sodium or potassium hydroxide?

[paulanderko]

I have a new post that I could use some help with. I need to determine whether I have potassium hydroxide or sodium hydroxide. (I have a 50lb barrel of half-round pellets that is not labeled that was left by the previous chemist...) It is used for degrease and/or paint stripping... Is there a simple reaction experiment the can determine the chemical?

[hermanntrude]

dissolve a small amount in as little water as possible, add a bunch of alcohol (ethanol) till the solution is flammable but still dissolving the unknown sample, then set light to a small quantity of it. If the flame is bright orange, you have sodium hydroxide, if it's blue or lilac, it's potassium.

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or you could titrate a small amount against a strong acid (HCl perhaps) and do the math to see how many moles were in your sample, then you'll be able to calculate it's molar mass.

[John Cuthber]

If you are using it as a paint stripper/ cleaner it doesn't matter much if it's KOH or NaOH

[insane_alien]

perhaps he wants to use it for something else as well depending on the composition.

[hermanntrude]

or perhaps he's required to label it for beaurocratice purposes.

[John Cuthber]

or perhaps he's required to label it for beaurocratice purposes.

 

"This substance is caustic".

[baxtonduglonn]

Just heat a small sample in a bunsen burner flame. No need to dissolve and add ethanol.

[UC]

so, you're sure that it's either NaOH or KOH? I'm only asking because Trisodium phosphate and Sodium carbonate are both frequently used for prepping and degreasing surfaces prior to painting. However, neither of the alternatives become rapidly "wet" from absorbed mositure when exposed to the air.

 

My initial guess would be that you have NaOH since it's cheaper and just as effective for that kind of use. The problem with using a flame test is that the potassium emission line is very readily obscured by even traces of sodium. it's still worth a shot, because you can't get a false positive for potassium. If the flame is violet, it's definitely KOH. if yellow, it may or may not be NaOH, which brings me to this test...

 

One of the quickest ways to get a definitive answer is going to be with a moderately concentrated sodium perchlorate solution. Take your mystery hydroxide and make a moderately strong solution. Mix the two. if it turns to the consistency of yogurt with gelatinous precipitate, you've got KOH. If nothing happens, you've got NaOH. This occurs because potassium perchlorate is soluble at a rate of 15g/L at room temperature. Sodium perchlorate is soluble on the order of two kilograms per liter of water at room temperature.

 

If sodium perchlorate is unavailable, but you can get an authentic sample of NaOH, combine about equal weights of the mystery hydroxide and NaOH, melt, and take note of the freezing temperature, or remelt and take note of the melting temperature. Compare this to a plain sample of NaOH. KOH and NaOH form a eutectic, which melts at a significantly lower temperature than either one alone. If the unknown is simply NaOH, not much, if any difference should be noted.

 

You could run a sample in solution (perhaps add an organic acid to lower the pH) through an AA spectrometer and look for K and Na. That's rather less accessible though.