Tutton's salts are a family of salts with the
formula M
2M'(SO
4)
2(H
2O)
6 (sulfates) or M
2M'(SeO
4)
2(H
2O)
6 (selenates). These materials are
double salts
A double salt is a salt that contains two or more different cations or anions. Examples of double salts include alums (with the general formula ) and Tutton's salts (with the general formula ). Other examples include potassium sodium tartrate, am ...
, 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 (BeF
42−),
hydrogenphosphate (HPO
42−) or
monofluorophosphate (PO
3F
2−). Tutton's salts crystallize in the
monoclinic
In crystallography, the monoclinic crystal system is one of the seven crystal systems. A crystal system is described by three vectors. In the monoclinic system, the crystal is described by vectors of unequal lengths, as in the orthorhombic s ...
space group
In mathematics, physics and chemistry, a space group is the symmetry group of an object in space, usually in three dimensions. The elements of a space group (its symmetry operations) are the rigid transformations of an object that leave it uncha ...
''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
Ammonium iron(II) sulfate, or Mohr's salt, is the inorganic compound with the formula (NH4)2Fe(SO4)2(H2O)6. Containing two different cations, Fe2+ and NH4+, it is classified as a double salt of ferrous sulfate and ammonium sulfate. It is a commo ...
, 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
In chemistry, metal aquo complexes are coordination compounds containing metal ions with only water as a ligand. These complexes are the predominant species in aqueous solutions of many metal salts, such as metal nitrates, sulfates, and perchlorat ...
'(H2O)6sup>2+.
Related to the Tutton's salts are the
alums, 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 bonding 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
*
Sulfates