Sciencemadness Discussion Board - Zinc Carbonate

Zinc Carbonate

Arthur Dent - 3-12-2011 at 10:46

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:

ZnCl2(aq) + Na2CO3(aq) → ZnCO3(s) + 2 NaCl(aq)

So my first step will be to dissolve some Zinc turnings in concentrated hydrochloric acid. I've synthetized ZnCl2 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 ZnCl2 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

blogfast25 - 3-12-2011 at 13:29

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 CO32- 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]

unionised - 3-12-2011 at 13:35

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]

blogfast25 - 4-12-2011 at 07:09

Quote: Originally posted by unionised

is the wrong way round.

[Edited on 3-12-11 by unionised]

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-] / [CO32-] ratio making it more prone to precipitating the hydroxide or a mixed hydroxide/carbonate. Where am I going wrong?

unionised - 4-12-2011 at 11:26

If there were hydroxide present in a bicarbonate solution it would react.
OH- + HCO3- ----> H2O + CO3 --

That's a straightforward acid/ base reaction.

blogfast25 - 4-12-2011 at 11:37

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.

Arthur Dent - 5-12-2011 at 04:42

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

blogfast25 - 5-12-2011 at 07:43

Hmmm… I’m now not so convinced what you’ll obtain though, with bicarbonate OR with carbonate. The solubility product Ksp = 3 x 10-17 = [Zn2+] x [OH-]2 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.

Arthur Dent - 5-12-2011 at 12:14

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

blogfast25 - 5-12-2011 at 12:50

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.

blogfast25 - 6-12-2011 at 13:36

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]

Arthur Dent - 7-12-2011 at 05:00

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

blogfast25 - 7-12-2011 at 06:07

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]