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I've bought a white powder. The label on the package states that the powder contains sodium chloride, potassium chloride and potassium citrate. How can I make sure that the powder does indeed contain potassium chloride? I'm mainly interested in the presence of potassium ions.

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    $\begingroup$ What kind of equipment do you have available, I assume you can't perform atomic absorption spectroscopy? $\endgroup$ – Mad Scientist Mar 11 '13 at 20:56
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    $\begingroup$ I don't know. The standard college lab test was for years a flame test. One took a platinum wire inserted into an insulated handle and stuck the wire into a solution containing the potassium ion. One then stuck the wire into a Bunsen burner flame. The last step was to look at the flame using a deep red piece of glass. If potassium was present a rather pretty lilac color was detected. The red glass was needed because the bright yellow flame from sodium otherwise masked the color of the potassium. $\endgroup$ – Paul J. Gans Mar 13 '13 at 1:25
  • $\begingroup$ @PaulJ.Gans That could be a great answer, really. $\endgroup$ – jonsca Mar 13 '13 at 2:49
  • $\begingroup$ @PaulJ.Gans I agree. The flame test would be the easiest method to detect K. You should turn that into an answer. But I've heard that blue glass is usually used to block out the colour of sodium and not red. I've also read somewhere that Potassium Hexachloroplatinate (IV) $K_2[PtCl_6]$ is insoluble in water. Maybe if it is specific enough to Potassium, that could be used as a rough (though quite expensive) test. $\endgroup$ – kaliaden Mar 13 '13 at 21:08
  • $\begingroup$ $KClO4$ is very weakly soluble and $HClO_4$ can be used to detect $K$. It will form weakly soluble salts with big low-charge cations too, of course. $\endgroup$ – permeakra Mar 14 '13 at 17:13
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  • What do you want?

If you're just looking for a source of potassium ions you could just buy $\ce{KCl}$ or some other compound, they're relatively cheap if not pro analysis. If you actually want to test for $\ce{K+}$:

  • What else do you know about the product?

Tests are completely different for different amounts of $\ce{K+}$. Does the vendor specifies the amount of each compound on the product's label?

  • How sure do you need to be?

Do you need to do a QUALITATIVE analysis or a QUANTITATIVE analysis? If you need to be absolutely sure about the concentration then you need to guess it's ridiculously low - you'll probably need at least AAS (Atomic Absorption Spectroscopy) equipment, which can't be accessed by anyone nor find anywhere, but you can order an analysis. Alternatively, you can use an electrode - keep reading .

Some ways of testing that I can think of:

(Of course you'll need to dissolve the powder in at least distilled water to do all the following tests, and do a blank with the same water - the more pure the water the more accurate your results.)

  • For big amounts of $\ce{K+}$

The flame test suggested by Paul J. Gans on the comments above could work, although I never actually did it.

Another way is doing the following titration:

$$\ce{KCl(aq) + AgNO3(aq) -> AgCl(s) + KNO3(aq)}$$

$\ce{AgNO3}$ can be found at any store that sells chemicals. We used 0.1 M for small amounts of $\ce{Cl-}$.

What you are actually testing for is $\ce{Cl-}$ ions, but if the amount of K = the amount of $\ce{Cl}$ the job is done. Now, since the powder contains potassium citrate this is just a qualitative test, although it can be very accurate for small concentrations if properly executed (see Mohr method). Silver chloride is a white solid that is barely visible in small amounts, and a similar titration's end point is just a few drops (Mohr method), and there are a lot of possible interference so you should not expect precision unless you do some pro work.

  • For small amounts of $\ce{K+}$

I think the best way of doing this is using a potassium-selective electrode. I never used one of those, but other ion-selective electrodes work great on water analysis, detection levels were a few mg/L on our old ones!

http://en.wikipedia.org/wiki/Potassium_selective_electrode

And of course, you can use AAS.

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