Question:"Sulphur formation on reduction with SO2,or SMB

[Mojeho]

Hello,

Does someone know what is causing fine sulphur particles formation during reduction of gold from AR with SO2, or SMB?

My guess is there's still some free nitric acid, but what i've tested in dissolved gold in AR with excess HNO3, it only fumed NOx gas, yet it didn't make any colloidal sulphur.
It's not a showstopper problem, just annoying(prolongs sedimentation time, needs to be sublimed away @300 deg celsius)
Thanks for any help/leads.

Best Regards
Marek "Moje"

 

[lazersteve]

Sounds like a sulfurous acid related side reaction.

When SMB is dissolved in acid solution and base metal(s) more reactive than copper are present (activity series), a combination of hydrogen sulfide and sodium sulfide can form in situ. These compounds can further react with other dissolved metals to ultimately yield elemental sulfur.

I posted a short sulfurous acid document on my website that deals with some of this chemistry. The same reaction is responsible for the infamous false positive (brown result with rotten egg smell) with stannous chloride after SMB is added to acidic solutions.

Steve

 

[Mojeho]

Sounds like a sulfurous acid related side reaction.

When SMB is dissolved in acid solution and base metal(s) more reactive than copper are present (activity series), a combination of hydrogen sulfide and sodium sulfide can form in situ. These compounds can further react with other dissolved metals to ultimately yield elemental sulfur.

I posted a short sulfurous acid document on my website that deals with some of this chemistry. The same reaction is responsible for the infamous false positive (brown result with rotten egg smell) with stannous chloride after SMB is added to acidic solutions.

Steve

Hello,

Confirming hydrogen sulfide -> elemental sulphur pathway, though I've not been able to get exact reactions so far.

Thanks for advice.
Best Regards
Moje

 

[Lino1406]

In my opinion your SMB may contain
STS (sodium thio sulphate)

 

[Mojeho]

In my opinion your SMB may contain
STS (sodium thio sulphate)

Unlikely, SMB we are buying is new, pure , directly from laboratory equipment and chemical distributor, packed in protective atmosphere.

Best Regards
Moje

 

[lazersteve]

Hello,

Confirming hydrogen sulfide -> elemental sulphur pathway, though I've not been able to get exact reactions so far.

...Moje

Check out this document form my website: SO2 Instructions: Sulfurous Acid

The author proposes an equation to the production of sulfur on pages 3 and 4 of the document. A few years back I reseached his assumptions and found a very old chemistry text that confirmed the reaction takes place, albeit with different reagents. I was also able to experimentally produce a light yellow/white powder which easily melted into a deep orange material with heating. This confrmed sulfur production in my mind.

The sodium and hydrogen sulfide produced in situ via the routes explained in the document are also responsible for the brown false positives accompanied by a rotten egg smell (Deadly H2S !!) commonly seen when stannous testing dirty solutions after precipitation with SO2 generating reagents in acid medium.

Steve