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Check my math re: lye solution

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  • Check my math re: lye solution

    If I add three pounds of sodium hydroxide to a gallon of water I will have a 9% solution.

    I am building a de-greasing tank from Type I PVC. I have (plastic) welded the tank but am experiencing several very tiny leaks and want to seal the tank with either silicone or epoxy. Both of these show excellent chemical compatibility but silicone is somewhat dependent on concentration of the lye solution so I need to know what I’ve got.

    The formula I found indicates the concentration is mols/liter. Sodium hydroxide is 40 grams per mole.

    3 lbs. = 1361g
    1361/40 = 34 mols
    34/3.79 = 9

  • #2
    Why not keep plastic welding? Or make up some filler goop by dissolving fine shavings of PVC into some PVC plumbing glue?

    It took me a couple of searches because the High School chem classes are well in the past. But something was nagging at me at seeing your use of molarity to express the concentration as a percentage. I was able to find this;

    Percent Concentration Weight/Volume (w/v): used with one solid and one liquid
    This means there is a certain mass, in grams, in every 100 mL of solution.
    Another page said it was a mass to mass thing. So mass of solute per 100 gms of solvent. This makes more sense since a solvent can have a different specific gravity from water. But in your case a milliliter of water is VERY close to 1 gram. Which is clearly what the first page was referring to.

    So.... your 3.79liters has 37.9 units of 100ml. So it's 37.9 x 9gms of lye to give you a weight of 341gms of lye per US gallon of water to get your 9% solution.

    And frankly that makes FAR more sense than trying to dissolve 3lbs into a gallon of water for a 9% solution. From my early days where I was known to make hydrogen balloons using lye and aluminium foil in a pop bottle I knew that 3lbs into a gallon of water would make for a pretty darn nasty mixture. And certainly one which would become HIGHLY "interesting" if it got a piece of aluminium dropped into the tank
    Chilliwack BC, Canada

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    • #3
      Add it slowly, it's exothermic, I melted a bucket on my first go(I hadn't done chemistry then), it's nasty stuff,
      Mark

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      • #4
        Originally posted by Horst View Post
        If I add three pounds of sodium hydroxide to a gallon of water I will have a 9% solution.

        The formula I found indicates the concentration is mols/liter. Sodium hydroxide is 40 grams per mole.

        3 lbs. = 1361g
        1361/40 = 34 mols
        34/3.79 = 9
        Yes, but...

        Sodium hydroxide is used in laboratories as a desiccant to suck up water from enclosed vessels (among other things). NaOH grabs water and holds on to it tenaciously. Normal commercial lye contains a lot of water... up to 30%. Even laboratory grades contain 6 to 8 % water. This makes it hard to calculate the molarity based on weight of original sodium hydroxide. In the lab, we make up a roughly X molar solution, and then use a method of neutralizing it with a well defined acid of well known composition to accurately determine what we actually ended up with (titration).

        There are more factors ... Old samples of sodium hydroxide slowly absorb CO2 out of the air and become sodium carbonate. This happens to a tiny extent with all NaOH, but any jar that has had a loose lid can have substantial amounts of NaCO3. There is a separate type of nitration to distinguish between alkalinity arising NaOH and the alkalinity that arises from NaCO3. However, unless your NaOH has been exposed to air it's normal to assume that the amount of carbonate is somewhere between 0.5 to 1%.

        But wait... there's MORE. When you dissolve 3 lbs of NaOH in 1 gallon of water you will not end up with 1 gallon of final solution. It will be a little bigger (very very rare that the final volume shrinks, but it does happen). So in fact you shouldn't divide by 3.79 liters because the final solution will probably be more on the order of 3.85 or 3.9 liters.. Not a huge change, but it's there. So that is a small reduction in the molarity. Not a lot, I'm mostly mentioning it to be complete.


        If I were doing this, I'd make a SWAG that the NaOH is 10% water, and adjust my weight accordingly.

        If my goal is to create a 9 molar solution I'd calculate how much extra NaOH to use in order to compensate for the water (and ignore the carbonate and final volume problems). I'd guess 10% water if I had no other data.

        If my goal is to calculate the molarity of a solution I get by dissolving 3 lbs of lye in 1 gallon of water, I'd enter a number for the weight of NaOH that was 10% less... 2.7 pounds, to make up for the fact that the 3 pounds of lye probably contained about 0.3 pounds of water.

        If my goal was to know the value of an existing solution exactly, I'd titrate the NaOH solution using a primary reference acid such as potassium acid phthalate.

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        • #5
          Originally posted by BCRider View Post
          Another page said it was a mass to mass thing. So mass of solute per 100 gms of solvent. This makes more sense since a solvent can have a different specific gravity from water. But in your case a milliliter of water is VERY close to 1 gram. Which is clearly what the first page was referring to.

          Chemists use both molarity and molality... the former is moles / liter of final solution. Molality is moles per Kg of solvent.

          As you note percents can be mass to mass. There are in fact many different ways of specifying percents, not all of which make sense in all cases. Imagine mixing Scotch with water. You can mix 50 ml of each, or 50 grams of alcohol in 50 mls of water, or 50 mls of alcohol in 50 grams of water, or 50 grams of each. We call those 50% v/v, 50% w/v, 50% v/w, and 50% w/w. I call it a travesty if it's good Scotch. Some of these are not used when mixing two different phases... If one is dissolving salt in water, then it is possible to measure 5 ml of salt and 90 grams of water, but most people would weigh the solid and measure the volume of the liquid. However, w/w percents do make sense when looking at solids. We are all used to steel alloys, and those are solid w/w measures.

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          • #6
            You're overthinking it.

            341 g NaOH/3790 mL H20 = about 9% solution (w/w)

            Dan is certainly right about the hydration of the lye, and you could correct for that, but how much accuracy do you really need for this application?

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

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            • #7
              Even more fun with percents...

              In the previous posting I mention the four different types of percents... w/w, w/v, v/w, v/v. I should add that there is "original w/w" and "mixed w/w", etc...

              If you take 25 ml of pure ethanol and 75 ml of pure water, and mix them, then that is a 25% v/v mixture, right ? Well... sorta. The problem is that the final solution is only about 98.8 mls. The ethanol and water mixture packs a little better (for reasons unexplained here) than the ethanol solution... and so it's a little denser and occupies less volume.

              SO... while it is 25% original v/v, it's a little funny to discuss what the original/mixed v/v percentage is - yes, you have 25 original ml of ethanol in 98.8 mls of final mixture, and you have 75 original ml in 98.8 mls of final mixture... which means that if you specify original v/final v percentages... you get more than 100% . WOW.

              So, that is why chemists try to use mass / final volume (molarity) as much as possible. We have one standard way of discussing it because once you go off of that standard it opens a lot of cans of worms.

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              • #8
                It's a good thing you qualified that Scotch example. If you mixed MY Scotch at that sort of ratio I'd ask you to leave my damn single malts to hell alone....

                But you do make my own point all the better. That being that Horst is mixing up molarity with percentage. Even with the flake lye being friendly with moisture from the air anything near his 9M for solution would be WAY over his goal of a 9% solution.

                The page I found that presented the definitions of percentages for solutions did cover all those other options. But since he's working with solid lye and water I just quoted the part he needed.

                Of course he may end up with some bottles of some such concentration of NaOH in solution at such and such a percentage or molarity. In that case the dilution needs to be calculated quite differently.

                Horst, if that occurs then there are online dilution calculators that showed up during my own searches. It would be easy enough for you to just run a search on "dilution calculator" and find them.
                Chilliwack BC, Canada

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                • #9
                  for rough calculations % = no. grams in 100ml. 1g/100ml = 1%. It's as simple as that. If you want to be a bit more precise, you add your chemical to a slightly smaller volume of liquid, dissolve and then make it up to the final desired volume.

                  Other than that you're getting into the world of science labs in which case most of the time you'll be using molarity rather than %.

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                  • #10
                    For about ten bucks you can get some good quality pH strips, probably enough to last a home shop guy a lifetime. Can you tell the difference between 13.6 and 13.7? No. But you can certainly add lye until it's a lot closer to 14 than 13.

                    metalmagpie

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                    • #11
                      As you say though, there's hardly the need to nail the pH or % down that closely. Anything close will do the job. He's cleaning greasy engine and machine parts, not calculating a balanced reaction. So I can't see the need for pH strips myself.
                      Chilliwack BC, Canada

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                      • #12
                        Of course true purists mix scotch with ice and coca-cola in quantities of each that is pure art in the making.

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                        • #13
                          The molarity is really not that important. It affects the time needed more than the amount of cleaning that it can do on a part (so long as you do not exhaust the lye).

                          AND, the lye is not the only important ingredient.... some other materials significantly improve performance.....
                          CNC machines only go through the motions

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                          • #14
                            Yep.. Purple cleaner is lye plus a surfactant... the secret to how damn good it works is the surfactant allowing the lye to do it's job.

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                            • #15
                              Percentage can be weight/weight or weight/ volume or Volume/ volume. For weight/ weight use the grams of sodium hydroxide ( lye) and water weighing 1 gm per milliter about 8360 gm per gallon. There remains some fudge factor as if your making a gallon of 9%, you need to understand the lye occopies volume. Thus using a gallon of water and the lye, you end up with a slightly weaker solution as your final volume is more than a gallon. Tom

                              Sent from my Nexus 10 using Tapatalk

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