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Author: Subject: CdS from CdSO4 and KCN?
National Hazard

Posts: 285
Registered: 11-11-2013
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[*] posted on 11-5-2019 at 07:58
CdS from CdSO4 and KCN?


So I'm currently supervising an analytical course at my university and one of my students wanted to test his sample (As2O3 + CdSO4 + CuSO4) for any oxidized Arsenic by melting the dry compound with KCN according to:

3 KCN + As2O3 -> 2 As + 3KCNO

It worked and in a few seconds, he got a black metallic mirror on the test tube wall. Inbetween that Arsenic and the sample on the bottom another layer formed, a bright red one. When he stopped heating it turned yellow immediately.

So I went home and tried this reaction myself, starting with CdSO4 as one possible component. I heated dry CdSO4 until the test tube started to deform under the heat and nothing happened. Then I added some KCN and heated again. The mass started to melt, Cadmium metal was formed, sublimated and formed a black metallic precipitate and brown gases, probably CdO also rose from the sample.

The black mass started to boil forming bubbles and then it stopped and turned bright red. When I removed the flame it turned orange, then bright yellow. When you heat it again it immediately turns red again. So it's reversibly thermochromic.

It hot boiling water nothing happens besides some of the insoluble yellow mass getting dispersed in the water. When I added some HCl it formed a white, cloudy solution and smelled a lot like H2S mixed with HCN.

So it seems like the hot KCN, acting as a reducing agent also reduced the Sulfate to either Sulfur or Sulfide. Therefore I looked into the Gmelin on Cadmium and it says there
"When KSCN is melted until it turns blue and then CdO is added you will get a mixture of KCN and CdS". So perhaps something similar is happening here.

Would be interesting to see other Sulphates under similar conditions. Due to Ostwald's rule, the kinetic products usually form and therefore precipitating sulfides often just give dark compounds instead of the colorful natural minerals. But maybe this method could help to equilibrate it to the thermodynamic product.

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