Rust, Iron II or III
How do you know whether rust is iron(II) oxide or iron
My impression is that Fe+3 is red, Fe+2 is black.
"Rust converter" paints tend to turn your red rust black,
and add some more black stuff of their own.
So you can see that they are forcibly reversing some of the oxidation.
However it might not be entirely clear-cut.
Iron oxides with mixed or intermediate oxidation numbers exist,
such as Fe3O4. What kind of oxidation number is 8/3??
I think they are mostly black, too.
I have had hints that the "color density" of
Fe+3 red is higher than that of Fe+2 black.
Both are clear in extremely thin perfect films, and as they get thicker,
Fe2O3 would turn reddish before Fe+2 got darkish.
Fe+2's first color in that case might be dull dark greenish or bluish.
Looking at the color of dilute solutions of Fe+x in acids gives you a hint.
You can probably change the color by adding oxidizing or reducing agents.
You can precipitate iron by adding base to those solutions.
Drying several such solutions onto filter paper would be smart, too.
That gives you two dimensions to survey: oxidation and pH.
Of course, oxygen in the air could push oxidation up gradually.
Artist's oil paints (sold in "toothpaste tubes")
typically include two kinds of black.
One of course is carbon black.
The other is "iron oxide black". I would bet it is not +3.
Adding some un-oxidized fine iron powder would make it even blacker,
because metallic iron is opaque in thinner films than FeO.
I do not think "rust" is ever really Fe+2.
But I am not a rust expert, and I think there are a few things I do not
know about it.
Hydration levels, growth pH, and crystal forms might influence the color too.
Jim S. Swenson
PS- my CRC says:
Fe(OH)2 - pale green to white
FeO - black
Fe2O3 hematite - red-brown to black
Fe2O3 magnetite - black or red-black
Fe2O3.xH2O - red-brown
Rust is usually Fe(III). Under some conditions the oxides are complex and
mixed, but the usual state is the (III) state. This oxidation state has
the characteristic red-brown color, where Fe (II) is "greenish". In
addition, in the presence of water and oxygen (air) it is rapidly oxidized
to the Fe (III) oxidation state.
The chemical formula of rust is always iron(III) oxide.
While we can imagine that there are two possibilities:
2Fe + O2 = 2FeO
4Fe + 3O2 = 2Fe2O3
under normal conditions of exposing iron to oxygen gas and water, we
should imagine that this is a galvanic process ( a series of spontaneous
reactions governed by the potential energies of the system) and not as a
simple combination as written above.
As such (and looking up a table of Standard Reduction Potentials): we find
1) O2 + 4H+ + 4e- = 2H2O (E = 1.23V)
2) Fe3+ + e- = Fe2+ (+0.77V)
3) Fe3+ + 3e- = Fe (-0.036)
4) Fe2+ + 2e- = Fe (-0.44V)
We find that the greatest potential difference is between eq1 and eq 4 to
5) 2Fe + O2 + 4H+ = 2Fe2+ + 2H2O (E = Ered - Eoxid = 1.67V)
So we expect that the most spontaneous reaction should form Fe2+ and not Fe3+.
However, since eq2 is also below that of eq1, then any Fe2+ that forms
from the reaction described in eq5 will undergo further reaction with eq1
6) 4Fe2+ + O2 + 4H+ = 4Fe3+ + 2H2O (E = +0.36V)
This reaction of Fe2+ with O2 is sometimes written as
4Fe2+ + O2 + (4+2x)H2O = 4Fe2O3.xH2O + 8H+
to show that rust is indeed formed in a hydrated form. In either case, O2
comes from the air, water is from humidity, and H+ is from the dissolved
CO2 in water which forms H2CO3 (carbonic acid).
The bottom-line is that the galvanic process controls spontaneity, and our
understanding of this tells us that Fe must go all the way to Fe3+ in this
Iron prefers to be in the more stable III state rather than the II
state. When left out in the atmosphere where oxygen is present, iron
will typically all convert to the III state. The reddish-orange color we
associate with rust.
Iron II is more prevalent under very acidic conditions (pH < 2). At
higher pH's, iron will convert. Iron II is a nice greenish color.
Rust is the reddish corrosion product formed by electrochemical
interaction between iron and atmospheric oxygen. It is Fe2O3; with
iron(Fe) with a valence of 3.
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Update: June 2012