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Author: Subject: Any way to prepare anhydrous ZnCl2 without using gaseous Cl2 or HCl?
S.C. Wack
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[*] posted on 14-6-2019 at 13:45


Distillation of the dehydrated hydrate would work, for a pure and anhydrous product. BTW Inorganic Chemical Preparations says any basic chloride produced by the dehydration can be converted back by adding conc. HCl. While stirring at maybe 240C. Thorp advises caution.



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maldi-tof
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[*] posted on 16-6-2019 at 13:33


There is a patent (if you search it you will find it easily), where NH3 (g) is bubbled through a solution of ZnCl2. The precipitate is separated from the solution (it being Zn(NH3)2Cl2). The solid is then heated in order to obtain ZnCl2 (and liberating NH3 as a sub-product, which can be re-used in the previous phase).

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AJKOER
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[*] posted on 17-6-2019 at 14:48


Perhaps one does even need a starting aqueous ZnCl2 just Zinc metal, CaCl2, NH3 (aq) and H2O2,. Albeit, I suspect it is likely hard to produce large quantities this way unless one uses NH3/CO2 to dissolve metals as perform in commercial leaching applications.

I suggest reviewing the following path based on my experience of working with Cu, dilute aqueous ammonia, 3% H2O2 (or possibly pumping in air, which I have not tried) and an electrolyte (usually sea salt or ordinary NaCl). I usually jump start this electrochemical reaction in a microwave (which removes the otherwise obvious inception period before the reaction commences). The product is [Cu(NH3)x(H2O)6-x](OH)2 with x varying depending on the concentration of ammonia. I successfully added MgY2 to this salt (which only exists in aqueous solutions) to form salts of the form [Cu(NH3)x(H2O)6-x]Y2 with a precipitate of Mg(OH)2, where Y is an anion of choice.

With Zinc, the path may again be dissolving Zn in dilute aqueous ammonia, 3% H2O2 and an electrolyte (sufficient aqueous NH4Cl from treating aqueous CaCl2 with NH3(aq), see equation below). Again jump start the electrochemical reaction in a microwave. The expected equilibrium product is assuming a simple substitution of Cu with Zn (? see https://www.physicsforums.com/threads/exact-formula-of-tetra... and also this 2010 research work reported at https://www.researchgate.net/publication/44669385_The_stabil... ):

[Zn(NH3)4(H2O)2](OH)2 + 2 NH4Cl = Zn(NH3)4(H2O)2]Cl2+ 2 NH3 + 2 H2O

Adjusting the pH to between 6 and 8 by adding ammonia gas may result in a precipitate of [Zn(NH3)4(H2O)2]Cl2, or not. Perhaps better is add CaCl2 producing a precipitate of Ca(OH)2, removing the Ca(OH)2 and then try the pH adjustment between 6 and 8.

A search of the internet does assert that my tentative rendition of a tetraammine aqua complex for Zinc is likely NOT accurate, that is, the salt is not a hydrate, reputedly like [Zn(NH3)4]Cl2. So, this is how heating this compound avoids the usual problems of working with hydrates to water free salts, and may NOT be a path workable for associated Copper salts.
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[Edit] Did come across this reference citing a low hydrate for a related salt: [Zn(NH3)4](NO3)2 . 1/5 H2O

Link: http://ddbonline.ddbst.com/DDBSearch/onlineddboverview.exe?s...

Even if this is correct for the related chloride, this may not be an issue on obtaining the dry ZnCl2, as with mild heating the water is probably loss first, before the underlying tetrammine complex is decomposed.

[Edited on 18-6-2019 by AJKOER]
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