chornedsnorkack
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Which alums exist?
Well, which do?
An alum requires:
A monovalent cation of proper size
A trivalent cation of proper size
A divalent anion compatible with the above
Known monovalent cations compatible to form some alums are:
Na+
K+
Rb+
Cs+
Tl+
NH4+
CH3NH3+
HONH3+
NH2N3+
Monovalent cations not known to form alums:
Li+ - too small
Ag+ - liable to precipitate with eligible anions?
Trivalent cations known to form alums:
Al3+
Ga3+
In3+
Ti3+
V3+
Cr3+
Mn3+
Fe3+
Co3+
Mo3+
Ru3+
Rh3+
Ir3+
Trivalent cations not known to form alums:
Tl3+
Bi3+
Sc3+
Rare earths
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:
(SO4)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?
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DraconicAcid
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I'm pretty sure I've read about chromate-doped alums, if not pure ones.
Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
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Bezaleel
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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.
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soreff
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I'd though that the divalent anion could also be (PO3F)2- but I haven't been able to find a reference :-(
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soreff
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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"
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Diachrynic
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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
we apologize for the inconvenience
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Bezaleel
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That's interesting. However, H2PO3F is not a substance I feel comfortable playing with.
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.
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Bedlasky
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Tetralkylammonium is bigger, isn't it?
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Bezaleel
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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!
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