Which is a dye: Ferric Acetate or Ferrous Acetate?

I’ve been trying to make Iron Acetate to dye leather and I cannot, for the life of me, figure this out experimentally.

I’ve been allowing steel wool to react with pure white vinegar in a sealed glass vessel with a pressure release for about a month. It turns dark black to green which I assume is due to the ferrous acetate ($$\ce{Fe(C2H3O2)2}$$). It also contains orange particulates that I assume are the ‘insoluble in water’ salt complex basic iron acetate, Ferric Acetate $$\ce{[Fe3O(OAc)6(H2O)3]^+}$$.

The solution has dyed leather black and acted as a mordant, and I assume it is a reducing agent. I’ve been trying to separate the iron acetates but I don’t know what properties they each have. Here’s the question:

Which is the dye? Which one turns leather and wood black? Any information anyone has on the properties of either one would be immensely helpful.

The gist is in the solubility products for the hydroxide precipitates of $$\ce{Fe^{2+}}$$ and $$\ce{Fe^{3+}}$$.

$$\ce{Fe(OH)3}$$ doesn't really exist but is rather $$\ce{FeO(OH)}\cdot x \ce{H2O}$$. However the $$K_\mathrm{sp}$$ is calculated as $$\ce{Fe(OH)3}$$.

Now for the vinegar solution let's consider a somewhat "spent" solution where a lot of the acid has reacted. So consider a nominal pH of 5.5, so pOH = 8.5 and $$\ce{[OH] = 3.16\times 10^{-9}}$$. Let's calculate the equilibrium molarity of Fe species as "Mol(Fe) @ pH=5.5" in the table below.

$$\begin{array}{|c|c|c|}\hline & K_\mathrm{sp} & \text{Mol(Fe) @ pH=5.5}\\ \hline \ce{Fe^{2+}} & 8.0 \times 10^{−16} & 80.12 \\ \hline \ce{Fe^{3+}} & 2.79 \times10^{−39} & 8.84\times 10^{-14}\\ \hline \end{array}$$

A $$\ce{Fe^{2+}}$$ solution can't be 80 molar of course, but what that does mean is that all the $$\ce{Fe^{2+}}$$ formed will stay in solution.

The calculations also show that a substantial amount, if not all, of the $$\ce{Fe^{3+}}$$ created will precipitate. The precipitation does lower the pH, so the equilibrium values are indeterminate without knowing the concentrations of the acetate anion and the nominal concentration of $$\ce{Fe^{3+}}$$ (amount dissolved plus amount in the precipitate). However looking at the overall reactions, it wouldn't seem that the pH could be lowered much.

$$\begin{array}{c c c c} & \ce{2Fe + 6H+} & \ce{<=>} & \ce{2Fe^{3+} + 3H2} \\ & \ce{2Fe^{3+} + 6OH- } &\ce{<=>} & \ce{2Fe(OH)3} \\ & \ce{6H2O} & \ce{<=>} & \ce{6H+ + 6OH-} \\ \hline \text{Overall} &\ce{2Fe + 6H2O} & \ce{<=>} & \ce{2Fe(OH)3 + 3H2}\\ \end{array}$$

A further consideration here is that oxygen from the air, which dissolves into the solution, can oxidize $$\ce{Fe^{2+}}$$ to $$\ce{Fe^{3+}}$$.

Now I'd assume that it is the $$\ce{Fe^{2+}}$$ salt that you'd really want since that salt could diffuse into (soak into) whatever you're coating the solution with. The $$\ce{Fe^{2+}}$$ would then be oxidized to $$\ce{Fe^{3+}}$$ in the interior of the item. The $$\ce{FeO(OH)}\cdot x \ce{H2O}$$ on the other hand would just sit on the surface of the item like a paint.

• When the metallic iron reacts with an acidic solution, it does not produce the ferric ion $\ce{Fe^{3+}}$ as MaxW states. It always produces the ferrous ion $\ce{Fe^{2+}}$ according to $$\ce{Fe + 2 H^+ -> Fe^{2+} + H2}$$ The ferric ion is produced by air oxidation of the ferrous ion. Oct 21 '20 at 18:36

Without getting into the chemistry which has been so clearly discussed by @MaxW, from my personal experience in "ebonizing woods" [particularly Oak and Black Walnut due to their tannin content] the light yellow solution you get as soon as the iron has dissolved, works the best. I am assuming that the red solution obtained after allowing the iron acetate to set for a month is iron in a higher oxidation state and possibly trimeric [Cotton and Wilkenson, 1st edition], and hence more stable or slower to react with tannins. Bottom line: Use the light yellow iron-acetate on the leather rather than the dark red solution.