Percentage of copper in "copper" coins

For my A2 chemistry investigation I have finally decided to calculate the concentration of copper in "copper" coins.

One of the stages includes adding Potassium iodide solution so that a precipitate of copper(I)iodide and iodine solution is gained. In order to perform a titration I need to liberate the iodine solution. Is the best way to do this by vacuum filtration? My problem, however, is that when I do this I will need a perfectly accurate volume of iodine solution to titrate to (25cm^3 or 50cm^3). When I vacuum filtrate how can I ensure that I only gain either exactly 25cm^3 or 50cm^3?

It needs to be highly accurate becuase I am calculating the concentration of copper, and therefore nothing can be lost en-route.
The answer is probably really simple, and i just cant see it becuase Ive been looking at Chemistry too much! :tongue:

. If anyone could help, I'd be really grateful :smile:

Reply 1

vinny2256

6

Erased Citizen

For my A2 chemistry investigation I have finally decided to calculate the concentration of copper in "copper" coins.

One of the stages includes adding Potassium iodide solution so that a precipitate of copper(I)iodide and iodine solution is gained. In order to perform a titration I need to liberate the iodine solution. Is the best way to do this by vacuum filtration? My problem, however, is that when I do this I will need a perfectly accurate volume of iodine solution to titrate to (25cm^3 or 50cm^3). When I vacuum filtrate how can I ensure that I only gain either exactly 25cm^3 or 50cm^3?

It needs to be highly accurate becuase I am calculating the concentration of copper, and therefore nothing can be lost en-route.
The answer is probably really simple, and i just cant see it becuase Ive been looking at Chemistry too much! :tongue:

. If anyone could help, I'd be really grateful :smile:

Hey there. My recipe to this would be to dissolve a fixed/known mass of copper coins in conc. nitric acid in a fume cupboard. The conc nitric acid should be in excess and of a known volume This will dissove the coins totally thus liberating the copper ions 2+. I would then carry out a back titration with the excess unreacted nitric acid using perhaps a strong base and phenolpthaplen sp? as the indicator. The equations would be:

Cu + 2HNO3---> Cu(NO3)2 + H2 1st equ with excess acid

HNO3 + Na2CO3 --> NaNO3 +CO2 +H2O Back titration.

To work out the percentage of copper you would find the number of moles of nitric that reacted with the sodium bi in excess in the back titration and subtract this from the original number of moles of nitric acid ie the molarity of it and the known volume you reacted the copper with initially. Then simply divide by 2 and multipy by the molar mass of copper and divide this value by the original weighed mass of copper coins and muliply by 100 to get the percentage purity.

Hope this helps?

Ask me anything that sounds vague.

PS I dont think you could use vacuum filtration as it would give highly inaccurate results. Too much of the chemicals involved would adhere to the sides of the filter thus being lostto give innacurate results. As far as I know the only way to get accurate volumes would be to use pipettes and burettes thus my method may work much easier?

Reply 2

ChemistBoy

16

vinny2256

Hey there. My recipe to this would be to dissolve a fixed/known mass of copper coins in conc. nitric acid in a fume cupboard. The conc nitric acid should be in excess and of a known volume This will dissove the coins totally thus liberating the copper ions 2+.

How do you know that the other alloying metals are unreactive towards Nitric acid?

The iodide solution should be roughly homogeneous so if you filter it you can take a portion titrate that and extrapolate back the the whole solution (you will know how much KI solution you had in the first place.

Reply 3

vinny2256

6

ChemistBoy

How do you know that the other alloying metals are unreactive towards Nitric acid?

The iodide solution should be roughly homogeneous so if you filter it you can take a portion titrate that and extrapolate back the the whole solution (you will know how much KI solution you had in the first place.

Good question. I understand my idea would cause inaccurate results due to ion complexes present in the copper coins etc, especially the 1971 species! But I'm quite sure the presence of alloys and other ion complexes in the coins shouldnt sway the percentage purity that much. What you think chemboy?

Reply 4

ChemistBoy

16

vinny2256

Good question. I understand my idea would cause inaccurate results due to ion complexes present in the copper coins etc, especially the 1971 species! But I'm quite sure the presence of alloys and other ion complexes in the coins shouldnt sway the percentage purity that much. What you think chemboy?

Well post '92 coins are mostly steel with a thin copper plate. Pre '92 are bronze with zinc and tin.

If we are talking pre-'92 coins then the zinc and tin are definately going to be reactive in conc nitric (thus giving you a perceived purity of 100% at all alloy mixtures).

Reply 5

vinny2256

6

ChemistBoy

Well post '92 coins are mostly steel with a thin copper plate. Pre '92 are bronze with zinc and tin.

If we are talking pre-'92 coins then the zinc and tin are definately going to be reactive in conc nitric (thus giving you a perceived purity of 100% at all alloy mixtures).