Can someone enlighten me on why the reaction could go in reverse?
Its been a long time since I was in chemistry class. but something
tells me that this ought not to happen.
It’s not going in reverse. It’s just that the reaction you expect
isn’t happening at all.
Here’s the deal. Iron in the pickle dissolves, displacing copper
ions which plate back out. In the absence of anything else to plate
on, they’ll coat the iron. Absence you say? What about the silver
that’s there? Well, it only counts if it’s in electrical contact with
the iron. The whole reaction is an electrolytic one, like a battery.
If the two metals are touching, then the less reactive silver will
attract the copper and it will get plated, while the iron will stay
relatively clean as it dissolves. Silver just dropped into the
pickle without touching the iron simply is not part of the equation
at all. In order to get that copper plating reaction on the silver,
it must be in contact with the metallic iron or steel that is
available to dissolve in the acid.
Which of course explains why you use copper tweezers with the
pickle. If you used your steel soldering tweezers to hold the work in
the pickle, or fish it out, etc, then you’d have that electrical
contact and the electrolytic cell would be set up, and the silver
would plate with copper. Same thing happens if you’ve got iron
binding wire still on the work, or if you pickle a piece of work
that happens to have a bit of steel tumbling shot stuck in some
Also, note that you’ll only get copper plating out on your silver if
there is actually some copper already in solution. Used pickle has
dissolved plenty of prior copper oxides, so the copper ions are
there, ready to be displaced by entering iron ions. If your pickle is
new, or not all that old, there may not be enough available copper in
the solution to cause noticable copper plating. Similarly, if you put
iron in the pickle, dissolving some and then remove the metallic
iron, you still now have some dissolved iron in the pickle. But once
dissolved like this, it no longer can cause the copper plating
reaction. It’s only when you have iron and silver in contact, and the
iron is able to dissolve (ionize) in the solution, that in order to
maintain electrical balance, copper then comes out of the solution to
replace the lost iron.