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Thread: How can I dissolve hardened caustic soda?

  1. #1

    Default How can I dissolve hardened caustic soda?

    My mum just died and I bought her house but one of the sinks was slow to drain. My brother in law said he would unblock it and poured caustic soda crystals down the plughole. This seems to have set and now the sink is completely blocked. Is there any way to dissolve the caustic soda that has set? I was going to try boiling water but my brother in law has poured a bottle of vinegar down the plughole instead. Nothing is happening - but the room stinks of vinegar. I would call a plumber but the mortgage payments are taking up all my wages!

    Any advice would be greatly appreciated!

  2. #2


    Some brother in law. Chemicals should never be used.


    The solution may be that you have to remove the sink trap and clean out the "junk" that has collected in there over the years.
    Last edited by Ladiesman271; 02-02-2009 at 09:22 AM.

  3. #3


    Thanks Ladiesman271! I will try to remove the trap and see how I get on! Here's hoping...........

  4. #4
    DIY Senior Member gardner's Avatar
    Join Date
    Apr 2007


    Be VERY careful as caustic soda/lye/sodium hydroxide is very dangerous.
    You need rubber gloves, old clothes that cover your arms, safety glasses.

    Lye will get VERY HOT when it disolves in water and when mixed with an acid like vinegar. Make sure the pipes/trap have cooled off before messing with them. Be prepared for clots of undisolved lye to suddenly heat up when you disturb them.

    If you engage a plumber, make sure you tell her/him what chemicals have been put down the sink and when.
    Last edited by gardner; 02-02-2009 at 10:39 AM.

  5. #5


    Thanks gardner. I will take precautions as best I can. Hopefully it won't be too difficult and I will manage to resolve the problem without getting caustic soda on myself or anything else!

  6. #6


    Just an update to everybody who has helped me! I thought I had solved the problem as when I poured water down the sink it disappeared. However, I could hear water running so pulled up the floorboards. The plastic waste pipes were joined to a lead pipe, which in the presence of a huge clog of caustic soda had split. At least I only had to remove 2 floorboards and the problem wasn't too far from the sink! I have bought all the parts I think I need so hopefully I will be able fix it.

    I am never letting caustic soda over my doorstep again!

    I may be back for advice on fixing the pipes if I run into any more problems!!


  7. #7
    Plumber Cass's Avatar
    Join Date
    Nov 2005


    Never mix Lye / sodium hydroxide with vinegar...never.

    When lye mixes with water it causes an exothermic reaction and will heat the water...when mixed with vinegar it does the same but the reaction can happen so fast it can explode and cover you and / or others with lye causing severe burns and blindness if it gets in your eyes...when mixing water with lye always add the lye to the water...not the other way around...read all directions on the container.

  8. #8
    Moderator & Master Plumber hj's Avatar
    Join Date
    Aug 2004
    Cave Creek, Arizona

    Default trap

    Don't even bother trying to remove the caustic soda. It will be like concrete. Just install a new trap, and whatever else is plugged with it.


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