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The percentage of copper in “copper” coins

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  • 31 augustus 2009
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The percentage of copper in “copper” coins
Using AAS to determine the percentage of copper in a coin


Introduction
The motive of this experiment is to find the percentage of copper in a copper coin of 10 eurocent. In the past the copper percentage used to be high in coins, as much as 97% (Mishler, 1981). However, the percentages are changed through the years. According to the “Richtlijnen voor euromunten” the copper contents in coins are now as following:

10 eurocent | € 0,10 19.75 mm 1.93 mm 4.10 g 89% Copper5% Aluminium5% Zinc1% Tin

20 eurocent | € 0,20 22.25 mm 2.14 mm 5.74 g
Table 1. Shows the percentages of metals in coins, published by Richtlijnen Voor Euromunten

Some other researches done on this topic indicated different percentages such as 93.6% and 70%( Reusch, 1999). The aim of this experiment is to find the exact percentage of the copper and it should be near to the percentages of “Richtlijnen voor euromunten” publishment. The experiment will be on “ten” Eurocent, because the other experiments were also done on the same cent. So, the aim of this experiment, to compare the results to other experiments would be otherwise not possible as each different cent might gave different percentages. The previous experiments have used three-stage procedure to find the copper contents.
Stage 1 A weighed coin is dissolved in concentrated nitric acid. This forms a solution of copper(II) ions.
Stage 2 The solution of Cu2+ ions is treated with aqueous potassium iodide. Copper(I) iodide is precipitated as a white solid and iodine is produced, according to the equation below: 2Cu2+(aq) + 4I–(aq) . 2CuI(s) + I2(aq)
Stage 3 The amount of liberated iodine is found by titrating with standard natrium thiosulphate solution. Knowing the amount of iodine formed, the mass of copper present in the original coin can be calculated.
However, in this experiment only stage 1 will be used and the results will be determined by checking with Atomic Absorption Spectroscopy (AAS), which is a method to determine the concentration of one element at a time. This method does require sample preparation and dilution and calibration series. Once more, the hypothesis of this experiment is as following: the percentage of copper in a copper coin of 10 eurocent is 89% with a deviation of about 1-4 %.


Equipment and supplies
· 1x 10 Eurocent
· 1x 250-cm3 beaker
· 1x 250 cm3 conical flask
· 1x 250cm3 volumetric flask with stopper
· 7x disposable sample storage containers (50 ml)
· Syringes and Millipore filters to fit on syringe for taking samples
· 15 M Concentrated nitric acid (HNO3)
· 0,1M HCl(aq)
· 1000 ppm Stock from Copper sulfate pentahydrate solution
CuSO4(H2O)5 (mass = 249.69g/mol)
· 10 ml automatic pipettes and tips
· Distilled water
· Weigh apparatus
· AAS apparatus (Atmic absorption Spectroscopy)

Safety Considerations
· Glasses are all the time compulsory

· Nitric acid (concentrated) may be fatal if swallowed or inhaled. Extremely corrosive. Contact with skin or eyes may cause severe burns and permanent damage.
· HCL: Extremely corrosive. Inhalation of vapour can cause serious injury. Ingestion may be fatal. Liquid can cause severe damage to skin and eyes..

Experimental Procedure:
Stage 1
1. Weigh the 10eurocent, record its mass and put it into a 250cm3 beaker.

2. Wearing eye protection and gloves, add about 9ml of 15M concentrated nitric acid to your beaker to start dissolving the coin. Do this in the fume cupboard (as toxic nitrogen dioxide is evolved). Allow the coin to dissolve, which should take about 10 minutes. Record what happens when the coin is treated with concentrated nitric acid. The reaction that takes place:
Cu(s) + 4H3O+ + 2NO3-(aq) → Cu2+(aq) + 2NO2(aq) + 6H2O

Cu(s) → Cu2+(aq) + 2e-
2NO3-(aq) + 4H+ + 2e- → 2NO2(aq) + 2H2O
Cu(s) + 2NO3-(aq) + 4H+ → Cu2+(aq) +2NO2(aq) + 2H2O

3. After the reaction seems to stall, add distilled water of about 4cm above the solution to make it 50ml and dilute it to 250 mL in a volumetric flask.

Stage 2
4. Now a 1000 ppm standard for Copper from CuSO4.5H2O should be made. Then make a standard series (0-8 ppm) from this stock solution.

Mass of 1 mole CuSO4(H2O)5 is 249.69g
And mass of 1 mole of Cu is 63. 55g
250 (CuSO4(H2O)5 ) / 63.55 (Cu) = 3.95g of CuSO4(H2O)5 needed in 1 liter distilled water for the stock solution.
1/63.55= 0.015736 M
0.015736 moles Cu in 1 litre water

(0-8) Standard series

samples Amount of stock (ml) Amount of water (ml)

0 ppm 0 50
1ppm C1xV1=C2xV21000 x V1 = 1 x 50ml V1 = 0.05ml 50-0.05 = 49.95
2ppm 0.1 49.90
3ppm 0.15 49.85
4ppm 0.2 49.80
5ppm 0.25 49.75
6ppm 0.3 49.70
7ppm 0.35 49.65
8ppm 0.4 49.60
Table 2. The amounts needed to make standard series from the stock solution and distilled water.

5. From the standard solution now should be the calibration series dilution prepared. In 6 containers of 50ml the samples should be made (automatic pipettes should be used).

6. The samples are brought to the AAS apparatus. First calibrate with standard series and then measure the unknown sample with eurocent.

How you will assess how the price for copper in the coin relates to the actual price of the coin.

According to “de Pers krant” 1 kilo of Cu is 5.16 eurocent (De pers, 2007). In the introduction it is mentioned that 10 eurocent has a percentage of 89% Cupper, but, the amount of cupper will be found in coin with the above experiment and the following will be calculated:

1000g = 5.16 eurocent
Xg Cu (found in 10 eurocent) x 5.16 euros = Y eurocent
1000g

Y eurocent = relation of the prize of copper in coin and the actual prize of the coin
10 eurocent

References
C. Mishler (1981). Standard catalog of 20th century world coins. Illustrated. University of Virginia.
W. Reusch (1999) Virtual Textbook of Organic Chemistry. Retrieved on 28 April 2009 from http://www.cem.msu.edu/~reusch/VirtualText/intro1.htm




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