Sciencemadness Discussion Board

sulfur charring?

woelen - 22-6-2021 at 03:22

Recently, I did some experiments with sulfur. I did the well-known experiment of heating it to just above its melting point. This gives a mobile yellow liquid. On stronger heating, the liquid turns dark red and becomes more viscous. At a certain point, the liquid is so viscous that you can keep the small beaker upside down without pouring out the sulfur. On cooling down, the liquid becomes more mobile again.
All of this is perfectly according to theory in the text books . . .

However, on cooling down, and finally on solidification, the color of the sulfur does not revert to its original color. It gets a more ochre color and if the heating was very strong (such that brown/red gaseous sulfur is formed), then the molten sulfur reverts to a dirty looking green (pale yellow, contaminated with a color like olives). On very long standing of the solidified sulfur (several days), the color somewhat comes closer to the original yellow, but it never becomes as nice bright yellow as the original solid.

I have done this experiment with so-called flowers of sulfur of high purity (99.99% or so, according to seller) and also with another sample, sold specifically for the purpose of using it as fuel in pyrotechnic mixes. This sulfur has a purity of 99.95% according to its label. But none of my samples of sulfur give the nice pure color of the sulfur on cooling down and solidifying.

Is this also the observation of others over here? What kind of impurities could cause this change of color? Organics? Absorption of oxygen from air, leading to colored compounds, contaminating the sulfur?

On the internet there are several pages about melting sulfur, but none of them covers this topic. Probably most people do not observe that closely and only look at the behavior of melting, becoming viscous and that kind of well-known things.



[Edited on 22-6-21 by woelen]

Sulaiman - 22-6-2021 at 04:18

https://en.wikipedia.org/wiki/Allotropes_of_sulfur
You will probably find the answer there.

pantone159 - 22-6-2021 at 06:31

I understood the greenish looking sulfur after melting to be the monoclinic crystal form, instead of the stable, and bright yellow, orthorhombic form that you started with.

macckone - 22-6-2021 at 12:44

Sounds like S6 instead of S8.
I didn't think that would form without a chlorine catalyst.
It should not be stable at room temperature and should eventually revert to S8 alpha form.

Darien - 18-8-2021 at 14:55

All commercial sulfur has trace amounts of hydrocarbons in it. When heated the sulfur takes the hydrogen and leaves the carbon behind. I’ve tried getting hold of carbon free sulfur but with no luck. Even the good stuff in the lab turns dark when heated strongly.

Triflic Acid - 18-8-2021 at 16:46

Actually, I don't think this is carbon. I made SCl2 the other day, and in the boiling flask where the sulfur was, there was a bunch of black residue. This was not attacked by piranha solution, so I doubt its carbon. It only slightly flaked off with hot piranha. It did seem to be slightly affected by concentrated sodium hydroxide, creating a slightly yellow solution full of black chunks. The NaOH did not react with the chunks either, just separated them from the glass. Maybe it is some form of a carbon sulfur polymer?

Deathunter88 - 18-8-2021 at 20:12

Yes, you can see the effect more clearly if you pour the molten sulfur into cold water. What you get is a gooey material that is orange/red and is a different allotrope than S8.

metalresearcher - 19-8-2021 at 00:16

@woelen, did you try to dissolve the greenish crap in toluene and recrystallize it ?

On your (excellent) site I found an experiment on crystallizing sulfur: https://woelen.homescience.net/science/chem/exps/S+toluene/i...

Sulaiman - 19-8-2021 at 08:03

https://en.wikipedia.org/wiki/Allotropes_of_sulfur, catena sulfur forms, insoluble sulfur
So recrystalisation from CS2 looks to be a good first step.

rockyit98 - 19-8-2021 at 08:28

did you try it in inert atmosphere? I think what happening here is Oxygen atoms replacing some Sulfur atoms in S8 / S6 or any poly-sulfur thereof.
maybe that's why it happens mostly or prevalently on the surface.

unionised - 19-8-2021 at 09:46

It's not especially complicated; hot molten sulphur is black.
This fact is well documented.

e.g.
"Just above its melting point, sulphur is a yellow, transparent, mobile liquid. Upon further heating, the viscosity of the liquid decreases gradually to a minimum at about 157 °C (314.6 °F), but then rapidly increases, reaching a maximum value at about 187 °C (368.6 °F); between this temperature and the boiling point of 444.6 °C (832.3 °F), the viscosity decreases. The colour also changes, deepening from yellow through dark red, and, finally, to black at about 250 °C (482 °F). The variations in both colour and viscosity are considered to result from changes in the molecular structure. "
From
https://www.britannica.com/science/sulfur

Morgan - 19-8-2021 at 14:44

This tiny amount of carbonaceous impurity was said to produce a black color.

"Sulfur can also vary in color and blackens upon boiling due to carbonaceous impurities. Even as little as 0.05% of carbonaceous matter darkens sulfur."
Found as this title on Google - Chemistry of Sulfur (Z=16)

[Edited on 20-8-2021 by Morgan]

woelen - 19-8-2021 at 23:59

Reading through all different replies I indeed am inclined to think that the presence of small amounts of hydrocarbons is the issue I observe. Even after weeks, the color of strongly heated sulfur does not revert to the original yellow color. It remains olive green. Indeed, tiny amounts of very finely divided carbon or carbon-rich black tar-like stuff can cause a strong coloration. I have seen similar things with sulfuric acid, which turned dark brown with just a tiny drop of organic contamination in a liter of acid.

The most important reason why I believe that the coloration is due to hydrocarbon contamination is the fact that the change of color is not reversible. All explanations, relying on allotropes of sulfur, do explain the observations at the moment of heating (e.g. turning dark red, nearly black, getting viscous, etc.), but if there were only allotropic changes, then on cooling down and waiting for weeks I really would expect the original color to reappear.

I have done another experiment on this, but now I heated the sulfur very carefully, so that it just melts. This leads to formation of a mobile yellow liquid. I heated this liquid carefully, until red 'schlieren' appear and the liquid becomes more viscous. Then I stopped heating, so it never became really hot. This sulfur, on cooling down, becomes yellow again after a few days. There still is some slight irreversible change of color, but it only is faint. In a final experiment I did heat even more carefully, so that the red 'schlieren' did not appear and the liquid remained mobile and yellow. Sulfur, treated that way, reverts to its original pure yellow color, albeit slightly darker. The latter can be explained, because originally it was a powder, and now I have little lumps. Larger particles usually have a somewhat darker color.

Bedlasky - 20-8-2021 at 03:53

I think, that cooling of the sulfur is still too much quick and you end up with amorphous sulfur. It slowly converts in to the cyclo-alfa-S8, but I don't know how quick this conversion is.

[Edited on 20-8-2021 by Bedlasky]

unionised - 20-8-2021 at 04:31

It would be fairly easy to distil some sulphur and then see if the distillate went black on heating.

pneumatician - 22-8-2021 at 14:06

Disable Smilies?

uh?

if you burn a man toy of plastic, how you can return the burned down plastic mass again to a man toy with same color, size...??

putting it again in the mold!!!

or search, good luck, the impurities in a 0,01% changing Sulfur color :-=



[Edited on 23-8-2021 by pneumatician]

woelen - 23-8-2021 at 00:30

Quote: Originally posted by unionised  
It would be fairly easy to distil some sulphur and then see if the distillate went black on heating.
Distilling? Can be done, but is not easy. Boiling point of sulfur is well over 400 C, so you don't do this in a glass apparatus with normal heating mantles and so on. You need to make some metal distilling device, with all its own engineering difficulties and the risk of contaminating the sulfur with metal sulfides.

Sulaiman - 23-8-2021 at 00:57

I think that ('disposable') borosilicate glass may be just able to boil sulfur (444.6oC)
and some member that has quartz glassware may want to try,
just to settle the question ?

An idea ...
if I seal the end of a borosilicate tube (7mm od, 5mm id) and bend the tube at about 90 degrees I could have a go at distilling sulfur.
If 'char' is produced by the boiling of the original sulfur charge
and not by heating then slowly cooling the distillate that should indicate that the charring is due to (carbon) contaminants.
If the distillate produces char then its probably an allotrope thing. (my suspicion)

Is this logical ?

unionised - 23-8-2021 at 06:01

Quote: Originally posted by woelen  
Quote: Originally posted by unionised  
It would be fairly easy to distil some sulphur and then see if the distillate went black on heating.
Distilling? Can be done, but is not easy. Boiling point of sulfur is well over 400 C, so you don't do this in a glass apparatus with normal heating mantles and so on. You need to make some metal distilling device, with all its own engineering difficulties and the risk of contaminating the sulfur with metal sulfides.

I think you are overestimating the sophistication needed.
Put some sulphur in the bottom of a test tube.
Heat it until it evaporates and then condenses in the upper part of the tube.
Then heat the stuff in the upper part of the tube and see if it goes black.

I wonder if anyone has ever made a still from gold- pretty much all the other metals react with sulphur.

woelen - 25-8-2021 at 00:32

That's right and seems doable. I'll try something like this. I once made a bent test tube for distilling white P (by strongly heating red P and then distilling the decomposing red P as white P). A similar thing may be possible for sulfur. I'll give it a try and spend one of my cheap frosty test tubes on it.