Metallus
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Could you help me figure out what happened here?
Hi there
I had some lead powder and put it in nitric acid; lead nitrate was formed with the production of NO2 and, since not soluble in the acid, it
precipitated. I filtered it, waited for it to dry and put a little amount in water; it took some seconds before totally dissolving but, like other
nitrates, it was soluble.
After that i was wondering whether i could oxidize the Pb2+ ion to Pb4+, and performed 3 different tests with hydrogen peroxide:
1) Lead nitrate in water and sulfuric acid, added H2O2, nothing happened
2) Lead nitrate in water, added H2O2, nothing happened
3) Lead nitrate in water and NaOH (At first the solution got whitey but then the lead dissolved again; i suppose it got soluble as Pb(OH)3-), added
H2O2 and this time some brownish stuff formed. There were no bubbles. I filtered the brownish stuff, dried it and added it to an acid solution of H2O2
and bubbles formed.
Now, what was that browninsh powder that formed? I read somewhere that PbO is yellow and Pb3O4 is red, but this was more yellow/brownish; do you have
any idea of what i obtained? Is it possible for H2O2 in OH- to oxidize lead(II) to lead(IV)? No bubbles formed so i suppose H2O2 got reduced to H2O.
In the last reaction, the fact that the brownish product managed to oxidize H2O2 to O2 in acid somehow led me to think that there was an oxydizing
agent in there (perhaps Pb(IV) ?).
Thanks for your attention and sorry for the improvised "scientific" language; i'll get used to it.
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unionised
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PbO2 is brown and is a very reasonable product from oxidation of Pb(II) in the presence of OH-
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woelen
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Yes, that brown material must be PbO2. PbO2 is a very strong oxidizer (e.g. mix some with Mg-powder or Al-powder and you have a nice flash powder,
unfortunately quite toxic though).
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blogfast25
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Quote: Originally posted by Metallus  | 3) Lead nitrate in water and NaOH (At first the solution got whitey but then the lead dissolved again; i suppose it got soluble as Pb(OH)3-), added
H2O2 and this time some brownish stuff formed. There were no bubbles. I filtered the brownish stuff, dried it and added it to an acid solution of H2O2
and bubbles formed.
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At first Pb(OH)2 formed but that is highly amphoteric [dissolves both in acid and alkaline solutions], so when you added more NaOH, sodium
plumbite (Na2Pb(OH)4) was formed. On addition of H2O2, PbO2 then formed. That too is amphoteric but requires much more
alkaline conditions to form plumbates [e.g. Na2Pb(OH)6], so it crashed out in your conditions.
Commercial bleach (sodium hypochlorite) is commonly used to oxidise plumbites to lead (IV) dioxide but hydrogen peroxide works too.
The precise colour of oxidic precipitates (in general) depends on a lot of factors, granulometry being an important one. I've seen PbO2 start
precipitate to yellow, only to age quickly to choc brown and with PbO2 on older car batteries nearly black. So colour alone is a poor guide to
identity.
I would strongly advise against using PbO2 in flash powders with Mg or Al because very finely divided lead is being formed, which is quite hard to
clean up.
In acid conditions, PbO2 will easily oxidise the O (-I) in peroxide to O (0), i.e. oxygen gas, which is what you observed.
[Edited on 16-5-2013 by blogfast25]
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Metallus
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Thanks for the answers; i'll try to perform a further test on a solution of Mn2+ ions just to confirm if this brownish product, supposedly PbO2, is
able to oxidize it to permanganate.
Thanks again for the fast replies; sadly, at my university, i'm probably one of the very few (so far i didn't meet any other, tho) who carries out
some experiment and most of the times not even professors are that reliable, considering that i get different answers depending on the professor. The
ones studying chemistry don't even seem interested at all...
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