Bezaleel - 27-10-2020 at 16:03
Hi, I made some NiCl2.xH2O by dissolving grey-black nickel oxide in dilute HCl solution and refluxing. At one point, no more oxide dissolved and the
pH was then between 6 and 7. I added half a ml of HCl, refluxed again and the remaining oxide dissolved. This solution was left to stand and it dried
up over time. A light green crystalline substance had formed. The crystals did not seem very transparent and I wondered whether the crystals contained
Ni(OH)2.
I dissolved some of the assumed nickel chloride in a test tube with water and a bit of precipitate settled in the course of a few hours. By
comparison, I dissolved a crystal of NiCl2.6H2O which I had produced earlier and it did give a clear solution with no precipitate.
This all seems to point to hydrolysis of the NiCl2.xH2O which I intended to make. I cannot find any mention of NiCl2.6H2O being subject to hydrolysis
though, or that it should be crystallised from an acid solution.
Is NiCl2 prone to hydrolysis, or does anyone have a similar experience making NiCl2.6H2O?
Bedlasky - 27-10-2020 at 19:57
Every metal hexaaqua complex hydrolyse in some extent. I never got a precipitate in nickel sulfate solution. Maybe is precipitate rest of the oxide.
Try add small amount of HCl in to your solution. It redisolves precipitate and prevents hydrolysis. I added some acid in to my stock solutions of
NiSO4, CoCl2 and CuSO4.
[Edited on 28-10-2020 by Bedlasky]
Bezaleel - 28-10-2020 at 15:37
Thanks, I will do so. I've neither seen it with NiSO4 solution (although in CuSO4 solution I have, when it stands for a longer period of time).
The solution was completely clear when it was left to crystallise. The precipitate is green, not black, so I don't think it's undissolved nickel
oxide, since I used the black oxide as a starting material.
I now added a drop of 36% HCl solution to the test tube with the precipitate and after heating the solution became completely clear and a bit more
yellowish in colour.
My conclusion: have your solution somewhat acidic when cystallising NiCl2 hydrates from it.