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16MillionEyes
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The purple precipitate is very strange though. What's your source for Na2CO3? I've done the Al and OH- many times before and I tell you no purple
precipitate forms and being that the OH- is the "active" reactant your reaction shouldn't either. I think I'll try doing the same exact set up and see
if I get any purple precipitate but I highly doubt it. I just can't see where that stuff would come from.
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497
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i agree it is a little surprising. i think it must come from the Al foil but i may be wrong. as far as my exact setup goes, it was pretty crude.
well.. actually very very crude... i realllly need to get some good glassware... it consisted of a quart mason jar with maybe 2 inches of water in the
bottom, in it was dissolved maybe 3 to 5 grams Arm and Hammer brand washing soda (about a year or two old.) then i added a 2 inch by 18 inch piece of
Reynolds Wrap brand Al foil (made into a spiral that would fit in the jar.) the jar is set in a pot of water on the stove, simmering. it starts
bubbling, increasing steadily. only after several minutes does a dark grey-blue gel like ppt start forming in noticeable amounts, eventually the foil
disappears and the reaction stops. i filtered it through a paper towel, and squeezed out as much water from the ppt as i could. the remaining liquid
was clear, when evaporated in an oven it crystallized out some white ppt, which im assuming is NaCO3. i cant tell if it crystallized out the same
amount as i added, it looks roughly the same but i never measured it. the purplish Al hydroxide becomes lighter colored as it dries. as soon as i have
time i'll try it again, this time measured and hopefully with some more pure Al. anyone know where i can get some other than the internet? have fun
trying to reproduce the purple ppt.
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16MillionEyes
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Ahhhh, well, now that you mention it that way then it seems very likely that your purple precipitate is some sort of impurity from the Arm and Hammer
washing soda. From what I found just now it contains all sorts of things raging from enzymes to Bentonite clay so I really find it very likely to get
unexpected results like that. In other words, you're basically shooting in the dark here.
You should try getting some Na2CO3 from somewhere else with reliable at least discernible purity. One easy OTC way that I'd recommend is to get
regular baking soda and oven heat at 250C or so for a while, this will get you relatively pure Na2CO3. Once you've done this then try again, I'm sure
no purple precipitate will form this time.
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497
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hehh that makes so much more sense. the reason i never considered it is i seemed to remember seeing that it was pure. i apparently remembered wrong, i
should have checked. thanks that clears things up alot.
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497
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im going to go try it right now, ill get back you you in a while.
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UnintentionalChaos
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I've used arm and hammer washing soda before. It is fairly pure, probably with a bit of crud in it since the stuff I have dissolves to a pale tan
solution. I use it for washing certain stains off glassware (used extensively while doing curcumin extractions). The hotter the oven, the faster the
baking soda (which is quite pure and very wonderfully does not form hydrates like sodium carbonate) decomposes. Spread a thin layer, breaking up lumps
and ramp up the oven. I give it an hour at 500F which is massive overkill most likely, but does the job well.
Department of Redundancy Department - Now with paperwork!
'In organic synthesis, we call decomposition products "crap", however this is not a IUPAC approved nomenclature.' -Nicodem
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497
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hmm yeah my oven only goes to 450F but it should work. its cooking right now. so if its not the washing soda where could the damn purple color come
from???
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UnintentionalChaos
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Give it a shot with a different brand of aluminum foil. I usually get grayish aluminum hydroxide from very finely divided metal dispersed in it, but
purple seems odd.
Department of Redundancy Department - Now with paperwork!
'In organic synthesis, we call decomposition products "crap", however this is not a IUPAC approved nomenclature.' -Nicodem
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chemkid
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Have you been able to replicate the purple precipitant?
Chemkid
[Edited on 9-10-2007 by chemkid]
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16MillionEyes
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With my set up certainly not, I did obtain the grayish precipitate from the NaAlO2 though. My sources were pharmaceutical grade NaHCO3 decomposed to
Na2CO3 and regular aluminum foil. The reactants were left over night and then heated for about 5 minutes the day after. About 8 hours later I came
back to find the precipitate on the bottom however a quick pH test revealed that the solution is still mostly basic (~10-11) and thus suggests that
the reaction is still not done. I think that these results, nonetheless, prove the purple precipitate is not from the reaction itself but from some
sort of impurity in his arm and hammer reactant.
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497
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well i just finished trying it again, this time with food grade NaHCO3 decomposed to Na2CO3, different brand Al foil. other than that the setup was
the same as before. got the normal grey ppt. must have been the washing soda that made things go differently. there was also much less ppt than
before. it reacted quickly for a few minutes then slowed down until it almost came to a stop. there as excess Al foil. according to Unintentional
Chaos's explanation the Na2CO3 is not used up in the reaction, so what would have stopped it? after it slowed down i added more water but it had
little effect. but youre saying its making NaAlO2 which would explain the stop in the reaction.
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UnintentionalChaos
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Some degree of it does get tied up in solution as sodium aluminate and some simply gets trapped in the precipitate's matrix. Did you use identical
quantities of washing soda and foil as before? Adding more water would actually slow it down since the formation of sodium aluminate is an equilibrium
reaction pushed foward by a concentrated solution.
Use the original aluminum foil with the new sodium carbonate and see if you get purple. Simple elimination of possible factors.
arm and hammer + brand 1 = purple
NaHCO3 derived + brand 2 = gray
NaHCO3 derived + brand 1 =?
arm and hammer + brand 2 =?
One of those ? should be purple and the other should be gray. Test one of them.
Did you consider the fact that the new foil may be coated differently than the first foil and this may be stopping the reaction?
[Edited on 10-9-07 by UnintentionalChaos]
Department of Redundancy Department - Now with paperwork!
'In organic synthesis, we call decomposition products "crap", however this is not a IUPAC approved nomenclature.' -Nicodem
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497
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well as far the purple color goes i am pretty sure its from the arm and hammer. i might do some more on that later, but we'll see. after finishing the
the reaction with baking soda derived Na2CO3, filtering out the grey sludge and leftover Al. something interesting is that the clear liquid began to
precipitate a very fine white ppt. so i put it in a ice water and precipitated as much as i could but it was too fine to be easily filtered. then i
boiled off most of the water and filtered out the now clumpy ppt. so i decided to try the same steps on straight NaCO3 to hopefully help determine if
the reaction changes things. so i dissolved roughly the same amount of NaCO3 as i used in the earlier reaction (i dont have the time or energy to
measure it right now) and am cooling it, but its not precipitating... maybe because of impurities gained in the reaction, im not sure. i'll go boil
the unreacted solution and how it precipitates that way. thanks for all the help and suggestions btw.
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16MillionEyes
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Well, my take in it is since the reaction of the Al is directly dependent on the presence of OH- anions once all the CO3(2-) has converted itself (due
to change in equlibirum) to HCO3- the reaction will stop almost completely (or too slow to be noticeable) due to the reduced amount of OH- present in
solution. Also if you consider the equation it balances out rather nicely:
Na +2CO3(2-) + 2H2O + Al ---> 2HCO3- + NaAlO2(s) + H2(g)
A quick test of the presence of CO3(2-) in solution confirms its presence (CO2 gas) and it would follow that it's from the HCO3-. In other words we're
getting a NaHCO3 solution with NaAlO2 as precipitate. I would also discard Al(OH)3 as the source of this precipitate due to its amphoteric nature it
would dissolve in a strong base and it does not.
Anyway, I hope this helps.
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497
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well is there a good test to dertermine if it is NaAlO2? and/or a good way to test if the dissolved Na compound is bicarb or carbonate? also after the
reaction completed and was filtered the liquid had a pH ~11.5 if thats any help.
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497
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i boiled to the straight Na2CO3, it didnt begin to ppt untill there was very little water left (the post reaction solution began to ppt soon after it
began boiling.) it formed a crust on the surface of the water, which did not occur with the other solution. ill try the same thing with NaHCO3 as soon
as i have time.
[Edited on 12-10-2007 by 497]
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16MillionEyes
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You know, I have to retract what I said about NaAlO2. What I did report was in fact true in the sense that the precipitate didn't dissolve right away
(as I would expect from Al(OH)3) in the presence of a strong base but after a day or so the precipitate had completely dissolved. As
UnintentionalChaos said, the reaction depends on an equilibrium and this base might have thrown it off to the right but it still doesn't answer
whether what forms in the precipitate is Al(OH)3 or NaAlO2.
Your reported pH is very similar to mine but I'd like to know just how much water is in your solution. In my case water is very limited so it's very
probable that highly concentrated NaHCO3 is giving the relatively high pH.
I really can't think of a good way of determining whether there are CO3 or HCO3 ions in solution other than taking in consideration the basic
properties of the CO3 over the HCO3. I still believe that a stoichiometric result is the best choice and would be very elucidating at this point so
perhaps you would like try that. I would try it myself but I have no balance.
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497
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yeah i lack a decent scale also... hmmm actually i might be able to get ahold of my grandma's gold scale  . it should be accurate enough. on another note, i doubt the post reaction solution is HCO3 mainly because the pH of a
solution of HCO3 similar to the quantity of CO3 i started out with in the earlier reaction was only about 8.5. the pH of the post reaction liquid was
about 11.5. i got a pH of about 12 with a similar aount of straight CO3 dissolved. but i find it interesting that neither the known CO3 or HCO3 ppt
out at room temp while the mystery post reaction solution does... maybe my quantities weren't as close as i had hoped. i think i'll finally have time
to do a somewhat more accurate expirement this weekend.
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Ozone
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IIRC, some Al° materials are Mn alloys (beer cans, for example). I have observed these becoming pink (purple at high c) under oxidative conditions...
Maybe the Al is not a pure as you think?
just pondering,
O3
[Edited on 12-10-2007 by Ozone]
-Anyone who never made a mistake never tried anything new.
--Albert Einstein
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497
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certainly possible. but after switching to Na2CO3 made from NaHCO3 i dont get the purple ppt so i suspect the Arm and Hammer is the culprit. i would
still like to use so good pure Al though, for good measure.
so i did a little crude testing of the soluble reaction product. well it is not very soluble at all. i added ~30 ml water to a teaspoon of it, just
clouds up the water, eventually settles, very little dissolves (dissolves much better at boiling temp). i also tested carbonate and bicarbonate. a
teaspoon of carbonate completely dissolves in 30 ml at room temp, a teaspoon of bicarb nearly completely dissolves in 30 ml. so what could it be?
maybe impurities? just a plain old white powder, you couldn't tell it from bicarb or carbonate by looking.
[Edited on 12-10-2007 by 497]
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Antwain
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You should also be aware that sodium aluminate is rather easily hydrolysed. Hence diluting a solution of it will result in the precipitation of
hydrated aluminum hydroxide.
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497
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well i have never heard that. interesting...
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497
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could it be that my mystery white powder is Al(OH)3? it is insoluble at room temp as far as i can tell. 30 ml of water with a teaspoon of it reads a
pH of about 10.5. pH of a teaspoon of carbonate in 30 ml is 11.5 and a teaspoon of bicarb is 9.5.
[Edited on 13-10-2007 by 497]
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497
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i have managed to acquire the balance, the good news is it can weigh up to 12 kg.. bad news is it has 1 g resolution . but it'll do.
so what should my molar ratios be? i was thinking .5m Al, but how much Na2CO3, since its not just simple Al to Al(OH)3? well i suppose i will just
have to try something.
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chemkid
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flame test will distinguish between sodium aluminate and aluminum hydroxide.
Chemkid
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