Clarification on proper potassium dichromate disposal?

rmc201 · Aug 19, 2023

Hi everyone!

I have been researching and learning how to do gum printing. I have enjoyed the few prints I have made, however I was interested in hearing clarification about potassium dichromate. I know that it is very toxic in powder form especially, safety protections need to be followed when using it in liquid form (gloves, safety glasses). But, is it toxic to wash out prints and then dump that developing tray down the drain? I have seen different answers from "no absolutely not, save it for proper disposal" to "it's too diluted to make a difference".

If it helps, I mixed mine at about 5% strength. I use a 1:1 ratio, 1 teaspoon of gum/color mix and 1 teaspoon of dichromate.

Any help or advice would be appreciated!

koraks · Aug 20, 2023

Welcome to Photrio!

rmc201 said:
But, is it toxic to wash out prints and then dump that developing tray down the drain?

Yes, the dichromate you wash out of prints is toxic. It's just as toxic as any other dichromate - there's just relatively little of it in a print. Although gum bichromate isn't all that efficient with dichromate, so it's still a significant quantity.

rmc201 said:
I have seen different answers from "no absolutely not, save it for proper disposal" to "it's too diluted to make a difference".

I'm going to go out on a limb and say that both are right, in a way. When it comes to dichromates, arguably, the only reasonable discharge in a waste system would be zero. At the same time, if you're on a regular sewer system, I doubt that the tiny amount of dichromate you wash away into your drain will be detectable once it arrives at the sewage treatment facility, let alone harmful in a significant way - even if it makes it all the way there without being reduced to far less harmful chromium-III.

But still - I personally consider dumping chromium-VI into a sewage system as bad practice, by definition. One might choose to do so in the knowledge that it's really not a great idea to begin with. That's an individual call to make. In the end, I don't think it can really be justified, even though I can see the argument that it's not very harmful as long as the quantities are tiny.

If you want to do this in a more sensible way, I would suggest the following:

1: Consider using a dichromate-free process. There are other pigment-printing processes that give more or less the same result and do not rely on chromium-VI. For instance, the recently published Zerochrome approach (https://zerochrome.org/) uses far more benign chemistry and will yield very similar results to gum bichromate. There's also Calvin Grier's PrintMaker's Friend (https://printmakersfriend.com/), which is expected to hit the market in a few months and probably similar/identical to Zerochrome.

2: If you must use a chromium-VI based process, consider saving your dichromated waste water in a tank or jerrycan. After doing a print, drop some sodium bisulfite into the tank and stir. You should see the color of the water change from (probably pale) yellow to nearly colorless with a green tinge. At higher concentrations, you'll see bright yellow or even orange give way to emerald green. For a single print, you'll probably need only one or two teaspoons of bisulfite to completely convert all chromium-VI in the water into chromium-III. While the latter is not entirely harmless, it's considerably less harmful than chromium-VI and I personally would not worry about dumping a small quantity of it down a typical drain into a sewer system. Again, it's a personal judgement call; even better would be to convert the chromium-VI to chromium-III and then take the waste water to a proper facility for disposal. Instead of bisulfite, vitamin C (ascorbic acid) might also be used.

3: Collect the chromium-VI containing wastewater and offer it to a disposal facility as is. They should (hopefully) dispose of it in a responsible manner. Given how easy it is to convert VI to III, I would still consider option (2) though, in case the waste disposal facility doesn't take into account the nature of the waste offered to them (which I find very likely).

4: The last resort is to just dump the wastewater into the drain. How bad this is, nobody can really answer. It's certainly at the bottom of the list for a good reason.

C-130 Nav · Aug 20, 2023

My short answer would be to take it to your municipality’s household hazardous waste location (if you’re lucky enough to have one - many places do not) or a city/county cleanup event. Storing the waste in a tank until you have enough to make it worthwhile can be done.

I’d use the chromium-III conversion trick posted by @koraks above. Additionally, since most photo chemicals are water soluble, evaporation can be used to reduce volume if you have to wait for a cleanup event. If you do use evaporation, definitely convert to chromium-III. I wouldn’t reduce the volume by evaporation on any of the more toxic chemicals because any precipitate will effectively be at a higher concentration. My 1940 Kodak manual gives the formula for uranium chloride toning - probably don’t want to be pouring that down the drain or concentrating it! Not so strangely, the post-WWII Kodak manual deleted that formula. Also, accomplish the evaporation in a place where it won’t be accidentally consumed by pets, kids, etc.

Most photographic chemicals are pretty safe but all have some level of toxicity (water can kill you if you drink too much). I recommend looking up Material Safety Data Sheets (MSDS) or Safety Data Sheets (SDS, the newer term for the same thing) for your specific photographic agents. These will tell you the harmful agents in the chemical and tell you how to dispose of it properly. Some folks cue in on the fact that when something is labeled at any level of toxicity, they run from it. No need to do so for photography. Toxicity has varying degrees and, for our hobby/profession, the minor concerns on photo chemicals can be fully mitigated by a few basic handling techniques.

Lastly, some people go to 4 years of college to get a degree in hazardous waste control and, in many cases, all that education can be summed up in one sentence:

“The solution to pollution is dilution.”

So keep that in mind for any of the stuff you deem appropriate to pour down the drain.

Have fun and welcome to the forum.

koraks · Aug 20, 2023

C-130 Nav said:
Lastly, some people go to 4 years of college to get a degree in hazardous waste control and, in many cases, all that education can be summed up in one sentence:

“The solution to pollution is dilution.”

Without wanting to start an argument on this (because if memory serves, we've had those on here on this very statement before): this is not an accurate summary of any degree. Firstly, there's no college degree in 'hazardous waste control', and secondly, current scholastic thinking and teaching on waste control no longer rely on a mantra of dilution. Perhaps this was accurate in the 1950s or 1960s, but it certainly is not accurate today. If it applies to any college currently offering a curriculum in environmental science, chemistry or a related field, steer clear of this college - they're not worth the tuition fees. Moreover, the dilution argument will not be accepted by any authority that may impose restrictions or penalties on discharge practices. Simply put, the argument "but I washed it down with a bucket of water" isn't going to win over any working professional active today.

Vaughn · Aug 20, 2023

I use Vitamin C (ascorbic acid) to convert the chromium-VI to chromium-III.

And agree with Koraks..."The solution to pollution is dilution." is a way-out-of-date way of thinking.

C-130 Nav · Aug 20, 2023

In response to my previous post, I concur with the follow-on comments. It was a humorous attempt to simplify a complex issue that fell flat. Note that I did not recommend pouring chromium-VI or any other hazardous substance down the drain but left that call to the reader for chemicals deemed appropriate to dispose of as such. I won’t edit the original comment as the follow-on comments will then lack context. But I acknowledge I stand corrected, or at least, clarified. Thank you.

I still stand by my recommendations to dispose of hazardous material at a collection site and to review applicable MSDS/SDS for appropriate handling/disposal methodologies which I don’t believe is contentious with anyone.

koraks · Aug 20, 2023

Thank you for your response, @C-130 Nav - much appreciated. Let it be noted that you indeed never recommended irresponsibly dumping any hazardous chemical down the drain!

To an extent, I can even agree with the pollution/dilution saying, but it's very context-dependent. For instance, I act on it when it comes to chromium-III (i.e. I dump tiny amounts of that down the drain alright - not that I recommend it, but I admit to doing it), but not when it comes to chromium-VI (i.e. I 'neutralize' that first).

rmc201 · Aug 20, 2023

koraks said:
Welcome to Photrio!

Yes, the dichromate you wash out of prints is toxic. It's just as toxic as any other dichromate - there's just relatively little of it in a print. Although gum bichromate isn't all that efficient with dichromate, so it's still a significant quantity.

I'm going to go out on a limb and say that both are right, in a way. When it comes to dichromates, arguably, the only reasonable discharge in a waste system would be zero. At the same time, if you're on a regular sewer system, I doubt that the tiny amount of dichromate you wash away into your drain will be detectable once it arrives at the sewage treatment facility, let alone harmful in a significant way - even if it makes it all the way there without being reduced to far less harmful chromium-III.

But still - I personally consider dumping chromium-VI into a sewage system as bad practice, by definition. One might choose to do so in the knowledge that it's really not a great idea to begin with. That's an individual call to make. In the end, I don't think it can really be justified, even though I can see the argument that it's not very harmful as long as the quantities are tiny.

If you want to do this in a more sensible way, I would suggest the following:

1: Consider using a dichromate-free process. There are other pigment-printing processes that give more or less the same result and do not rely on chromium-VI. For instance, the recently published Zerochrome approach (https://zerochrome.org/) uses far more benign chemistry and will yield very similar results to gum bichromate. There's also Calvin Grier's PrintMaker's Friend (https://printmakersfriend.com/), which is expected to hit the market in a few months and probably similar/identical to Zerochrome.

2: If you must use a chromium-VI based process, consider saving your dichromated waste water in a tank or jerrycan. After doing a print, drop some sodium bisulfite into the tank and stir. You should see the color of the water change from (probably pale) yellow to nearly colorless with a green tinge. At higher concentrations, you'll see bright yellow or even orange give way to emerald green. For a single print, you'll probably need only one or two teaspoons of bisulfite to completely convert all chromium-VI in the water into chromium-III. While the latter is not entirely harmless, it's considerably less harmful than chromium-VI and I personally would not worry about dumping a small quantity of it down a typical drain into a sewer system. Again, it's a personal judgement call; even better would be to convert the chromium-VI to chromium-III and then take the waste water to a proper facility for disposal. Instead of bisulfite, vitamin C (ascorbic acid) might also be used.

3: Collect the chromium-VI containing wastewater and offer it to a disposal facility as is. They should (hopefully) dispose of it in a responsible manner. Given how easy it is to convert VI to III, I would still consider option (2) though, in case the waste disposal facility doesn't take into account the nature of the waste offered to them (which I find very likely).

4: The last resort is to just dump the wastewater into the drain. How bad this is, nobody can really answer. It's certainly at the bottom of the list for a good reason.

Thank you for the well written response! I have been following news on the printmaker's friend so I am very excited to try it out.

This is a side question for number 2, I got vitamin c to try and convert the waste water. I did a few small 7x9 prints earlier, saved the water and added vitamin c. However the color of the waste water is already a dark gray color (I think something is going wrong both in exposure and development for it to do that). So how do I know it is the correct amount of vitamin c in that case? I did about a teaspoon for 500mL of waste water.

MattKing · Aug 20, 2023

The "solution is dilution" reference still makes sense in one circumstance - when the pollutant is a pollutant because of excess concentration.
As an example, ocean water with excess amounts of salt is a pollutant, but dilution that lowers the salt concentration to normal levels renders it non-polluting if it is then returned to the ocean.

koraks · Aug 20, 2023

rmc201 said:
So how do I know it is the correct amount of vitamin c in that case?

Well, you could calculate it, and thus approximate it. I'm on my phone now, but maybe I can do an example later tonight. Anyway, short answer: your teaspoon is ample.

Finn lyle · Aug 20, 2023

Cr (III) compounds can also be precipitated out of solution into the insoluble Cr(OH)3 by adding dilute solutions of caustic soda, which would (theoretically) reduce the amount of soluble chromium available to dump into a wastewater system. One could then collect the precipitate until it can be disposed of at a hazardous waste facility. IIRC it is best to err on the side of excess Cr(III) as too much hydroxide will re-dissolve the Chromium.

koraks · Aug 20, 2023

Finn lyle said:
Cr (III) compounds can also be precipitated out of solution into the insoluble Cr(OH)3 by adding dilute solutions of caustic soda, which would (theoretically) reduce the amount of soluble chromium available to dump into a wastewater system. One could then collect the precipitate until it can be disposed of at a hazardous waste facility. IIRC it is best to err on the side of excess Cr(III) as too much hydroxide will re-dissolve the Chromium.

That's a neat trick! Thanks for sharing that, and also the caution about overdoing it, although it would just result in the original situation with a little waste of hydroxide.

rmc201 said:
So how do I know it is the correct amount of vitamin c in that case?

Ok, so the calculus exercise - and this is mostly for entertainment purposes, because your teaspoon approach is really amply sufficient. But to get a feeling for magnitudes and whatnot, it can be helpful to do the paper napkin exercise once in a while.

I'll assume potassium dichromate in my calculations; you could correct for this if you use ammonium dichromate instead based on the difference in molar mass, ammonium dichromate being about 15% lighter than the potassium salt. That's a small difference for this purpose, so don't worry about it, I'd say.

Going by Calvin Grier's Gum Printing manual (recommended if you haven't already read it), roughly 10g sensitizer solution (in a total of ca. 20ml coating solution) is sufficient for a 40x50cm print - at least in his workflow. That's 0.010ml 10% dichromate per cm2, or 1mg potassium dichromate per cm2.

You mention 7x9" (ca. 18x23cm, or 414cm2) prints and roughly 1 teaspoon of 5% dichromate. A teaspoon is roughly 5ml, so you're coating about 0.012ml 5% dichromate per cm2, or 0.6mg potassium dichromate per cm2, or a total of ca. 250mg potassium dichromate in your entire print.

Let's assume that virtually all of this dichromate will wash out of your print. That's not entirely correct, but it's close enough since very little will remain in the gum layer and only a small amount tends to print out (converting into chromic acid) during exposure.

Potassium dichromate is around 295g/mol, so we have around 0.85 mmol (milli-mol) of dichromate to dispose of. Reacting the dichromate ion with vitamin C (ascorbic acid) takes 3 moles of vitamin C for each mole of dichromate, so we need around 2.55mmol vitamin C. Vitamin C is around 176g/mol, which results in some 0.45g of vitamin C being needed to 'neutralize' all the dichromate you put into the print.

A teaspoon will hold roughly 5g of powder, which works out as about a tenfold excess for this purpose. So your teaspoon guesstimate was perfectly reasonable - albeit somewhat wasteful, but that's a reasonable price to pay, I'd say.

Conversely, you could collect the was water of, say, five prints and neutralize all that dichromate with the same teaspoon of vit.C.

If you want to work out the math for bisulfite: the dichromate reaction with bisulfite requires two moles of bisulfite for each mole of dichromate. The molar mass of bisulfite can easily be googled. See if you can work out how much bisulfite you'd need to convert the same dichromate from that print into less harmful Cr(III). If you're so inclined, you could then determine which is cheaper, and go with that. Or just don't worry about it and keep adding a teaspoon of vitamin C to the wash water of each print!

Maris · Aug 20, 2023

Decades ago I was a scientist working in the field of tracing toxins in the environment and had quite a bit to do with hazardous waste management and services. Do you ever wonder what a municipal hazardous waste facility would do with your small amount of chromium?

Even today there are only four basic pathways for hazardous waste:

1. Sealed landfill. Solves the problem unless the seal leaks in the next few hundred years.
2. Disposal in an ordinary sewage treatment stream where dilution is ultimately the solution.
3. High temperature incineration on land. But not for chromium which as an inorganic does not burn.
4. High temperature incineration at sea. Again not for chromium.

My guess is that small amounts of chromium are put down a sewage treatment system and the final discharge to a river or the sea is monitored for a spectrum of nasties including Cr.

Quantity is critical. The government laboratory where I worked used chromic acid for decontaminating glassware. Exhausted chromic acid, kilograms per week, was routinely put down the drain with lots of water not to save the environment but to save the pipes.
The local tannery used chromates in ton quantities. Their waste now lies in sealed landfill. Maybe a ticking time-bomb for the future.

I analysed trace amounts of chromium in people using a Hitachi Z-7000 Zeeman effect atomic absorption spectrophotometer. Nothing approaching a toxic level was ever found. This seriously expensive research was funded by a sugar company hoping that trace amounts of chromium would protect people against diabetes. It didn't.

Pardon my reminiscences.

rmc201 · Aug 22, 2023

koraks said:
That's a neat trick! Thanks for sharing that, and also the caution about overdoing it, although it would just result in the original situation with a little waste of hydroxide.

Ok, so the calculus exercise - and this is mostly for entertainment purposes, because your teaspoon approach is really amply sufficient. But to get a feeling for magnitudes and whatnot, it can be helpful to do the paper napkin exercise once in a while.

I'll assume potassium dichromate in my calculations; you could correct for this if you use ammonium dichromate instead based on the difference in molar mass, ammonium dichromate being about 15% lighter than the potassium salt. That's a small difference for this purpose, so don't worry about it, I'd say.

Going by Calvin Grier's Gum Printing manual (recommended if you haven't already read it), roughly 10g sensitizer solution (in a total of ca. 20ml coating solution) is sufficient for a 40x50cm print - at least in his workflow. That's 0.010ml 10% dichromate per cm2, or 1mg potassium dichromate per cm2.

You mention 7x9" (ca. 18x23cm, or 414cm2) prints and roughly 1 teaspoon of 5% dichromate. A teaspoon is roughly 5ml, so you're coating about 0.012ml 5% dichromate per cm2, or 0.6mg potassium dichromate per cm2, or a total of ca. 250mg potassium dichromate in your entire print.

Let's assume that virtually all of this dichromate will wash out of your print. That's not entirely correct, but it's close enough since very little will remain in the gum layer and only a small amount tends to print out (converting into chromic acid) during exposure.

Potassium dichromate is around 295g/mol, so we have around 0.85 mmol (milli-mol) of dichromate to dispose of. Reacting the dichromate ion with vitamin C (ascorbic acid) takes 3 moles of vitamin C for each mole of dichromate, so we need around 2.55mmol vitamin C. Vitamin C is around 176g/mol, which results in some 0.45g of vitamin C being needed to 'neutralize' all the dichromate you put into the print.

A teaspoon will hold roughly 5g of powder, which works out as about a tenfold excess for this purpose. So your teaspoon guesstimate was perfectly reasonable - albeit somewhat wasteful, but that's a reasonable price to pay, I'd say.

Conversely, you could collect the was water of, say, five prints and neutralize all that dichromate with the same teaspoon of vit.C.

If you want to work out the math for bisulfite: the dichromate reaction with bisulfite requires two moles of bisulfite for each mole of dichromate. The molar mass of bisulfite can easily be googled. See if you can work out how much bisulfite you'd need to convert the same dichromate from that print into less harmful Cr(III). If you're so inclined, you could then determine which is cheaper, and go with that. Or just don't worry about it and keep adding a teaspoon of vitamin C to the wash water of each print!

Thank you again for the informative response! I do use potassium dichromate and while I agree it is pretty wasteful to use that much vitamin c, I would probably prefer to be little excessive compared to using too little. At least for the beginning while I continue to research the topic. I'll also look into the bisulfite so thank you for that recommendation!

Calvin Grier's book is next on my list to purchase. I have been looking at Christina Anderson's book and more videos/websites detailing the process. One area I have found is using ferric ammonium citrate instead of the dichromate and then washing in a hydrogen peroxide bath. I have not attempted that yet, but I'll keep looking into that and the zerochrome as well.

koraks · Aug 22, 2023

rmc201 said:
One area I have found is using ferric ammonium citrate instead of the dichromate and then washing in a hydrogen peroxide bath

I've tried this (once) for carbon transfer, and I'd like to follow up that line of experimentation one day. The drawback at least in carbon transfer is that the tissue needs to be exposed through a transparent substrate, which creates sharpness/resolution problems due to the substrate sitting in between the tissue (print) and the negative. I don't know how and if the process works with gum; I've never looked into this.

Donald Qualls · Aug 22, 2023

FWIW, sodium sulfite will also accomplish the conversion of Cr+6 into Cr+3, and is significantly less costly than ascorbate. When I've used dichromate in the past (as reversal bleach), I just poured the bleach clearing bath (sodium sulfite solution, don't offhand recall the strength) into the used bleach and it went from orange to brown to green in a minute or less.

Clarification on proper potassium dichromate disposal?

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