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Author: Subject: Phosphate, hydrogen phosphate or dihydrogen phosphate?
AndreiChim
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[*] posted on 22-6-2011 at 08:19
Phosphate, hydrogen phosphate or dihydrogen phosphate?


I have a sample of a "phosphate" containing salt. When i performed the test for phosphate ions with magnesium sulphate/ammonia solution, a white precipitate was observed. The precipitate was soluble in dilute sulphuric acid. Although the test was positive, i'm still not surtain that the salt in phosphate.



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spotlightman1234
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[*] posted on 22-6-2011 at 09:21


Could you tell us more abou the salt, like what the cation is and what its properties are. Then i'd be able to help you a lot better.
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AndreiChim
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[*] posted on 22-6-2011 at 09:36


I've been not able to identify the cation. The flame test gives no result and the solid melts when heated.



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[*] posted on 22-6-2011 at 09:46


If the solid melts it is most probably ammonium phosphate. Does it release NH3 on contact with akalis?

To test to see if it is H2PO4-, HPO4- or PO4- use PH.
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AndreiChim
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[*] posted on 22-6-2011 at 10:11


I don't think it's ammonium. It does not release ammonia when concentrated NaOH is added and even heated. The pH is about 5.



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[*] posted on 23-6-2011 at 06:12


ammonium dihydrogenphosphate is common in crystal growing sets but it has a pH very close to 7, not 5.

The low melting point points to a hydrate.

Your only chance may be to carefully titrate a solution of the salt with a 0.1 N solution of NaOH, carefully recording the obtained pH, ml by ml. A dihydrogen phosphate should give two distinct (and quite alkaline) endpoints, monohydrogen phosphate only one. A phosphate should simply increase steadily in pH as more NaOH is added.

H2PO4- + OH- === > HPO4 2- + H2O

HPO4 2- + OH- === > PO4 3- + H2O

[Edited on 23-6-2011 by blogfast25]
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AndreiChim
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[*] posted on 23-6-2011 at 06:49


Ok. I will try the titration and I'll post when I have the results.



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[*] posted on 24-6-2011 at 00:47


A pH of about 5 is consistent with Na H2 PO4 (with or without water of crystallisation).
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