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.

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.