Rumoden
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White precipitate from NaOH + Al reaction?
The other day, I was just trying to make hydrogen gas by reacting NaOH with some aluminum foil. Out of interest, I decided to leave the solution
overnight. The once black solution turned colorless with white precipitate on the bottom. What precipitate might this be?
I thought this is NaAlO2 but according to Wikipedia NaAlO2 is soluble in water.
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12thealchemist
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How much precipitate is there relative to the quantity of sodium hydroxide/aluminium you used? i.e. could it be an impurity in the foil? Aluminium
foil is usually ~98% aluminium and 2% alloying agents/impurities
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Ozone
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An over-simplification, but aluminum hydroxide. It is amphoteric, so increasing pH further should cause it to dissolve.
Attachment: aluminum hydroxide complexes report.pdf (2.5MB)
This file has been downloaded 526 times
-Anyone who never made a mistake never tried anything new.
--Albert Einstein
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AJKOER
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Ozone, that’s a great paper, but perhaps a bit dated (1967).
In addition to the reaction occurring on standing, I further suspect also if one actually leaves out a solution of sodium aluminate, in contact with
air, that any dissolving CO2 may react (see below) resulting in a precipitate of Al(OH)3 and the introduction of aqueous NaHCO3:
Al(OH)4- (aq) = OH- (aq) + Al(OH)3 (s)
CO2 (aq) + OH- (aq) = HCO3- (aq)
Net: Al(OH)4- (aq) + CO2 --> HCO3- (aq) + Al(OH)3 (s)
To quote a study from 1997 (https://inis.iaea.org/collection/NCLCollectionStore/_Public/... ):
"....aluminate in high level waste. Care was taken to minimize exposure of the samples to air since aluminate solutions and solids are extremely
hygroscopic and deliquescent, in addition, absorption of atmospheric CO2 occurs readily in solutions exposed to air and therefore such exposure was
minimized. Solutions prepared from commercial grade sodium aluminate (method B) were contaminated with carbonate as well as fluorescent impurities
that interfered with measurement of Raman spectra."
Also:
"Kinetics of aluminate precipitation:
Precipitation of sodium aluminate from supersaturated solutions is extremely slow at room temperature (22 C). In fact, very concentrated solutions
(5-6 M Al) can be prepared as long as they are not heated above about 40 C. At 60 C, all supersaturated' solutions precipitated monosodium aluminate
over a period of hours to days. At 22 C, these solutions were stable for several months."
[Edited on 20-11-2018 by AJKOER]
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