I'm trying to create a high school level lab in which we separate the metals in an alloy. The objective is to understand and apply standard reduction potentials, and also to do some extra redox reactions to reclaim solid metals. Plus I think it might be fun!

My plan was to simply submerge nickels in an acidic solution. Theoretically, as long as I don't use nitric acid, the nickel will dissolve and leave the solid copper. I tried it out in the fume hood (1 nickel, 100mL 5M HCl solution), but didn't see the telltale blue/green of nickel ions. I tried sanding the nickel, in case there was an inert oxide layer. For a few minutes, I saw a tiny bit of hydrogen evolution, then the sanded surface turned black.

I read that magnesium is sometimes added to coins for corrosion resistance, which is probably why I'm having problems. Does anyone have suggestions to make this lab possible? Maybe some electrochemistry?

  • $\begingroup$ Compare solubilities of Nickel and Copper salts. In particularm consider acetate. Also consider reduction of copper salts by iodide-ion $\endgroup$ – permeakra Jul 24 '15 at 18:02
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    $\begingroup$ Such destruction of coins may by prohibited by law also some coins have added stuff to prevent obtaining pure copper electrolytically as pure metals may be more valuable than coin. $\endgroup$ – Mithoron Jul 24 '15 at 20:24

I recommend to dissolve the copper-nickel alloy in nitric acid and determine the dissolved $\ce{Cu(II)}$ by iodometric titration.

  • Start by weighing the coin, then dissolve it by placing it in nitric acid solution (under a fume hood). Continue to boil the solution for 20 min after complete dissolution of the coin (to remove any traces of remaining $\ce{NO2}$ in soution. The volume of the solution is about $40 \,\mathrm{mL}$. $$\ce{Cu(s) + 4HNO3(aq)->Cu^{2+}(aq) + 2NO3^- (aq) + 2NO2(g) + 2H2O(l)}$$ $$\ce{Ni(s) + 2H3O^+(aq)->Ni^{2+}(aq) + H2 (g) + 2H2O(l)}$$
  • Prepare a fresh solution of sodium thiosulfate of precise concentration $\approx 0.04\,\mathrm{M}$
  • Add $1.00 \,\mathrm{mL}$ of the diluted copper-containing solution to $15 \,\mathrm{mL}$ of $10\% (W/V)$ potassium iodide solution. $$\ce{Cu^{2+}(aq) + 5I^- (aq)->CuI(s) + I3^- (aq) }$$
  • Titrate the yellow-orange suspension with sodium thiosulfate solution until the solution color has become yellow. To detect the end-point add about $1 \,\mathrm{mL}$ of starch solution. $$\ce{I3^- (aq) + 2S2O3^{2-} (aq)->S4O6^{2-} (aq) + 3I^- (aq) }$$
  • Repeat the titration to precisely determine the end-point.

Yomen Atassi has given you a anayltical method for the measurement of the copper content. I will give you a chemical method for the separation of the two metals.

If you want to separate the metals then I would use the following method, I would dissolve the coins by immersing them in hydrochloric acid with air present. This will convert the nickel / copper alloy into a solution of copper chloride and nickel chloride.

This might take a while as you need to wait for the coins to undergo oxidation in the acid.

Next dilute the mixture to a suitable concentration and filter it to get a nice and clear liquid. To separate the copper, I would suggest that you shake the copper / nickel mixture in hydrochloric acid with 30 % aliquat 336 diluted with aromatic kerosene (or ethyl benzene). Do this in a fume hood.

The copper will be extracted by this solution of an ionic liquid in the aromatic solvent and the nickel will be left behind in the aqueous solution. To recover the copper from the organic layer, shake it with dilute hydrochloric acid or an EDTA solution.

In a school lab the use of toluene and aliquat 336 might not be viewed as reasonable for health and safety reasons. If you want to do a method which avoids volatile organics and a toxic ionic liquid. Then use DOWEX 1, this is a cross linked polystyrene which has tetralakyl ammonium salt groups on it. This will function in the same way as aliquat 336 in toluene.

When the chloride concentration is high the copper will bind to the resin in the form of [CuCl4]2- anions. So by passing the copper / nickel solution through a pad of resin you can remove the copper onto the resin. To obtain the copper, I would first wash the resin with hydrochloric acid to wash off the nickel solution, then I would elute the column with water to get the copper back.

I my lab I would use ICPOES to measure the copper in the solution, but I think you could use the titration method of Yomen.

To get ready to do the elution separation method, you may have to do some batch experiments. In these you put a known amount of the copper containing solution in a vial with a known amount of resin. After allowing the system to come to equilibrium you measure the copper content of the solution. You also compare the copper content with the original solution.

In this way you determine how much copper was adsorbed per gram of resin, you should divide the concentration of the copper on the resin (moles Cu per gram of resin) by the concentration of the copper in solution (moles Cu per ml of solution) to obtain the Rd (Kd) value.

I would want to make this measurement at differnet resin to liquid ratios with a moderate amount of copper present, this would allow you to determine the amount of copper which the resin is able to bind. This is an important thing to know.

With a small amount of copper, I would want to make the measurements for copper with different amounts of sodium chloride added. This will allow you to measure Rd as a function of the chloride concentration. It will allow you to better design the copper separation process. You could use the titration method to make the copper measurements.

For a dilute solution of hydrochloric acid you can assume that the acid is totally dissociated, but for very strong hydrochloric acid the chloride concentration is lower than the anayltical acid concentration.


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