# 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.

## 2 Answers

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. – Maurice 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.