crystalised "theoretical acids"

[Chemistoftheelements]

There seems to be much uncertainty of the structure of the oxoacids of certain elements, particularly metalloids, but also of metals such as molybdenum, tungsten (i.e. "molybdic" and "tungstic" acids), and such terms as "perrhenic acid" and "rhenium heptoxide" are frustratingly used interchangeably. Orthosilicic acid certainly exists in solution, but when crystallized from aqueous solution under standard conditions, no-one really knows what happens- perhaps the acid polymerizes, perhaps a hydrous silica is produced.

 

An example of this uncertainty can be illustrated by the compound "Arsenic acid" when crystallized; wikipedia* talks of the structure H₃AsO₄ ½ H₂O, but how do we know that the structure isn't actually the stoichiometrically identical As₂O₅.4H₂O? What are the limitations of Raman spectra, X-ray crystallography and other techniques for determining the structure of those substances which result from crystallization from aqueous solution of such acids?

 

What my question is is this: How can we be sure of the structure of these substances who's structures have so far proved so elusive?

 

* http://en.wikipedia.org/wiki/Arsenic_acid

 

 

[John Cuthber]

"What are the limitations of Raman spectra, X-ray crystallography and other techniques for determining the structure of those substances which result from crystallization from aqueous solution of such acids?"

Why assume there are "limits"?

If you can crystallise the material you can generally get a structure by XRD.

[mississippichem]

"What are the limitations of Raman spectra, X-ray crystallography and other techniques for determining the structure of those substances which result from crystallization from aqueous solution of such acids?"

Why assume there are "limits"?

If you can crystallise the material you can generally get a structure by XRD.

 

Is he saying that some of these compounds display XRD disorder?

 

I'm not sure I understand the question.

[Chemistoftheelements]

Is he saying that some of these compounds display XRD disorder?

 

I'm not sure I understand the question.

 

 

For an example of what I mean: http://www.nrcresearchpress.com/doi/abs/10.1139/v72-106 if XRD etc, cannot answer the question of the structure of this compound, what can? Also, although I have limited knowledge of this sort of thing, the structure H₃AsO₄.½ H₂O (http://en.wikipedia.org/wiki/Arsenic_acid) seems a little strange to me; is a water molecule shared between two H₃AsO₄ units? Do the XRD experiments on this compound (if any have been done) rule out As₂O₅.4H₂O ?

[John Cuthber]

That article fairly precisely defines the structure of the stable hydrated antimony oxide.

Specifically it says" Sb6O13 has a cubic structure of the defect pyrochlore type, a0 = 10.303(1) Å, x(O2) = 0.4304(14)."

 

On the other hand, "H3AsO4.½ H2O" isn't a structure- it's a molecular formula. The same is true of As2O5.4H2O

The problem is that you can get all sorts of odd species present

http://www.oldenbourg-link.com/doi/abs/10.1524/zkri.2007.222.7.321?journalCode=zkri

aand even getting a sample of the "obvious" oxides can be tricky

http://onlinelibrary.wiley.com/doi/10.1002/anie.197703142/abstract

[Chemistoftheelements]

That article fairly precisely defines the structure of the stable hydrated antimony oxide.

Specifically it says" Sb6O13 has a cubic structure of the defect pyrochlore type, a0 = 10.303(1) Å, x(O2) = 0.4304(14)."

 

On the other hand, "H3AsO4.½ H2O" isn't a structure- it's a molecular formula. The same is true of As2O5.4H2O

The problem is that you can get all sorts of odd species present

http://www.oldenbour...ournalCode=zkri

aand even getting a sample of the "obvious" oxides can be tricky

http://onlinelibrary...703142/abstract

 

This admitedly isn't the best example, but it concludes on a probable structure, i.e. there still remains an uncertainty. However this may well be explained by the difficulty of producing material which is sufficiently crystalline and uniform in composition, as your second link hints, thanks.

 

 

I take your point about the molecular formulas, but the H₃AsO₄. 1/2H2O in the wikipedia link I took to imply a compound with As single bonded to 3 OH groups, and double bonded to an O, analogous to an orthophosphoric acid unit, apart from the ½ H20 bit. I read it this way as wikipedia phrases it as the "hemihydrate form"(of arsenic acid). The structure As₂O₅.4H₂O which I suggested is hydrous As₂O₅, i.e with no OH groups. This highlights the confusion, and part of my frustration, that hydrous acids and hydrous oxides are banded together with the same name- cf "perrhenic acid": http://en.wikipedia....Perrhenic_acid.

Edited January 28, 2012 by Chemistoftheelements

[John Cuthber]

In many cases- probably the overwhelming majority, the names were chosen before the structures were known.

If one scientist thought it was a hydrated oxide he would call it that. Another scientist might consider it to be an acid.

 

It gets worse when you consider that some acids (which themselves are made by hydrating an oxide) form hydrates.

[Chemistoftheelements]

It surprises me just how lasting these names are, and even chemical manufacturers occasionally perpetuate this confusion, as I’ve witnessed with a bottle of “tungstic acid”.

 

 

[John Cuthber]

well, perhaps that's what it is.

http://en.wikipedia.org/wiki/Tungstic_acid

it reacts with alkalis to produce tungstates.

[Chemistoftheelements]

Yes, I've figured since that its was some tungstic oxide hydrate. Wasn't at all sure at the time, though.