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 donĀ“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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