Naked Science Forum
Non Life Sciences => Chemistry => Topic started by: solesasa on 06/03/2015 14:33:42
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I am looking for help to find the chemical reactions at the anode and the cathode for electrolysis of a coin as one electrode and a graphite rod as the second electrode. The reaction would be taking place in a nacl solution. Specifically we are interested in how the metal components of the coins react, as well as the reactions involving the h2o and nacl. Case 1 for a penny 95% copper, 5% zinc, and case 2 quarter 91.7 % copper and 8.3 % nickel. Thanks for any help!!!
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Is the coin the anode or the cathode? What is the applied voltage? Any observations of chemical reactions (bubbles, color change etc.) at the coin?
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The coin is the cathode, and initially the anode a stainless steel spoon , but changed the stainless steel to a graphite electrode because of concerns re hexavalent chromium production. Using an AC adapter with ~12-18 volt , and 300-500 milliwatt output. Thanks for your interest!
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Also when current applied bubbling occurs at the coin surface
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I heard that leaving coins in ****Cola for a day can clean them up. No electricity required.
Apparently the phosphoric acid etches away the surface layer. Just don't leave them in for too long...
Try it on everyday coins before you try a rare coin!
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Given the electrolyte used, the voltage applied and that the coin is at the cathode, I think the main reaction going on at the coin is reduction of water to hydrogen gas and hydroxide ions:
2 H2O + 2e– → H2 + 2 OH–
The produced hydrogen bubbles agitate the solution at the surface of the coin, scrubbing it gently, while the hydroxide ions help to dissolve any grease and gunk on the coin surface. If the coin was highly corroded, the surface metal oxides might also be reduced to neutral metal (or dissolved away with the hydroxide and chloride in solution).
At the anode oxidation of the graphite is likely the dominant reaction:
C + 2 H2O → CO2 + 4 H+ + 4 e–
Notice that overall the major reactions are pH neutral (one H+ produced per electron removed at the anode, and one OH– produced per electron added at the cathode) but unless you are stirring very vigorously and the electrodes are almost in contact there is probably a fairly significant pH gradient through your electrolyte.
As per evan's suggestion, I recommend looking into using solutions of dilute acid (like vinegar or diet cola of your choice) or dilute base (10% ammonium hydroxide) and isopropanol (rubbing alcohol). Wet a paper towel or soft rag with the solution of choice and give the coin a gentle rub. Maybe not as fun as electricity, but should be about as effective. Different solutions will remove different contaminants.
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If the coin is of any value get advice from professional coin people the slightest damage can make a vast difference to its value.
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professional coin people
Why does this sound so funny? [^]
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"Professional coin people" sounds almost as funny as "numismatists" (though not nearly as funny as "philatelists"--those who collect stamps)