AndreiChim - 22-6-2011 at 08:19
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.
spotlightman1234 - 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.
AndreiChim - 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.
Picric-A - 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.
AndreiChim - 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.
blogfast25 - 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]
AndreiChim - 23-6-2011 at 06:49
Ok. I will try the titration and I'll post when I have the results.
unionised - 24-6-2011 at 00:47
A pH of about 5 is consistent with Na H2 PO4 (with or without water of crystallisation).