# Tutton's salt

Tutton's salts are a family of salts with the formula M$_2$M'(SO$_4$)$_2$(H$_2$O)$_6$ (sulfates) or M$_2$M'(SeO$_4$)$_2$(H$_2$O)$_6$ (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, caesium, 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$_4$$^{2−}$), tetrafluoroberyllate (BeF$_4$$^{2−}$), hydrogenphosphate (HPO$_4$$^{2−}$) or monofluorophosphate (PO$_3$F$^{2−}$). Tutton's salts crystallize in the monoclinic space group P2$_1$/a. The robustness is the result of the complementary hydrogen-bonding between the tetrahedral anions and cations as well their interactions with the metal aquo complex [M(H$_2$O)$_6$]$^{2+}$.

## Examples and related compounds

Perhaps the best-known is Mohr's salt, ferrous ammonium sulfate (NH$_4$)$_2$Fe(SO$_4$)$_2$$^.$(H$_2$O)$_6$). Other examples include the vanadous Tutton salt (NH$_4$)$_2$V(SO$_4$)$_2$(H$_2$O)$_6$ and the chromous Tutton salt (NH$_4$)$_2$Cr(SO$_4$)$_2$(H$_2$O)$_6$. In solids and solutions, the M'$^{2+}$ ion exists as a metal aquo complex [M'(H$_2$O)$_6$]$^{2+}$.
Related to the Tutton's salts are the alums, which are also double salts but with the formula MM'(SO$_4$)$_2$(H$_2$O)$_{12}$. The Tutton's salts were once termed "false alums".
* Unit cell of ferrous ammonium sulfate (N is violet, O is red, S is orange, Fe is large red).
* The same structure with hydrogen bonding network highlighted.

## History

Tutton salts are sometimes called Schönites after the naturally occurring mineral called Schönite (K$_2$Mg(SO$_4$)$_2$(H$_2$O)$_6$). 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

| M$^1$ | M$^2$ | formula | name | a Å | b Å | c Å | β° | V Å$^3$ | colour | Nα | Nβ | Nγ | Biaxial | 2V | other |
| K | Cd | K$_2$[Cd(H$_2$O)$_6$](SO$_4$)$_2$ | Potassium cadmium sulfate hexahydrate | | | | | | | | | | | | |
| Cs | Cd | Cs$_2$[Cd(H$_2$O)$_6$](SO$_4$)$_2$ | caesium cadmium sulphate hexahydrate | | | | | | | | | | | | |
| NH$_4$ | Cd | (NH$_4$)$_2$[Cd(H$_2$O)$_6$](SO$_4$)$_2$ | Ammonium Cadmium Sulfate Hydrate | 9.395 | 12.776 | 6.299 | 106°43' | 727.63 | colourless | l.486 | 1.488 | 1.494 | Biaxial(-f) | large | density=2.05 Slowly loses water in dry air. |
| K | Co | K$_2$[Co(H$_2$O)$_6$](SO$_4$)$_2$ | Potassium cobaltous sulfate | 6.151 | 9.061 | 12.207 | 104.8° | 657.78 | red | | | | | | density=2.21 |
| Rb | Co | Rb$_2$[Co(H$_2$O)$_6$](SO$_4$)$_2$ | Rubidium hexaaquacobalt(II) sulphate | 6.24 | 9.19 | 12.453 | 105.99° | 686.5 | ruby-red | | | | | | desnsity=2.56 |
| Cs | Co | Cs$_2$[Co(H$_2$O)$_6$](SO$_4$)$_2$ | Caesium hexaaquacobalt(II) sulphate | 9.318(1) | 12.826(3) | 6.3650(9) | 107.13(1)° | 727.0 | dark red | | | | | | |
| NH$_4$ | Co | (NH$_4$)$_2$[Co(H$_2$O)$_6$](SO$_4$)$_2$ | Cobaltous ammonium sulfate hexahydrate | 6.242 | 9.255 | 12.549 | 106.98° | 693.3 | purple | | | | | | density=1.89 |
| Tl | Co | Tl$_2$[Co(H$_2$O)$_6$](SO$_4$)$_2$ | Cobaltous thallium sulfate hexahydrate, Thallium hexaaquacobalt(II) sulfate, | 9.227(1) | 12.437(2) | 6.220(1) | 106.40(1)° | 684.7 | light red | | | | | | |
| Tl | Co | Tl$_2$[Co(H$_2$O)$_6$](SO$_4$)$_2$ | dithallium cobalt sulfate hexahydrate | 9.235(1) | 12.442(2) | 6.227(1) | 106.40(1)° | | yellowish pink | 1.599 | 1.613 | 1.624 | biaxial(-) | medium large | density=4.180 g/cm$^3$ |
| Rb | Cr | Rb$_2$[Cr(H$_2$O)$_6$](SO$_4$)$_2$ | dirubidium chromium sulfate hexahydrate | | | | | | | | | | | | |
| Cs | Cr | Cs$_2$[Cr(H$_2$O)$_6$](SO$_4$)$_2$ | dicaesium chromium sulfate hexahydrate | | | | | | | | | | | | |
| ND$_4$ | Cr | (ND$_4$)$_2$Cr(SO$_4$)$_2$ · 6 H$_2$O | dideuterated ammonium chromium sulfate hexahydrate | | | | | | bright blue, | | | | | | formed from ${\ce {CrSO4.3H2O}}$ with ammonium sulfate in minimal water under nitrogen gas. Stable in air from oxidation, but may dehydrate. |
| K | Cu | K$_2$[Cu(H$_2$O)$_6$](SO$_4$)$_2$ | cyanochroite | 9.27 | 12.44 | 6.30 | 104.47 | 663.0 | pale green blue | | | | | | density=2.21 within unit cell 7.76 between two Cu atoms |
| Rb | Cu | Rb$_2$[Cu(H$_2$O)$_6$](SO$_4$)$_2$ | Dirubidium hexaaquacopper sulfate | 9.267 | 12.366 | 6.228 | 105°19' | 686.8 | brilliant greenish blue | 1.488 | 1.491 | 1.506 | biaxial (+) | medium | density=2.580g/cm3 Cu-O 2.098 Å Rb-O 3.055 Å. |
| Cs | Cu | Cs$_2$[Cu(H$_2$O)$_6$](SO$_4$)$_2$ | dicaesium hexaaquacopper sulfate | 9.439 | 12.762 | 6.310 | 106°11' | 718.5 | brilliant greenish blue, | 1.504 | 1.506 | 1.514 | biaxial (+) | | density=2.864g/cm3 |
| NH$_4$ | Cu | (NH$_4$)$_2$[Cu(H$_2$O)$_6$](SO$_4$)$_2$ | ammonium hexaaquacopper(II) sulfate | 6.31 | 12.38 | 9.22 | 106.16° | 691.25 | | | | | | | density=1.921; heat of formation=-777.9 kcal/mol Jahn-Teller distortion axis switches under pressure of ~1500 bars, a,b axis shrinks 3.3% and 3.5% and c axis extends 4.5%. |
| Tl | Cu | Tl$_2$[Cu(H$_2$O)$_6$](SO$_4$)$_2$ | Thallium copper sulfate hydrate | 9.268 | 12.364 | 6.216 | 105°33' | | brilliant greenish blue | 1.600 | 1.610 | 1.620 | biaxial | very large | density=3.740 g/cm3 |
| K | Fe | K$_2$[Fe(H$_2$O)$_6$](SO$_4$)$_2$ | dipotassium iron sulfate hexahydrate | | | | | | | | | | | | |
| Rb | Fe | Rb$_2$[Fe(H$_2$O)$_6$](SO$_4$)$_2$ | Rubidium iron sulfate hydrate | 9.218 | 12.497 | 6.256 | 105°45' | | pale green | 1.480 | 1.489 | 1.501 | biaxial (+) | large, | density=2.523g/cm3 |
| Cs | Fe | Cs$_2$[Fe(H$_2$O)$_6$](SO$_4$)$_2$ | Caesium hexaaquairon(II) sulphate | 9.357(2) | 12.886(2) | 6.381(1) | 106.94(1)° | 736.0 | dark yellow very pale green | 1.501 | 1.504 | 1.516 | biaxial (+) | medium | density=2.805 |
| NH$_4$ | Fe | (NH$_4$)$_2$[Fe(H$_2$O)$_6$](SO$_4$)$_2$ | mohrite | 6.24(1) | 12.65(2) | 9.32(2) | 106.8(1) | 704.28 | vitreous pale green | | | | | | density=1.85 named after Karl Friedrich Mohr |
| Tl | Fe | Tl$_2$[Fe(H$_2$O)$_6$](SO$_4$)$_2$ | Thallium hexaaquairon(II) sulfate | 9.262(2) | 12.497(1) | 6.235(2) | 106.15(1)° | 693.2 | light green | 1.590 | 1.605 | =1.616 | biaxial (-) | large | density=3.662g/cm3 |
| K | Mg | K$_2$[Mg(H$_2$O)$_6$](SO$_4$)$_2$ | picromerite | 9.04 | 12.24 | 6.095 | 104° 48' | | colourless or white | 1.460 | 1.462 | 1.472 | biaxial (+) | medium | density=2.025g/cm3; expanded second coordination sphere around Mg. |
| Rb | Mg | Rb$_2$[Mg(H$_2$O)$_6$](SO$_4$)$_2$ | rubidium magnesium sulphate hexahydrate | 9.235 | 12.486 | 6.224 | 105°59' | | colourless | 1.467 | 1.469 | 1.476 | biaxial | | |
| Cs | Mg | Cs$_2$[Mg(H$_2$O)$_6$](SO4)$_2$ | Caesium hexaaquamagnesium sulphate | 9.338(2) | 12.849(4) | 6.361(2) | 107.07(2)° | 729.6 | colourless | 1.481 | 1.485 | 1.492 | biaxial(+) | medium | density=2.689 |
| NH$_4$ | Mg | (NH$_4$)$_2$[Mg(H$_2$O)$_6$](SO$_4$)$_2$ | boussingaultite | 9.28 | 12.57 | 6.2 | 107°6' | | | | | | | | |
| NH$_4$ | Mg | (NH$_4$)$_2$[Mg(H$_2$O)$_6$](SO$_4$)$_2$ | Ammonium Magnesium Chromium Oxide Hydrate | 9.508±.001 | 12.674 | 6.246 | 106°14' | | bright yellow | 1.637 | 1.638 | 1.653 | biaxial(+) | small | density=1.840 g/cm $^3$ |
| Tl | Mg | Tl$_2$[Mg(H$_2$O)$_6$](SO$_4$)$_2$ | dithallium magnesium sulfate hexahydrate | 9.22 9.262(2) | 12.42 12.459(2) | 6.185 6.207(1) | 106°30' 106.39(2)° | 687.1 | colourless | | | | | | density=3.532 g/cm$^3$ |
| Rb | Mn | Rb$_2$[Mn(H$_2$O)$_6$](SO$_4$)$_2$ | Dirubidium hexaaquamanganese sulfate(VI) | 9.282(2) | 12.600(2) | 6.254(2) | 105.94(2) | 703.3Å$^3$ | | | | | | | |
| Cs | Mn | Cs$_2$[Mn(H$_2$O)$_6$](SO$_4$)$_2$ | Caesium hexaaquamanganese(II) sulphate | 9.418(3) | 12.963(2) | 6.386(3) | 107.17(4)° | 744.9 | pale pink purplish white | 1.495 | 1.497 | 1.502 | biaxial(+) | large | density=2.763 |
| NH$_4$ | Mn | (NH$_4$)$_2$[Mn(H$_2$O)$_6$](SO$_4$)$_2$ | manganese ammonium sulfate hexahydrate | 9.40 | 12.74 | 6.26 | 107.0° | | pale pink | 1.482 | 1.456 | 1.492 | biaxial(+) | large | density=1.827 |
| Tl | Mn | Tl$_2$[Mn(H$_2$O)$_6$](SO$_4$)$_2$ | Thallium manganese sulfate hexahydrate | 9.3276(6), 9.322(2) | 12.5735(8), 12.565(2) | 6.2407(4), and 6.233(1) | 106.310(3)° 106.29(2)°, | 700.8 | light pink | | | | | | |
| K | Ni | K$_2$Ni(SO$_4$)$_2$ · 6 H$_2$O | Potassium Nickel Sulfate Hexahydrate used as UV filter | | | | | | | | | | | | |
| Rb | Ni | Rb$_2$[Ni(H$_2$O)$_6$](SO$_4$)$_2$ | Rubidium Nickel Sulfate Hexahydrate | 6.221 | 12.41 | 9.131 | 106.055° | 677.43 | | | | | | | 001 surface has step growth of 4.6 Å, optical transmission bands at 250, 500 and 860 nm which are the same as nickel sulfate hexahydrate, but UV band transmits more. Heavy absorption 630-720 nm and 360-420 nm$^3$ density 2.596 g cm$^{−3}$. stable to 100.5 °C solubility in g/100ml=0.178t + 4.735 MW=529.87 |
| Cs | Ni | Cs$_2$[Ni(H$_2$O)$_6$](SO$_4$)$_2$ | Caesium hexaaquanickel(II) sulphate, Caesium Nickel Sulfate Hexahydrate | 9.259(2) | 12.767(2) | 6.358(1) | 107.00(2)° | 718.7 | greenish blue | 1.507 | 1.512 | 1.516 | biaxial(-) | very large | density=2.883 used as UV filter |
| NH$_4$ | Ni | (NH$_4$)$_2$[Ni(H$_2$O)$_6$](SO$_4$)$_2$ | nickel-boussingaultite | 9.186 | 12.468 | 6.424 | | 684.0 | blueish green. | | | | | | density=1.918 cas=51287-85-5 |
| Tl | Ni | Tl$_2$[Ni(H$_2$O)$_6$](SO$_4$)$_2$ | Thallium hexaaquanickel(II) sulfate | 9.161(2) | 12.389(2) | 6.210(2) | 106.35(2)° | 676.3 | greenish blue | 1.602 | 1.615 | 1.620 | biaxial(-) | large | density=3.763 |
| K | Ru | K$_2$[Ru(H$_2$O)$_6$](SO$_4$)$_2$ | | 8.950 | 12.268 | 6.135 | 105.27 | 644 |
| Rb | Ru | Rb$_2$[Ru(H$_2$O)$_6$](SO$_4$)$_2$ | | 9.132 | 12.527 | 6.351 | 106.30 | |
| K | V | K$_2$[V(H$_2$O)$_6$](SO$_4$)$_2$ | Vanadium(II) potassium sulfate hexahydrate | | | | | | | | | | | | |
| Rb | V | Rb$_2$[V(H$_2$O)$_6$](SO$_4$)$_2$ | Rubidium vanadium(II) sulfate | | | | | | | | | | | | |
| NH$_4$ | V | (NH$_4$)$_2$[V(H$_2$O)$_6$](SO$_4$)$_2$ | Vanadium(II) ammonium sulfate hexahydrate | 9.42 | 12.76 | 6.22 | 107.2° | 714.2 | amethyst | | | | | | density=1.8 V-O length 2.15Å |
| K | Zn | K$_2$[Zn(H$_2$O)$_6$](SO$_4$)$_2$ | dipotassium zinc sulphate hexahydrate | 9.041 | 12.310 | 6.182 | 104.777° | | colourless | 1.478 | 1.481 | 1.496 | biaxial | large | density=2.242g/cm3 Thermal decomposition at 252K. |
| Rb | Zn | Rb$_2$[Zn(H$_2$O)$_6$](SO$_4$)$_2$ | Rubidium Zinc Sulphate Hexahydrate | 9.185 | 12.450 | 6.242 | 105°54' | | colourless | 1.483 | 1.489 | 1.497 | biaxial | large | |
| Cs | Zn | Cs$_2$[Zn(H$_2$O)$_6$](SO$_4$)$_2$ | zinc caesium sulphate hexahydrate | 9.314(2) | 12.817(2) | 6.369(2) | 106.94(2)° | 727.3 | colourless | 1.507 | 1.610 | 1.615 | biaxial(-) | large | density=2.881 |
| NH$_4$ | Zn | (NH$_4$)$_2$[Zn(H$_2$O)$_6$](SO$_4$)$_2$ | | 9.205 | 12.475 | 6.225 | 106°52' | 684.1 | | | | | | | heat of fusion 285 J/g |
| Tl | Zn | Tl$_2$[Zn(H$_2$O)$_6$](SO$_4$)$_2$ | Thallium hexaaquazinc(II) sulfate | 9.219(2) | 12.426(2) | 6.226(1) | 106.29(2)° | 684.6 | colourless | | | | | | |
| selenates |
| Cs | Ni | Cs$_2$[Zn(H$_2$O)$_6$](SeO$_4$)$_2$ | Dicaesium nickel selenate hexahydrate | 7.4674 | 7.9152 | 11.7972 | 106.363 | 669.04 | light green | | | | | | |
| Rb | Cu | Rb$_2$[Cu(H$_2$O)$_6$](SeO$_4$)$_2$ | Dirubidium copper selenate hexahydrate | 6.363 | 12.431 | 9.373 | 104.33 | 718.3 | | | | | | | |

## Organic salts

Some organic bases can also form salts that crystallise like Tutton's salts.
| formula | name | a Å | b Å | c Å | β° | V Å$^3$ | colour | Nα | Nβ | Nγ | Biaxial | 2V | other |
| (C$_4$H$_{12}$N$_2$)[Zn(H$_2$O)$_6$](SO$_4$)$_2$ | piperazinediium hexaaquazinc(II) bis(sulfate) | 12.9562 | 10.6502 | 13.3251 | 114.032 | 1679.30 | Colourless |
| | cadmium creatininium sulfate | 6.5584 | 27.871 | 7.1955 | 110.371 | 1232.99 | colourless |