Dichromate-Waste-Treatment

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BJ68

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I posted this text at the altphotolist and the LF Forum, too
http://www.largeformatphotography.info/forum/showthread.php?129525-Dichromate-Waste-Treatment


Even if the amounts of dichromate are very tiny, there is a problem with the dichromate waste which is accumulated during gum printing.

In the literature there are some methods reported to reduce the toxicity of that stuff. One way is reducing the Cr(VI) to Cr(III) with some reduction reagents. In the web you will find protocols for that e.g.
- sodium sulfite Dead Link Removed
- sodium thiosulfate http://infohouse.p2ric.org/ref/19/18012.pdf
- iron filings Dead Link Removed
- sugar http://inpressco.com/wp-content/uplo...aper524-27.pdf

So far I see all this methods have a few disadvantages:
a) They are working only in a small range of pH. between 1 and 2.
b) To get to this pH you have to use sulfuric acid
c) With the first one, you get toxic gas and with the second the chemical (fixing agent) is expensive.

So I searched further and found this nice paper:

Reduction of hexavalent chromium by ascorbic acid in aqueous solution.
Chemosphere 57:609
https://www.researchgate.net/profile...ication_detail

It works at neutral or even at slight alkaline pH, it goes fast, you see a color change and if you use a excess of ascorbic acid you are in the acid pH-region and it´s cheap because ascorbic acid is a food supplement.

In short for 3 g dichromate you use 10 g of ascorbic acid, which is twice the amount according to the paper above, to be on the safe side and to get the pH more in the acidic direction.

Here you find pictures and a description in German of the experiment what I have done:
http://illumina-chemie.de/dichromatv...ung-t4201.html

Additionally you can use the sodium salt of ascorbic acid, too and you can thread all stuff with it. E.g. I use now a little bit of a ascorbic acid solution for washing my brushes and even here I can see the color change.

Reduction to Cr(III) is a good way to reduce the toxicity of the Cr(VI) compounds and if you use a ascorbic acid solution you can decontaminate your workspace, the bench and other equipment which was in contact with dichromate-containing solutions.

Although I have a chemistry background (but I am more practical), please can somebody with more knowledge check my suggestions....Thank you


Bj68
 

removed account4

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i commend you in not dumping nasty photochemistry down the drain.
i wish more people would follow your lead and not use the drain pipe in their sink
or outdoors as their waste bin ... there is an awful lot of mis and DIS information about
many things related to photography, i'm glad you are helping people do the right thing by posting this.

best of luck !
john
 
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Well the best would be to replace the dichromate by diazo. In carbon print this works well with Disodium 4,4'-Diazidostilbene-2,2'-disulfonate (CAS 2718-90-3). I don’t know if somebody already found some method to use diazo for gum prints.
 
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