Distinguishing potassium and sodium hydroxide

[Trypstar]

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?

[Vitus_Verdegast]

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..  

[Bond_DoubleBond]

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?

[Trypstar]

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.

[Nicodem]

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.

[methyl_ethyl]

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

[hest]

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

[_mu_]

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 :-)

[methyl_ethyl]

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

[Rhodium]

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.

[Polverone]

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.

[Trypstar]

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

Thanks for all the replies.

[Vitus_Verdegast]

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..

[Smilaxium]

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

[Vitus_Verdegast]

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

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