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Author: Subject: How do Zn hydroxides behave in MeOH
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[*] posted on 30-12-2020 at 20:57
How do Zn hydroxides behave in MeOH


I had just finished a dissolving metal reduction with Zn in HCl and MeOH. Next, I had to basify the reaction mixture and did so using KOH saturated MeOH. Using pH papers I observed the mixture go from acidic, to neutral, to basic, and then back to neutral! That last part surprised me.

My understanding is that all of the HCl would have been neutralised by the KOH, and then the solution would have become basic by further addition of KOH. Is the reacidification of the solution by the addition of KOH explained by some of the Zn(OH)2 behaving like an acid in an increasingly alkaline system?

Can anyone explain the chemistry, or point to relevant pieces of literature to help explain what happened?
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Corrosive Joeseph
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[*] posted on 30-12-2020 at 22:03


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After adding NaOH or KOH solution (and Zn(OH)2 precipitation occured) you can add ammonium hydroxide solution (aq. conc.) or NH4Cl to redissolve the precipitate and form well soluble and colorless zinc – ammonia aqua complex [Zn(NH3)4(H2O)2]2+.





/CJ




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[*] posted on 30-12-2020 at 22:26


Fantastic. That oughta take care of this sludge of MeOH/Zn(OH)2

CJ - do you have a reference/link for that extract?
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[*] posted on 30-12-2020 at 23:04


If zinc precipitates all hydroxide, it's logical that there isn't any leftover hydroxide, so solution is neutral. If you add excess of hydroxide, solution become basic (and zinc hydroxide will dissolve to form soluble [Zn(OH)4]- complex).



If you are interested in aqueous inorganic chemistry look at https://colourchem.wordpress.com/main-page/

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[*] posted on 30-12-2020 at 23:14


https://hyperlab.info/inv/index.php?s=&act=ST&f=17&a...

The forum is in Russian, you will need to use Chrome browser with Google Translate browser extension to view it.



/CJ




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[*] posted on 31-12-2020 at 04:30


pH paper is not designed to work in methanol.

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[*] posted on 4-2-2021 at 19:20


right you are, unionised! Damn, how embarassing. Thanks for the heads up.

I think I'll take an aliquot and basify that to see if my problem is solved that way.

Regardless of which route I take I think I've got some titrating to do..
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