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?Corrosive Joeseph - 30-12-2020 at 22:03
Quote:
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 sffap - 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?Bedlasky - 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).Corrosive Joeseph - 30-12-2020 at 23:14
