guy
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Lead sulfate and NH4CH3COO
How does ammonium acetate dissolve lead sulfate? The only thing that makes sense is that acetate would complex with it.
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chemoleo
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lead sulfate --> lead ammonium sulfate --> lead ammonium acetate?
Never Stop to Begin, and Never Begin to Stop...
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guy
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Do double salts increase solubility? I've seen them decrease solubilities.
I don't know much about the chemistry of double or triple salts, they seem really interesting. I saw an example of K2CuPb(NO2)6 precipitating and
that got me wondering about how they work.
Edit:
http://www.public.asu.edu/~jpbirk/qual/qualanal/lead.html
PbSO4(s) + 2CH3CO2-(aq) <==> Pb(CH3CO2)2(aq) + SO42-(aq)
Does this mean lead acetate is a covalent molecule? Lead is has a high electronegativity for a metal so it would probably be a covalent complex which
is soluble.
[Edited on 8/4/2006 by guy]
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chemoleo
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>Do double salts increase solubility? I've seen them decrease solubilities.
A prominent case is NiSO4 and K2SO4, both are reasonably soluble, and the double salt precipitates if you mix saturated solutions of both. There are
many other examples.
Increased solubility - why would that be conceptually impossible? I don't recall examples but I dont see why not. Ammonium ions are very well capable
of solubilising ions. Didnt you do this experiment of Cu solubilisation in the presence of ammonia/ammonium sulfate and O2 yourself, while this doesnt
work well without ammonia? I think it's a similar principle.
The principle of double salts is easy once you get your head around it.
Just think of the divalent anion SO4(2-) as a bridge.
So you essentially get in the case of Ni-K sulphate:
K-Ni-SO4-Ni-K
or in the case of potassium alum
K-SO4-Al=SO4
This is why they have properties, their are essentially ionic mixes with different connectivities.
In the case of the lead salts - you could even see mixed ionic 'polymers' forming...however, they arent double salts, they are complexed salts, which
is a different thing.
On the covalent character of lead acetate - I seem to remember something on it, it may well be so partially, facilitating its absorption through the
skin, making it toxic in particular. But I could be wrong on this, I couldnt find anything on it.
[Edited on 4-8-2006 by chemoleo]
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guy
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| Quote: | Originally posted by chemoleo
Increased solubility - why would that be conceptionally impossible? I don't recall examples but I dont see why not. Ammonium ions are very well
capable of solubilising ions. Didnt you do this experiment of Cu solubilisation in the presence of ammonia and O2 yourself, while this doesnt work
fast without ammonia? I think it's a similar principle.
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That was because NH3 (ammonia not ammonium) complexed with the Cu(OH)2 and the OH- reacted with (NH4)2SO4 to form more NH3. Without it the Cu(OH)2
would be stuck cover the rest of the copper and form a protective layer.
~~~~
I think my first question should have been why do ACETATE ions dissolve PbSO4 (the ammonium part was unnecessary).
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chemoleo
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Try this with Na acetate and PbSO4 and see if it works. I bet if it is freshly preciptiated PbSO4, you'll have a small amount of PbSO4 dissolving to
form PbAc2.
It likely is an equilibrium issue where you have a certain fraction of Na2SO4 and PbAc2 together with PbSO4 and NaAc. My guess is that the NH3 in
NH4Ac helps solubilising the PbSO4 through complexation, thereby shifting the equilibrium.
Never Stop to Begin, and Never Begin to Stop...
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guy
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NH3 doesn't complex with Pb2+ as far as I know, if so adding NH3 would dissolve it. Here is my hypothesis:
NH4OAc <---> NH3 + HOAc .............(1)fairly to the right
NH3 + H2O <---> NH4+ OH-.............(2)small but will shift as being used up
2OH- + PbSO4 ---> Pb(OH)2..............(3)Pb(OH)2 is less soluble than PbSO4 so it shifts right
2HOAc + Pb(OH)2 ---> Pb2+ + 2OAc- + 2H2O......(4)Acid base reaction so stays to right
Pb2+ + SO42- ----> PbSO4..........(5)Also large K
~~~~~~~~~~~
BUT there seems to be a paradox since reaction 5 and 4 compete with each other. I think that the 4th one wins because K of 3 is bigger than 5.
[Edited on 8/4/2006 by guy]
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Magpie
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I can't resist this opportunity to show off the double salt I just made: Rochelle salt (KNa tartrate*4H2O) - photo attached. It is made by
dissolving Cream of Tartar (KH tartrate) in water, then adding washing soda (Na2CO3) until CO2 evolution stops. What amazed me was that a large
crystal (~1/2 cm on a side) had formed within 2 hours! This is the first time I have grown any crystals since I was a child.
The solublizing of PbSO4 via NH4CH3COO seems like it could work by NH3 complexing with Pb++. Even though PbSO4 is insoluble there are going to be
some Pb++ and SO4-- ions available. If they are complexed they are removed from the Ksp equilibrium, so more PbSO4 will dissolve. This could
continue until all the PbSO4 is dissolved. This is just my speculation on the mechanism - I don't really know.
The single most important condition for a successful synthesis is good mixing - Nicodem
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Polverone
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Those are some beautiful crystals, Magpie. A favorite of mine is calcium tartrate, made by combining solutions of calcium chloride and rochelle salt.
The solubility is quite low, but for some reason it takes considerable time for the pretty little needles to form. Then those needles can be ground
while still wet with oxalic acid to put pure tartaric acid in solution, which forms some beautiful crystalline patterns of its own when evaporated
down.
PGP Key and corresponding e-mail address
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not_important
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Just to give a little additional data
NH4Cl will also solubilize lead sulfate
Ammonium salts will solubilize CaSO4, and prevent chromate from precipitating Ca and Sr.
Zinc can be prevented from precipitating by ammonium salts.
NH3 (aq) will complex with Cu, Cd, and Zn (all 2+ state), but not Bi(III)
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