Zinc Sulphate Monohydrate stock solution turning brown

Discussion in 'Reef Chemistry by Randy Holmes-Farley' started by Charterreefer, Mar 13, 2018.

  1. Charterreefer

    Charterreefer Active Member

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    I mixed up a stock solution using Zinc sulphate Monohydrate (from Alpha chemical) to dose zinc. I mixed two batches already and have had the same thing happen i.e. the solution turning slightly brown tint with what looks like a flaky precipitate laying on the bottom on the glass container the stock solution is stored in. I use one gram of the zinc compound dissolved into 250 mL of DI water.
    The first time I thought I cross contaminated it with another additive. I mixed a second batch and after 2 weeks have the same problem. I use the same eye dropper but rinse it in between the Iodine and Manganese that I also dose daily.
     
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  2. Randy Holmes-Farley

    Randy Holmes-Farley Reef Chemist Staff Member Team R2R R2R Supporter R2R Excellence Award Article Contributor Expert Contributor

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    I think it must be some other chemical that is an impurity. Maybe iron. Or the manganese.

    There are no colored forms of zinc that are likely to form.
     
  3. Christoph

    Christoph Active Member R2R Supporter Gold Sponsor R2R Excellence Award

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    Since imo all Zinc salts are colorless i would assume its some contaminant in your zinc sulfate. - Possibly iron (II) that oxidized in sokution to iron(III) giving you the brownish coloration.

    Strange is the precipitate since also iron sulfates should be readily soluble in water.

    cheers,
    Christoph

    Edit: Randy was quicker :)
     
  4. Randy Holmes-Farley

    Randy Holmes-Farley Reef Chemist Staff Member Team R2R R2R Supporter R2R Excellence Award Article Contributor Expert Contributor

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    lol
     
  5. Charterreefer

    Charterreefer Active Member

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    Thanks for the reply.
    Where would the iron come from? How would it get into my solution?
     
  6. Randy Holmes-Farley

    Randy Holmes-Farley Reef Chemist Staff Member Team R2R R2R Supporter R2R Excellence Award Article Contributor Expert Contributor

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    Contamination in the zinc. :)
     
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  7. Charterreefer

    Charterreefer Active Member

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    Can that be a contaminant in the zinc sulphate?
     
  8. Charterreefer

    Charterreefer Active Member

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    Is there a specific grade that is free of contaminants? I bought it from Alpha chemicals. I believe they only had one grade. Can you tell me of a better place to get chemicals at a reasonable price?
     
  9. Randy Holmes-Farley

    Randy Holmes-Farley Reef Chemist Staff Member Team R2R R2R Supporter R2R Excellence Award Article Contributor Expert Contributor

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    There is nothing you can afford that is "free" of contaminants. All materials have contaminants. The question is how much you dose and how much is present in the additive.

    In this case, you are dosing a trace element which implies quite a small amount of dosing. Let's suppose it is 98% pure. Then at most, if all of that other 2% was some other contaminant, you are dosing 50x as much zinc sulfate as anything else. Is there any ion that we would worry about if dosed at 50x lower than the amount of zinc appropriately added to a reef tank? I doubt it. Iron certainly isn't a concern. :)

    So I would not worry about it if you bought any decent grade of zinc sulfate.

    By comparison, you dose vastly much more calcium and alk additives, and those all have contaminants in them too. Maybe ten thousand times more calcium (10 ppm per day??) is added than zinc (1 ppb or less per day, likely, I never added any and maintained NSW levels) to maintain a reef tank. So contaminants in those big additives, even if far lower than in your zinc additive, will be adding much more.
     
  10. Charterreefer

    Charterreefer Active Member

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    Thanks Randy
     
  11. Randy Holmes-Farley

    Randy Holmes-Farley Reef Chemist Staff Member Team R2R R2R Supporter R2R Excellence Award Article Contributor Expert Contributor

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    You’re welcome.

    Happy Reefing!
     
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