[Alt-photo] Dichromate-Waste

udo.schnitzbauer at access.uzh.ch udo.schnitzbauer at access.uzh.ch
Tue Mar 15 07:20:50 UTC 2016

Dear List, 

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 http://www.wrc.org.za/Knowledge%20Hub%20Documents/Water%20SA%20Journals/Manuscripts/1999/03/WaterSA_1999_03_jul99_p363.pdf 
- sodium thiosulfate http://infohouse.p2ric.org/ref/19/18012.pdf 
- iron filings http://www.must.edu.ph/mjst/wp-content/uploads/2014/02/1.-Hazardous-Waste-Chemicals-from-Dichromate-Chemical.pdf 
- sugar http://inpressco.com/wp-content/uploads/2013/03/Paper524-27.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

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. 

Here you find pictures and a description in German of the experiment what I have done:

Additionally you can use the sodim salt of ascorbic acid, too and you can thread all stuff whit 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. 


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