NaOH in solution is precipitating

Hello all,

I made my first NaOH solution at a strength of 5mol by following the steps shown in this online calculator:

https://www.periodni.com/preparation_of_solutions.php?scq=NaOH

I used distilled water. 

It was all fine for at least the first few days if not a couple weeks, but then I noticed a white crust building up along the inside walls of the clear plastic bottle I have it in, but only above the water line. Then some time after that it seems the NaOH is actually separating within the solution and there are now bits of it floating around in there, with more and more showing up each time I check. 

Does any one know why this is happening and what I can do next time to prevent it? 

Thanks. 

Comments

  • PerryPerry Administrator, Professional Chemist
    It’s possible the sodium hydroxide is reacting with some compound in the plastic container. Normally, NaOH is stored in a glass bottle.
  • Thanks, I’ll make it in glass next time.
  • The same thing happens if you use glass bottles. I believe that crust is sodium carbonate, formed by reacting sodium hydroxide with CO2 from the air inside the bottle.
  • @MariaGarcia interesting, so what can i do to prevent this? I suppose i could just make it as i need it and use it all up every time or discard the rest, but that is not ideal. There must be some way to store it without this happening...maybe i could use an airless bottle.
  • PerryPerry Administrator, Professional Chemist
    We stored sodium hydroxide in glass bottles with a glass stopper and didn’t have this problem.
  • ketchitoketchito Member
    The same thing happens if you use glass bottles. I believe that crust is sodium carbonate, formed by reacting sodium hydroxide with CO2 from the air inside the bottle.
    Sodium carbonate is actually quite soluble in water. 

    @GeorgeBenson Maybe you are using hard water, and what you're getting is just calcium hydroxide?
  • AbdullahAbdullah Member
    How do you make it and what materials it contact with during making? 

    In my experience the sweet spot is making no more than 10% solution and it should not contact any type of metal. 

    I store it in HDPE plastic.
  • MariaGarciaMariaGarcia Member
    edited May 16
    ketchito said:
    Sodium carbonate is actually quite soluble in water. 

    @GeorgeBenson Maybe you are using hard water, and what you're getting is just calcium hydroxide?
    Yes, the crust disolves if you shake.

    If you use destilled water, avoid contact with metals, and store un glass containers , it appears too. 
  • The more concentrated the solution, the more abundant the precipitate and crust. There comes a time when it stops forming, I guess when most of the co2 has reacted. (I was soaper for almost 18 years, used to make 1:1,5 solutions).

    NaOH is used to reduce the atmospheric CO2 concentration, given that when they come into contact  transform into Na2CO3.
  • @MariaGarcia interesting, so what can i do to prevent this? I suppose i could just make it as i need it and use it all up every time or discard the rest, but that is not ideal. There must be some way to store it without this happening...maybe i could use an airless bottle.
    The best solution I know of is to use the right size bottle to minimize the amount of air available. If you use an airless container, make sure that the mechanism resists pH 14.
  • I used distilled water. it’s quite possible, even likely, that it came into contact with metal when making it but i dont remember. I was not aware of that issue before now. Next time i will make sure it doesnt. Thanks for the tips!
  • However, with stainless steel you shouldn't have any problem. As far as I know it is the only metal that doesn't react with sodium hydroxide.
  • Perry said:
    We stored sodium hydroxide in glass bottles with a glass stopper and didn’t have this problem.
    And what concentration was the solution?
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