Discarding Cyanotype senitizer; best method?

Kino · Feb 5, 2024

Yes, I know you should use it all, but I have a small amount that is probably expired or near expiring and I'd rather toss it than use it.

Wondering if you could take some junk paper and coat it, let it dry, set it in the sun for a day and run it through a water bath.

Would that not be the best method, rather than store it for a year until our local haz waste day happens. (yes it's only a day a year)

gmikol · Feb 5, 2024

Could you just add a strong alkali to the solution to get the prussian blue to precipitate out? And save yourself the hassle of coating/drying/washing the paper before discarding it?

titrisol · Feb 5, 2024

Kino said:
Yes, I know you should use it all, but I have a small amount that is probably expired or near expiring and I'd rather toss it than use it.

Wondering if you could take some junk paper and coat it, let it dry, set it in the sun for a day and run it through a water bath.

Would that not be the best method, rather than store it for a year until our local haz waste day happens. (yes it's only a day a year)

It depends what is a "small amount" if you are talking a few ml or liters or gallons?

For tiny amounts (ml) just soak paper towels and expose them to light, no need to wash afterwards as the blue is very stable and is non-toxic (AFAIK) , and those them in the trash.

Kino · Feb 5, 2024

titrisol said:
It depends what is a "small amount" if you are talking a few ml or liters or gallons?

For tiny amounts (ml) just soak paper towels and expose them to light, no need to wash afterwards as the blue is very stable and is non-toxic (AFAIK) , and those them in the trash.

Yes, just talking about a few ounces.

BJ68 · Feb 6, 2024

Which kind of sensitizer is it or what is the composition?

Ammonium iron(III) citrate, Potassium ferricyanide or the New Cyanotype sensitizer solution https://www.alternativephotography.com/new-cyanotype-process/ can go down the drain, if it is a small amount (under 1 L) and if you dilute it.
But if you have added Ammonium dichromate you should reduce it to Chrome(III) with addition of ascorbic acid first.

bj68

Kino · Feb 6, 2024

It is only about two ounces of the first type mentioned; not the new type.

I have not added any of the 1% Potassium Dichromate to this batch.

Is there a rule of thumb for how much ascorbic acid is required to reduce the dicromate to safe level, should I add it in the future?

BrianShaw · Feb 6, 2024

If you are on a municipal sewer system, perhaps ask the agency for guidance. It would not surprise me to hear that they would consider dilution of the sewer system as adequate for such small quantities

titrisol · Feb 6, 2024

Kino said:
It is only about two ounces of the first type mentioned; not the new type.

I have not added any of the 1% Potassium Dichromate to this batch.

Is there a rule of thumb for how much ascorbic acid is required to reduce the dicromate to safe level, should I add it in the future?

These remediation articles used 3:1 to 5:1 Ascorbic to Chromium ratio, with sunlight, pH close to neutral (acid is better) and time in the range of hours
Since ascorbic is not expensive, I'd add at least 10X to provide plenty of ascorbic ions and to lower the pH which should work on your favor. Plus I don't know if the ascorbates are going to react wit the ferro/ferri cyanides.

Reduction of hexavalent chromium by ascorbic acid in aqueous solutions - PubMed

Hexavalent chromium is a priority pollutant in the USA and many other countries. Reduction of Cr(VI) to Cr(III) is environmentally favorable as the latter species is not toxic to most living organisms and also has a low mobility and bioavailability. Reduction of Cr(VI) by ascorbic acid (vitamin...

pubmed.ncbi.nlm.nih.gov

https://www.researchgate.net/publication/8227753_Reduction_of_hexavalent_chromium_by_ascorbic_acid_in_aqueous_solutions

Some rough math
K2Cr2O7 has ~35% of Cr in weight
1% potassium dichromate = 0.35% Cr(VI) or 35mg/ml
For every ml (approx 1g) you'd need 35*5 = 175 mg of ascorbic
Drops are about 0.05-0.1 ml so you can make the math from there
AND
1 teaspoon ~3g of Vitamin C

Kino · Feb 6, 2024

Thank you, Titrisol. Great information for the future!

I'll do it outside when the need arises.

koraks · Feb 6, 2024

titrisol said:
These remediation articles used 3:1 to 5:1 Ascorbic to Chromium ratio, with sunlight, pH close to neutral (acid is better) and time in the range of hours

When doing this in the photo-lab, the process is virtually instantaneous. I often neutralize small amounts of dichromate; I prefer using bisulfite for this because it's even cheaper than ascorbic acid and it dissolves quicker. The downside is that it releases some sulfur dioxide gas, but this amount is so miniscule that I never even smell it. It's only a concern when trying to neutralize (reduce) large amounts of dichromate - for which there is no practical reason in photography anyway. It's also perfectly safe to do indoors, regardless if you're using ascorbic acid or bisulfite.

I'd have no qualms flushing your little bit of cyanotype through the sink. Just run the tap for a minute or so in order to dilute.

BrianShaw said:
If you are on a municipal sewer system, perhaps ask the agency for guidance.

I'd be surprised if any official agency in the 21st century would say "oh no problem, just flush it down". I bet they won't even give a straight answer if you ask if it's OK to flush some bleach down the toilet. It has nothing much to do with what is technically or environmentally sane, and everything to do with liability...Of course, asking for guidance is the 'correct' thing to do. Can't argue with that...

Kino · Feb 6, 2024

I am on a septic system, so I will probably try to avoid dichromate if at all possible.

koraks · Feb 6, 2024

When neutralized into chromium(III), I doubt it'll affect your septic system at all.
Heck, even if you don't neutralize it, it'll probably reduce from (VI) to (III) in the complex chemical system of a septic tank anyway. It just takes a little longer.

Of course, it's good practice to avoid using hexavalent chromium.

titrisol · Feb 6, 2024

Kino said:
Thank you, Titrisol. Great information for the future!

I'll do it outside when the need arises.

I rarely have any sensitizer left over but the paper towels (its hard to find newspaper anymore) that I use to protect the table go into the normal trash. And I guess if I had a few ml I'd do the same.

Andrew O'Neill · Feb 6, 2024

koraks said:
When doing this in the photo-lab, the process is virtually instantaneous. I often neutralize small amounts of dichromate; I prefer using bisulfite for this because it's even cheaper than ascorbic acid and it dissolves quicker. The downside is that it releases some sulfur dioxide gas, but this amount is so miniscule that I never even smell it. It's only a concern when trying to neutralize (reduce) large amounts of dichromate - for which there is no practical reason in photography anyway. It's also perfectly safe to do indoors, regardless if you're using ascorbic acid or bisulfite.

I'd have no qualms flushing your little bit of cyanotype through the sink. Just run the tap for a minute or so in order to dilute.

I'd be surprised if any official agency in the 21st century would say "oh no problem, just flush it down". I bet they won't even give a straight answer if you ask if it's OK to flush some bleach down the toilet. It has nothing much to do with what is technically or environmentally sane, and everything to do with liability...Of course, asking for guidance is the 'correct' thing to do. Can't argue with that...

I've been using Ascorbic Acid for a while now. I don't think it's so cheap like some have mentioned here, and I would prefer using it for the various film developers that I use. How much bisulfite are you using to neutralise dichromate? Thanks!

koraks · Feb 6, 2024

Andrew O'Neill said:
How much bisulfite are you using to neutralise dichromate?

A pinch, and if it doesn't clear to pale green, then add another pinch. The color is a great indicator; a yellow/orange dichromate solution turns emerald green first and ultimately a pale, muted green when all the chromium VI has been converted to III.

Andrew O'Neill · Feb 6, 2024

koraks said:
A pinch, and if it doesn't clear to pale green, then add another pinch. The color is a great indicator; a yellow/orange dichromate solution turns emerald green first and ultimately a pale, muted green when all the chromium VI has been converted to III.

Excellent! Thank you!

Kino · Feb 6, 2024

I was given a full pound of Potassium Dichromate by a friend who was going to attempt to 16mm reversal processing, but decided otherwise.

It's still viable and rattles freely in the bottle from Photographers Formulary.

That's a lot of dichromate...

Discarding Cyanotype senitizer; best method?

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Reduction of hexavalent chromium by ascorbic acid in aqueous solutions - PubMed

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