I was recently asked why I use copper tweezers to place and remove
sterling silver items from my pickle pot (sodium Bisulfate pickle).
As an explanation, I placed an iron nail head down in the pickle and
dropped in a piece of scrap sterling plate. I then stood back and
waited for the sterling to start turning copper coloured. To my
surprise, the silver stayed a new and the iron nail started copper
plating. In the past when I have accidentally dropped silver and
iron into the pickle it has always been the silver which took the
copper plate (much to my annoyance). 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 didn’t reverse: it’s always the iron and anything in electrical
contact with it that gets copper plated. Go ahead, grab that chunk
of silver with iron tweezers. Watch what happens…
The critical bit is the “anything in contact” part. If you’ve got a
whole pile of stuff in the pickle, and there’s one little scrap of
sawblade in there, the sawblade gets it, along with anything else
that’s touching anything that’s touching it. Since your slug of
silver wasn’t touching the nail, there wasn’t any net charge on it,
so it didn’t plate.
Other things to keep in mind, that run counter to many of the common
’pickle myths’ are that once there’s iron in the pickle, the plating
reaction stops as soon as you get the iron out. It doesn’t trash the
whole bath, permanently, it just causes a reaction as long as it’s
there, and only on things that are in electrical contact with it
somehow.
Also, a new pickle bath won’t plate anything: there’s no dissolved
copper to plate out, so no way to plate anything, regardless of
what’s in there. This state of grace won’t last long, however.
To my surprise, the silver stayed a new and the iron nail started
copper plating. In the past when I have accidentally dropped silver
and iron into the pickle it has always been the silver which took
the copper plate (much to my annoyance). Can someone enlighten me
on why the reaction could go in reverse?
The answer is that an electrochemical reaction is what causes copper
to plate out on iron or steel dipped into a solution of copper
sulfate. If an electrical circuit is completed by an iron wire or
similar object coming into contact with sterling, the copper will
plate out onto the silver. This only happens if copper ions are
present in the electrolyte (the pickle). Your latest experience
probably occurred that way because the silver piece was not in
electrical contact with the nail.
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
crevice.
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.