Manganic and Cobaltic Alums(I): Theory

About the ions

Although +3 oxidation state exists for all transition metals between Sc and Co, they are almost always unfriendly. While the common Cr3+ is stable and Fe3+ is mildly oxidizing and highly acidic, the closely related Mn3+ and Co3+ are exceedingly unstable at normal ranges.

Instead of directly telling you the phenomena I'd like to show thermodynamic data first.

• E0(Mn3+/Mn2+)=+1.49V
• E0(MnO2/Mn3+)=+0.95V
• E0(Co3+/Co2+)=+1.92V
• E0(O2/H2O)=+1.229V

The first two lines may need an explanation: if we make a battery of the first minus the second, it turns out that Mn3+ can release much electricity(energy) when it becomes Mn2+ and MnO2, as the net potential is highly positive, about +0.5V. That is to say, Mn3+ is very prone to disproportionation.

The third line could just be called horrible: Co3+ is much higher than O2, so oxidizing water is quite easy!

In fact this is just at pH=0 or [H+]=1mol/L, and things become much worse at neutral pH as H+ lies on the right side of the equations:

2Mn3+ + 2H2O = Mn2+ + MnO2 + 4H+

4Co3+ + 2H2O = 4Co2+ + O2 + 4H+

Now let me tell you what happens if you try to produce them at pH~0: Mn3+ immediately becomes brown stuff and Co3+ can't even be seen.

On the other hand, if we can move pH to the negative region, allow ions on the right side to be very concentrated, we might be able to make and save them for enough time for experiments.

The colors are very deep so you can see them immediately when they are produced:

Mn2+=very pale pink

Mn3+=deep purple

Mixed Mn2+/Mn3+="wine red"

Co2+=red

Co3+=dark blue

Mixed Co2+/Co3+=brown

When acidity is extremely high, it can be shown that Mn3+ becomes quite thermodynamically stable, while Co3+ can still destroy water. Now let me tell you a secret: unlike any other homoleptic aquo complexes of first row transition metals, but similar to later rows, Co3+ is low-spin. This is surely due to its d6 configuration and its high charge. As a result, reducing it to high-spin Co2+ is quite complex and kinetically slow. According to some paper it is shown that decomposition of Co3+ is strongly suppressed by H+.

Ionic radii

As you can imagine, putting a ball into a hole smaller than it is very painful. Indeed, this is exactly what we are doing when we make alums of large trivalent cations and mixed alums. Usually, larger monovalent cations(Cs+, Rb+) produce more stable alums, while larger trivalent cations(Sc3+, Tl3+) produce less stable or no alums. CsMn and CsCo alums are stable, but the first one still decomposes rapidly upon warming and Cs is expensive. NH4Mn and NH4Co alums are very unstable or non-existent at room temperature. How about confining them in normal alums? You know, decomposition of these alums generates water as alums contain much water which decomposes more, while mixed crystals are largely immune to this.

You can check wiki and get some ionic radii data(Co3+ is LOW spin!), and you can see that Co3+ is nearly the same size as Al3+, the perfectly compatible Cr3+ is larger, and Mn3+ is still larger, the same size as Fe3+, you know the famous 1240g/L solubility ferric alum!

Later you will see how this affects us.