# NaOH solution not stable

$\ce{NaOH}$ solution can react with $\ce{CO2}$ to form sodium carbonate. Is there any other reason why $\ce{NaOH}$ solutions for accurate titrations cannot be stored for long (or the titre has to be updated every time). Wouldn't filling the vessel with argon fix the problem.

• This one reason is quite enough. As to filling the vessel with argon, it will surely solve this problem and create many more. Oct 28, 2015 at 22:01
• NaOH is hygroscopic, ie it absorbs water from the atmosphere. So over time your solution will become more dilute so can't be used over a period of time. Therefore it cannot be used as a primary standard. Usually you determine the molarity of your NaOH solution just prior to its use. If you carry out your titration under argon or other dry atmosphere, sure use it but not hta feasible! Oct 28, 2015 at 22:20
• Thank you Leeser, Is NaOH solution still hygroscopic in solution as it is already dissolved in water or it does not matter. Oct 28, 2015 at 22:26

(1) How would you get the $\ce{NaOH}$ out of a sealed bottle?
(2) A concentrated stock solution can be made for $\ce{NaOH}$. In the top of the bottle you use a drying tube stuffed with solid $\ce{NaOH}$ to remove $\ce{CO2}$ from air. On the bottom you have an outlet from which you pour the $\ce{NaOH}$. The gist is that $\ce{Na2CO3}$ is the least soluble carbonate. So most of the $\ce{CO3^2-}$ in the original solution participates. So if you let the solution sit for a couple of days it will be very stable.
The point with the stock solution as outlined is that $\ce{Na2CO3}$ "sticks" to inside of the container and doesn't flow out with solution. (You don't shake bottle up like you do with tomato juice that you drink to resuspend the solids.)
So for (1) it is high near impossible to make a $\ce{NaOH}$ solution without some $\ce{CO3^2-}$ in it. The $\ce{NaOH}$ itself has $\ce{CO3^2-}$ unless it is specially made, sealed in argon and so on.
So your stock solution is concentrated. I boil some distilled water to get rid of $\ce{CO2}$ and seal it till it cools. I then take $10~\mathrm{ml}$ of my stock $\ce{NaOH}$ solution and dilute to $250~\mathrm{ml}$ (or whatever ratios) to get the desired concentration of $\ce{NaOH}$. (That I can check with KHP titration.)
• $\ce{NaOH}$ also attacks glass and quartz containers. Before the advent of plastic bottles, that was an additional reason to prepare a solution just before use. BTW, I've had to deal with glass bottles in which solid $\ce{NaOH}$ pellets absorbed enough moisture to fuse the cap onto the bottle! Oct 28, 2015 at 23:46