On Wed, 12 May 2004 13:43:42 -0700, Larry Blanchard
Do you not know what it does?
I've got a fairly reasonable handle on chemistry, and I can't see what
the salt is there for. The acetic acid will dissolve rust, and slowly
dissolve iron. No need for the salt unless you know of a good reason.
As a chemist my first choice is hydrochloric acid as a solvent for
rust. Muriatic acid is readily available from the local hardware
stores. Yes it will also dissolve iron so you don't want to dump the
parts in and come back next week to see if the rust is dissolved.
I guess I want to mildly disagree with this. Vinegar is about 5%
acetic acid and also contains various organic stuff to provide some
taste but which are not particularly essential to the derusting.
Acetic acid is a weak acid and is going to behave differently than
The acetate ion forms soluble complexes with iron in solution which
aids in the dissolution. Since acetic acid is a weak acid there will
be a lot of the acetate ion tied up as undissociated acetic acid.
Hydrochloric acid is a strong acid and will be essentially completely
dissociated. The chloride ion forms a stable complex with trivalent
iron. It is the stability of this complex which aids in dissolving the
iron oxide (rust).
The addition of sodium chloride to the vinegar provides the extra
complexing power of the chloride to this mixture.
The chloride and/or the acetate ions form stable complexes with iron
ions which aids in the dissolution of rust. The trivalent iron forms
much more stable complexes which would lead one to believe that the
solutions would be more effective with e.g. Fe2O3 than with lower
oxides. Those tenacious black oxides are more dense and usually
contain divalent iron oxide.
Well sort of but there will also be a fair amount of undissociated
Yes but the buffering is not important in this case.
In principle it is the same but it is the complexing power of the
anions (chloride and acetate) which drive the reaction. It needs to be
acidic enough to prevent the precipitation of the very insoluble iron
hydroxides. Other than that the hydrogen ion does not participate in
the dissolution of the rust. The hydrogen ion concentration does
contribute to the speed at which the metallic iron disappears.
There have been extensive discussions of an electrolytic method in the
past which I have stayed clear of. In that case it appears that the
rust removal is accomplished by creating hydrogen gas underneath the
rust coating and 'blasting" it off. This would explain why some of
those black dense oxide coatings are not removed.
The electrolytic method should not remove much metallic iron but on
the other hand what ever is on the surface as rust and gets "blasted"
off is not likely to be redeposited as metallic iron back on the piece
from the same location from which it originated.
In other words what is turned to rust ain't gonna get put back where
it once was.
Search for Google's copy of this article
Well simply a salt of a strong acid and a weak base.
H+ + Fe --->
H+ + Fe + O2 ---> Rust :)
Fine, but what does the chloride contribute?
Of course, but as vinegar is so harmless and cheaply available...
I just want to know the rationale for the addition of neutral chloride
ions. They don't increase the dissociation of the acetic acid, I would
have thought. Iron chlorides are no more soluble than iron acetates.
Sandy - I provided that rather long post that you responded to yesterday. I
deleted it because of its size, and most of it is unrelated to your
question. Also, you should realize (if you didn't) that I was passing on an
article written by someone and didn't add any of my own thoughts, since I
have forgotten so much chemistry I didn't want to dust off the brain cells.
However, after reading your correct responses to his article, I had some
thoughts that might possibly help, although I don't have a proven answer to
your question. :(
Let me say that I agree that the driving force in the reaction to dissolve
the iron oxide is the hydronium ion, and formation of water on the right
side of the equilibrium. I think the answer to why you have to add salt to
the mix is that the salt increases the ionic strength of the solution. This
improves the conductivity of the solution, and might have the effect of
improving the rate of reaction by allowing an easier transfer of charges
among the reacting ions. I know this is a bit vague, but I don't know if
anybody has a really good answer on this.
On the other hand, I recall that the equilibrium constant is really based on
the activity of the ions in solution, and not their concentration. At
higher ionic strengths (due to salt addition), I believe the activity, and
therefore, the equilibrium constant, is lower than it would be without the
salt. It seems to me this would tend to lower the dissociation of the
acetic acid, and bind up even more H+ than without the salt. I think if
this is correct, that it isn't the prevailing factor as I believe the salt
does increase the reaction rate.
I think the case of cleaning your copper pots is similar to the rust issue.
In that case, if you pour vinegar on the copper pot with the oxides on it,
you won't see much if anything happen. When you sprinkle salt on the
surface wetted with vinegar, you quickly see the oxides disappear from the
spots where the salt is dissolving. It is very clear that salt does speed
the rate of reaction in the case of copper oxides, and I have to think it
does the same with iron oxides.
If you don't believe salt does anything, it is simple enough to test for
yourself on two equal rusty spots. As to "why" it works, I have to think it
is because of improved migration of electrons through solution due to the
higher ionic strength of the solution. You might find, for example, that it
takes two weeks to do the job with acetic acid alone, and one day with the
salt added. Whatever problems you say the salt may cause later on may be
outweighted by the time factor. Of course it is also possible (probable?)
that the acid alone will not only take longer, but might in fact not remove
as much oxide in the end.
I had one other thought, improbable as it may be: I'd say the Fe3+ ion is
relatively large compared to the Cl- ion. On the other hand, the acetate
ion is of course a molecule and not a single ion. Maybe there is also a bit
of steric hindrance going on as well. With no Cl- present, all the Fe3+ has
to bond with the acetate molecule (3 of them). If there is some difficulty
fitting 3 of these molecules on one Fe ion, the reaction could be inhibited.
If free Cl- is present, it could more quickly and easily neutralize the Fe
ion and move the molecule away from the reacting area.
But steric hindrance is not a phenomenon reserved only to the crystalization
of compounds. There are several kinds of hindrance, and they don't all have
to do with crystallization. I don't think it really matters anyway because
the acetate ion probably isn't nearly large enough. It was just a thought.
Interesting thoughts, Dan. This discussion has certainly disturbed
some old cobwebs in my attic :)
Gotta be over 40 years since that happened, although my daughter is
doing some elementary chem at night school and my helping her is also
raising the dust.
I don't disbelieve that it helps matters, but I just can't quite
understand why, although you have given me some food for thought.
I have never tested it, although as it happens, I had a rusty steel
band on a hose fiting that I wanted to remove and so stuck it in a
glass of white vinegar overnight. It cleaned the rust off enough to
show me that there was so much metal left that it needed the grinder
to get it off.
I can't really buy the conductivity argument, as what needs to happen
is the migration of H+ to the rust and the migration away from it of
I also can't buy the spatial problem of the size of acetate ions, coz
the stuff is all in solution, and not forming crystals.
BTW, the problems later with soaking a piece of rusty steel in salt
solution is that the salt gets trapped in the fine pits of the rusted
surface and later attracts moisture and THEN the conductivity of this
solution exposed to oxygen continues the corrosion at a pace.
Anyways, thanks for the stimulating discussion :)
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.
I think Sandy's point on this was that the H+ ions are already in solution
from the acetic acid. The presence of Cl- ions doesn't change anything
related to acid strength. For instance, it does not change the dissociation
constant for acetic acid, thereby increasing the H+ concentration. It is
the H+ concentration that determines how strong the acid is. My point was
that the activity of the ions in solution is decreased by the addition of a
lot of ions like Cl- and might actually lower the acidity. This does not
seem to be the predominant outcome, however, because the salt does increase
the speed of the reaction.
On Thu, 13 May 2004 09:04:26 -0700, Larry Blanchard
Yep it makes what might be regarded as a "weak" hydrochloric acid.
This doesn't mean "dilute" BTW.
And guess what? Acetic acid is "weak" so there is effectively no
difference on this count. If you could make acetic acid "strong" you
would have the equivalent of hydrochloric acid.
Strong and Weak wrt acids means that they dissociate forming the H+
and anions to a greater or lesser extent.
On Thu, 13 May 2004 09:04:26 -0700, Larry Blanchard
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
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
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?
Your mechanisms sound interesting, but it's hard to know if this is the
actual path without knowing the driving forces mathematically.
Both ferrous ions and ferric ions form stable chloride complexes.
Stable enough so that the rust does dissolve in the strong chloride
solution by breaking iron-oxygen bonds. When the rust dissolves in a
chloride solution 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.
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
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. 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.
It is nice that the correct chemistry is also 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.
OK I missed your original post. I wrote the article you refer to. I am
a chemist and do know what is happening in this procedure so let's
What is it that needs more clarification on this topic? You ask the
questions and I will try to give a reasonable explanation.
HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.