Sciencemadness Discussion Board

Which alums exist?

chornedsnorkack - 27-12-2020 at 10:02

Well, which do?
An alum requires:
  1. A monovalent cation of proper size
  2. A trivalent cation of proper size
  3. A divalent anion compatible with the above

Known monovalent cations compatible to form some alums are:
  1. Na+
  2. K+
  3. Rb+
  4. Cs+
  5. Tl+
  6. NH4+
  7. CH3NH3+
  8. HONH3+
  9. NH2N3+

Monovalent cations not known to form alums:
  1. Li+ - too small
  2. Ag+ - liable to precipitate with eligible anions?

Trivalent cations known to form alums:
  1. Al3+
  2. Ga3+
  3. In3+
  4. Ti3+
  5. V3+
  6. Cr3+
  7. Mn3+
  8. Fe3+
  9. Co3+
  10. Mo3+
  11. Ru3+
  12. Rh3+
  13. Ir3+

Trivalent cations not known to form alums:
  1. Tl3+
  2. Bi3+
  3. Sc3+
  4. Rare earths
  5. Au3+

While rare earths are explained to be too big, is absence of scandium alums actually confirmed?
Divalent anions that are known to form alums:
  1. (SO4)2-
  2. (SeO4)2-

Note that since all alums are acids due to the trivalent cations, other similar anions like (MnO4)2- and (FeO4)2-, are not stable in conditions where alums are. Even chromate seems to turn into dichromate in the relevant pH.

So, which of the combinations don´t work? Any more cations or anions suitable for alums?

DraconicAcid - 27-12-2020 at 10:24

I'm pretty sure I've read about chromate-doped alums, if not pure ones.

Bezaleel - 27-12-2020 at 15:01

K2SO4.Pr2(SO4)3.2H2O and its ammonium congener actually exists, but it is not usually considered an alum, because of its structure and because it has only 1 H2O per KPr(SO4)2. In contrast to alums, it's neither very soluble.

soreff - 29-12-2020 at 19:57

I'd though that the divalent anion could also be (PO3F)2- but I haven't been able to find a reference :-(

soreff - 30-12-2020 at 12:43

Found: https://www.nature.com/articles/126916b0
which, amongst other things, states:
"monofluorphosphoric acid H2PO3F resembles sulphuric acid H2SO4 so closely that it even gives alums, which are isomorphous with ordinary sulphate alums"

Diachrynic - 3-1-2021 at 08:13

Guanidinium is another known cation for alumn, and the water of crystallization they all form with can be replaced by D2O. This german paper gives a good overview over their crystallography: https://doi.org/10.1524/zkri.1961.116.16.371

Bezaleel - 4-1-2021 at 07:30

Quote: Originally posted by soreff  
Found: https://www.nature.com/articles/126916b0
which, amongst other things, states:
"monofluorphosphoric acid H2PO3F resembles sulphuric acid H2SO4 so closely that it even gives alums, which are isomorphous with ordinary sulphate alums"
That's interesting. However, H2PO3F is not a substance I feel comfortable playing with.
Quote: Originally posted by Diachrynic  
Guanidinium is another known cation for alumn, and the water of crystallization they all form with can be replaced by D2O. This german paper gives a good overview over their crystallography: https://doi.org/10.1524/zkri.1961.116.16.371
Tetramethyl- and tetrabutylammonium will do as well, I guess. What would really interest me is a water soluble cation which is bigger than caesium, since the caesium alums are most easily formed.

Bedlasky - 4-1-2021 at 08:07

Tetralkylammonium is bigger, isn't it?

Bezaleel - 5-1-2021 at 15:55

Quote: Originally posted by Bedlasky  
Tetralkylammonium is bigger, isn't it?
From Wiki:
The ionic radius for TMA is given as 0.322 nm (doi:10.1021/ja00418a002)
Cs+ has an ionic radius of 174 pm (Wells, A. F. (1984). Structural Inorganic Chemistry (5th ed.). Oxford Science Publications. ISBN 978-0-19-855370-0)

So I misread and you're right. Thanks!