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AJKOER
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Blogfast:
Assume we heat up aqueous Na2CO3 to boiling in a wide mouth vessel in air, then we have the reactions:
2 Na2CO3 + 2 H2O = 2 NaHCO3 + 2 NaOH
And possibly to a limited extent as well the breakdown of the aqueous Sodium bicarbonate on boiling (although the supplied reference claims completion
on boiling):
2 NaHCO3 + H2O ---Boiling → CO2(g) + 2 H2O + Na2CO3 (see http://antoine.frostburg.edu/chem/senese/101/inorganic/faq/c... )
Or, on net, an optimistic, but likely inaccurate assessment of the max possible NaOH formation:
Na2CO3 + H2O ---Boiling → CO2(g) + 2 NaOH
And indeed a limiting one as the following reaction has come to my attention in the presence of say Al(OH)3, the NaHCO3 is possibly consumed. To quote
https://books.google.com/books?id=FijWBgAAQBAJ&pg=PA149&...
Al(OH)3 + NaHCO3 = NaAl(CO3)(OH)2 + H2O
Now, we bring into play some electrochemistry, to quote Wikipedia on the Aluminum-air battery (https://en.m.wikipedia.org/wiki/Aluminium–air_battery#Elec... :
"The anode oxidation half-reaction is Al + 3OH− → Al(OH)3 + 3e− +2.31 V.
The cathode reduction half-reaction is O2 + 2H2O + 4e− → 4OH− +0.40 V.
The total reaction is 4Al + 3O2 + 6H2O → 4Al(OH)3 + 2.71 V.
About 1.2 volts potential difference is created by these reactions, and is achievable in practice when potassium hydroxide is used as the electrolyte.
Saltwater electrolyte achieves approximately 0.7 volts per cell."
Note the overall consumption of water which could important implications for pH and solubility.
Now, in our case upon adding Aluminum, to boiled Na2CO3 in the presence of oxygen (or an oxygen source), we have a better electrolyte than saltwater
but not as effective as the KOH. Note, it is O2 and H2O attacking the Al and not the electrolyte!
At cooler temperatures (under 50 C) and agreeable pH, some Sodium aluminate could form via the equilibrium reaction:
Al(OH)3(s) + NaOH = NaAl(OH)4
With respect as to whether the Sodium aluminate is stable see https://www.google.com/url?sa=t&source=web&rct=j&...
A pH related comment on the article page number 195 is, to quote:
"The optimum pH during precipitation of aluminum hydroxide is within the range 8.7–9.3 [5,6]. "
at least for the purposes of that article.
To speed things up in the electrochemical half reactions, one could use dilute H2O2 as an oxygen source and I suspect microwave heating will also be
benefical.
------------------------
I have since run an experiment and for courtesy, have decided to place the results with pictures in a soon to be created new thread (exists link: http://www.sciencemadness.org/talk/viewthread.php?tid=63150 ).
[Edited on 29-7-2015 by AJKOER]
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blogfast25
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AJ:
Your first ‘reaction; is nothing but the first hydrolysis of the carbonate ion:
CO<sub>3</sub><sup>2-</sup>(aq) + H<sub>2</sub>O(aq) < === >
HCO<sub>3</sub><sup>-</sup>(aq) + OH<sup>-</sup>(aq)
pKb = 3.67
Approximately:
pOH = ½(pKb + pC)
So for carbonate concentration of 1 M:
pOH = 1.8, pH = 12.2
And [HCO3-] = 0.015 M
For higher concentrations of carbonate, this increases approx. with the square root of that concentration.
Sure, bicarbonate slowly loses CO<sub>2</sub> but boiling a carbonate solution is a near infinitely slow way of preparing an NaOH
solution!
As regards the oxidation of Al by water and oxygen to Al(OH)3, bear in mind that gases like oxygen are very poorly soluble in boiling water.
[Edited on 28-7-2015 by blogfast25]
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unionised
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Ajoker, to put it simply, this reaction
2 Na2CO3 + 2 H2O = 2 NaHCO3 + 2 NaOH
does not happen. In fact the opposite reaction goes practically to completion.
And this reaction
Na2CO3 + H2O ---Boiling → CO2(g) + 2 NaOH
is even less plausible.
However at very high concentrations sodium carbonate will dissolve a little aluminium hydroxide.
Who cares?
It's a lousy way to test for Al in the presence of Zn.
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AJKOER
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Unionised:
On boiling, Aluminum in hot Washing soda is a known hazard for Aluminum cookware!
As Na2CO3 is a common ingredient in dishwasher detergent, an ongoing one at that.
It literally dissolves the Al, so please verify it if you don't believe.
The simplest chemical argument is that the reaction is moved to the right with the NaOH attacking the Aluminum.
Na2CO3 + H2O --boiling--) NaHCO3 + NaOH
It is a vigorous reaction, but if you want to see insane, replace H2O with dilute H2O2, heat treat your Al foil in a flame before employing, and jump
start the reaction in a microwave. For more details on the chemistry, see my latest new thread on Dawsonite.
I have decided to edit the second reaction to make it clearer it is likely an inaccurate expression giving limited guidance as to the max amount of
NaOH possibly formed.
Also, Al(OH)3 apparently can react with NaHCO3, especially in the presence of Na2CO3, to form a mixed carbonate hydroxide (there is a Patent on
Dawsonite that so claims), which limits the effective aqueous decomposition of heated NaHCO3 here.
[Edited on 29-7-2015 by AJKOER]
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unionised
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Why do you think it matters that washing soda attacks aluminium?
How much of the metal dissolves and how much is converted into a suspension of hydrated Al2O3?
Anyway, while you are rewriting your post you might want to take out the stuff about electrochemistry- it's got nothing much to do with the subject.
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AJKOER
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Unionised:
A quote:
"So the alkaline sodium carbonate in the dishwasher detergent does indeed attack aluminum, at the very least eating deeply enough into the surface to
make it dull and pewter-gray with aluminum compounds. For this reason most manufacturers of quality aluminum cookware advise against putting it in the
dishwasher."
Link: http://superbeefy.com/why-does-aluminum-cookware-corrode-and...
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blogfast25
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AJ:
NOBODY disputes sodium carbonate solutions corrode Al objects. But that is not an argument that proves sodium carbonate can dissolve SIGNIFICANT
amounts of aluminium. Mild surface corrosion does simply not equate to significant dissolution. Corrosion is also affected by other factors. To equate
corrosion so insistently with dissolution as you do is simply silly.
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AJKOER
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Yes, a fair question.
Perhaps looking at the starting to finish pictures (sorry, not the best quality) in my new thread (link: http://www.sciencemadness.org/talk/viewthread.php?tid=63150 ) where to the heat treated Aluminum foil (albeit more reactive) and washing soda,
some dilute H2O2 (again better than air) was also added (that is, in place of just boiling aqueous Na2CO3 in an open vessel in air) may still give one
cause to be astonished given that it was accomplished in around 10 minutes with only 30 seconds in the microwave!
If the Al foil was more heat treated via a methane flame a little more consistently (that is, made to glow red creating the weak gamma Al2O3 over the
Aluminum), I would expect near total destruction in short order. Otherwise, the process is slower for sure, but the ultimate result is the same.
[Edit] Also interesting is the Aluminum-air battery electrochemistry involved (discussed at the top of this page), implying that Na2CO3 acts in one
role as an electrolyte, which is a catalytic role only! So as long as there is water and an oxygen source, the Aluminum could continue to be attacked!
[Edited on 30-7-2015 by AJKOER]
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unionised
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Ajkoer,
That piece of aluminium you were looking at as it dissolved.
Do you know that the metal was made by electrolytic reduction of Al2O3 dissolved in molten cryolite?
I guess you did.
Do you know where the Al2O3 comes from?
Most of it is from an ore called bauxite.
i guess you knew that too.
The Bauxite isn't very pure- it has quite a lot of Fe2O3 in it.
You may well have known that too.
Do you know how they separate the Alumina from the iron oxides?
They dissolve the alumina by digesting it with hot NaOH solution then filter off the "red mud" of (mainly) Fe2O3.
Do you know how they get the alumina back again?
There are a couple of ways.
One is to add CO2 to the solution of sodium aluminate.
So, the fact that you can find a bit of cheap aluminium to dissolve is a consequence of the fact that the reaction you are relying on goes
backwards.
A multi million dollar industry says one thing and you say the other .
Which one do you think I should believe?
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