Expert: George Maxwell - 9/1/2007

Question
Hi, I'm attempting to make a standard solution of NaOH. I am aware that I need to use anhydrous Sodium Hydroxide in order to accurately measure it. The container of my caustic soda says 99.9% purity, does this mean that it is anhydrous? If not is there a way I can remove the water in the pellets?

Answer
Thank you for your question.

Firstly, purity indicators don't usually include solvents, so you can have a solution of "99% pure" NaOH: this means that the NaOH that was used to make the solution was 99% pure. If your NaOH is in solid form, it's probably "anhydrous" by normal standards

NaOH wil absorb water readily from the atmosphere (try leaving a pile of pellets in an open beaker, and they will gradually form a slush as they absorb moisture). Therefore, the "Anhydrous" NaOH sold commercially is not totally dry, just solid rather than in solution.

Because it absorbs moisture so readily, there's little point in drying it (e.g. using a dessicator or heating it on a watchglass). This is particularly true when you're making up a solution from solid: the small amount of water in each crystal is not important when you're planning to dissolve the crystal anyway.

Normally, you'd make up the solution using 99.9% pellets and then perform a titration to find out the exact concentration.

Hope this helps, thanks again.