Copper (II) and thiosulfate (was Product of HNO3 and NaCN?)
Wilco Oelen
> -------------------------------------------------------------------------
> Before attempting to discuss the original problem, Cu(II)-thiosulfate
> reaction caught my attention. The questions were.
>
> What is the yellow precipitate?
>
>
> Most likely colliodal sulfur. Copper sulfate is intrinsically acidic
> and you know any acid as weak as "carbonic acid" can initiate sulfur
> precipitation. The dissolution of yellow precipitate is indeed complex.
> Just check if sulfur has the property of dissolving in concentrated
thiosulfate solutions
>
>
> What is the colorless compound, when excess sodium thiosulfate is
added?
> Could be Cu(I) and different polythionates.
>
>
> Is the dark compound indeed copper (II) oxide or copper (II) sulfide?
> Most likely. As you say it takes time for blackening. Did you try to
isolate it?
> -------------------------------------------------------------------
I doubt that the yellow stuff is sulphur. It is very bright. I have
made sulphur from thiosulfate solutions before, by simply adding some
acid, and in those cases, the solutions become much lighter, more like
milk. This is really bright yellow.
I also added some sulphur to a solution of sodium thiosulfate and the
sulphur definitely does not dissolve. This yellow stuff dissolves at
once when more thiosulfate is added. No need to shake vigorously and so
on, just adding of more thiosulfate, a single swirl and it is 'gone'.
I left the colorless liquid standing for a longer time, and this
precipitated a dark brown solid. So, it probably means that I was
mislead, when I wrote the text in my website. Over there, it is stated
that the top layer of the yellow precipitate has become brown, but now
I believe that a thin layer of precipitate from the colorless liquid
has settled on top of the yellow precipitate. Also, the yellow
precipitate has become somewhat less bright (look carefully at the
pictures and compare them). This can also be explained. The liquid, in
which the precipitate is immersed, precipitates the dark brown stuff.
The darker stuff settles on the yellow solid, making it slightly less
bright. The top layer receives dark stuff from a much thicker layer of
liquid, so it can easily be understood that a brown/black layer is
formed on top of it.
I'll change my webpage, based on these new observations. Still, the
riddles remains unsolved :)
http://woelen.scheikunde.net/science/chem/riddles/copper+thiosulfate
muha
hydrolyses to brown CuS (Cu2S).
I do not know if Cu(2+) gets reduced first to Cu(+) by thiosulfate.
Wilco Oelen
Somebody was kind enough to try the experiment this and mail the
results to me. I'll put the results at this place.
What is the yellow precipitate?
Quote from his email:
> My guess: as Cu2+ is a relative strong oxidiser, one gets tetrathionate out
> of thiosulfate; so: Cu-tetrathionate. I did not yet test if it is colloidal S.
> That seems unlikely to me because on heating the colour rapidly changes to brown and ultimately black.
> I have isolated the black precipitate: IS coppersulfide ( tested positive on Cu & sulfide)
What is the colorless compound, when excess sodium thiosulfate is
added?
> Could be Cu(I) and different polythionates.
So, the chance that the yellow precipitate is sulphur is not that high
anymore for the following reasons:
1) it has a very bright color, much brighter than when sulphur is made
from Na2S2O3 and an acid.
2) it dissolves quickly in excess solution of sodium thiosulfate.
3) it blackens on heating. This last point of course does not
completely rule out sulphur, it might be that the sulphur oxidizes a
copper (I) compound in solution, forming copper (II) sulfide. If this
is the case, then the yellow precipitate should not blacken on heating,
when it is rinsed very well and put in clean water.
Farooq W
Wilco Oelen wrote:
> An update on this one:
>
> Somebody was kind enough to try the experiment this and mail the
> results to me. I'll put the results at this place.
>
> What is the yellow precipitate?
>
> Quote from his email:
> > My guess: as Cu2+ is a relative strong oxidiser, one gets tetrathionate out
> > of thiosulfate; so: Cu-tetrathionate. I did not yet test if it is colloidal S.
> > That seems unlikely to me because on heating the colour rapidly changes to brown and ultimately black.
> > I have isolated the black precipitate: IS coppersulfide ( tested positive on Cu & sulfide)
>
> What is the colorless compound, when excess sodium thiosulfate is
> added?
> > Could be Cu(I) and different polythionates.
>
> So, the chance that the yellow precipitate is sulphur is not that high
> anymore for the following reasons:
I was thinking of another test, if this is really colliodal sulfur or
not. Do you have access to carbon disulfide (or a solvent which would
dissolve sulfur), filter the precipitate and see if it dissolves in CS2
or not. If it does not, chances are it is something else.
> 1) it has a very bright color, much brighter than when sulphur is made
> from Na2S2O3 and an acid.
> 2) it dissolves quickly in excess solution of sodium thiosulfate.
> 3) it blackens on heating. This last point of course does not
> completely rule out sulphur, it might be that the sulphur oxidizes a
> copper (I) compound in solution, forming copper (II) sulfide. If this
> is the case, then the yellow precipitate should not blacken on heating,
> when it is rinsed very well and put in clean water.
Also see, www.gauss.ffii.org/PatentView/EP1200136 which says that
copper thiosulfate is yellow and decomposes to black copper sulfide on
heating.
fvp
In order to try to get some more answers :
Prepd. fresh sol of CuSO4 & Na2S2O3 of +/- 0.8 M ( = nearly satd. sol of
CuSO4.5H2O in Aq.)
I mixed approx 5 ml of each sol. at RT with exclusion of more air. A
light-green colour is observed. After about 10 min precipitate is forming.
After a 24 h standing a bright yellow precipitate is observed.
Following rxn seems tempting: CuSO4 + Na2S2O3 -> CuS2O3 (s) + Na2SO4
Filtered and rinsed multiple times with aq. demin.
The filtrate remains slightly turbid, even after multiple rinses. On heating
this filtrate it quickly becomes brown, most likely CuS. Neither Na2S2O3 or
CuSO4 sol. change in colour or give a precipitate when heated to boiling
point.
Perhaps the precipitate CuS2O3 is somewhat water-soluble?
The yellow residu loses somewhat of its brightness on dryring in air at RT
and some brown colour is obseved at the edges.(CuS?)
I tried to dissolve +/- 100 mg of the yellow prec. in 20 ml CS2: no obvious
dissolution took place, but as the particles are very small it looked as if
the CS2 was yellow after shaking. However upon standing the yellow colour
diminished. I added 20 ml of water, shook vigourously and after resting for
a while the upper = aqeous layer was colourless, the lower = CS2 layer was
also colourless, and in between "floated" the yellow stuff. It obviously
also not dissolves in CS2. Very unlikely behaviour if it was indeed sulfur.
The dried precipitate does NOT burn with a weak blue flame. Not sulfur!
CuS2O3 seems to me the safest bet. I think it is very sensitive towards
degradation on exposure to air/heat to coppersulfide(s).
"Wilco Oelen" <ph...@woelen.nl> schreef in bericht
news:1128064343.0...@z14g2000cwz.googlegroups.com...
Wilco Oelen
solution) and blue aqueous Cu(2+). Over time more of the Cu(2+) would
react. But, the liquid above the yellow precipitate would be colorless
in that case. Is it? Or does it turn blue or yellow again? I also would
expect some side reaction, with formation of a colorless copper (I)
compound.
A possible way to check out, whether the yellow precipitate is CuS2O3
is to heat a suspension of a rinsed/cleaned sample of this precipate in
water. The water then should be strongly acidic after heating, because
I would expect the following (simplified) reaction:
CuS2O3 + H2O ---> CuS + H2SO4
I'll try to repeat the experiment one of these days and post pictures
over here. The ratio of CuSO4 and Na2S2O3 has a strong influence on the
results.