Arthur Dent
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Zinc Carbonate
Hi folks,
I would like to synthetize some Zinc Carbonate from metallic Zinc, and after a few searches, I discovered that Zinc Carbonate can be easily obtained
from Zinc Chloride:
ZnCl<sub>2</sub>(aq) + Na<sub>2</sub>CO<sub>3</sub>(aq) → ZnCO<sub>3</sub>(s) + 2 NaCl(aq)
So my first step will be to dissolve some Zinc turnings in concentrated hydrochloric acid. I've synthetized ZnCl<sub>2</sub> a while ago
and remember that the Zinc Chloride solution is quite acidic and tends to dissolve cellulose easily so no paper filter for this filtering step. Enter
Mr Büchner funnel!
After the solution of ZnCl<sub>2</sub> is filtered and warmed a bit, i'll add some warmed Sodium Carbonate solution.
The cool part is that the Zinc Carbonate precipitate is not soluble, and all the other reagents are quite soluble, so after washing the precipitate a
few times in hot distilled water, I should be left with a fairly pure cake of Zinc Carbonate.
Now some questions. After I have obtained my precipitate, i'll decant as much of the liquid as I can and then dump the precipitate in my Büchner
funnel. I'll use my vacuum aspirator with an erlenmeyer w. sidearm to accelerate the process, but I fear that the extremely fine precipitate might
clog up the filter and take an eternity to filter off, especially if I add ample amounts of warm distilled water a few times to remove the soluble
traces of Zinc Chloride, Sodium Chloride and Sodium Carbonate.
Is it preferable to use straight Sodium Carbonate or could I substitute it with Sodium Bicarbonate?
I'll probably go ahead with this synthesis next weekend. So if anyone has any advice or experience with this synthesis, I'll appreciate your input.
Thanks
Robert
--- Art is making something out of nothing and selling it. - Frank Zappa ---
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blogfast25
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Firstly, filtering even a quite acidic solutions (say, HCl) on paper filtering media is usually not much of a problem (*). I do it all the time.
It’s different when using really strong HCl, which does attack paper quite readily. But zinc is very reactive, so you don’t need to use a huge
excess of HCl. Try starting with about 10 - 20 % extra HCl with respect to stoichiometric, heat gently when reaction starts to tail off. Ask if you
don’t know how to do the stoichiometry.
Carbonate v. bicarbonate usually depends on the solubilities of the target carbonate and its equivalent hydroxide. Here, Zn(OH)2 is far more insoluble
than ZnCO3 which means that by using the bicarbonate you’ll probably precipitate Zn(OH)2, not ZnCO3. By using sodium carbonate, zinc carbonate
should form and precipitate. Again, try not to use a vast excess of Na2CO3 because that tends to promote occlusion.
It’d best to allow the precipitate to stand overnight as usually this improves filterability. Then wash profusely to pH about 9 - 10. You can test
the filtrate for free CO<sub>3</sub><sup>2-</sup> with CaCl2 solution (turbidity means CaCO3) and free carbonate means you
haven’t washed out all the excess sodium carbonate.
(*) at a pH = 3 the approx. free HCl concentration is only about 0.001 M.
[Edited on 3-12-2011 by blogfast25]
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unionised
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Solutions of some salts (cuprammonium sulphate is the best known) will dissolve cellulose. I suspect ZnCl2 might be one of them.
(it is according to WIKI which says
"In textile processing
Concentrated aqueous solutions of zinc chloride (more than 64% weight/weight zinc chloride in water) have the interesting property of dissolving
starch, silk, and cellulose. Thus, such solutions cannot be filtered through standard filter papers.")
I would dilute the acid 50:50 with water first, partly to moderate the rate of reaction, but mainly to keep the HCl fumes down a bit.
Use of bicarbonate rather than carbonate will ensure that you precipitate the carbonate rather than a basic carbonate ( a mix of hydroxide and
carbonate)so this bit
"Zn(OH)2 is far more insoluble than ZnCO3 which means that by using the bicarbonate you’ll probably precipitate Zn(OH)2, not ZnCO3."
is the wrong way round.
[Edited on 3-12-11 by unionised]
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blogfast25
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Well, that surprises me. I’d have thought, w/o any calculations, that for equivalent solutions of sodium carbonate and sodium bicarbonate, the
latter would show a much higher [OH<sup>-</sup>] / [CO<sub>3</sub><sup>2-</sup>] ratio making it more prone to
precipitating the hydroxide or a mixed hydroxide/carbonate. Where am I going wrong?
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unionised
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If there were hydroxide present in a bicarbonate solution it would react.
OH- + HCO3- ----> H2O + CO3 --
That's a straightforward acid/ base reaction.
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blogfast25
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Quote: Originally posted by unionised  | If there were hydroxide present in a bicarbonate solution it would react.
OH- + HCO3- ----> H2O + CO3 --
That's a straightforward acid/ base reaction. |
So is this:
HCO3- + H2O === > H2O + CO2 + OH-
... which explains why bicarbonate solutions react alkaline (as do straight carbonate solutions, of course). No 'hydroxide present' needed to explain
the slightly alkaline nature of bicarbonate solutions. HCO3- is the conjugated base of a weak acid, thus a weak base.
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Arthur Dent
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Excellent! Thanks for the info, I think I'll go ahead with the Sodium Bicarbonate then, which will save me the step of gradually roasting it over
several hours in order to turn it into Carbonate (it'll save electricity!) I have about 40 g of zinc in the form of small cut-up bits, gathered from
C/Zn battery casings, thoroughly cleaned with a brush and then dipped briefly in HCl to clean off impurities.
i'll prepare the Zinc chloride solution during the week so that i'll be able to perform the synthesis during the weekend. i'll post my results.
Thanks!
Robert
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blogfast25
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Hmmm… I’m now not so convinced what you’ll obtain though, with bicarbonate OR with carbonate. The solubility product K<sub>sp</sub>
= 3 x 10<sup>-17</sup> = [Zn2+] x [OH-]<sup>2</sup> for Zn(OH)2.
Simply put, this means that for a solution of about 0.1 M in [Zn2+], Zn(OH)2 will start to form from a pH of about 6 or higher (if that sounds
surprising then bear in mind that so called ‘neutral’ salts of zinc - zinc chloride or sulphate - form solutions that are slightly acidic, thus
preventing Zn(OH)2 to form).
I think you’ll be precipitating a zinc hydroxy carbonate. One that’s described in the literature is 2(ZnCO3).Zn(OH)2.2H2O.
If you’re using NaHCO3, beware that it’s poorly soluble in water, you’ll slowly, with much stirring, have to sprinkle it into your solution.
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Arthur Dent
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So it's going to be a "basic" carbonate composed of hydroxyde and carbonate? Oh well... :S
As for the Sodium Bicarbonate, don't worry, i'll dissolve it in hot water and then add it to the warm Zinc Chloride solution and stir thoroughly the
resulting mixture with a stirring bar.
I'll label the resulting dried compound "Zinc Carbonate-ish" 
Robert
--- Art is making something out of nothing and selling it. - Frank Zappa ---
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blogfast25
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The only real way to determine what you’ve concocted is to dry it properly and determine precisely how much CO2 it contains per mole of product,
either by releasing the CO2 with acid or by strong heating. Obviously ZnCO3 will generate 1 mole CO2 per mole ZnCO3, 2(ZnCO3).Zn(OH)2.2H2O will
generate 2 moles CO2 per mole of 2(ZnCO3).Zn(OH)2.2H2O.
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blogfast25
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Acc. to this patent ZnCO3 is precipitated from a dilute solution of a zinc salt with carbonate but when a strong solution of a zinc salt is treated with a
strong solution of sodium carbonate, a zinc hydroxycarbonate is formed.
And if sodium carbonate will precipitate (in part) Zn(OH)2 from a zinc salt solution, we should see bubbles:
ZnCl2(aq) + Na2CO3(aq) +H2O(l) === > Zn(OH)2(s) + 2NaCl(aq) + CO2(g)
[Edited on 6-12-2011 by blogfast25]
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Arthur Dent
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Good thing to know. I'll proceed with the synthesis using dilute solutions to avoid making hydroxide. My initial reflex would have been to use more
concentrated solutions (faster, more reactive, less water to decant, etc) but I would have ended up with the wrong compound.
I'll use my widemouth 1-liter erlenmeyer to let the precipitate settle, decant as much liquid as I can and add again some distilled water and repeat a
few times. The final precipitate should then be free of most impurities, but i'll still make one last wash through my Büchner funnel to make sure
it's pristine and sodium-free.
Robert
--- Art is making something out of nothing and selling it. - Frank Zappa ---
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blogfast25
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Watch this here UToob on mixing ZnCl2 with soda.
Although it says nothing about concentrations used or acidity of the initial ZnCl2 solution, going by the amount of precipitate I’d say his
solutions were quite concentrated. The whole thing also fizzes like mad, even after the first precipitate has formed and right up to the end, an
indication that CO2 is being lost and a zinc hydoxycarbonate is being formed.
If we look at the carbonates of trivalent cations we know that these cannot from in aqueous media because solvated trivalent cations act as weak acids
(typically Al, Fe):
[M(H2O)6]3+(aq) + CO3(2-)(aq) === > [MOH(H2O)6-1]2+(aq) + HCO3-(aq)
And:
[MOH(H2O)6-1]2+(aq) + HCO3-(aq) === > [M(OH)2(H2O)6-2]+(aq) + H2O(l) + CO2(g)
Further hydrolysis then precipitates M(OH)3.nH2O. Aluminium sulphate (for instance) + soda = hydrated alumina + CO2
For the divalent solvated cations a similar scheme can be imagined but due to the smaller charge these are generally even weaker acids. Some neutral
carbonates of divalent D-block cations do exist: see MnCO3, CoCO3, FeCO3, NiCO3 and others. In other cases like Cu, a hydroxycarbonate is formed
(Cu(OH)2.CuCO3). Zinc seems to be sitting on the fence somewhat.
[Edited on 7-12-2011 by blogfast25]
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