We do that here industrially. The cheap, fast method is reducing with FeCl2. Go buy some clorhidric acid and put some iron in it (nails, etc) fort a couple of days in a ventilated area (outside). You will get a green solution on FeCl2. You can use it to reduce the CrVI to CrIII.
Yes, that should work, too, but you have to extend the reaction time, because the concentration of the Cr(VI) is very low or you use more ascorbic acid.Originally Posted by jp
I made a test with a big amount of water (10 L) where I put a few grains of potassium dichromate in it. The color of the liquid was slight yellowish. After addition of a bigger amount (about 10 g) ascorbic acid the water was completely decolorized.
So my suggestion where: Collect the first rinse water (where the main part of the dichromate gets washed out) in a bin, add about 1 g ascorbic acid for every liter and let it sit for a while (best would be until your next cyanotype session, but 1 hour should be enough).
One disadvantage or important fact: The treatment changed the toxicity of the chromic compound, but it do not change the fact, that chromium itself is a heavy metal and the cumulation of this stuff e.g. in soil if you have a cesspit (Sickergrube in German) should be avoided. According to this https://en.wikipedia.org/wiki/Chromium_toxicity (the first lines) even Cr(III) has its own problems.
Edit: You can get a side reaction with the other compounds of the cyanotype rinse water...so expect some precipitation....
The other aspect can be how stable is the ferricyanide against the ascorbic acid. From memory I think that there is no risk to produce prussic acid https://en.wikipedia.org/wiki/Hydrogen_cyanide which is very, very toxic, because ferricyanide reacts only with mineral acid to release hydrogen cyanide see https://en.wikipedia.org/wiki/Ferricyanide So I think ascorbic acid is to weak for that reaction.
Bj68
Bj68; thank you for answering! You are great!
From this response I think I will not dump the first rinse down the drain.
Short update: It seems that citric acid is not working as reducing agent see my quick shot at
Minimizing wash water containing dichromate?
Bj68
The common method in darkrooms is (or was...) mixing the dichromate solution with used developer, inmediately the orange shifts to green indicanting the Cr III presence.
It's the way I did it when I was making reversal process with dichromate.
Note: this should be done in open air because some "sulfur dioxide" may be released.
Last edited by Pere Casals; 12-Oct-2018 at 12:51. Reason: Note
Mixing with used developer sounds better.
Do you know if color change indicates toxicity reduction?
Tin Can
Yes... the Cr (VI) solution is orange, the Cr (III) is green, I drop used developer until the the color does not change anymore, so not much Cr III remains, then I add more to be sure that enough developer is added to exhaust the VI as possible.
IIRC there is a "Predominance diagram" that tells the remaining Cr (VI) depending on pH and Cr concentration...
Docs posted by OP are quite interesting...
Last edited by Pere Casals; 13-Oct-2018 at 01:13.
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