Mixell
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Sulfur color variation
I've tried producing sulfur from thiosulfate and various acids (HCl, NaHSO4, formic and acetic).
Concentrations and volumes used: 5ml 2M acid, 10 ml 0.15M sodium thiosulfate solution.
I have noticed that the color of the sulfur formed is proportional to the speed of the reaction (which is proportional to the strength of the acid).
HCl and NaHSO4 produced a clearly yellow precipitate (reaction time around 40-60 sec).
But the formic and acetic acids (reaction time around 12-15 minutes) produced a completely white precipitate (especially the acetic acid), may be very
slightly yellowish when put next to extremely white objects. Could the speed of the reaction affect the type of allotrope formed?
I know S7 more deeply yellow then S8, so may be the faster the reaction, the more S7 rings are formed? But the slow reaction precipitate was
practically white, so may be there are other allotropes in play here?
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Bot0nist
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"The range of molecular allotropes possessed by sulfur can in part be ascribed to the wide range of bond lengths (180–260 pm) and bond angles
(90–120°) exhibited by the S–S bond and its strength (the unrestrained S–S single bond has a high bond energy of 265 kJ mol−1).[1]
Early workers identified some forms that have later proved to be allotropes, i.e. pure forms, whilst others have proved to be mixtures.[2] Some forms
have been named for their appearance, e.g. "mother of pearl sulfur", or alternatively named for a chemist who was pre-eminent in identifying them,
e.g. "Muthmann's sulfur I" or "Engel's sulfur".[3] A commonly used naming system uses Greek suffixes (α, β, etc.); however, this system
predates the discovery of the new forms that have been synthesized rather than prepared from elemental sulfur.[3]"
~wiki
I'm sure you have already looked here, but there is some info on wiki that I found interesting. I know a lot more about sulfur than I did ten minutes
ago. That's funny because I thought I knew a lot about sulfur, as I use it often. I guess that shows what I know...
http://en.wikipedia.org/wiki/Allotropes_of_sulfur
[Edited on 25-1-2012 by Bot0nist]
U.T.F.S.E. and learn the joys of autodidacticism!
Don't judge each day only by the harvest you reap, but also by the seeds you sow.
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ScienceSquirrel
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I suspect that the white particles are just very small so it is hard to see the colour.
The state of division of a substance can affect the colour. For example iron alum is pale violet in large crystals but small crystals can appear
colourless.
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woelen
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In general the following is true: the smaller the particles the paler the color.
In the slow reaction you most likely have very small particles. It might be that if you wait even longer (hours), that the color deepens somewhat.
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neptunium
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thiosulfates are usually used in physics to demonstrate the scattering of visible light by a chemical, which usually leads to a description of
spectrometers...
if thiosulfates remain in the solution with a weak acid its possible that a scattering occur and the color appear to change,
when ever this happen add a little bit of HCl and let me know if the color remain...(i understand it kind messes up your experiment..)
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Mixell
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Woelen, I forgot to mention that I've let it stand for about 4 days (the acetic one), the sulfur settled at the bottom and it was pretty much white.
Neptunium, so I should add the HCl after the reaction is completely done (a few hours or so)?
I don't have the thoisulfate in my possession at the moment, but I will try to do this experiment in the up-coming week.
If assuming that the color is due to different allotropes of sulfur, The best candidate is: γ-sulfur.
ω-sulfur may have a white color too.
Also, I've read that the S6 ring is orange-red, the S7 is bright yellow, and the S8 is light yellow in color. So it seems that the sulfur is getting
paler as the number of sulfur atoms in the ring increase. It is also stated that the S12 ring is the most stable after the S8 ring, so if the above
mentioned trend continues up to the S12 ring, it also may be a viable candidate.
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neptunium
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4 days? ok but if there was not enough Acetic acid to begin with then it doesnt matter how long you let it stand for....
maybe add mor Acetic acid just to see if the coloration remains?
i am looking at this from a physics stand point but i understand i could be all wrong and alotropes of sulfur could very well be present.
hell !! thats the point of research right?
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Mixell
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Neptunium, I've mentioned that in my first post.
The acid : thoisulfate ratio was 100:15, I think that is more than enough.
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neptunium
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i stand corrected
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