hodges
National Hazard
Posts: 525
Registered: 17-12-2003
Location: Midwest
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Reducing H2SO4
While reading about qualitative analysis online I ran across a quick test for iodide. When concentrated H2SO4 is added to a solution containing
iodide, the iodide is oxidzed to iodine and the H2SO4 is reduced to a mixture of H2S, SO2, and S. This didn't seem right to me, but I tried
adding a drop of full-strength H2SO4 drain cleaner to a bit of KI. Sure enough I got bubbling and a smell of H2S (as well as some free S - I
didn't detect any SO2 smell though).
How can iodine, which is more of an oxidising agent than a reducing agent, reduce H2SO4 so strongly? Why don't metals, which are certainly
better reducing agents than iodine, reduce H2SO4 to H2S instead of producing the normal hydrogen reaction? Would anything else besides iodine reduce
H2SO4 to H2S?
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Pyrovus
Hazard to Others
Posts: 241
Registered: 13-10-2003
Location: Australia, now with 25% faster carrier pigeons
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Mood: heretical
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I think you're confusing iodINE and iodIDE. Iodine is indeed an oxidising agent. However, once it has gained an electron to form iodide, it has
no tendency to further gain electrons. This is also true of fluorine - in the elemental state, the strongest oxidising agent known, but when converted
to fluoride, no oxidising properties whatsoever. As iodine isn't a terribly strong oxidising agent, it follows that iodide doesn't have a
terribly strong hold over the extra electron and is easily oxidised back to iodine, and sulphuric acid is a strong enough oxidising agent to achieve
this. However, once the iodine is back in the elemental form, sulphuric acid isn't strong enough to oxidise it further to hypoiodite etc.
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