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I am not expert in chemistry. I need to find a way to identify a $\ce{KNO3}$ and $\ce{KH2PO4}$ powder at home. I suspect the seller gave me the wrong powder. What is a simple way to test between these compounds at home?

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    $\begingroup$ KNO3 is much better soluble in water. $\endgroup$ – Mithoron Feb 27 at 1:16
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    $\begingroup$ Given the number of votes and views gained in such a short time, I hope that you will edit this question to be more clear, As-is I don't know how to answer it. $\endgroup$ – A.K. Feb 27 at 16:36
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In the absence of other chemicals you can utilize the fact that nitrates(V) are much stronger oxidizing agents than phosphates(V). You can soak a piece of newspaper in a saturated solution of the salt, let it dry well in a ventilated place and try to ignite it.

If the salt was $\ce{KNO3}$, it will burn vigorously and fast (that's why kids are often using paper soaked in sodium nitrate as primitive fuse for home-made firecrackers and rockets).

$\ce{KH2PO4}$ doesn't promote combustion of paper, so it will burn normally or even slower.

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    $\begingroup$ Okay. The burning technique is surely simple for detecting $\ce{KNO3}$. Thanks! $\endgroup$ – sancho21 Feb 27 at 15:11
  • $\begingroup$ @sanch21 It looks like you used an unregistered account to ask this question. I just want to point out that you can request to have your accounts merged and be able to accept this answer. $\endgroup$ – A.K. Mar 19 at 4:28
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Well, given that one is a weak acid and the other is the salt of a strong base and a strong acid, the pH of a solution of the powder should tell you which it is. If it's $\ce{KNO3}$ it should neutral, and if it's $\ce{KH2PO4}$ it should be acidic. Just need some pH paper, which you could probably buy at a pool supply store.

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    $\begingroup$ This could work, but KH2PO4 solution would have pH somewhere around 4.7 - water used at home might be as acidic as this, I guess. $\endgroup$ – Mithoron Feb 27 at 1:13
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    $\begingroup$ @Mithoron so be sure to use the pH-paper to test regular water too. $\endgroup$ – JAD Feb 27 at 10:56
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Touch medium samples of both salts with a glowing ember. No noticable effect would be observed on contact with $\ce{KH2PO4}$. Contact with $\ce{KNO3}$ shall produce noticeable flame.

Video https://www.youtube.com/watch?v=dJRqJ145dyM

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