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  1. #1
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    Default OT Caustic soda solution lose its potency after it dries out

    A while ago I made up a tub of mild paint stripper, 50 litres of water and maybe a kg of 100% NaOH, and used it to remove the paint from 3 or 4 small items. Recently I needed to use the paint stripper again but over time the water had evaporated leaving behind a thick skin of white crystals on the floor of the tub.
    Not a problem, refill the tub with water, break up the crystals and stir them back into solution. Put painted item into tub, remove a day later, hose off the paint except that the paint does not budge. The paint hasn't softened or wrinkled it doesn't look too much different to when I put it in the tub.
    This is probably not a surprise to the chemists on here, but what has happened to the NaOH?

  2. #2
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    Probably consumed in the chemical reaction the previous time it was used - what you have left is likely not NaOH but a different sodium salt.
    For paint stripping these days (and softening hardening brushes too) I use a 50:50 mix of Meths and Turps. Works surprisingly well

    Michael

  3. #3
    BobL is online now Member: Blue and white apron brigade
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    What you have done is scrub CO2 from the air and made sodium carbonate (foot bath salts)

    2NaOH(aq) + CO2(g) → Na2CO3(aq) + H2O(l)

  4. #4
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    What does aq mean?

  5. #5
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    Aqueous, but what does (l) mean?

  6. #6
    BobL is online now Member: Blue and white apron brigade
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    Quote Originally Posted by Oldbikerider View Post
    Aqueous, but what does (l) mean?
    It means "in a solution" ie not a solid.

    Na2CO3(aq) consists of Na^+ ions and (CO3)^-2 ions and water, same for the NaOH (aq)

    This reaction will take place continuously while the water evaporates until the water is gone and all you have left is the Na2Co3
    2NaOH(aq) + CO2(g) → Na2CO3(aq) + H2O(l)

    Please don't use it as foot bath salts as there could be some residual NaOH present.

    BTW the Na2CO3 makes an excellent safe electrolysis salt/

  7. #7
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    Quote Originally Posted by BobL View Post

    BTW the Na2CO3 makes an excellent safe electrolysis salt/
    and what could I do with this electrolysis salt?

  8. #8
    BobL is online now Member: Blue and white apron brigade
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    Quote Originally Posted by eskimo View Post
    and what could I do with this electrolysis salt?
    Use it to remove rust by electrolysis.

    If a carbon rod is used as the other electrode the used solution doesn't go foamy orange and can be safely disposed of down the sewage.

    Clean, safe, Na2CO3 costs about $5 for a 500g bag at most supermarkets so it's not worth making it using caustic.

  9. #9
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    Default Supermarket caustic soda

    I've noticed that 500gm packs of supermarket drain cleaner tend to be around 50/50 NaOH and NaCl. Is that to make it work better as a drain cleaner and/or make it cheaper and/or make it less hazardous to the user and/or ???.

  10. #10
    BobL is online now Member: Blue and white apron brigade
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    Quote Originally Posted by bob ward View Post
    I've noticed that 500gm packs of supermarket drain cleaner tend to be around 50/50 NaOH and NaCl. Is that to make it work better as a drain cleaner and/or make it cheaper and/or make it less hazardous to the user and/or ???.
    Could be both.

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