process was duplicated to produce a second calibration curve.Note: care was taken to hold the cuvettes by the opaque sides and insert with the clear sides along the light path.


Calibration graph 1

Sample calculation (formula mass for Cu(NO3)2.3H20=241.6g):

n= mFm

=1.210241.6

=0.005mol

C=1000nV

=1000∗0.00550

=0.1mol . l−1

Weighed mass of copper nitrate (g)

Calculated concentration (mol.l-1)

Absorbance (A)

Run 1 Run 2 Run 3 Average1.210 0.100 No data 0.35 0.29 0.322.395 0.198 0.66 0.66 0.62 0.653.617 0.299 0.99 0.97 0.94 0.974.815 0.399 1.29 1.21 1.23 1.246.006 0.497 1.58 1.51 1.53 1.547.265 0.601 1.86 1.81 1.82 1.838.441 0.699 off top of scale off top of scale off top of scale off top of scale

Calibration graph 2

Weighed mass of copper nitrate (g)

Calculated concentration (mol.l-1)

Absorbance (A)

Run 1 Run 2 Run 3 Average1.203 0.100 0.35 0.35 0.33 0.342.390 0.198 0.67 0.69 0.69 0.683.577 0.296 0.97 1.13 1.00 1.034.836 0.400 1.07 1.10 1.26 1.146.066 0.502 1.63 1.65 1.65 1.647.242 0.600 1.90 1.97 1.98 1.958.448 0.699 off top of scale off top of scale off top of scale off top of scale

Calibration graphs


3) Colorimetric determination of copper content of coins

Equipment:

100cm3 glass beakers (pyrex glass) Colorimeter with 680nm filter 2 Matching Cuvettes

Reagents:

Pre-1992 1 pence coins Deionised water as required

Method:

Three 1p coins were weighed separately on a tared balance. Then each coin was placed in a separate beaker containing approximately 25cm3 of concentrated nitric acid and allowed to react completely (when no more NO2 fumes were given off). The solution was then diluted with deionised water and transferred into a 250cm3 volumetric flask with washings. The flask was then made up to the 250cm3 mark then stoppered and inverted (this procedure is analogous with that on pg12) A reference value was set by inserting a cuvette filled with deionized water into the colorimeter and setting it as the reference value. A small volume of coin/copper nitrate solution was then poured into a matching cuvette. The cuvette was placed into the colorimeter and its absorbance was taken. 3 cuvettes were prepared for each solution. The previous procedure was completed for each coin. The absorbance results were then inserted into the equations from the calibration curves to produce concentration values as seen on the table.

Results:

Raw Data:

Coin no.

Coin mass (g)

Absorbance (A)Run 1 Run 2 Run 3 Average

1 3.544 0.91 0.90 0.92 0.912 3.620 0.80 0.79 0.79 0.793 3.510 0.72 0.72 0.72 0.72

Coin no. Using Calibration Curve 1 Using Calibration Curve 2

Concentration (mol.l-1)

Mass (g)

% Copper Content

Concentration (mol.l-1)

Mass (g)

% Copper Content

1 0.29 4.58 129 0.28 4.42 125

2 0.22 3.50 96.5 0.22 3.41 94.1

3 0.20 3.08 87.9 0.19 3.04 86.5


To calculate the % of copper, the following sequence of calculations was used:

1. Concentration was calculated using the average absorbance and the line of best fit equation from the

calibration curve: Concentration= Absorbance− y interceptGradient

2. Number of moles was calculated using n=V∗C1000

where n is the number of moles in a solution, V is the

volume of the solution and C is the concentration of the solution.

3. Then the mass is calculated using m=Fm∗n where m is the mass of the solution component, Fm is the relative formula mass of the compound and n is the number of moles in a solution.

4. Finally percentage is calculated by %=100mmcoin

where % is the percentage, m is the mass of copper in

the coin and mcoin is the initial mass of the coin.

E.g: coin 1, using calibration graph one:

C=0.91−0.04422.9998

=0.29mol .l−1

n=0.29∗2501000

=0.073mol

m=0.073∗63.5=4.58 g

%Cu=4.583.544

∗100=129%

To find the mean value for this method, all of the final percentages were used giving 103.2±1.6%


Volumetric Analysis

Equipment:

100cm3 glass beakers (pyrex glass) 25cm3 bulb pipette dropper 250cm3 volumetric flask 50cm3 burette 10cm3 measuring cylinder

Reagents:

Deionised water as required 1mol.l-1 calcium carbonate solution 50cm3 per coin Dilute (1mol.l-1) ethanoic acid (25cm3) 1mol.l-1 potassium iodide solution, 50cm3 per coin Sodium Thiosulphate solution, 0.22mol.l-1 as required (see appendix 2 for preparation) 0.2% starch solution, 10cm3 per coin

Method:

Three 1p coins were weighed separately on a tared balance. Then each coin was placed in a separate beaker containing approximately 25cm3 of concentrated nitric acid and allowed to react completely (when no more NO2 fumes were given off). The solution was then diluted with deionised water and transferred into a 250cm3 volumetric flask with washings. The flask was then made up to the 250cm3 mark then stoppered and inverted (this procedure is analogous with that on pg12).For each solution 25cm3 was pipetted into a conical flask this was repeated a further 3 ties to give 4 flasks of Copper(II)Nitrate solution. A 1 mol.l-1 solution of calcium carbonate was carefully added to each conical flask until a small mass of precipitate was formed and remained in the flask indicating any excess nitric acid had been neutralised. A volume of dilute ethanoic acid was then added to the conical flask until any remaining precipitate had dissolved. 10cm3 of 1 mol.l-1 potassium iodide measured in a 10cm3 measuring cylinder was added to the solution which produced a colour change from transparent blue to cloudy brown. A fresh solution of sodium thiosulphate was then produced as the titre. The burette was prepared for titration by rinsing out with the prepared thiosulphate solution then filling with the same solution and noting the start point. The titration was then carried out by adding thiosulphate until the solution becomes very pale then 1cm3 of starch indicator was added producing a purple/blue colour at the point of contact and turning the solution a pale pink colour. Small volumes of thiosulphate should continue to be added followed by drops of starch indicator until no more purple colour is produced. The volume of thiosulphate required to complete the reaction is then noted from the end point on the burette. The difference between start and end volumes is the Titre. The titration solution in conical flask and titration was then repeated as necessary to produce concordant results. Each set of results was repeated for each coin.


Results:

Coin No. Run no Start Vol (cm3)

End Vol (cm3)

Titre (cm3) Average Titre (cm3)

1 1 0.0 24.0 23.0->24.0 24.72 24.0 48.7 24.73 0.0 24.7 24.74 n/a

2 1 24.8 48.8 23.0->25.0 24.952 0.0 24.9 24.93 24.9 49.9 25.04 n/a

3 1 0.0 23.0 22->23 23.452 23.0 45.3 22.33 0.0 23.4 23.44 23.4 46.9 23.5

Coin no.

Mass of coin (g)

Number of moles of copper in 25cm3

Number of moles of copper in 250cm3

Mass of copper in coin (g)

% copper in coin

1 3.611 0.0054 0.054 3.43 95.02 3.598 0.0055 0.055 3.49 96.93 3.584 0.0052 0.052 3.30 92.1

Sample Calculation (coin 1):

As the ratio of Thiosulphate ions to Copper ions is 1:1, nthiosulphate=ncopper

n= VC1000

=24.7∗0.221000

=0.0054mol

this value for n applies to the copper in the 25cm3 pipette, n for the 250cm3 volumetric flask is 0.054mol as its volume is 10 times that of the pipette.

m=n∗FM=0.054∗63.5=3.43 g

%Cu=mCu

mcoin

∗100= 3.433.611

=95.0%

Conclusion

The average percentage of copper in a 1p coin is 94.7±1.1%


Discussion

Conclusion

Method Average percentage copper content of a coin

Colorimetric Analysis 103.2±1.6%

Volumetric Analysis 94.7±1.1%

When compared with the royal mint value for the percentage of copper in 1 penny coins of 97% it can be seen that volumetric analysis is both a more accurate and precise method as it is closer to the expected value and the uncertainty in this value is also lower.


Evaluation

‘Dissolving’ the Coins

Because this procedure is common to both techniques the errors involved can be disregarded for the purpose of comparison. The errors that could contribute however which are not covered by the measurement uncertainties are as follows: The coin did not entirely react with the nitric acid, however this was avoided by adding excess nitric acid to the beaker. Any splashes from the beaker which could result in lost copper, though none were observed. Had any of these other errors occurred they would likely have caused a decrease in the copper in the solution, causing a decrease in the concentration and a lower result overall for the percentage of copper in the coin.


1) The errors involved in determining the maximum absorbance come only from the colorimeter scale as all other factors were kept constant. The difference between results was far larger than the inaccuracy in the scale on the colorimeter giving my greater confidence that the red filter was the correct one to use.

2) The percentage errors in the mass of copper nitrate salt weighed out decreases as the mass of salt increases. Making it unusual that the lines in the calibration curve begin to drift apart, however this can be explained by assumptions taken by the graphing software treating all points equally. The lines are however very close together allowing me to be confident they are accurate.

3) Though the procedure uncertainty for this was ±1.6%, the random uncertainty for the results was ±7.2% suggesting that other errors were present causing the result to err significantly above the expected value. The possible reasons for this include: The cuvettes being inserted incorrectly or selecting a non-identical set of cuvettes (both of which I took care not to do), any contact with greasy fingerprints against the clear sides of the cuvette (which was possible while removing the cuvettes from the packaging) this error would cause an increase in the read absorbance value and lead to an increased value for concentration and final percentage of copper in the coin. This causes me to believe that it could at least in part be responsible for the overall result being higher than expected. To remove this error in a future experiment more care could be taken when handling the cuvettes. The other primary factor that could possibly influence the result would be interactions by the zinc and tin ions in the solution. These ions have either full or empty outer d orbitals making d-d transitions unlikely to be the cause for the colour effect. However permanganate ions are able to produce a strong purple colour despite having an empty d orbital. The presence of zinc and tin ions may cause the result for absorbance become higher and in turn lead to an increased result for copper content in the coin making it probable that it plays a part in producing results that do not conform to the stated figures. To determine of this uncertainty in future I would create the calibration curves with a mixture similar to that of a coin’s composition (97% copper, 2.5% zinc, 0.5% tin).


Volumetric Analysis

The procedure uncertainty for this procedure was ±1.1% while the random uncertainty was ±1.4% based on

the results. The expected value (97%) however does not fall within the range generated by either uncertainty

from the average result. This suggests that many other factors were at play that could be not controlled. This could include side reactions in the conical flask prior to titration (such as excess acid reacting with iodine) as well as not correctly identifying the end point due to a colour change that was hard to determine.

When determining the end point it was likely that I pre-empted this, causing a lower volume of thiosulphate to be recorded and therefore a lower percentage copper calculated.

Other effects that could influence the result were impurities in the reagents used. For example the sodium thiosulphate crystals may have been impure causing the expected concentration to be incorrect. This would have decreased the number of moles in solution. This would mean that the calculated value should have been lowered further still.

As sodium thiosulphate is not a primary standard due its variable water content, this could have either inceased or decreased the concentration of the solution based on ambient conditions. It would have been possible to verify the concentration of the solution using the primary standard Potassium Iodate. This was not done during my experiment due to time constraints and because the thiosulphate solutions were made up fresh on the day of use from the crystals to minimise any errors that may occur during storage. The titrations were all carried out in a single afternoon, and so the ambient conditions were constant. However this error cannot be quantified.

When adding the Potassium iodide used to create the colour, the reaction relies on an excess volume of KI solution being added to ensure all the copper was reacted. The actual volume added was not important and so a measuring cylinder could be used. This solution was added using a 10cm3 measuring cylinder. The minimum volume for this was calculated, in retrospect, to be 11cm3 on average. So it is very likely that an inadequate volume of potassium iodide was added to solution leading to a likely source of error. Therefore the mass of copper would be less than was present.

The major errors in this procedure were in determining the end point and in using insufficient KI solution, with the use of insufficient KI having the most marked effect on the results, resulting in a calculated value of copper content being lower than was actually present.


Appendices:

Appendix 1: UncertaintiesThe procedure uncertainties were derived from the valued stated on the equipment and the specifications stated for class B scientific glassware (jaytecglass.co.uk)

Item Tolerance

3d.p. Weighing balance ±0.001g

50cm3 volumetric flask (class B)

±0.12cm3

2d.p. colorimeter ±0.01A

250cm3 volumetric flask (class B)

±0.3cm3

25cm3 pipette ±0.06cm3

50cm3 burette ±0.1cm3

Using the values from the table above:

Uncertainty in colorimetric analysis:Uncertainty in weighing 0.001

Coinstartingmasses∗100=average0.03%

Uncertainty in 50cm3 volumetric flask

0.1250

∗100=0.24%

Uncertainty in absorbance value 0.01Absorbances

=1% ,1.3% ,1.4%

Overall uncertainty 1.27%, 1.57%, 1.67% average 1.50%Uncertainty in % copper content 1.50% of average copper content = 1.6%

Uncertainty in volumetric analysis:Uncertainty in weighing 0.001

Coinstartingmasses∗100=average0.03%

Uncertainty in 250cm3 volumetric flask

0.3250

∗100=0.12%

Uncertainty in pipette 0.0625

=0.24%

Uncertainty in burette readings 0.2titrevolumes

∗100=0.8% ,0.8% ,0.9%

Overall uncertainty 1.19%, 1.19%, 1.29% average 1.22%Uncertainty in % copper content 1.22% of average copper content = 1.1%

The random uncertainty in the results was calculated using the formula Uncertainty=∆Valuesnvalues

, this gave

random uncertainties of 7% for colorimetric analysis and 1.6% for volumetric analysis.

e.g. for volumetric analysis Unc=96.9−92.13

=1.6%



Appendix 2: Preparation of sodium thiosulphate solution

Equipment:

100cm3 glass beaker (pyrex glass) Balance (accurate to 3d.p. or more) 250cm3 volumetric flask

Reagents:

Crystals of Sodium thiosulphate Deionised water as required

Method:

A mass of sodium thiosulphate was weighed in a weighing boat on a tared balance and recorded. This was then transferred to a beaker and the weighing boat washed with deionised water to provide washings. The crystals were then partially diluted and transferred to the 250cm3 volumetric flask then made up to the 250cm3 mark, stoppered then inverted.

The mass of sodium thiosulphate used was: 13.656gThe formula mass stated on this container of sodium thiosulphate crystals was: 248g/molTo calculate the concentration, I first fount the number of moles:

n= mFm

=13.656248

n=0.055mol

C=1000nV

=1000∗0.055250

C=0.22mol . l−1

Appendix 3: Calculation of the required volume of KI solution (retrospectively)

To predict the average number of moles of copper in each conical flask I used the following calculation:Mass of copper is 97% the mass of each coin (average mass 3.6g)Mass copper: 3.49gNumber of moles of copper=3.49/63.5

=0.055molesNumber of moles of copper per conical flask=0.0055molesThe ratio of Cu2+ ions to potassium iodide in the equation 2Cu2+¿¿ ¿¿¿¿is 2:4 or 1:2, so the number of moles

required of KI is 0.011moles

The minimum required volume of 1mol.l-1 KI can now be calculated:

V=1000n/C=1000*0.011/1

V=11cm3


Appendix 4: References

http://www.royalmint.com/discover/uk-coins/coin-design-and-specifications/one-penny-coin, visited 29/01/15

http://chemwiki.ucdavis.edu/Inorganic_Chemistry/Coordination_Chemistry/Ligands, visited 29/01/15

http://www.docbrown.info/page07/appendixtrans09.htm, visited 29/01/15

Garvie D, Reid J & Robertson A, (1976) Core Chemistry, Oxford University Press, London, Page 158.

http://home.freeuk.net/concord2/Transition_elements/Transition_Elements_notes_files/image032.jpg, visited 30/1/15

http://dwb.unl.edu/dwb/Meetings/Oct-8-99/ComplexIonsTutor.html, visited 30/1/15

http://blog.asmartbear.com/color-wheels.html, visited 4/3/15

http://www.titrations.info/iodometric-titration-copper, visited 4/2/15

http://www.csudh.edu/oliver/che230/textbook/ch04.htm, visited 4/2/15

Gibb A & Hawley D, (2010) BrightRED Revision Advanced Higher CHEMISTRY, Bright Red Publishing, Edinburgh, page 25

http://www.jaytecglass.co.uk/laboratory-glassware/general-volumetric-glassware/volumetric-flask/class-b-volumetric-flasks/, visited 21/2/15


Documents

Investigation into the copper content of penny coins