On Fri, 14 May 2004 06:02:58 -0500, Unknown
posted:

On Fri, 14 May 2004 03:56:58 GMT, "Dan White"
wrote:

,;
,;"Unknown" wrote in message
m...
,; On Thu, 13 May 2004 09:04:26 -0700, Larry Blanchard
,; wrote:
,;
,; ,;In article ,
,; says...
,; ,; Ditto! All I can say is that the presence of ions in solution, ionic
,; ,; strength, does definitely affect how species in solution react.
,;Maybe there
,; ,; is some physical chemistry website or ng you can visit and ask this
,; ,; question. I'd be interested to know, too!
,; ,;
,; ,;Wow! I didn't mean to start such a learned discussion :-). My
,; ,;knowledge of chemistry is limited to making various explosive
,; ,;compounds, learned long ago in my juvenile days. And lately, I
,; ,;think I've forgotten most of that - CRS seting in :-).
,; ,;
,; ,;But what I meant by "ask a chemist" is that a friend of mine who
,; ,;is a chemist said that the vinegar and salt combined to form a
,; ,;weak hydrochloric acid. I took his word for it.
,;
,; You shouldn't have as he was wrong.
,;
,; Vinegar is approximately 5% acetic acid plus some other goodies to
,; provide some taste. The hydrogen ion concentration is not sufficient
,; to react with metallic iron and therein lies one of the keys to the
,; process. The other key is the fact that chloride ions form a stable
,; complex with iron ions in solution. The iron chloride complex is
,; strong enough so that iron oxide will dissolve and form that complex.
,; Since there is no oxidant strong enough to react with iron metal the
,; net result is that the iron oxide goes into solution as the chloride
,; but the iron metal does not react.
,;
,;Can you explain exactly what this iron-chloride complex is? Are you saying
,;that the iron oxide (rust) is preferentially breaking it's molecular bonds
,;and is reforming as some kind of complex, or as iron chloride? I take it
,;that it is not iron chloride because you say below that if oxygen is
,;introduced, then ferric chloride will form. Second question: What is the
,;reaction that transforms this "iron chloride complex" into ferric chloride?

Both ferrous ions and ferric ions form stable chloride complexes.

Much like stable acetate complexes?

Stable enough so that the rust does dissolve in the strong chloride
solution by breaking iron-oxygen bonds.

Just like in acetic acid solutions?

When the rust dissolves in a
chloride solution

Where does this ever happen? Surely the rust dissolves in a solution
of H+ with a non-precipitating anion?

one will get a solution which contains both species.
In the presence of metallic iron the ferric chloride (from dissolution
of iron III in rust) will be reduced to ferrous chloride so when the
reaction is done we have a ferrous chloride in solution. If one adds
oxygen (from air) then the ferrous is oxidized to ferric and this
ferric immediately reacts with the metallic iron. The result is that
one dissolves more metallic iron than is necessary and probably more
than one wants.

Yep, the acidic solutioss speed up the atmospheric corrosion process.

In general you don't usually find three different oxidation states of
an element present in solution at the same time. The highest oxidation
state (ferric in this case) tends to react with the lowest (iron
metal) to equilibrate with the one in the middle (ferrous). If you
keep adding more air to form more ferric it should be obvious that the
above reaction will continue until you run out of iron or oxygen. You
can run out of oxygen by keeping the pot covered and the piece
immersed.

Yep. Basic measures to stop atmospheric corrosion.

Usually the quantity of rust dissolved is small in comparisons to the
mass of iron metal so one doesn't notice the fact that some iron metal
is sacrificed in this procedure.

Which is why I personally would prefer mechanical derusting procedures
in ethe medium of non-polar solvents that tend to prevent further
corrosion.

If you allow air into the mix the
effect of dissolved oxygen can be very apparent. You can demonstrate
this by letting a piece of iron be partially immersed in the solution.
You will get an etch line at the liquid surface. If it is some antique
you are restoring this etch line will not be a pleasant sight and will
be almost impossible to fix.

Yep! Sad!

,;Your mechanisms sound interesting, but it's hard to know if this is the
,;actual path without knowing the driving forces mathematically.

It is nice that the correct chemistry is also interesting.

Isn't the correct chemistry always the most interesting?

If you need the math lookup some coordination chemistry text books at
a technical library. The topic is probably not of that much interest
for this group. Google "coordination chemistry" with the quotes will
give you a good start. Probably more than you ever wanted to know.

Ayup.

,;dwhite
,;
,;
,;
,;
,; It is essential that the solution be kept oxygen free or the metal
,; will dissolve. This is particularly noticeable if you allow the metal
,; to be "derusted" to stick out of the solution into air e.g. you will
,; find that there has been a dissolution of iron metal at the air liquid
,; interface.
,;
,; The role of the acetic acid is to keep the solution acidic enough to
,; prevent the precipitation of iron oxide but low enough so that iron
,; metal does not react with hydrogen ions. It is the high concentration
,; of chloride that removes the rust not a "weak hydrochloric acid".
,;
,; If one used a concentrated salt solution without the acetic acid then
,; one would get a preciptate of hydrous iron oxide at the surface. This
,; would slow the reaction to a crawl.
,;
,; A weak acid such as acetic acid allows one to put a lot of acid in the
,; solution but maintain a relatively low hydrogen concentration.
,;
,; The solution if kept covered can be used repeatedly until the amount
,; of dissolved iron reaches a point where the hydrous oxide begins to
,; precipitate. If the used solutions are left open to the air then it
,; will accumulate ferric chloride as a result of air oxidation. That
,; ferric chloride is an oxidizing agent strong enough to react with iron
,; metal which is the reason one gets an "etch line" at the liquid
,; surface.
,;
,;
,;