Tutton's salt

Tutton's salts are a family of salts with the formula M₂M'(SO₄)₂(H₂O)₆ (sulfates) or M₂M'(SeO₄)₂(H₂O)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.^[1] The univalent cation can be potassium, rubidium, caesium, ammonium (NH₄), deuterated ammonium (ND₄) 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 (CrO₄²⁻), tetrafluoroberyllate (BeF₄²⁻), hydrogenphosphate (HPO₄²⁻)^[2] or monofluorophosphate (PO₃F²⁻). Tutton's salts crystallize in the monoclinic space group P2₁/a.^[3] 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(H₂O)₆]²⁺.

Examples and related compounds

Perhaps the best-known is Mohr's salt, ferrous ammonium sulfate (NH₄)₂Fe(SO₄)₂^.(H₂O)₆).^[4] Other examples include the vanadous Tutton salt (NH₄)₂V(SO₄)₂(H₂O)₆ and the chromous Tutton salt (NH₄)₂Cr(SO₄)₂(H₂O)₆.^[5] In solids and solutions, the M'²⁺ ion exists as a metal aquo complex [M'(H₂O)₆]²⁺.

Related to the Tutton's salts are the alums, which are also double salts but with the formula MM'(SO₄)₂(H₂O)₁₂. The Tutton's salts were once termed "false alums".^[6]

• 
  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 (K₂Mg(SO₄)₂(H₂O)₆). They are named for Alfred Edwin Howard Tutton, who identified and characterised a large range of these salts around 1900.^[7]
Such salts were of historical importance because they were obtainable in high purity and served as reliable reagents and spectroscopic standards.

Table of salts

M1	M2	formula	name	a Å	b Å	c Å	β°	V Å3	colour	Nα	Nβ	Nγ	Biaxial	2V	other
K	Cd	K2[Cd(H2O)6](SO4)2	Potassium cadmium sulfate hexahydrate[8]
Cs	Cd	Cs2[Cd(H2O)6](SO4)2	caesium cadmium sulphate hexahydrate[9]
NH4	Cd	(NH4)2[Cd(H2O)6](SO4)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[10]	density=2.05[11] Slowly loses water in dry air.[12]
K	Co	K2[Co(H2O)6](SO4)2[13]	Potassium cobaltous sulfate[14]	6.151	9.061	12.207	104.8°	657.78[15]	red						density=2.21
Rb	Co	Rb2[Co(H2O)6](SO4)2	Rubidium hexaaquacobalt(II) sulphate	6.24	9.19	12.453	105.99°	686.5[12]	ruby-red[16]						desnsity=2.56
Cs	Co	Cs2[Co(H2O)6](SO4)2	Caesium hexaaquacobalt(II) sulphate	9.318(1)	12.826(3)	6.3650(9)	107.13(1)°	727.0[17]	dark red
NH4	Co	(NH4)2[Co(H2O)6](SO4)2[18]	Cobaltous ammonium sulfate hexahydrate	6.242	9.255	12.549	106.98°	693.3[19]	purple[20]						density=1.89
Tl	Co	Tl2[Co(H2O)6](SO4)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[21]
Tl	Co	Tl2[Co(H2O)6](SO4)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[22]	density=4.180 g/cm3
Rb	Cr	Rb2[Cr(H2O)6](SO4)2[23]	dirubidium chromium sulfate hexahydrate
Cs	Cr	Cs2[Cr(H2O)6](SO4)2[23]	dicaesium chromium sulfate hexahydrate
ND4	Cr	(ND4)2Cr(SO4)2 · 6 H2O[23]	dideuterated ammonium chromium sulfate hexahydrate						bright blue,						formed from ${\displaystyle {\ce {CrSO4.3H2O}}}$ with ammonium sulfate in minimal water under nitrogen gas. Stable in air from oxidation, but may dehydrate.[24]
K	Cu	K2[Cu(H2O)6](SO4)2	cyanochroite[14]	9.27	12.44	6.30	104.47[25]	663.0[25]	pale green blue						density=2.21[25] within unit cell 7.76 between two Cu atoms[26]
Rb	Cu	Rb2[Cu(H2O)6](SO4)2	Dirubidium hexaaquacopper sulfate	9.267	12.366	6.228	105°19'	686.8	brilliant greenish blue	1.488	1.491	1.506	biaxial (+)[27]	medium	density=2.580g/cm3[10] Cu-O 2.098 Å Rb-O 3.055 Å.[27]
Cs	Cu	Cs2[Cu(H2O)6](SO4)2[28]	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[29]
NH4	Cu	(NH4)2[Cu(H2O)6](SO4)2	ammonium hexaaquacopper(II) sulfate[30]	6.31	12.38	9.22	106.16°	691.25[31]							density=1.921;[31] heat of formation=-777.9 kcal/mol[31] 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%.[30]
Tl	Cu	Tl2[Cu(H2O)6](SO4)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[32]	density=3.740 g/cm3
K	Fe	K2[Fe(H2O)6](SO4)2	dipotassium iron sulfate hexahydrate[14]
Rb	Fe	Rb2[Fe(H2O)6](SO4)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[33]
Cs	Fe	Cs2[Fe(H2O)6](SO4)2	Caesium hexaaquairon(II) sulphate	9.357(2)	12.886(2)	6.381(1)	106.94(1)°	736.0	dark yellow[17] very pale green	1.501	1.504	1.516	biaxial (+)	medium[34]	density=2.805
NH4	Fe	(NH4)2[Fe(H2O)6](SO4)2	mohrite[14]	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[35]
Tl	Fe	Tl2[Fe(H2O)6](SO4)2	Thallium hexaaquairon(II) sulfate	9.262(2)	12.497(1)	6.235(2)	106.15(1)°	693.2[21]	light green	1.590	1.605	=1.616	biaxial (-)	large	density=3.662g/cm3[36]
K	Mg	K2[Mg(H2O)6](SO4)2	picromerite	9.04	12.24	6.095	104° 48'[14]		colourless or white	1.460	1.462	1.472	biaxial (+)	medium	density=2.025g/cm3;[37] expanded second coordination sphere around Mg.[14]
Rb	Mg	Rb2[Mg(H2O)6](SO4)2	rubidium magnesium sulphate hexahydrate[38]	9.235	12.486	6.224	105°59'		colourless	1.467	1.469	1.476[39]	biaxial
Cs	Mg	Cs2[Mg(H2O)6](SO4)2	Caesium hexaaquamagnesium sulphate	9.338(2)	12.849(4)	6.361(2)	107.07(2)°	729.6	colourless[17]	1.481	1.485	1.492	biaxial(+)	medium	density=2.689[40]
NH4	Mg	(NH4)2[Mg(H2O)6](SO4)2	boussingaultite	9.28	12.57	6.2	107°6'[14][18]
NH4	Mg	(NH4)2[Mg(H2O)6](SO4)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[10]
Tl	Mg	Tl2[Mg(H2O)6](SO4)2[41]	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[21]						density=3.532 g/cm3
Rb	Mn	Rb2[Mn(H2O)6](SO4)2	Dirubidium hexaaquamanganese sulfate(VI)	9.282(2)	12.600(2)	6.254(2)	105.94(2)	703.3Å3[42][43]
Cs	Mn	Cs2[Mn(H2O)6](SO4)2	Caesium hexaaquamanganese(II) sulphate	9.418(3)	12.963(2)	6.386(3)	107.17(4)°	744.9	pale pink[17] purplish white[44]	1.495	1.497	1.502	biaxial(+)	large	density=2.763[44]
NH4	Mn	(NH4)2[Mn(H2O)6](SO4)2	manganese ammonium sulfate hexahydrate	9.40	12.74	6.26	107.0°[45]		pale pink	1.482	1.456	1.492	biaxial(+)	large	density=1.827 [46]
Tl	Mn	Tl2[Mn(H2O)6](SO4)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)°[47] 106.29(2)°,	700.8[21]	light pink
K	Ni	K2Ni(SO4)2 · 6 H2O[13]	Potassium Nickel Sulfate Hexahydrate[14] used as UV filter[48]
Rb	Ni	Rb2[Ni(H2O)6](SO4)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 nm3 density 2.596 g cm−3.[48] stable to 100.5 °C solubility in g/100ml=0.178t + 4.735 MW=529.87
Cs	Ni	Cs2[Ni(H2O)6](SO4)2	Caesium hexaaquanickel(II) sulphate, Caesium Nickel Sulfate Hexahydrate	9.259(2)	12.767(2)	6.358(1)	107.00(2)°	718.7[17]	greenish blue	1.507	1.512	1.516	biaxial(-)	very large	density=2.883 [49] used as UV filter[48]
NH4	Ni	(NH4)2[Ni(H2O)6](SO4)2	nickel-boussingaultite[14][50]	9.186	12.468	6.424		684.0	blueish green.[51][52]						density=1.918 cas=51287-85-5
Tl	Ni	Tl2[Ni(H2O)6](SO4)2	Thallium hexaaquanickel(II) sulfate	9.161(2)	12.389(2)	6.210(2)	106.35(2)°	676.3	greenish blue[21]	1.602	1.615	1.620	biaxial(-)	large	density=3.763[53]
K	Ru	K2[Ru(H2O)6](SO4)2	[54]	8.950	12.268	6.135	105.27	644
Rb	Ru	Rb2[Ru(H2O)6](SO4)2	[54]	9.132	12.527	6.351	106.30
K	V	K2[V(H2O)6](SO4)2	Vanadium(II) potassium sulfate hexahydrate [55]
Rb	V	Rb2[V(H2O)6](SO4)2	Rubidium vanadium(II) sulfate
NH4	V	(NH4)2[V(H2O)6](SO4)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[56]
K	Zn	K2[Zn(H2O)6](SO4)2[13][14]	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[57] Thermal decomposition at 252K.[58]
Rb	Zn	Rb2[Zn(H2O)6](SO4)2	Rubidium Zinc Sulphate Hexahydrate[59]	9.185	12.450	6.242	105°54'		colourless	1.483	1.489	1.497	biaxial	large [60]
Cs	Zn	Cs2[Zn(H2O)6](SO4)2	zinc caesium sulphate hexahydrate [61]	9.314(2)	12.817(2)	6.369(2)	106.94(2)°	727.3	colourless[17]	1.507	1.610	1.615	biaxial(-)	large	density=2.881 [62]
NH4	Zn	(NH4)2[Zn(H2O)6](SO4)2		9.205	12.475	6.225	106°52'[18]	684.1							heat of fusion 285 J/g[63]
Tl	Zn	Tl2[Zn(H2O)6](SO4)2	Thallium hexaaquazinc(II) sulfate [64]	9.219(2)	12.426(2)	6.226(1)	106.29(2)°	684.6	colourless[21]
selenates
Cs	Ni	Cs2[Zn(H2O)6](SeO4)2	Dicaesium nickel selenate hexahydrate[65]	7.4674	7.9152	11.7972	106.363	669.04	light green
Rb	Cu	Rb2[Cu(H2O)6](SeO4)2	Dirubidium copper selenate hexahydrate[66]	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 Å3	colour	Nα	Nβ	Nγ	Biaxial	2V	other
(C4H12N2)[Zn(H2O)6](SO4)2	piperazinediium hexaaquazinc(II) bis(sulfate)[67]	12.9562	10.6502	13.3251	114.032	1679.30	Colourless
	cadmium creatininium sulfate[68]	6.5584	27.871	7.1955	110.371	1232.99	colourless