fluorescence - 15-8-2017 at 00:17
Hi,
I was roasting a small amount of Pyrite when I saw the formation of a thick yellow layer on the colder part of the test tube. I'm a bit confused how
this could happen. I thought it would be oxidized to SO2 or could it be that due to the low air conc. in the test tube the FeS2
turned to S + FeS and the S sublimed?
Σldritch - 15-8-2017 at 11:54
Yes it is probably sulfur:
FeS2 = FeS + [S]
It is more complex than that though becasue there is a lot of iron sulfides between FeS and FeS2 but that should be the over all reaction if it goes
to completion.
https://www.google.com/patents/US2872294
[Edited on 15-8-2017 by Σldritch]
AJKOER - 16-8-2017 at 04:47
Keep some of your FeS2, however!
There is some interesting advanced chemistry surrounding surface reactions on it. No need to thermally decompose as with some moisture and O2/air the
cited net reaction is:
FeS2 + 7/2 O2 + H2O = Fe(ll) + 2 SO4- + 2 H+
which is basically an ionic mix of FeSO4 and H2SO4. Note, the presence of a cupric salt (like CuSO4) is said to be catalytic to the above reaction.
Source: see for example, "Pyrite Oxidation Mechanism by Oxygen in Aqueous Medium", by Egon Campos Dos Santos, Juliana Cecília de Mendonca Silva, and
He ̧ lio Anderson Duarte ́, link: http://pubs.acs.org/doi/pdf/10.1021/acs.jpcc.5b10949 .
If an excess of O2 is presence, an electrochemical reaction may ensue with the net reaction:
Fe(ll) + O2 + H+ = Fe(lll) + .HO2 (see, for example, http://pubs.acs.org/doi/abs/10.1021/ja01600a004 )
My take on one path for the above reaction would be as follows:
Fe(ll) --> Fe(lll) + e-
which is an electrochemical half cell reaction followed by either:
e- + H+ --> .H
.H + O2 --> .HO2
or:
e- + O2 --> .O2-
H+ +.O2- = .HO2
with both cases producing the cited net reaction above, but with varying rates. Note, my explicit introduction of solvated electrons is supportive of
the variation of reaction rates, as claimed in the above cited source, as some anions (and cations) are noted to impact the ionization potential
(IPs) and electron affinities (EAs) of aqueous solutions. See, for example, discussion at http://advances.sciencemag.org/content/3/6/e1603210/tab-pdf where the author notes in water, which is in essence a wide band gap insulator, that
simple anions/cations perform the same role as defects in insulators and semiconductors.
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After confirming a change in pH, one could add some Copper powder to test for SO2 if the H2SO4 is sufficiently highly concentrated.
Also, spray on a mix of NaCl and some liquid chlorine bleach (NaOCl) to see if one could make a small amount of Cl2 (note, chlorine is an irritating
acidic and toxic gas).
[Edited on 16-8-2017 by AJKOER]
Σldritch - 16-8-2017 at 08:26
I should try that i have some pyrite from an old mine and a need for sulfuric acid. Might be better to just try oxidizing H2SO3 with air/copper or
something.