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Author: Subject: Detecting phosphates (chlorides too) without Mo
MeshPL
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[*] posted on 25-4-2016 at 04:49
Detecting phosphates (chlorides too) without Mo


So assume I want to detect phosphates.
I have a sample containing (probably) ClO3-, Cl-, PO4-, OH-, CO3 (2-) and Na+. It is somewhat basic. What is the best way of detecting phosphates in this sample? I have no acces to molybdenum salts. It would be easy to acidify the solution to drive off carbonates as CO2, but phosphates dissolove in acidic enviroment. Magnesia mixture is also not good because the sample is to basic and Mg(OH)2 may also percipitate. What should I do, to detect phosphates? Good method of detecting chlorides also appreciated.

ClO3- was easy, just add a bunch of acid, and in fact is aboundant enough to be used as a part of explosive (tested separately with Zn, S and sugar)
Na is confirmed by flame test.
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solitanze
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[*] posted on 25-4-2016 at 06:23


I would add excess sulphuric acid, boil for a while to drive off all the HCl, chlorine and carbon dioxide gases. Dilute the solution and precipitate the phosphate with magnesium and ammonium salts.

The magnesium ammonium hydrogen phosphate hexahydrate precipitate formed can then be filtered off, washed thoroughly, dried and weighed.
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MeshPL
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[*] posted on 25-4-2016 at 08:37


Thanks! Good idea. Drive of the HCl... will do. Thanks again!:)

I will ensure full neutrality of pH by boiling to dryness. That should work ok.

I may try adding some silver nitrate as well, as silver phosphate is very insoluble and suppossedly yellow-ish. But I wouldn't count on it turning out to be intensly enough coloured to be clearly identifiable. So I will do boiling anyway.

Actually, I remember having SrCl2, so maybe I could acidify the solution to drive away carbonates, than basify with NH3 and just add SrCl2, as Sr(OH)2 is somewhat soluble, unlike strontium phosphate. In fact NH3 is too weak base to percipitate even Mg(OH)2. I'll have to rethink all of that...
[Edited on 25-4-2016 by MeshPL]

[Edited on 25-4-2016 by MeshPL]

[Edited on 25-4-2016 by MeshPL]

[Edited on 25-4-2016 by MeshPL]

[Edited on 25-4-2016 by MeshPL]
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[*] posted on 27-4-2016 at 04:57


I think I overestimated he basicity of the tested sample as it was a mere 9 pH. I added a bit of sulfuric acid and boiled for a while. Then I added some MgSO4, NH4NO3 and a little bit of NH3 (a drop or so), all in solution. The amount of percipitate of ammonium magnesium phosphate was so minute, that actually I am not sure, if thare was any percipitate at all as the sample was already a bit turbid.

Test with silver also didn't confirm presence of phosphates, the percipitate was clearly white and not even a bit yellow, but relatively quickly became grayish.

So I think I overestimated the presence of OH-, PO4 (3-) and CO3 (2-) i a sample, yet chlorides were surely present.

[Edited on 27-4-2016 by MeshPL]

[Edited on 27-4-2016 by MeshPL]

[Edited on 27-4-2016 by MeshPL]
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