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neutralizing sodium hydroxide

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  • neutralizing sodium hydroxide

    I’m planning on using muriatic acid from a big box store. Does anyone know how much to use to neutralize 1 lb. of lye? Ditto the procedure. The lye is dry. Just tossing it in the garbage seems like a really bad idea.

  • #2
    Slowly add the lye to a few (many?) gallons of water then pour it down your sink/ bath/ toilet/ drain. It'll give them a good clean out and no harm to the environment those concentrations. I certainly wouldn't want to be messing around mixing strong bases and acids.


    • #3
      It will depend on the strength of the acid.

      We do "elementary neutralization" at work to get rid of both Sulfuric acid and Sodium Hydroxide. We aim for a PH of about 6-7; the EPA rules are greater than 2 and less than 12.5 (your local rules may vary but generally tighter, not looser). The result is water and and Sodium Sulfate. In your case it will be Sodium Chloride. Careful with "just pour it down the sink... Even though it's sold in some states as drain cleaner for HOME use, you can't do "treatment by generator" after anodizing or electroplating, and dispose of it the same way. Your rules may vary.

      First.... Make the lye into a weak solution (less than 5-10 %), and the same with the acid. SLOWLY combine the two COLD solutions, acid solution into the hydroxide, stirring as you go. Pause and wait for the bubbling to stop. Watch for temperature buildup (one reason to use a lot of water in the solutions). No more bubbling and you are done. Check with Litmus strips (cheap); repeat as required

      Here we can't pour the "water" and/or precipitate into the drain so we evaporate off the water, recover the salts and dispose of that ($$$). Wear eye protection and have hose or faucet nearby to wash out your eyes or face! Lye is nasty stuff (way worse than acid, but both are bad.).
      Last edited by lakeside53; 08-11-2019, 03:30 PM.


      • #4
        Mixing lye and muriatic solutions, there shouldn't be any bubbling or precipitate, but just a neutral brine if neutralization is complete. A little excess acid or base shouldn't be a big problem if it is dilute enough.

        For just a little more, you can do it yourself!


        • #5
          sorry, I didn't mean to give inaccurate advice. I've used sodium hydroxide a bunch (handy in anodising) and I simply pour it down the sink when I'm done with it. It's usually around a 5% solution, max 10%, which is similar to drain cleaners. Whatever you do, when you dissolve the sodium hydroxide do it a little at a time as the reaction is exothermic and the solution will get hot.


          • #6
            lye is not pure NaOH. It is typically between 80% to 90% NaOH, with the remainder being carbonates, water, etc. So, lets assume it's 85%

            So 1 pound of 85% lye is .85 * 454 g = 386 g NaOH
            Molecular Mass of NaOH = 40 (40 grams/mole), so 386 g / 40 g/ mole = 9.65 moles of NaOH

            Muriatic acid is typically around 27 - 30 % HCl, although it could be even less. Lets assume 28%
            The density of 27% muriatic acid is about 1.135 g/ml, so 1 liter would weigh 1135 grams
            28% would contain .28 * 1135 = 318 grams of HCl in 1 liter.
            Molecular mass of HCl = 36.5 g/mole, so 318 g / 36.5 g/mole = 8.70 moles of HCl in 1 liter of 28%

            Since you have roughly 9.7 moles of NaOH, you will need roughly 9.7 moles of HCl
            So to get that using 28% which contains 8.7 moles / liter you will need 9.7 / 8.7 = 1.11 liters of muratic acid.
            You can convert that to cups, or ounces, or firkins as you choose.

            Since the lye might be anywhere between 90 to 95%, and the muriatic acid might be anywhere from 15% to 30%, this is obviously fraught with uncertainty. You will need anywhere from 1 to 2 quarts of muriatic acid to neutralize the lye.

            Having figured that out... how does one do this?
            Pouring straight muriatic acid on solid NaOH is probably a great way to get a trip to the chemical burns unit ...

            Do the following wearing EYE PROTECTION and rubber gloves. An apron would be nice.

            If I had to do this with minimal equipment, I'd put the lye into about 2 gallons of water in a 5 gallon bucket. Then I'd add small bits of muriatic acid to that solution with stirring, making sure not to add it too quickly. I'd check the side of the bucket to see if it is heating up. If it is heating up, then stop and go do something else until it cools back down. You don't want to melt the plastic.

            ALSO... use a wooden stick to stir the mixing bucket before and during the addition. You don't want to pour a reactive liquid of one density into a bucket containing a reactive liquid of a different density and accidentally create two layers sitting on top of each other. Adding two liquids with stirring generates all of the heat in a controllable fashion - if it is heating up too much you stop adding. But if you have two layers sitting on top of each other you have no way to control it if the mixing starts to generate too much heat. It's gonna mix.


            The suggestions about just dumping the lye down the drains to give them a good clean out are okay suggestions. Lye is one of the least harmful of all the industrial chemicals to dump down a drain. It's commonly done on purpose to clean the drains. Just do it in several smallish batches
            and run a lot of water afterwards.
            Last edited by Dan_the_Chemist; 08-11-2019, 02:14 PM.


            • #7
              Why do you want to get rid of it? It's a great paint stripper - 1lb per gallon of water more or less.


              • #8
                I can attest to the lye making the water hot. I'd opt for a big bucket of water and add the lye in around 8 or 10 even size amounts stirring to dissolve for each portion. Once dissolved check the outside of the bucket for the temperature. If still cool or no more than lukewarm add the next portion. If it's quite warm leave it for a while to cool off. Or switch to another bucket of water.

                Silly teenage trick. I used to dissolve about a heaping teaspoon full of lye in about 3 oz of water in a pop bottle.... it was really fun getting the lye in through the neck. Once the solution was fully dissolved I'd make up three or four aluminium foil "rollies". In quick succession the foil rollies were dropped into the solution and a big party balloon snapped over the lid. Bubble, bubble, toil and trouble and the balloons would fill with hydrogen and the bottle would get hot. The balloons would provide entertainment for a couple of hours as I floated weights in equilibrium around the house. And the dog would go positively bananas at that floating thing. Finally I'd tie a long string around the neck and light it and let the balloon float away at night. At around a minute there would be a blue flash in the sky followed by a firecracker like BANG! and I'd go to sleep with a grin on my face....

                And that was about 50 years ago so don't bother bringing on the safety warnings.....
                Chilliwack BC, Canada


                • #9
                  Originally posted by Bob Engelhardt View Post
                  Why do you want to get rid of it? It's a great paint stripper - 1lb per gallon of water more or less.
                  I never knew that.



                  • #10
                    You can also use it to turn fat (human or otherwise) into soap.

                    Take care with lye... it may not fume like acid, but it can "burn" bad and take out your eyes in an instant.


                    • #11
                      I thought maybe vinegar (acetic acid) might be safer to neutralize the lye, but apparently that is not so good, either. Apparently copious amounts of water is recommended to dilute the caustic, and then vinegar for final clean-up.


                      Paul , P S Technology, Inc. and MrTibbs
                      USA Maryland 21030


                      • #12
                        I work in waste water. Dumping lye down the drain to a sewer system is not bad. We always have a problem with low pH not high and use a lot of it to correct the pH. We call it Caustic and it is EXTREMELY dangerous for your eyes! It will dissolve them in an instant.


                        • #13
                          Boy, there's a lot of armchair chemistry on this thread. (Not you, Dan.)

                          For just a little more, you can do it yourself!


                          • #14
                            Does your county not have a household Hazmat day, when you can dispose of pool chemical, paint, solvents, etc. at no cost?


                            • #15
                              Originally posted by garyhlucas View Post
                              I work in waste water.
                              You work .. IN ... waste water?