Manganic and Cobaltic Alums(II): Experiment

 Welcome back! Now, let's first dissolve some CoSO4 or MnSO4 into 5M sulfuric acid, together with ammonium alum. To do so, add 0.02 mol of ammonium sulfate and aluminium sulfate to 100mL of acid, then 0.02 mol of the transition metal one. Now heat to dissolve these all, then cool down for a night to let excess alum crystallize. They are just alum of course.

From now on different strategies are needed for them.

Mn: Mn3+ is stable when H+ is very concentrated and Mn2+ is much more concentrated than Mn3+, so we should electrolysis without partitioning, and only a little part of Mn2+ is oxidized. Finally solution looks like this:

Finely red. Cool this solution down to 0 degree celcius or so gives orange crystals, and throwing alum into initial solution gives it a cover. All crystals MUST be dried carefully!

Solution can be used again of course. It seems that crystals are unstable when [Mn3+] is too high. This is hard to control. Reduction is simple though, just carefully add oxalic acid.

Co: Co3+ is unstable, but as said before its decomposition is slow. It lasts a few hours. Do as Mn but never try to keep the result.

Straw color. Cooling gives cyan to blue crystals. Both kinds are stable when kept at dark place and dried before sealing.

TODO: Any way to make big ones?