Error in Sandy King's article about Vandyke Brown on unblikingeye.com ?

[largo]

I was reading Sandy King's article "Vandyke Brown: A Method for Making Permanent Prints in Gold Metal" about Vandyke Brown prints on unblinkingeye.com : https://unblinkingeye.com/Articles/GTV/gtv.html ...

As usual, great article, but something caught my eye...

Sandy King writes the following :

The Vandyke process is based on ferric ammonium citrate, which contains ferric iron, Fe (3+). On exposure to ultraviolet light, ferric iron is reduced to ferrous iron, Fe (2+). To make a silver-iron print ferrous iron must be further reacted with silver nitrate.

I of course totally agree with this, the chemical reactions involved being the following (correct me if I'm wrong) :

(in French, ferriques means ferric and ferreux means ferrous...)

Then Sandy King writes the following :

The major danger to long-term permanence of a Vandyke print image is residual ferrous iron, Fe (2+). If even very small quantities of residual ferrous iron is left in the paper it will eventually oxidize the silver, and the image will fade.

This is what caught my eye... : I don't think this is possible for ferrous iron to oxydize the silver metal, but once again correct me if I'm wrong...

A redox reaction between Fe²⁺ and metallic silver Ag cannot occur spontaneously under standard conditions. If we examine the standard reduction potentials of the species involved:

- Fe²⁺ + 2e⁻ → Fe, with E° = -0.44 V
- Ag⁺ + e⁻ → Ag, with E° = +0.80 V

The higher the reduction potential, the more likely the species is to gain electrons (i.e., be reduced), right ? Here, silver ions Ag⁺ are much more easily reduced to Ag than Fe²⁺ is to Fe.

For Fe²⁺ to oxidize metallic silver Ag, silver would have to be oxidized to Ag⁺ and Fe²⁺ would have to be reduced to Fe. However, metallic silver is very stable, and because of the high reduction potential of Ag⁺, it does not easily oxidize back to Ag⁺.

Thus, if I understand correctly, there is no driving force for Fe²⁺ to be reduced to Fe while oxidizing Ag to Ag⁺ and I come to the conclusion that no spontaneous redox reaction can occur between Fe²⁺ and metallic silver Ag. The potential difference between the species indicates that Fe²⁺ cannot oxidize silver metal.

On the contrary, a redox reaction can occur between (ferric) Fe³⁺ and metallic silver (Ag). If we look at the standard reduction potentials of the species involved:

- Fe³⁺ + e⁻ → Fe²⁺, with E° = +0.77 V
- Ag⁺ + e⁻ → Ag, with E° = +0.80 V

In this case, the reduction potential of Ag⁺ is slightly higher than that of Fe³⁺, meaning Ag metal can be oxidized to Ag⁺, and Fe³⁺ can be reduced to Fe²⁺.

The reaction would look like this:
Fe³⁺ + Ag → Fe²⁺ + Ag⁺

Since the reduction of Fe³⁺ to Fe²⁺ and the oxidation of Ag to Ag⁺ are close in potential, this reaction can happen spontaneously.

So, is Sandy King's article wrong, with confusion between ferrous and ferric ions ?

I'd be tempted to answer yes, since one can read in Mike Ware's article about "The Argyrotype Process" (another silver-iron process) : https://www.mikeware.co.uk/mikeware/Argyrotype_Process.html :

The inherent problem of the iron-based silver processes lies in the danger of leaving residual ferric iron in the print - to its ultimate undoing, because iron(III) will oxidise silver with consequent degradation of the image. It is this problem that the Argyrotype process has been designed to avert.

Your thougths ?

 

[revdoc]

I noticed this a while ago, and it looks wrong to me, too. The same error appears in his kallitype article on the same site.

For what it's worth, I've seen it in many other places. It's not always the case; e.g., his book on kallitypes, Dick Stevens refers correctly to ferric hydroxide. Others are less specific. E.g., Clerc just refers to "iron salts". I'd take Mike Ware's account as accurate.

 

[koraks]

So, is Sandy King's article wrong, with confusion between ferrous and ferric ions ?

I think so, yes. Calling in @fgorga and @nmp to confirm.
Note that Sandy is an (evidently) accomplished photographer and a trailblazer in various alternative printing processes - but he's not a chemist. So it's entirely possible he slipped up here and there, and would probably happily admit to it, too.

 

[nmp]

I would think Fe(2) won't stick around as such and oxidize to Fe(3) anyhow and then the mechanism is the same.

So both are bad. When using FO, there is a greater propensity for the ferrous oxalate to not wash out completely - that might be why at least in kallitype it would be somewhat more applicable.

:Niranjan.

 

[largo]

Thanks for your replies and confirmation !