woelen
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Tellurium oxide and hydrogen halide acids
I finally prepared some pure TeO2 from technical grade TeO2 (see other thread from me on purification of TeO2). I did some interesting experiments
with this:
TeO2 when added to a solution of NaOH gives a colorless and clear solution. This is used as a test for purity. My technical grade TeO2 gives a brown
and turbid solution, so my purified TeO2 really is much better.
I added some of this TeO2 to the following acids:
30% HCl --> gives an intensely colored yellow solution. On addition of water this liquid becomes colorless again.
40% HBr --> gives a beautiful deep orange solution, somewhat like a concentrated solution of a dichromate. On addition of much water this also
becomes colorless and somewhat turbid (this must be TeO2).
impure (few tens of percent) HI (made by adding a 50% solution of HClO4 to a concentrated solution of KI and filtering the solid KClO4 out of the
liquid) --> Gives a black precipitate on addition of TeO2. Also gives a black precipitate when a solution of TeO2 in conc. HCl is added. The liquid
above the black precipitate is dark brown.
I expected the dark brown/red color to be due to dissolved iodine and the black precipitate to be due to elemental tellurium. So, I allowed the
preciputite to settle and decanted the dark brown liquid. I was left with quite some black precipitate. I added a lot of water in order to rinse this
precipitate in order to obtain more pure tellurium, but to my surprise the black precipitate dissolves in water, giving a very pale yellow solution
which becomes turbid (just like the diluted solution of TeO2 in HBr). So, the black precipitate definitely is not elemental tellurium, but it must be
some other compound. I am quite surprised to see this kind of behavior. I am quite sure that all of these three compounds with the different hydrogen
halide acids are complexes of tellurium(IV) with the halogens.
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Summarizing:
With HCl --> yellow soluble complex
With HBr --> orange soluble complex
With HI --> black insoluble complex
In all cases the complex hydrolyses when a lot of water is added.
What could these complexes be? According to my documentation, they cannot be TeCl4, TeBr4 and TeI4, because in the aqueous solutions these immediately
would be oxidized. Especially the black compound is stable up to quite some dilution (you can also make it by dissolving some TeO2 in conc. HCl and
adding a dilute solution of KI or NaI).
Is there anybody over here who has experience with tellurium/halogen compounds and complexes? This is again a piece of remarkable chemistry which
apparently gives rise to interestng and special compounds. I may make some pictures of this and make a web page about this, but before doing so I
first will try to find more theoretic background.
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PS: I see I placed this topic in the wrong subforum, can some of the moderators or administrators please move this topic to an approriate subforum
(e.g. chemistry in general)? My apologies for the extra workload on the mods.
[Edited on 3-7-10 by woelen]
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Polverone
Now celebrating 21 years of madness
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Thread Moved
3-7-2010 at 13:02 |
kmno4
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TeO2 in HX gives TeX6(-2) complexes, X=Cl, Br, I.
TeO2 is easily reduced to Te (often colloidal form) with iodode in acid sol.
TeI6(-2) is yellow.....
ps, there is a lot of available "electronic literature" covering Te chemistry.
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woelen
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I have been reading more on this subject and found some interesting information in an old book of Sidgwick. Iodide does not reduce tellurite or
tellurous acid to elemental tellurium, but instead a complex is formed of the form HTeI5.nH2O. A similar thing happens with the other halides.
I made a webpage about this interesting piece of chemistry, with pictures and an outline of how the experiments were done, so that others can repeat
it.
http://woelen.homescience.net/science/chem/exps/Te_halogen_c...
Have fun with this, but avoid exposure if you don't want to be smelly.
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