"Harold and Susan Vordos" wrote in message
...

"Grant Erwin" wrote in message
...
I got a little bit of 70% nitric acid, potent stuff. I cut the end off a
screw
of the same exact type as I previously dissolved in alum, put it in the
bottom
of a small glass, and added about 10 drops of nitric acid. Naturally, I
wore
face shield and gloves, and used an exhaust fan. Nitric is pretty nasty.
Anyway,
it was cooking along nicely for a few minutes but then it seems to have
slowed
down quite a bit. It's been going about 8 hours now, and I'm wondering if
there
is enough oomph in the little bit of acid I put on it, barely enough to
cover
the bottom of a small water glass.

Thing is, my actual screw, which is in an aluminum casting, won't have
much acid
around it either.

I suppose I could try drawing the liquor out carefully after it slows,
and
adding a few more drops.

Anyway, tomorrow I'll take the piece out and weigh it and I'll get a rate
of
dissolution and then I'll compare it to how the alum did.

GWE

I'm no chemist, but I think the it's the hydrogen in nitric that does the
work. Needless to say, just because you still have a liquid doesn't mean
you still have what you started with. Just like consuming gasoline in
your
car, you've used up the part that made it nitric acid (HN03), so it is now
something else. It's likely the action will have ceased completely, and
you'll have to add more, or pour out the old and start with new. When
I
refined gold, the amount of nitric used was critical, due in part to
having
to get rid of it before I could recover the gold from solution. I found
that an ounce of nitric in conjunction with 4 parts of HCL would dissolve
a
troy ounce of gold. Less would always leave some gold undisclosed.

I'm not a chemist either, but I wonder if H+ is the worker here. If it was
, it seems to me that sulfuric acid would work as well as nitric and that's
apparently not the case. Nitrate (NO3-) is known as a good oxidizer, so I'm
thinking that it is the active component in nitric acid, perhaps oxidising
the iron in steel to Fe(NO3)2 or some other form. Googling the subject has
not been productive for me, but a post to sci.chem would probably turn up
the correct reaction. Once that is known it should be a simple matter to
calculate how much nitric acid would be needed to dissolve the screw or tap,
as the case may be.

BTW, somewhere in the results of a Google search it was suggested that
concentrated nitric acid be diluted before application.

Assuming reaction to Fe(NO3)2, you'd have:

2 HNO3 + Fe = Fe(NO3)2 + H2

so you'd need 2 moles of ntric (126 grams) for every mole of iron (55.8
grams) in the tap, or roughly 2-1/4 gram of pure nitric per gram of iron.
For a 70% nitric acid solution that would be a litlle over 3 grams nitric
per gram of iron and a 1:1 dilution of nitric acid would require around 6
grams nitric acid per gram of iron. The approach is correct, I think, but
the reaction could be all wrong. Note that hydrogen is released in the
above reaction. If the solution bubbles, some sort of gas is being released
and hydrogen is probably more likely than oxygen.

Mike