Tutton's salts are a family of salts with the
formula
In science, a formula is a concise way of expressing information symbolically, as in a mathematical formula or a ''chemical formula''. The informal use of the term ''formula'' in science refers to the general construct of a relationship betwee ...
M
2M'(SO
4)
2(H
2O)
6 (sulfates) or M
2M'(SeO
4)
2(H
2O)
6 (selenates). These materials are
double salts, which means that they contain two different cations, M
+ and M'
2+ crystallized in the same regular ionic lattice.
The univalent cation can be potassium, rubidium, cesium, ammonium (NH
4), deuterated ammonium (ND
4) or thallium. Sodium or lithium ions are too small. The divalent cation can be magnesium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc or cadmium. In addition to sulfate and selenate, the divalent anion can be
chromate (CrO
42−),
tetrafluoroberyllate
Tetrafluoroberyllate or orthofluoroberyllate is an anion containing beryllium and fluorine. The fluoroanion has a tetrahedral shape, with the four fluorine atoms surrounding a central beryllium atom. It has the same size and outer electron struct ...
(BeF
42−),
hydrogenphosphate (HPO
42−) or
monofluorophosphate (PO
3F
2−). Tutton's salts crystallize in the
monoclinic space group ''P''2
1/''a''. The robustness is the result of the complementary hydrogen-bonding between the tetrahedral anions and cations as well their interactions with the metal aquo complex
(H2O)6sup>2+.
Examples and related compounds
Perhaps the best-known is
Mohr's salt, ferrous ammonium sulfate (NH
4)
2Fe(SO
4)
2.(H
2O)
6). Other examples include the vanadous Tutton salt (NH
4)
2V(SO
4)
2(H
2O)
6 and the chromous Tutton salt (NH
4)
2Cr(SO
4)
2(H
2O)
6. In solids and solutions, the M'
2+ ion exists as a
metal aquo complex '(H2O)6sup>2+.
Related to the Tutton's salts are the
alum
An alum () is a type of chemical compound, usually a hydrated double salt, double sulfate salt (chemistry), salt of aluminium with the general chemical formula, formula , where is a valence (chemistry), monovalent cation such as potassium or a ...
s, which are also double salts but with the formula MM'(SO
4)
2(H
2O)
12. The Tutton's salts were once termed "false alums".
File:MohriteD.jpg, Unit cell of ferrous ammonium sulfate (N is violet, O is red, S is orange, Fe is large red).
File:Mohrite-DwH-bonds.jpg, The same structure with hydrogen bond
In chemistry, a hydrogen bond (or H-bond) is a primarily electrostatic force of attraction between a hydrogen (H) atom which is covalently bound to a more electronegative "donor" atom or group (Dn), and another electronegative atom bearing a ...
ing network highlighted.
History
Tutton salts are sometimes called Schönites after the naturally occurring mineral called
Schönite (K
2Mg(SO
4)
2(H
2O)
6). They are named for
Alfred Edwin Howard Tutton, who identified and characterised a large range of these salts around 1900.
Such salts were of historical importance because they were obtainable in high purity and served as reliable reagents and spectroscopic standards.
Table of salts
Organic salts
Some organic bases can also form salts that crystallise like Tutton's salts.
References
{{Reflist, 30em
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Sulfates