Bezaleel
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Phosphoric acid for nickel(II) determination
Hello! I purchased some black nickel oxide and I'd like to determine the nickel content of the oxide.
Pure nickelous oxide is a green compound with the formula NiO. Most nickel oxide that is being sold, however, is black and has a somewhat higher
oxygen content, NiOx x=1.1 ... 1.4.
I can dissolve my oxide in HCl solution, with which it forms a clear dark green solution. The standard method for determining the Ni(II) content is
titration of such a soluiton with dimethylglyoxime in an ammoniacal solution, which precipitates Ni(II) quantitatively.
Almost all simple nickel compounds have the problem that they loose their water of crystallisation easily, or that they oxidise to a
non-stoichiometric oxide. Dehydrated compounds of nickel are either very hygroscopic or partly decompose during dehydration. As far as I have found,
the recommenden method to determine the nickel content of a compound by chemical means is to dissolve the compound in strong acid and precipitate the
nickel as its dimethylglyoxime complex by titration with dimethylglyoxime.
I don't have any dimethylglyoxime and neither wish to buy any.
Now, my question is: does phosphoric acid, H3PO4, precipitate all nickel(II) in solution as Ni3(PO4)2? Because, if so, I might weigh the dried
precipitate, and thus calculate the amount of Ni(II) that was in solution. (And thence the amount of Ni in the dissolved oxide.)
I would expect some basic phosphate to precipitate, but Treadwell (part 2) states that a solution of Na3PO4 will precipitate Ni(II) as Ni3(PO4)2. So
that's why I'm asking.
[Edited on 22-4-2014 by Bezaleel]
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blogfast25
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There are certainly references to Ni3(PO4)2, so lets assume for argument's sake that it's a stable, calcinable compound.
One important question then is, does it have a tendency to occlude/co-precipitate other species in solution? If it does, significant weighing errors
will result. The difference in molecular mass between NiO and NiO<sub>1.4</sub> being relatively small and all that.
Have you considered complexometric titration (EDTA)?
http://www.titrations.info/EDTA-titration-nickel
[Edited on 22-4-2014 by blogfast25]
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Magpie
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Have you considered making your own DMG?
FYI: http://www.sciencemadness.org/talk/viewthread.php?tid=8122#p...
The single most important condition for a successful synthesis is good mixing - Nicodem
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DraconicAcid
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If you do make DMG, remember that it's not a titration, but a gravimetric analysis.
Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
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Bezaleel
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Quote: Originally posted by blogfast25  | There are certainly references to Ni3(PO4)2, so lets assume for argument's sake that it's a stable, calcinable compound.
One important question then is, does it have a tendency to occlude/co-precipitate other species in solution? If it does, significant weighing errors
will result. The difference in molecular mass between NiO and NiO<sub>1.4</sub> being relatively small and all that.
Have you considered complexometric titration (EDTA)?
http://www.titrations.info/EDTA-titration-nickel
[Edited on 22-4-2014 by blogfast25] |
Thanks for thinking with me. I did consider alternative methods, but I do not have any EDTA and murexide available. I neither wish to use Magpie's
suggestion to synthesize DMG, although it is a nice synthesis as such. I'd just like to do a simple test for nickel content, not make it a project on
its own. If that's not possible with simple means or what I have at hand, I'll just accept the inaccuracy in the nickel content of my preparation.
I guess I'll make another trip to the library to learn more about Ni3(PO4)2, to see whether indeed occlusion may occur and whether there exists a good
drying temperature for it (without decomposition of any kind).
Again, thanks for the suggestions.
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