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Topic: 'Chemical test' for potassium ion?  (Read 9228 times)

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Offline Burner

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'Chemical test' for potassium ion?
« on: February 25, 2016, 02:27:56 AM »
We usually detect the presence of potassium ion by flame test, where species containing potassium ion will show a lilac colour flame, but this is a physical test. A lot of books' questions asked me to suggest a 'chemical test' for potassium ion but give 'flame test' in their answer key.

So... are there any 'chemical test' for potassium ions? I have found that potassium perchlorate KClO4 has a significantly lower solubility of 1.5g/100mL in water at r.t.p. compared to other potassium componds or perchlorates, however as far as I know 1.5g/100mL is not regarded as insoluble...Any ideas?

P.S. This question is asked just out of curiousity. I know that practically we detect potassium ion by flame test.
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Offline Borek

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Re: 'Chemical test' for potassium ion?
« Reply #1 on: February 25, 2016, 04:57:45 AM »
Old, classic books suggest precipitation of K3[Co(NO2)6] and K[B(C6H5)4].
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Offline Arkcon

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Re: 'Chemical test' for potassium ion?
« Reply #2 on: February 25, 2016, 06:58:13 AM »
The USP Pharmacopeia still contains the classical wet chemistry tests for many ions.  It was supposed to replace them with instrumental tests -- for example flame AA or even ICP, which come in tabletop models these days.  However, that;s been pushed back to 2018.  Therefore:

Quote
In neutral, concentrated or moderately concentrated solutions of potassium salts (depending upon the solubility and the potassium content), sodium bitartrate TS produces a white crystalline precipitate that is soluble in 6 N ammonium hydroxide and in solutions of alkali hydroxides and carbonates. The formation of the precipitate, which is usually slow, is accelerated by stirring or rubbing the inside of the test tube with a glass rod. The addition of a small amount of glacial acetic acid or alcohol also promotes the precipitation.

I've done this a number of times.  I suppose if someone sold me sodium sorbate instead of potassium sorbate, then the test would fail.  But really, this test won't help with a minor impurity, and a major contamination would alter the results of other, quantitative tests, so why do it at all?
Hey, I'm not judging.  I just like to shoot straight.  I'm a man of science.

Offline Burner

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Re: 'Chemical test' for potassium ion?
« Reply #3 on: February 25, 2016, 07:26:34 AM »
Thanks for all your replies.  :)

So in what circumstances will we use chemical test to detect potassium ion instead of using flame test? Maybe testing highly volatile or thermally unstable compounds?

And, just to double confirm, flame test is considered a physical test instead of a chemical test right?
Year 1 science student in HKUST and a Chemistry geek.
If I make any mistakes in the forum, please don't hesitate to correct me as I want to learn.

Offline AWK

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Re: 'Chemical test' for potassium ion?
« Reply #4 on: February 25, 2016, 07:54:23 AM »
Perchlorate test is good. It is performed with quite concentrated HClO4 (70-80 %), and the solubility of KClO4
is greatly diminished by the common ion effect.
AWK

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