Author Topic: Distinguishing potassium and sodium hydroxide  (Read 1696 times)

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Trypstar

  • Guest
Distinguishing potassium and sodium hydroxide
« on: June 28, 2004, 10:59:00 PM »
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

  • Guest
make the salt of gamma-hydroxybutyric acid!
« Reply #1 on: June 28, 2004, 11:30:00 PM »
8)

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

  • Guest
koh is more commonly found as flakes, while...
« Reply #2 on: June 29, 2004, 07:04:00 AM »
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

  • Guest
Well they are flakes.
« Reply #3 on: June 29, 2004, 01:16:00 PM »
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

  • Guest
The old flame test
« Reply #4 on: June 29, 2004, 01:25:00 PM »
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

  • Guest
Correct
« Reply #5 on: June 29, 2004, 07:52:00 PM »
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

  • Guest
Titrate
« Reply #6 on: June 29, 2004, 09:43:00 PM »
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_

  • Guest
IIRC, KOH is also more hygroscopic than NaOH.
« Reply #7 on: June 29, 2004, 09:57:00 PM »
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

  • Guest
What do you mean?
« Reply #8 on: June 29, 2004, 10:09:00 PM »
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

  • Guest
Sodium yellow is very intense
« Reply #9 on: June 29, 2004, 11:51:00 PM »
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

  • Guest
perchlorate solubility
« Reply #10 on: June 30, 2004, 04:22:00 AM »
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

  • Guest
Pinkish/violet
« Reply #11 on: July 03, 2004, 11:11:00 PM »
Well the flame test worked.
A nice pinkish/ light violet colour was seen.

Thanks for all the replies.

Vitus_Verdegast

  • Guest
flame test
« Reply #12 on: July 04, 2004, 03:31:00 PM »

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

  • Guest
Pair of glasses ?
« Reply #13 on: July 06, 2004, 04:02:00 PM »
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

  • Guest
it's quite known
« Reply #14 on: July 06, 2004, 06:00:00 PM »
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