Water of crystallization

Source: Wikipedia, the free encyclopedia.

In chemistry, water(s) of crystallization or water(s) of hydration are

cation
.

Upon crystallization from water, or water-containing solvents, many compounds incorporate water molecules in their crystalline frameworks. Water of crystallization can generally be removed by heating a sample but the crystalline properties are often lost.

Compared to inorganic salts, proteins crystallize with large amounts of water in the crystal lattice. A water content of 50% is not uncommon for proteins.

Applications

Knowledge of hydration is essential for calculating the masses for many compounds. The reactivity of many salt-like solids is sensitive to the presence of water. The hydration and dehydration of salts is central to the use of

phase-change materials for energy storage.[2]

Position in the crystal structure

Some hydrogen-bonding contacts in FeSO4·7H2O. This metal aquo complex crystallizes with water of hydration, which interacts with the sulfate and with the [Fe(H2O)6]2+ centers.

A salt with associated water of crystallization is known as a hydrate. The structure of hydrates can be quite elaborate, because of the existence of hydrogen bonds that define polymeric structures.[3] [4] Historically, the structures of many hydrates were unknown, and the dot in the formula of a hydrate was employed to specify the composition without indicating how the water is bound. Per IUPAC's recommendations, the middle dot is not surrounded by spaces when indicating a chemical adduct.[5] Examples:

  • CuSO4·5H2O – copper(II) sulfate pentahydrate
  • CoCl2·6H2O – cobalt(II) chloride hexahydrate
  • SnCl2·2H2O – tin(II) (or stannous) chloride dihydrate

For many salts, the exact bonding of the water is unimportant because the water molecules are made labile upon dissolution. For example, an aqueous solution prepared from CuSO4·5H2O and anhydrous CuSO4 behave identically. Therefore, knowledge of the degree of hydration is important only for determining the equivalent weight: one mole of CuSO4·5H2O weighs more than one mole of CuSO4. In some cases, the degree of hydration can be critical to the resulting chemical properties. For example, anhydrous RhCl3 is not soluble in water and is relatively useless in organometallic chemistry whereas RhCl3·3H2O is versatile. Similarly, hydrated AlCl3 is a poor Lewis acid and thus inactive as a catalyst for Friedel-Crafts reactions. Samples of AlCl3 must therefore be protected from atmospheric moisture to preclude the formation of hydrates.

Structure of the polymeric [Ca(H2O)6]2+ center in crystalline calcium chloride hexahydrate. Three water ligands are terminal, three bridge. Two aspects of metal aquo complexes are illustrated: the high coordination number typical for Ca2+ and the role of water as a bridging ligand.

Crystals of hydrated copper(II) sulfate consist of [Cu(H2O)4]2+ centers linked to SO2−4 ions. Copper is surrounded by six oxygen atoms, provided by two different sulfate groups and four molecules of water. A fifth water resides elsewhere in the framework but does not bind directly to copper.

Glauber's salt
, Na2SO4(H2O)10, is a white crystalline solid with greater than 50% water by weight.

Consider the case of nickel(II) chloride hexahydrate. This species has the formula NiCl2(H2O)6. Crystallographic analysis reveals that the solid consists of [trans-NiCl2(H2O)4] subunits that are hydrogen bonded to each other as well as two additional molecules of H2O. Thus one third of the water molecules in the crystal are not directly bonded to Ni2+, and these might be termed "water of crystallization".

Analysis

The water content of most compounds can be determined with a knowledge of its formula. An unknown sample can be determined through thermogravimetric analysis (TGA) where the sample is heated strongly, and the accurate weight of a sample is plotted against the temperature. The amount of water driven off is then divided by the molar mass of water to obtain the number of molecules of water bound to the salt.

Other solvents of crystallization

Water is particularly common solvent to be found in crystals because it is small and polar. But all solvents can be found in some host crystals. Water is noteworthy because it is reactive, whereas other solvents such as benzene are considered to be chemically innocuous. Occasionally more than one solvent is found in a crystal, and often the stoichiometry is variable, reflected in the crystallographic concept of "partial occupancy". It is common and conventional for a chemist to "dry" a sample with a combination of vacuum and heat "to constant weight".

For other solvents of crystallization, analysis is conveniently accomplished by dissolving the sample in a

NMR spectroscopy. Single crystal X-ray crystallography
is often able to detect the presence of these solvents of crystallization as well. Other methods may be currently available.

Table of crystallization water in some inorganic halides

In the table below are indicated the number of molecules of water per metal in various salts.[7][8]

Hydrated metal halides
and their formulas		 Coordination sphere
of the metal		 Equivalents of water of crystallization
that are not bound to M		 Remarks
Calcium chloride
CaCl2(H2O)6		 [Ca(μ-H2O)6(H2O)3]2+ none example of water as a bridging ligand[9]
Titanium(III) chloride
TiCl3(H2O)6		 trans-[TiCl2(H2O)4]+[10] two isomorphous with VCl3(H2O)6
Titanium(III) chloride
TiCl3(H2O)6		 [Ti(H2O)6]3+[10] none isomeric with [TiCl2(H2O)4]Cl.2H2O[11]
Zirconium(IV) fluoride

ZrF4(H2O)3		 (μ−F)2[ZrF3(H2O)3]2 none rare case where Hf and Zr differ[12]
Hafnium tetrafluoride
HfF4(H2O)3		 (μ−F)2[HfF2(H2O)2]n(H2O)n one rare case where Hf and Zr differ[12]
Vanadium(III) chloride
VCl3(H2O)6		 trans-[VCl2(H2O)4]+[10] two
Vanadium(III) bromide
VBr3(H2O)6		 trans-[VBr2(H2O)4]+[10] two
Vanadium(III) iodide
VI3(H2O)6		 [V(H2O)6]3+ none relative to Cl− and Br−, I− competes poorly
with water as a ligand for V(III)
Nb6Cl14(H2O)8 [Nb6Cl14(H2O)2] four
Chromium(III) chloride
CrCl3(H2O)6		 trans-[CrCl2(H2O)4]+ two dark green isomer, aka "Bjerrums's salt"
Chromium(III) chloride
CrCl3(H2O)6		 [CrCl(H2O)5]2+ one blue-green isomer
Chromium(II) chloride
CrCl2(H2O)4		 trans-[CrCl2(H2O)4] none square planar/tetragonal distortion
Chromium(III) chloride
CrCl3(H2O)6		 [Cr(H2O)6]3+ none violet isomer. isostructural with aluminium compound[13]
Aluminum trichloride

AlCl3(H2O)6		 [Al(H2O)6]3+ none isostructural with the Cr(III) compound
Manganese(II) chloride
MnCl2(H2O)6		 trans-[MnCl2(H2O)4] two
Manganese(II) chloride
MnCl2(H2O)4		 cis-[MnCl2(H2O)4] none cis molecular, the unstable trans isomer has also been detected[14]
Manganese(II) bromide
MnBr2(H2O)4		 cis-[MnBr2(H2O)4] none cis, molecular
Manganese(II) iodide
MnI2(H2O)4		 trans-[MnI2(H2O)4] none molecular, isostructural with FeCl2(H2O)4.[15]
Manganese(II) chloride
MnCl2(H2O)2		 trans-[MnCl4(H2O)2] none polymeric with bridging chloride
Manganese(II) bromide
MnBr2(H2O)2		 trans-[MnBr4(H2O)2] none polymeric with bridging bromide
Iron(II) chloride
FeCl2(H2O)6		 trans-[FeCl2(H2O)4] two
Iron(II) chloride
FeCl2(H2O)4		 trans-[FeCl2(H2O)4] none molecular
Iron(II) bromide
FeBr2(H2O)4		 trans-[FeBr2(H2O)4] none molecular,[16] hydrates of FeI2 are not known
Iron(II) chloride
FeCl2(H2O)2		 trans-[FeCl4(H2O)2] none polymeric with bridging chloride
Iron(III) chloride
FeCl3(H2O)6		 trans-[FeCl2(H2O)4]+ two one of four hydrates of
ferric chloride,[17]
isostructural with Cr analogue
Iron(III) chloride
FeCl3(H2O)2.5		 cis-[FeCl2(H2O)4]+ two the dihydrate has a similar structure, both contain FeCl−4 anions.[17]
Cobalt(II) chloride
CoCl2(H2O)6		 trans-[CoCl2(H2O)4] two
Cobalt(II) bromide
CoBr2(H2O)6		 trans-[CoBr2(H2O)4] two
Cobalt(II) iodide
CoI2(H2O)6		 [Co(H2O)6]2+ none[18] iodide competes poorly with water
Cobalt(II) bromide
CoBr2(H2O)4		 trans-[CoBr2(H2O)4] none molecular[16]
Cobalt(II) chloride
CoCl2(H2O)4		 cis-[CoCl2(H2O)4] none note: cis molecular
Cobalt(II) chloride
CoCl2(H2O)2		 trans-[CoCl4(H2O)2] none polymeric with bridging chloride
Cobalt(II) chloride
CoBr2(H2O)2		 trans-[CoBr4(H2O)2] none polymeric with bridging bromide
Nickel(II) chloride
NiCl2(H2O)6		 trans-[NiCl2(H2O)4] two
Nickel(II) chloride
NiCl2(H2O)4		 cis-[NiCl2(H2O)4] none note: cis molecular[16]
Nickel(II) bromide
NiBr2(H2O)6		 trans-[NiBr2(H2O)4] two
Nickel(II) iodide
NiI2(H2O)6		 [Ni(H2O)6]2+ none[18] iodide competes poorly with water
Nickel(II) chloride
NiCl2(H2O)2		 trans-[NiCl4(H2O)2] none polymeric with bridging chloride
Platinum(IV) chloride
[Pt(H2O)2Cl4](H2O)3[19]		 trans-[PtCl4(H2O)2] 3 octahedral Pt centers; rare example of non-first row chloride-aquo complex
Platinum(IV) chloride
[Pt(H2O)3Cl3]Cl(H2O)0.5[20]		 fac-[PtCl3(H2O)3]+ 0.5 octahedral Pt centers; rare example of non-first row chloride-aquo complex
Copper(II) chloride
CuCl2(H2O)2		 [CuCl4(H2O)2]2 none tetragonally distorted
two long Cu-Cl distances
Copper(II) bromide
CuBr2(H2O)4		 [CuBr4(H2O)2]n two tetragonally distorted
two long Cu-Br distances[16]
Zinc(II) chloride
ZnCl2(H2O)1.33[21]
 2 ZnCl2 + ZnCl2(H2O)4 none coordination polymer with both tetrahedral and octahedral Zn centers
Zinc(II) chloride
ZnCl2(H2O)2.5[22]
 Cl3Zn(μ-Cl)Zn(H2O)5 none tetrahedral and octahedral Zn centers
Zinc(II) chloride
ZnCl2(H2O)3[21]
 [ZnCl4]2− + Zn(H2O)6]2+ none tetrahedral and octahedral Zn centers
Zinc(II) chloride
ZnCl2(H2O)4.5[21]
 [ZnCl4]2− + [Zn(H2O)6]2+ three tetrahedral and octahedral Zn centers

Hydrates of metal sulfates

Substructure of MSO4(H2O), illustrating presence of bridging water and bridging sulfate (M = Mg, Mn, Fe, Co, Ni, Zn).

Transition metal sulfates form a variety of hydrates, each of which crystallizes in only one form. The sulfate group often binds to the metal, especially for those salts with fewer than six

aquo ligands. The heptahydrates, which are often the most common salts, crystallize as monoclinic and the less common orthorhombic forms. In the heptahydrates, one water is in the lattice and the other six are coordinated to the ferrous center.[23] Many of the metal sulfates occur in nature, being the result of weathering of mineral sulfides.[24][25] Many monohydrates are known.[26]

Formula of
hydrated metal ion sulfate		 Coordination
sphere of the metal ion		 Equivalents of water of crystallization
that are not bound to M		 mineral name Remarks
MgSO4(H2O) [Mn(μ-H2O)(μ4,-κ1-SO4)4][26] none kieserite see Mn, Fe, Co, Ni, Zn analogues
MgSO4(H2O)4 [Mg(H2O)4(κ′,κ1-SO4)]2 none sulfate is bridging ligand, 8-membered Mg2O4S2 rings[27]
MgSO4(H2O)6 [Mg(H2O)6] none hexahydrate common motif[24]
MgSO4(H2O)7 [Mg(H2O)6] one epsomite common motif[24]
TiOSO4(H2O) [Ti(μ-O)2(H2O)(κ1-SO4)3] none further hydration gives gels
VSO4(H2O)6 [V(H2O)6] none Adopts the hexahydrite motif[28]
VOSO4(H2O)5 [VO(H2O)4(κ1-SO4)4] one
Cr(SO4)(H2O)3 [Cr(H2O)3(κ1-SO4)] none resembles Cu(SO4)(H2O)3[29]
Cr(SO4)(H2O)5 [CR(H2O)4(κ1-SO4)2] one resembles Cu(SO4)(H2O)5[30]
Cr2(SO4)3(H2O)18 [Cr(H2O)6] six One of several chromium(III) sulfates
MnSO4(H2O) [Mn(μ-H2O)(μ4,-κ1-SO4)4][26] none szmikite see Fe, Co, Ni, Zn analogues
MnSO4(H2O)4 [Mn(μ-SO4)2(H2O)4][31] none Ilesitepentahydrate is called jôkokuite; the hexahydrate, the most rare, is called chvaleticeite with 8-membered ring Mn2(SO4)2 core
MnSO4(H2O)5 ? jôkokuite
MnSO4(H2O)6 ? Chvaleticeite
MnSO4(H2O)7 [Mn(H2O)6] one mallardite[25] see Mg analogue
FeSO4(H2O) [Fe(μ-H2O)(μ4-κ1-SO4)4][26] none see Mn, Co, Ni, Zn analogues
FeSO4(H2O)7 [Fe(H2O)6] one melanterite[25] see Mg analogue
FeSO4(H2O)4 [Fe(H2O)4(κ′,κ1-SO4)]2 none sulfate is bridging ligand, 8-membered Fe2O4S2 rings[27]
FeII(FeIII)2(SO4)4(H2O)14 [FeII(H2O)6]2+[FeIII(H2O)4(κ1-SO4)2]−
2		 none sulfates are terminal ligands on Fe(III)[32]
CoSO4(H2O) [Co(μ-H2O)(μ4-κ1-SO4)4][26] none see Mn, Fe, Ni, Zn analogues
CoSO4(H2O)6 [Co(H2O)6] none moorhouseite see Mg analogue
CoSO4(H2O)7 [Co(H2O)6] one bieberite[25] see Fe, Mg analogues
NiSO4(H2O) [Ni(μ-H2O)(μ4-κ1-SO4)4][26] none see Mn, Fe, Co, Zn analogues
NiSO4(H2O)6 [Ni(H2O)6] none retgersite One of several nickel sulfate hydrates[33]
NiSO4(H2O)7 [Ni(H2O)6] morenosite[25]
(NH4)2[Pt2(SO4)4(H2O)2] [Pt2(SO4)4(H2O)2]2- none Pt-Pt bonded Chinese lantern structure[34]
CuSO4(H2O)5 [Cu(H2O)4(κ1-SO4)2] one chalcantite sulfate is bridging ligand[35]
CuSO4(H2O)7 [Cu(H2O)6] one boothite[25]
ZnSO4(H2O) [Zn(μ-H2O)(μ4-κ1-SO4)4][26] none see Mn, Fe, Co, Ni analogues
ZnSO4(H2O)4 [Zn(H2O)4(κ′,κ1-SO4)]2 none sulfate is bridging ligand, 8-membered Zn2O4S2 rings[27][36]
ZnSO4(H2O)6 [Zn(H2O)6] none see Mg analogue[37]
ZnSO4(H2O)7 [Zn(H2O)6] one goslarite[25] see Mg analogue
CdSO4(H2O) [Cd(μ-H2O)2(κ1-SO4)4] none bridging water ligand[38]

Hydrates of metal nitrates

aquo ligands
. Nitrates are uncommon in nature, so few minerals are represented here. Hydrated ferrous nitrate has not been characterized crystallographically.

Formula of
hydrated metal ion nitrate		 Coordination
sphere of the metal ion		 Equivalents of water of crystallization
that are not bound to M		 Remarks
Cr(NO3)3(H2O)9 [Cr(H2O)6]3+ three octahedral configuration[39] isostructural with Fe(NO3)3(H2O)9
Mn(NO3)2(H2O)4 cis-[Mn(H2O)4(κ1-ONO2)2] none octahedral configuration
Mn(NO3)2(H2O) [Mn(H2O)(μ-ONO2)5] none octahedral configuration
Mn(NO3)2(H2O)6 [Mn(H2O)6] none octahedral configuration[40]
Fe(NO3)3(H2O)9 [Fe(H2O)6]3+ three octahedral configuration[41] isostructural with Cr(NO3)3(H2O)9
Fe(NO3)3)(H2O)4 [Fe(H2O)3(κ2-O2NO)2]+ one
pentagonal bipyramid[42]
Fe(NO3)3(H2O)5 [Fe(H2O)5(κ1-ONO2)]2+ none octahedral configuration[42]
Fe(NO3)3(H2O)6 [Fe(H2O)6]3+ none octahedral configuration[42]
Co(NO3)2(H2O)2 [Co(H2O)2(κ1-ONO2)2] none octahedral configuration
Co(NO3)2(H2O)4 [Co(H2O)4(κ1-ONO2)2 none octahedral configuration
Co(NO3)2(H2O)6 [Co(H2O)6]2+ none octahedral configuration.[43]
α-Ni(NO3)2(H2O)4 cis-[Ni(H2O)4(κ1-ONO2)2] none octahedral configuration.[44]
β-Ni(NO3)2(H2O)4 trans-[Ni(H2O)4(κ1-ONO2)2] none octahedral configuration.[45]
Pd(NO3)2(H2O)2 trans-[Pd(H2O)2(κ1-ONO2)2] none
square planar coordination geometry[46]
Cu(NO3)2(H2O) [Cu(H2O)(κ2-ONO2)2] none octahedral configuration.
Cu(NO3)2(H2O)1.5 uncertain uncertain uncertain[47]
Cu(NO3)2(H2O)2.5 [Cu(H2O)2(κ1-ONO2)2] one square planar[48]
Cu(NO3)2(H2O)3 uncertain uncertain uncertain [49]
Cu(NO3)2(H2O)6 [Cu(H2O)6]2+ none octahedral configuration[50]
Zn(NO3)2(H2O)4 cis-[Zn(H2O)4(κ1-ONO2)2] none octahedral configuration.
Hg2(NO3)2(H2O)2
 [H2O–Hg–Hg–OH2]2+ linear[51]

Photos

  • Hydrated copper(II) sulfate is bright blue.
    Hydrated copper(II) sulfate is bright blue.
  • Anhydrous copper(II) sulfate has a light turquoise tint.
    Anhydrous copper(II) sulfate has a light turquoise tint.

See also

References

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