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  1. #1
    holmburgers's Avatar
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    Does sodium bisulfite make dichromates any safer?

    So we all know that sodium bisulfite will clear the stain of dichromates and "neutralize" them. But does it actually make the dichromates safer? As in, environmentally neutral and inert.

    I ask this mainly because I've been turning my used sensitizer green with the addition of Na-Bisulfite, and then pouring it down the drain (very small amounts).

    My question is whether or not this is necessary, or if I'm just wasting Na-bisulfite.
    If you are the big tree, we are the small axe

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    Been curious myself about transforming waste dichromate Cr VI to something less hazardous. Did some reading a couple years ago and came across a treatment where a two zinc anodes are placed in the waste solution. A low voltage is passed between the anodes and ( now this part is sketchy from memory ) Cr III is then precipitated. Never did get around to testing anything, but I would be curious if there was a simple method to redox (?) the Cr VI to Cr III.

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    Upfront disclaimer: The amounts typically used by home carbon printers are probably insignificant compared to the volumes of water processed by your local sewer or water authority. But, your sewer authority may not be equipped to remove chromium from the waste stream before it is discharged into surface water supplies.

    Upfront disclaimer 2: I am not a chemist. I have no background in water / environmental science. I don't speak for any regulatory agency or sewer or water authority.



    My understanding is that just reacting the dichromate with bisulfite doesn't render the substance truly inert. It does convert the hexavalent chromium to trivalent chromium (Cr6+ to Cr3+). It can be precipitated by mixing that green solution with lye (or perhaps other strong alkalai) to form chromium hydroxide, which is a stable compound (and a solid) often used as a green pigment (?PG18?, CI 77289). I don't have a link, but I found a discussion of this somewhere in APUG, but I'm in front of the wrong computer to find the bookmark.

    The EPA (and in turn, municipal sewer and water departments) only regulate total chromium, AFAIK, so whatever form it is put into the drain in solution is a potential issue. From what I understand Cr3+ is less damaging to human tissue, and less readily absorbed by cells, so if faced with an option of putting Cr6+ or Cr3+ down the drain, Cr3+ is the better choice.

    I use brush sensitization so I have very little waste solution. But there is some, so I have been collecting it, plus the first rinse water of the brush, and plan on precipitating it once the container gets full.

    --Greg

  4. #4
    holmburgers's Avatar
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    Thanks for the informed responses guys.

    I suppose another idea would be throw it away instead of pour it down the drain. I too am going to start brush sensitizing and perhaps a ziplock bag with some cat litter would be sufficient for small amounts.

    I like the idea of evaporating the water and collecting the dried powder too, but this seems like a practicality nightmare.
    If you are the big tree, we are the small axe

  5. #5

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    Quote Originally Posted by gmikol View Post
    Upfront disclaimer: The amounts typically used by home carbon printers are probably insignificant compared to the volumes of water processed by your local sewer or water authority. But, your sewer authority may not be equipped to remove chromium from the waste stream before it is discharged into surface water supplies.

    Upfront disclaimer 2: I am not a chemist. I have no background in water / environmental science. I don't speak for any regulatory agency or sewer or water authority.

    --Greg
    I like your disclaimers.

    Most of my waste is from gum printing, I save that 1st bath where most of the dichromate leaches out.

    But it would be cool to make my own green pigment from my gum waste water which then becomes another gum print. I'd be going green with my green.

  6. #6
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    Quote Originally Posted by R Shaffer View Post
    But it would be cool to make my own green pigment from my gum waste water which then becomes another gum print. I'd be going green with my green.
    mind = blown
    If you are the big tree, we are the small axe

  7. #7

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    Quote Originally Posted by holmburgers View Post
    mind = blown
    I'm with you on that. Cool idea!

  8. #8
    DAP
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    Before I start let me echo Greg's disclaimers. I haven't tried this disposal method as of yet. If any of you screw up and die, cause a severe population die-off, blow up your house, raise the dead, create a black hole, become possessed, etc. as a result of this post I will deny everything

    I did some searching on the internet about Potassium Dichromate disposal (I am going to start doing some negative enlarging using the reversal method which uses a dichromate bleach). During my research I came across a book (via Google books) called "Hazardous Laboratory Chemicals Disposal Guide". The recommended disposal method for Potassium Dichromate that they provided is as follows (they are assuming that you are starting with the dry solid form):

    1) Add solid to water (100ml/5g)
    2) Acidify w/ Sulfuric acid (35-55ml) until you reach a pH of 1
    3) While stirring add 13.5g of sodium thiosulfate until the solution becomes cloudy and blue colored
    4) Neutralize w/ sodium carbonate
    5) After a few minutes a blue/gray flocculant precipitate will form
    6) Let mixture stand for 1 week or filter through Celite
    7) After 1 week the supernant can be decanted and placed down drain
    8) The remaining liquid is allowed to evaporate
    9) The remaining residue is washed w/ hot water and dried.
    10) Send the solid off to a hazardous waste facility

    Hopefully somebody who actually knows about chemistry will chime in on whether this is a realistic disposal method.

  9. #9

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    Any reducing agent like sodium sulfite or sodium bisulfite will convert dichromate ion to Cr (III). Just make sure that you use use an excess. The high sulfide environment in a septic tank or sewage system will then convert the Cr (III) to very insoluble chromium sulfide.
    A rock pile ceases to be a rock pile the moment a single man contemplates it, bearing within him the image of a cathedral.

    ~Antoine de Saint-Exupery



 

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