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Author: Subject: Oxidation of Solid Sulphides by Bleach
Aurus
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[*] posted on 20-11-2009 at 05:59
Oxidation of Solid Sulphides by Bleach


Recently, I was considering whether sulphides such as cadmium sulphide (CdS) and Mercury(II) Sulphide (HgS), which I have in my possession as artists' pigments would be converted to oxides or in the case of Mercury, to the metal, by addition of Sodium Hypochlorite solution.

I proceeded to add a small amount of these sulphides into a test tube for each and to add 5 ml of 5% sodium hypochlorite solution as household bleach. I was expecting something like:

2NaClO + HgS = 2NaCl + Hg +SO2
3NaClO + CdS = 3NaCl + CdO + SO2
or possibly sulphur formation.

After one day, the original yellow colour of cadmium sulphide had turnt brownish,the red color of mercury(II) sulphide had turned brown-black and the solution was yellowish from dissolved chlorine. While cadmium oxide is brown, and this would support my theory, the solid from mercuric sulphide does not behave like mercury (it does not stick together and is not shiny) and I am wondering what it could be.

I drained off the bleach solution from the cadmium and added hydrochloric acid. The solid dissolved to give a colourless solution. I am wary of interpreting these results as oxide formation through, although I imagine that cadmium oxide may have been formed and another reaction had occured with mercury(II) sulphide. Do you have any idea of what happened?




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woelen
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[*] posted on 20-11-2009 at 13:35


Indeed, cadmium oxide is brown and gives a colorless solution in acids. I have some CdO and it is a nice brown powder with a somewhat reddish hue. Nice to see that from CdS one can easily make other Cd-salts, simply by oxidizing the sulfide with bleach. I am quite sure that your theory on CdS is right. Beware with Cd-salts, they are VERY toxic and carcinogenic.

HgS certainly will not give Hg-metal with bleach, but most likely some basic oxychloride, or just HgO. Whatever the product, it also will remain contaminated with lots of HgS, due to the extremely low solubility of HgS. So, expect your powder to be something like Hg(Cl,O,S), a rather useless mix.




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merrlin
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[*] posted on 20-11-2009 at 22:46


I recall seeing the Ellingham diagrams for oxygen compared to that for sulfur, and in general, the free energy of formation of an oxide was greater than that for a sulfide of a particular metal. If you want to look at the equilibrium thermodynamics of oxides and sulfides, you might try this website. It also covers nitrides, carbides, and halides.

http://www.engr.sjsu.edu/ellingham/tutorial.html#step3
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Aurus
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[*] posted on 21-11-2009 at 02:32


@ Arthur's wizard. I looked at the link you sent. Unfortunately, I do not know what Ellingham diagrams are. Could you explain please?




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[*] posted on 21-11-2009 at 07:20


Quote: Originally posted by Aurus  
Unfortunately, I do not know what Ellingham diagrams are. Could you explain please?
It's good form to at least read the wikipedia article first: http://en.wikipedia.org/wiki/Ellingham_diagram
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[*] posted on 21-11-2009 at 13:00


Quote: Originally posted by watson.fawkes  
Quote: Originally posted by Aurus  
Unfortunately, I do not know what Ellingham diagrams are. Could you explain please?
It's good form to at least read the wikipedia article first: http://en.wikipedia.org/wiki/Ellingham_diagram


The wikipedia article pretty much says it all. In fact, it provides the same link I posted above. As an alternative, you could make a spreadsheet using thermodynamic data from the CRC Handbook of Chemistry and Physics.
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