View Full Version : Ferric Oxalate...Ferrous Oxalate??

Andrew O'Neill
21-Jan-2007, 16:40
I posted the same question over at apug. Are Ferric Oxalate and Ferrous Oxalate the same thing? I am getting into Kallitype printing and I need to clear this dilema up. I have a bottle of Ferrous Oxalate and I was told by the company I bought it from, that it is the same thing...Please say that it is.

Ed Richards
21-Jan-2007, 17:27
They are not the same:


Andrew O'Neill
21-Jan-2007, 18:00
Thanks Ed. Now my next question. Can Ferrous Oxalate substitute Ferric Oxalate for kallitype's part B?

21-Jan-2007, 18:17
Ferric and ferrous are two different oxidation states of iron ions. Ferrous is Fe2+ and ferric is Fe3+. This difference will determine how it forms salts with negatively charged ions such as oxalate.

Andrew O'Neill
21-Jan-2007, 18:43
So, you can't use it then as a substitute for Ferric Oxalate?

Andrew O'Neill
21-Jan-2007, 18:46
This difference will determine how it forms salts with negatively charged ions such as oxalate.

What about Ferric Oxide? Ferric Amm. Oxalate?
Is Alum potassium sulfate? Sorry for all the questions...

Ed Richards
21-Jan-2007, 19:07
> Ferric Oxide? Ferric Amm. Oxalate?

All different - oxalates are organic compounds, oxides are inorganic, i.e., ferric oxide is just rust.

I cannot help on the effect on the process - while they are chemically different, they may act the same, depending on the chemistry of the process. You really need a Kallitype expert. Failing that, I would get exactly what the recipe requires. I would find a different chemical supplier - if they are telling you that ferrous and ferric are the same, you probably cannot trust what is in the bottle.

Andrew O'Neill
21-Jan-2007, 19:47
My last question didn't make any sense...is Alum also potassium Sulfate?

21-Jan-2007, 20:39
I don't know how they behave in the contexts you're asking about. But they are certainly different entities chemically. The most important (but not sole) biological function of iron in our bodies is that iron is the oxygen-carrying moiety in hemoglobin. Oxygen can bind to Fe2+, but not Fe3+. Certain drug toxicities and poisons can cause a condition called methemoglobinemia, in which your hemoglobin-bound iron becomes oxidized to Fe3+, and this can be lethal without using a strong reducing agent (classically methylene blue, a sulfide-containing molecule that can reduce Fe3+ to Fe2+).

So there are important chemical differences between the two. All I can infer from that, though, is that they should not be assumed to be interchangeable for any given chemical reaction until you look into it.

Ed Richards
21-Jan-2007, 20:42
Alum is aluminium potassium sulfate. (Wikapedia is pretty good on chemicals.)

There has to be a good darkroom chemistry book out there - anyone, anyone?

My days as a chemist are long gone, but I have always had the view that it is risky to get into old processes unless you really figure out the chemistry. Some of the chemicals are very dangerous - remember, photograhers were considered an expendable reagent in the good old days.

21-Jan-2007, 22:42

There are several types od "alums" -- Ammonium alum, Potassium alum (potassium aluminium sulfate), Chrome alum (I've seen it labled as Potassium chrome alum), etc.

Kinda of good to know which one they are talking about in a formula! I think the Potassium aluminium sulfate is the most commonly used alum in alt photo formulas.


PS If I understand it properly, in platinum/palladium printing one uses the ferric oxalate as the sensitizer -- it is the exposure to UV light that changes it to ferrous oxalate -- which then leads to the creation of the metal forms of pt and pd. So if one starts out with ferrous oxalate, one would always end up with a totally black print -- expensive platinum and/or palladium coated paper!

Andrew O'Neill
21-Jan-2007, 23:02
I'm no chemist. The book I am reading says add Alum to gelatin to facilitate hardening, when sizing paper. It must refer to potassium Alum sulfate...

Ole Tjugen
22-Jan-2007, 01:47
1: Ferrous/Ferric:

When you expose a Kallitype, some of the ferric(3+) iron is reduced to ferrous(2+). This then reacts with the silver (1+), forming metallic silver(0) and ferrir iron(3+) again. If you use a ferrous iron compound, ALL the silver would react and the print would go totally black even without exposure. Same if your ferric compound is old - some of the ferric will have changed to ferrous, and you will at best get muddy highlights. Conclusion: No, you can't substitute one for the other.

2: Alum

"Alum" is an old "trade name" (= "unscientific name") for a sulfate of two metals, one of which is monovalent (1+) and the other trivalent(3+). If nothing else is specified, it usually means sodium aluminium sulfate - NaAl(SO4)2. In photographic use the most common is potassium aluminium sulfate - KAl(SO4)2, so "unspecified alum" in photographic texts might be this. That's the danger of trade names - it can be difficult to find out which of several different compounds is meant.

The monovalent ion can be sodium, potassium, ammonium, lithium, and so on; and the trivalent can be aluminium, chromium, or any one of several others. They are all different.