Contaminated cyanotype reagent?

Last night I finally got round to making up my own cyanotype solutions, rather than just using pre-made kits.

The ferricyanide went as I'd expect, but when I opened the 'ferric ammonium citrate (green)' that we brought several years ago I found a green gum in the tub rather than the powder I was expecting.
I'm a organic chemist so not fully up on inorganic compounds, but this isn't what I'd expect as either breakdown or moisture contamination!

Heating the tub in a water bath made the gum mobile enough to transfer & dissolve. If the stated citrate mass (100g) was correct, dissolving in 400ml of water should be about right.
The weight of the tub before & after transfer suggests there is 20g not accounted for, which I assume is the cause of the gum.

One evening this week I plan to try out my solutions to see if it still works with the additional unknown.

I have heard of gelatine being added to the cyanotype mix to improve it's coating of glass plates, but I've only heard of this done just prior to coating, after parts A & B are combined and the final concentration is not correct for this application. 20g/100ml mixture being recommended.

The citrate was brought by my wife from an Amazon art supplier, sold specifically for cyanotypes, rather than a general chemical supplier as I'm used to using for work.


Has anyone else come across deliberate additions to purchased ferric ammonium citrate (hopefully labeled as such which ours wasn't) or had a gum form on leaving old citrate solid around for years?

A response from a retired chemistry/biochemistry professor...

Ferric ammonium citrate is fairly hydroscopic. Thus I expect that your 'green gum' is just wet compound and that your mass discrepancy is the water that has been absorbed.

I would expect that the solution you made will work just fine.

The classic cyanotype recipe calls for a 25% (w/v) ferric ammonium citrate solution and a 10% (w/v) potassium ferricyanide solution. These are mixed in a 1:1 ratio to make the working sensitizer.

However, these days I use a 10% (w/v) solution of each component, mixed 1:1 to make the working solution. Most of my cyanotypes these days are toned with botanical dye stuffs, but the prints before toning are perfectly acceptable.

Gelatin is not needed for cyanotypes on paper or cloth. As you suggest, it is used to get the sensitizer to adhere to nonabsorbent surfaces.

You might consider 'developing' your cyanotypes in a dilute solution of vinegar. I use a tray of 10% (v/v) vinegar for my first wash. Subsequent washes are done in about 1% (v/v) vinegar except for the final wash which is plain water. I do this because my tap water (from a private well) tends to be a little basic. Prussian blue (the cyanotype pigment) is susceptible to alkaline hydrolysis. The use of vinegar helps with contrast, especially in the mid-tones.

fgorga said:

I would expect that the solution you made will work just fine.

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Sort of. The problem is that along with absorbing moisture from the air, it also tends to culture some forms of bacterial growth. This may or may not result in a non-functional soup after it's dissolved. Verify that the solution is a pure moss green with no black bits/strands floating around in it.

The tricky bit is working out how much FAC you've got once it has gained mass by drinking up water from the air.

Petrochemist said:

I'm a organic chemist so not fully up on inorganic compounds

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Correct me if I'm wrong, but ferric ammonium citrate is an organic chemical, no?
Either way, you're familiar with hygroscopic compounds, I trust. This is one of them.

Petrochemist said:

I have heard of gelatine being added to the cyanotype mix to improve it's coating of glass plates, but I've only heard of this done just prior to coating

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Makes sense; if you mix this stuff with gelatin and then keep it around, there's two things that will likely create problems: (1) the emulsion itself will fog; i.e. it'll create Prussian blue without considerable UV exposure. (2) As mentioned above, there will be bacterial and fungal problems that tend to make the gelatin break down into a liquid, stinky mess.

Petrochemist said:

or had a gum form on leaving old citrate solid around for years?

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Yup. I personally ended up discarding the solution I made from the 'gummed up' FAC. It wasn't much, and there were problems with it along the lines I mentioned above.

koraks said:

Sort of. The problem is that along with absorbing moisture from the air, it also tends to culture some forms of bacterial growth. This may or may not result in a non-functional soup after it's dissolved. Verify that the solution is a pure moss green with no black bits/strands floating around in it.

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Possible, but not likely. The amount of free water in this situation is very low and thus microbial growth is unlikely. A situation similar to that seen with honey.

Your idea of looking for "gunk" is a good one. If there is gunk, filtering it out of the solution will give a usable product.

koraks said:

The tricky bit is working out how much FAC you've got once it has gained mass by drinking up water from the air.

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Probably not a problem. As I said the standard solution is 25% (w/v) but some folks, including me, use a 10% solution. Thus the exact concentration does not seem to be critical.

koraks said:

Correct me if I'm wrong, but ferric ammonium citrate is an organic chemical, no?
Either way, you're familiar with hygroscopic compounds, I trust. This is one of them.

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Ferric ammonium citrate is probably most accurately categorized as an organometallic compound/complex.

koraks said:

Makes sense; if you mix this stuff with gelatin and then keep it around, there's two things that will likely create problems: (1) the emulsion itself will fog; i.e. it'll create Prussian blue without considerable UV exposure. (2) As mentioned above, there will be bacterial and fungal problems that tend to make the gelatin break down into a liquid, stinky mess.

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Agreed! The growth of microorganisms can be slowed/inhibited by adding thymol to most alt process solutions that support such growth. I keep a stock of 10% (w/v) thymol in iso-propanol (rubbing alcohol) and add 2 or 3 drops per 100 mL of solution. This is very effective at inhibiting fungus in ferric ammonium citrate solutions.

koraks said:

Yup. I personally ended up discarding the solution I made from the 'gummed up' FAC. It wasn't much, and there were problems with it along the lines I mentioned above.

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A very rational course of action!

Thank you both for your answers
I tried my reagents last night. No black bits visible & the reactions went as they should.
A second go today with better preparation has given a faint but promising print from my infra red digital negative. Hopefully it will darken up sufficiently in time. I now know the glass sheet I'd added increases exposure times by a factor of two to three...

I did hover find a few tweaks I need to make to my UV development box, now done

As frank pointed out ferric ammonium citrate, is on the border between inorganic & organic.
Citric acid is certainly organic so provides an organic ligand, but the main chemistry here is inorganic chemistry relating to the multiple oxidation states of iron.
Citric acid is fairly stable & easy to keep for years.

I have seen some definitions of organic chemistry suggesting the term applies to all carbon compounds (in my view this is nonsense carbonates,are not organic...) I would consider organic chemistry to relate to the covalently bonded chemistry of carbon with hydrogen & other non metals. Organometallics etc would them be more that are borderline

Petrochemist said:

I have seen some definitions of organic chemistry suggesting the term applies to all carbon compounds (in my view this is nonsense carbonates,are not organic...) I would consider organic chemistry to relate to the covalently bonded chemistry of carbon with hydrogen & other non metals. Organometallics etc would them be more that are borderline

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Thanks for the report back and glad everything is working well for you.

Your definition of organic chemistry is a pretty good one.

We do tend to sometime over simplify things when teaching beginners. The usual definition of organic compounds when teaching introductory courses (at least on this side of the pond) goes something like this... compounds containing carbon but not carbonates and cyanides. I've always considered this pretty insufficient.

We also seem to give beginners the impression that all compounds are either organic or inorganic, which is certainly not true. As we see here, organometallic compounds are a third class and an interesting one at that!!!