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Thread: Edta tetra sodium vs phosphonic acid

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    Join Date
    Apr 2009
    Burlington, ON, Canada

    Edta tetra sodium vs phosphonic acid

    Most of the metal stain and scale products I've seen at my local pool stores contain phosphonic acid or citric acid (Natural Chemistry with their phosphate free products). I found one today that has Edta tetra sodium. Does anyone know if one is better than the other?
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    Melt In The Sun's Avatar
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    Oct 2009
    Tucson, AZ

    Re: Edta tetra sodium vs phosphonic acid

    While I don't really understand the chemistry, I know that Chem Geek and JasonLion recommend the phosphonic acid types.
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    JasonLion's Avatar
    Join Date
    May 2007
    Silver Spring, MD

    Re: Edta tetra sodium vs phosphonic acid

    The full recommendation is "Sequestrants based on HEDP, phosphonic acid, or phosphonic acid derivatives are the most effective."

    Sequestrants based on EDTA work, but are significantly less effective.
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  4. Back To Top    #4

    Join Date
    Mar 2007
    San Rafael, CA USA

    Re: Edta tetra sodium vs phosphonic acid

    Also, HEDP is more chlorine resistant while EDTA breaks down from chlorine so quickly that you will notice an increased chlorine demand from using it and need to add it much more frequently.

    The more recent Natural Chemistry Metal Free appears to have citric acid as a reducing agent (similar to our recommendation of ascorbic acid), but the metal sequestrant appears to be some sort of anionic (negatively charged) polymer. They note that you can't use that with normal clarifiers in the water (they are normally positively charged polymers), but it's possible that the Metal Free product is effective and resistant to breakdown from chlorine -- we just don't know and haven't heard enough experiences from users to make an evaluation. So for now, HEDP-based products are the recommended metal sequestrant.
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