What would be the most feasable method of distinguishing the two metal hydroxides?
   Burn it with a torch? Sodium (ion) is supposed to give a yellow flame and potassium (ion) a lilac one, but will this difference be easy to tell?

One could ofcourse convert it (K+ or Na+ hydroxide) to the metal chloride and taste it  

Any thoughts, advices?

The sodium salt will have a soap/salt taste, the potassium salt will have a more sweetish licorice salty taste.

Of course you better make a control batch using NaOH or NaHCO3, to be sure..  

koh is more commonly found as flakes, while naoh is usually pellets or granules.  this info is based solely on swim's personal observations.

off topic, but is naoh or koh a better dessicant?

Well they are flakes.
The reason I'm asking this rather dubious question, is because the supplier of this supposed potassium hydroxide has screwed me before.

Just make a flame test on a pencil grafite stick. I think you are right on the colors. The flame of sodium is bright orange, while that of pottasium is violet. That is if I still remember it correctly. The difference is very easy to tell. You can't miss the color even with a very dilluted solution. You can use NaCl and a potassium salt as a control if you are still not satisfied.

The flame test, as simple as it may be, would be the best method for distinguishing between the two IMO.  It is correct that sodium imparts a yellow color to a non-luminous flame (USP jargon he he) and potassium imparts a violet color.  We always used a clean Pt inoculating loop, and made up solutions using de-ionized water.  It is a very simple test, and is recognized by the USP as an acceptable identity test.

regards,

m_e

Im sure that 98% KOH will give you a nice sodium flame.
NaOH 40g/mol
KOH  56g/mol
Disolve 4g into 100Ml wather and titrate with 1M HCl(aq)
Iff you get a 1M konc. you have NaOH
Iff you get a 0,7M konc. you have KOH

IIRC, KOH is also more hygroscopic than NaOH. Maybe you could calculate the heat of solvation of KOH vs NaOH, and see if they differ very much.


Last alternative: read the label from the box where it is stored in :-)

Im sure that 98% KOH will give you a nice sodium flame.

What do you mean by this?  Are you suggesting that a solution made from 98% KOH would impart a yellow color to a flame, not a violet color.  I am not sure if the flame test would be valid if the 2% of impurites of KOH contained sodium.  I guess you could look for interference by making a solution of equal parts of KOH and NaOH and noting the flame color.

Is 98% common for KOH flakes?  I assumed they were fairly pure, which is a big assumption as I have never even used such a product.

m_e

The sodium yellow flame is so distinct that it tends to take over the weak violet flame of potassium, so if you have 2% sodium and 98% potassium hydroxide, it is actually possible for the flame test to produce a yellow result.

If you have ammonium perchlorate or perchloric acid handy, react a bit of your MOH with it and look for a leftover solid. Potassium perchlorate has a very low aqueous solubility, sodium perchlorate a very high solubility. You can also do this with sodium chlorate; a strong solution of NaClO3 should give a precipitate with KOH but not NaOH.

But for what it's worth, my tech. grade KOH exhibits the desired/expected pinkish flame coloration, so it's certainly worth trying.

Well the flame test worked.
A nice pinkish/ light violet colour was seen.

Thanks for all the replies.

The sodium yellow flame is so distinct that it tends to take over the weak violet flame of potassium, so if you have 2% sodium and 98% potassium hydroxide, it is actually possible for the flame test to produce a yellow result.

There's a very simple solution for that: A piece of blue (cobalt) glass will mask the yellow flame of sodium entirely and show only the violet flame of potassium.

This is from a 1950s textbook, so maybe a piece of blue transparant plastic may also work..

You mean that you have to look at the flame through this piece of blue glass ? Interesting....which text book did you find this ?

I remember this from every chemistry booklet I had as a kid. Of course, things were different back then.

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Sodium, in particular, is present in most compounds and will color the flame. Sometimes a colored glass is used to filter out light from one metal. Cobalt glass is often used to filter out the yellow of sodium.

Primary Reference: Lange's Handbook of Chemistry, 8th Edition, Handbook Publishers Inc., 1952.

http://chemistry.about.com/library/weekly/aa110401a.htm

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