Ferric oxalate question

t5SQ · Dec 1, 2024

I prepared a 20% ferric oxalate solution for the first time today. I heated 40ml water to 180°F, added 10g ferric oxalate slowly on a stir plate over about 60 seconds, then topped it up to 50ml. It dissolved readily and completely, with no problem whatsoever.

I've read that this can be a terrible pain to dissolve, seen suggestions of leaving it on the stir plate for 8 hours, and that I should wait a day before using it. This leaves me concerned that I have the wrong stuff. It's labelled "ferric oxalate hexahydrate" and dry it's seafoam green and consists of crunchy/crumbly chunks and powder. I noticed the B&S and Artcraft listings just say "ferric oxalate" -- are they the anhydrous form? Do I need to dissolve a different amount to make an equivalent solution to what I would get with the B&S/Artcraft chemicals?

dpurdy · Dec 1, 2024

Ok this is my butt talking but it seems the hexahydrate version has twice the H2o molecules, so that might explain the quickness of dissolving. Since you are dissolving it in H2o anyway I am not so sure it would make any difference except that it might change its weight. It seems to me that when I am dissolving FO it is completely saturated and it takes a long time for it all to go into solution. My technique is to mix it with the water at room temperature and then keep it in a hot water bath and shaking it every time I think of it and leaving it over night and usually it needs a bit more shaking in the morning.

koraks · Dec 1, 2024

dpurdy said:
Ok this is my butt talking but it seems the hexahydrate version has twice the H2o molecules, so that might explain the quickness of dissolving.

I'd say that compared to the anhydrous variant, the hexahydrate has a higher molar mass and thus, a weight-for-weight substitution of hexahydrate for anhydrous will result in a significantly less concentrated solution.
Anyway, when it comes to ferric oxalate, isn't the main issue that it's one of those poorly characterized substances and you're effectively never quite sure *what* you get, even if it's labeled? IDK; maybe @fgorga can comment; I'm sure he has looked into this.

Alan9940 · Dec 1, 2024

I buy FO powder from B&S and its a yellow/green color. To mix, I toss 6.2g into a 25ml bottle, then add 25ml of distilled water at about 125F. I shake it for a bit, let it rest a bit, shake it a bit more, etc. I probably do this about a dozen times. Over the next 24 hours, I shake it a bit, then rest. The next morning, I shake it a bit more, then call it done. I don't remember using it that same day, but a couple of days later it's fine. Never had a problem.

A friend of mine starts with the FO + water in a 25ml bottle, shakes it some, then carries it around in his pocket all day. I guess the jostling from normal body movements throughout a day is enough to mix it satisfactorily.

t5SQ · Dec 1, 2024

So maybe I should see if it works first and only worry about it if I encounter problems? The "carry it around in the pocket all day" story is one of the stories that came to mind before I wrote this post!

koraks · Dec 1, 2024

I'd definitely try and see.

fgorga · Dec 1, 2024

Clearly the OP has the hexahydrate which is green, as they say it is labeled.

B&S sells the anhydrous form since they say their material is yellow.

Which form is sold by ArtCraft is unspecified and we can not make an educated guess since they do not provide any details. One could ask the folks at ArtCraft to specify what color their material is.

The real question here is which form is specified in the recipe/process the OP is intending to use the solution. If the recipe calls for the hexahydrate, all is well.

However, if the recipe calls for the anhydrous form one will probably need to account for the difference in molar mass (formula weight). Using that same weight of the hexahydrate when the recipe calls for the anhydrous from will result in a solution that has a lower concentration of iron oxalate than specified. I say "probably" because not knowing any of the details of its use, we can't say for sure that the exact concentration is critical.

The formula weight of the anhydrous form is 375.75. The formula weight of water is 18. Thus the formula weight of the hexahydrate is 375.75 + (18 x 6) = 483.75. Thus, to substitute the hexahydrate for the anhydrous, one needs to use 483.75/375.75 = 1.29 times as much by weight in order to compensate for the 'extra' weight of the water.

As to @koraks mention that ferric oxalate is poorly characterized in terms of its structure, my inexpert (remember I am a biochemist not an inorganic chemist) thought is that yes there are other hydrated forms of ferric oxalate but the two crystalline (solid) forms we are discussing here seem to be the most stable and the ones which are commercially available. Things get much more complicated when looking at exactly what complexes are present in aqueous solution but this is not relevant here.

I guess that I would say that @koraks advice of just trying the solution you have made is a good one. If it does not work as expected, try remaking the solution as you describe except use 12.9 g of the solid you have.

Lastly, with regard to the solubility, the anhydrous form is know to be slow to dissolve. However, I would expect that the hexahydrate would go into solution much faster. (That the hydrated crystalline form of a compound is quicker to dissolve than the anhydrous form, is generally, but not universally, true.)

dpurdy · Dec 1, 2024

My supply of Bostick and Sullivan FO is green. I keep it in the freezer. Probably don't have to.

fgorga · Dec 1, 2024

dpurdy said:
My supply of Bostick and Sullivan FO is green. I keep it in the freezer. Probably don't have to.

Was it green when you first got it?

If you have not stored it with a desiccant and especially if you have opened the jar before allowing the contents to warm completely to room temperature the material has most likely picked up water and is along the road towards the hexahydrate.

You are correct, there is no reason to store ferric oxalate in the freezer.

dpurdy · Dec 1, 2024

fgorga said:
Was it green when you first got it?

If you have not stored it with a desiccant and especially if you have opened the jar before allowing the contents to warm completely to room temperature the material has most likely picked up water and is along the road towards the hexahydrate.

You are correct, there is no reason to store ferric oxalate in the freezer.

It has always been green. I have been buying it from them since the early 1990s. I have bought several "jars" of it. Always almost florescent green.

t5SQ · Dec 2, 2024

fgorga said:
Clearly the OP has the hexahydrate which is green, as they say it is labeled.

B&S sells the anhydrous form since they say their material is yellow.

Which form is sold by ArtCraft is unspecified and we can not make an educated guess since they do not provide any details. One could ask the folks at ArtCraft to specify what color their material is.

The real question here is which form is specified in the recipe/process the OP is intending to use the solution. If the recipe calls for the hexahydrate, all is well.

However, if the recipe calls for the anhydrous form one will probably need to account for the difference in molar mass (formula weight). Using that same weight of the hexahydrate when the recipe calls for the anhydrous from will result in a solution that has a lower concentration of iron oxalate than specified. I say "probably" because not knowing any of the details of its use, we can't say for sure that the exact concentration is critical.

The formula weight of the anhydrous form is 375.75. The formula weight of water is 18. Thus the formula weight of the hexahydrate is 375.75 + (18 x 6) = 483.75. Thus, to substitute the hexahydrate for the anhydrous, one needs to use 483.75/375.75 = 1.29 times as much by weight in order to compensate for the 'extra' weight of the water.

As to @koraks mention that ferric oxalate is poorly characterized in terms of its structure, my inexpert (remember I am a biochemist not an inorganic chemist) thought is that yes there are other hydrated forms of ferric oxalate but the two crystalline (solid) forms we are discussing here seem to be the most stable and the ones which are commercially available. Things get much more complicated when looking at exactly what complexes are present in aqueous solution but this is not relevant here.

I guess that I would say that @koraks advice of just trying the solution you have made is a good one. If it does not work as expected, try remaking the solution as you describe except use 12.9 g of the solid you have.

Lastly, with regard to the solubility, the anhydrous form is know to be slow to dissolve. However, I would expect that the hexahydrate would go into solution much faster. (That the hydrated crystalline form of a compound is quicker to dissolve than the anhydrous form, is generally, but not universally, true.)

Wow, wonderful and comprehensive response, thanks a ton. I'll be using the ferric oxalate for kallitypes and was going to follow Sandy King's article at unblinkingeye.com.

fgorga · Dec 2, 2024

dpurdy said:
It has always been green. I have been buying it from them since the early 1990s. I have bought several "jars" of it. Always almost florescent green.

Interesting.

Their current webpage (https://www.bostick-sullivan.com/product/ferric-oxalate-powder-100-g/) says they are selling a "free-flowing, bright yellow" material.

The material you have is almost certainly the hexahydrate.

cliveh · Dec 2, 2024

Why do you need to dissolve it at 180°F? I dissolve it in cold water.

t5SQ · Dec 2, 2024

cliveh said:
Why do you need to dissolve it at 180°F? I dissolve it in cold water.

Perhaps I don't need to dissolve it at 180°F! Assuming I am able to make some images using this solution, next time I'll give it a whirl at room temperature

fgorga · Dec 2, 2024

t5SQ said:
Wow, wonderful and comprehensive response, thanks a ton. I'll be using the ferric oxalate for kallitypes and was going to follow Sandy King's article at unblinkingeye.com.

You are quite welcome.

Looking a King's article, he specifies the incomplete & ambiguous "ferric oxalate" which is always frustrating to those of us with a chemical bent but very common in the alt process world.

Given the other responses here about the slow dissolution of ferric oxalate and the text in this article, I would guess that most are using the anhydrous form.

However, as I said in my last response, the approx. 30% difference in concentration iron of a 20% (w/v) solution between the two forms may not matter. Only a test in your own dim room will tell for sure.

P.S. I just, for the heck of it, took a look in my chemical cabinet and found that I actually have some ferric oxalate from ArtCraft. It is a greenish yellow, free-flowing powder. Thus is is probably the anhydrous form.

I've not used it despite having bought it months ago. I keep thinking that I'll try kallitype at some point, but I am mainly a salted-paper guy when it comes to silver-based processes.

dpurdy · Dec 2, 2024

fgorga said:
Interesting.

Their current webpage (https://www.bostick-sullivan.com/product/ferric-oxalate-powder-100-g/) says they are selling a "free-flowing, bright yellow" material.

The material you have is almost certainly the hexahydrate.

No. I have mixed this stuff a hundred times for years and always followed B&S instructions. 6.5 grams to 25ml water.
I guess this is the twilight zone. Here is some of my work..

Dennis Purdy

Explore Dennis Purdy’s 140 photos on Flickr!

www.flickr.com

fgorga · Dec 2, 2024

cliveh said:
Why do you need to dissolve it at 180°F? I dissolve it in cold water.

Usually hot water is used in a case like this to speed up the dissolution of a slow to dissolve solid. The end result is the same whether you use hot water or room temperature water.

t5SQ · Dec 2, 2024

dpurdy said:
No. I have mixed this stuff a hundred times for years and always followed B&S instructions. 6.5 grams to 25ml water.
I guess this is the twilight zone. Here is some of my work..

Dennis Purdy

Explore Dennis Purdy’s 140 photos on Flickr!

www.flickr.com

These are beautiful, thanks for posting them.

fgorga · Dec 2, 2024

dpurdy said:
No. I have mixed this stuff a hundred times for years and always followed B&S instructions. 6.5 grams to 25ml water.
I guess this is the twilight zone. Here is some of my work..

Dennis Purdy

Explore Dennis Purdy’s 140 photos on Flickr!

www.flickr.com

Nice work!

Of course, I have no clue as to the discrepancy between what B&S says on their webpage and what they have delivered to you in the past. The 'evidence' still suggests that what you have is the hexahydrate.

Does the material you have dissolve easily or does it take a long time to go into solution?

Either way, I think that your results suggest that either a 26% (w/v) solution of the hexahydrate (6.5/25= 0.26) which is 1.3x more concentrated than the 20% (w/v) solution of the anhydrous compound specified in Kings article will work.

fgorga · Dec 2, 2024

To summarize all of the above...

The B&S instructions for kallitype specify a 26% (w/v) solution (6.5 g in 25 mL) of what seems to be ferric oxalate hexahydrate.

Sandy King's instructions in his kallitype article specify a 20% (w/v) solution (20g in 100 mL) of anhydrous ferric oxalate.

The first solution is probably easier to make as the hexahydrate is easier to dissolve than the anhydrous form.

However, these two solutions contain identical amounts of ferric oxalate on a molar basis. (Moles are what counts in chemistry!) Thus the two solutions are chemically interchangeable.

The outstanding question is whether the 20% (w/v) solution of the hexahydrate that @t5SQ made (as described in their initial post) will work for kallitype. It might, but it would probably be safer just to make a 26% (w/v) solution of the hexahydrate that they already have on hand.

koraks · Dec 3, 2024

fgorga said:
The outstanding question is whether the 20% (w/v) solution of the hexahydrate that @t5SQ made (as described in their initial post) will work for kallitype. It might

I'm sure it will, but the ratio between the ferric oxalate solution and the other ingredients (most importantly silver nitrate) in the sensitizer makeup should be adjusted accordingly. The net result will be a slightly thinner coating, but I suspect it'll still produce the desired densities.

t5SQ · Dec 4, 2024

OK, now I have a bonus question. First: convinced I have the hexahydrate form, I actually adjusted my solution by adding 5.5g more ferric oxalate powder and topping the solution up to 60ml total, for a total of 15.5g in 60ml, or about 26% w/v.

Now, today I went to coat my first sheet of paper, so I mixed equal volumes of 10% silver nitrate and this solution I made. A white precipitate formed immediately. After Googling, I see that this sometimes happens. I see: "If your sensitizer does form a precipitate, just be sure you do not transfer it to the paper you will be coating" (how?? actually this appears to be a slightly different approach where the precipitate could settle out?) and "Never mix the two solutions until right before using them, if you do the silver will quickly precipitate" (that's what I did...)

Any words of wisdom?

revdoc · Dec 4, 2024

You might actually have ferric ammonium oxalate, which will create a precipitate on mixing with silver nitrate (That might also explain why it dissolved so easily.)

Alternatively, I recall that free oxalic acid in the solution will also cause precipitation of silver oxalate. That issue can he diagnosed from the colour of the solution. It should be amber. Any other colour indicates a problem.

In any case, there should be no precipitate, even if you wait a while after mixing.

t5SQ · Dec 4, 2024

revdoc said:
You might actually have ferric ammonium oxalate, which will create a precipitate on mixing with silver nitrate (That might also explain why it dissolved so easily.)

Alternatively, I recall that free oxalic acid in the solution will also cause precipitation of silver oxalate. That issue can he diagnosed from the colour of the solution. It should be amber. Any other colour indicates a problem.

In any case, there should be no precipitate, even if you wait a while after mixing.

The bottle is labeled "ferric oxalate hexahydrate" with CAS 166897-40-1, but I suppose it could be mislabeled. I spread the juice, chunks and all, onto some paper and exposed a step wedge, and processed according to Sandy King's instructions. I got an image with yellowish brown shadows (though not very dark) and more neutral midtones and highlights. There are specks throughout the shadows (hello chunks).

Indeed, my ferric oxalate solution is not amber. It is more greenish. Perhaps it's time to buy my ferric oxalate from a photographic outfit instead of a random eBay seller.

revdoc · Dec 5, 2024

Take a look at the images labelled "mature ferric oxalate solution" towards the bottom of this page:

Manufacturing Ferric Oxalate

Introduction This article explains how to make high quality ferric oxalate for platinum/palladium printing. It is my attempt to document a consistent process for manufacturing ferric oxalate. The process is based upon the one described by Giuseppe (Josef) Pizzighelli and Baron Arthur von Hübl in...

www.ianleake.com

You can see the green solution with 5% free oxalic acid. The brown solution below it is pure ferric oxalate. The presence of oxalic acid is usually a by-product of the manufacturing process.

I read somewhere that free oxalic acid has no adverse effect in platinum printing, but for kallitypes you want none. Or at most, a very small excess of oxalic acid.

You can probably eliminate the acid hy adding an alkali.

Ferric oxalate question

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