Physics:Water of crystallization

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In chemistry, water(s) of crystallization or water(s) of hydration are water molecules that are present inside crystals. Water is often incorporated in the formation of crystals from aqueous solutions.[1] In some contexts, water of crystallization is the total mass of water in a substance at a given temperature and is mostly present in a definite (stoichiometric) ratio. Classically, "water of crystallization" refers to water that is found in the crystalline framework of a metal complex or a salt, which is not directly bonded to the metal 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. For example, in the case of sodium chloride, the dihydrate is unstable at room temperature.

Coordination sphere of Na+ in the metastable dihydrate of sodium chloride (red = oxygen, violet = Na+, green = Cl, H atoms omitted).[2]

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.


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.[3]


In molecular formulas water of crystallization is indicated in various ways, but is often vague. The terms hydrated compound and hydrate are generally vaguely defined.

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.[4] [5] 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. Examples:

  • CuSO
     · 5 H
    – copper(II) sulfate pentahydrate
  • CoCl
     · 6 H
    – cobalt(II) chloride hexahydrate
  • SnCl
     · 2 H
    – 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 CuSO
 · 5 H
and anhydrous CuSO4 behave identically. Therefore, knowledge of the degree of hydration is important only for determining the equivalent weight: one mole of CuSO
 · 5 H
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 RhCl
 · 3 H
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(H
centers linked to SO2−
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.[6] The cobalt chloride mentioned above occurs as [Co(H2O)6]2+ and Cl. In tin chloride, each Sn(II) center is pyramidal (mean O/Cl–Sn–O/Cl angle is 83°) being bound to two chloride ions and one water. The second water in the formula unit is hydrogen-bonded to the chloride and to the coordinated water molecule. Water of crystallization is stabilized by electrostatic attractions, consequently hydrates are common for salts that contain +2 and +3 cations as well as −2 anions. In some cases, the majority of the weight of a compound arises from water. 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".


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 deuterated solvent and analyzing the sample for solvent signals by 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]

Formula of
hydrated metal halides
sphere of the metal
Equivalents of water of crystallization
that are not bound to M
CaCl2(H2O)6 [Ca(μ-H2O)6(H2O)3]2+ none example of water as a bridging ligand[9]
TiCl3(H2O)6 trans-[TiCl2(H2O)4]+[10] two isomorphous with VCl3(H2O)6
VCl3(H2O)6 trans-[VCl2(H2O)4]+[10] two
VBr3(H2O)6 trans-[VBr2(H2O)4]+[10] two
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
CrCl3(H2O)6 trans-[CrCl2(H2O)4]+ two dark green isomer, aka "Bjerrums's salt"
CrCl3(H2O)6 [CrCl(H2O)5]2+ one blue-green isomer
CrCl2(H2O)4 trans-[CrCl2(H2O)4] none square planar/tetragonal distortion
CrCl3(H2O)6 [Cr(H2O)6]3+ none violet isomer. isostructural with aluminium compound[11]
AlCl3(H2O)6 [Al(H2O)6]3+ none isostructural with the Cr(III) compound
MnCl2(H2O)6 trans-[MnCl2(H2O)4] two
MnCl2(H2O)4 cis-[MnCl2(H2O)4] none cis molecular, the unstable trans isomer has also been detected[12]
MnBr2(H2O)4 cis-[MnBr2(H2O)4] none cis, molecular
MnI2(H2O)4 trans-[MnI2(H2O)4] none molecular, isostructural with FeCl2(H2O)4.[13]
MnCl2(H2O)2 trans-[MnCl4(H2O)2] none polymeric with bridging chloride
MnBr2(H2O)2 trans-[MnBr4(H2O)2] none polymeric with bridging bromide
FeCl2(H2O)6 trans-[FeCl2(H2O)4] two
FeCl2(H2O)4 trans-[FeCl2(H2O)4] none molecular
FeBr2(H2O)4 trans-[FeBr2(H2O)4] none molecular, hydrates of FeI2 are not known
FeCl2(H2O)2 trans-[FeCl4(H2O)2] none polymeric with bridging chloride
FeCl3(H2O)6 trans-[FeCl2(H2O)4]+ two one of four hydrates of ferric chloride,[14] isostructural with Cr analogue
FeCl3(H2O)2.5 cis-[FeCl2(H2O)4]+ two the dihydrate has a similar structure, both contain FeCl4 anions.[14]
CoCl2(H2O)6 trans-[CoCl2(H2O)4] two
CoBr2(H2O)6 trans-[CoBr2(H2O)4] two
CoI2(H2O)6 [Co(H2O)6]2+ none[15] iodide competes poorly with water
CoBr2(H2O)4 trans-[CoBr2(H2O)4] none molecular
CoCl2(H2O)4 cis-[CoCl2(H2O)4] none note: cis molecular
CoCl2(H2O)2 trans-[CoCl4(H2O)2] none polymeric with bridging chloride
CoBr2(H2O)2 trans-[CoBr4(H2O)2] none polymeric with bridging bromide
NiCl2(H2O)6 trans-[NiCl2(H2O)4] two
NiCl2(H2O)4 cis-[NiCl2(H2O)4] none note: cis molecular
NiBr2(H2O)6 trans-[NiBr2(H2O)4] two
NiI2(H2O)6 [Ni(H2O)6]2+ none[15] iodide competes poorly with water
NiCl2(H2O)2 trans-[NiCl4(H2O)2] none polymeric with bridging chloride
CuCl2(H2O)2 [CuCl4(H2O)2]2 none tetragonally distorted
two long Cu-Cl distances
CuBr2(H2O)4 [CuBr4(H2O)2]n two tetragonally distorted
two long Cu-Br distances
ZnCl2(H2O)1.33[16] 2 ZnCl2 + ZnCl2(H2O)4 none coordination polymer with both tetrahedral and octahedral Zn centers
ZnCl2(H2O)2.5[17] Cl3Zn(μ-Cl)Zn(H2O)5 none tetrahedral and octahedral Zn centers
ZnCl2(H2O)3[16] [ZnCl4]2- + Zn(H2O)6]2+ none tetrahedral and octahedral Zn centers
ZnCl2(H2O)4.5[16] [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.[18] Many of the metal sulfates occur in nature, being the result of weathering of mineral sulfides.[19][20] Many monohydrates are known.[21]

Formula of
hydrated metal ion sulfate
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][21]   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[22]
MgSO4(H2O)6 [Mg(H2O)6] none hexahydrate common motif[19]
MgSO4(H2O)7 [Mg(H2O)6] one epsomite common motif[19]
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[23]
VOSO4(H2O)5 [VO(H2O)41-SO4)4] one
Cr2(SO4)3(H2O)18 [Cr(H2O)6] six One of several chromium(III) sulfates
MnSO4(H2O) [Mn(μ-H2O)(μ4,-κ1-SO4)4][21]   none   see Fe, Co, Ni, Zn analogues
MnSO4(H2O)7 [Mn(H2O)6] one mallardite[20] see Mg analogue
FeSO4(H2O) [Fe(μ-H2O)(μ41-SO4)4][21]   none   see Mn, Co, Ni, Zn analogues
FeSO4(H2O)7 [Fe(H2O)6] one melanterite[20] see Mg analogue
FeSO4(H2O)4 [Fe(H2O)4(κ′,κ1-SO4)]2 none sulfate is bridging ligand, 8-membered Fe2O4S2 rings[22]
FeII(FeIII)2(SO4)4(H2O)14]] [FeII(H2O)6]2+[FeIII(H2O)41-SO4)2]2 none sulfates are terminal ligands on Fe(III)[24]
CoSO4(H2O) [Co(μ-H2O)(μ41-SO4)4][21]  none   see Mn, Fe, Ni, Zn analogues
CoSO4(H2O)6 [Co(H2O)6] none morehouseite see Mg analogue
CoSO4(H2O)7 [Co(H2O)6] one bieberite[20] see Fe, Mg analogues
NiSO4(H2O) [Ni(μ-H2O)(μ41-SO4)4][21]   none   see Mn, Fe, Co, Zn analogues
NiSO4(H2O)6 [Ni(H2O)6] none retgersite   One of several nickel sulfate hydrates[25]
NiSO4(H2O)7 [Ni(H2O)6] morenosite[20] 
(NH4)2[Pt2(SO4)4(H2O)2] [Pt2(SO4)4(H2O)2]2- none Pt-Pt bonded Chinese lantern structure[26]
CuSO4(H2O)5 [Cu(H2O)41-SO4)2] one chalcantite sulfate is bridging ligand[27]
CuSO4(H2O)7 [Cu(H2O)6] one boothite[20]
ZnSO4(H2O) [Zn(μ-H2O)(μ41-SO4)4][21]  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 ringss[22][28]
ZnSO4(H2O)6 [Zn(H2O)6] none see Mg analogue[29]
ZnSO4(H2O)7 [Zn(H2O)6] one| goslarite[20] see Mg analogue
CdSO4(H2O) [Cd(μ-H2O)21-SO4)4] none   bridging water ligand[30]

Hydrates of metal nitrates

Transition metal nitrates form a variety of hydrates. The nitrate anion often binds to the metal, especially for those salts with fewer than six 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
sphere of the metal ion
Equivalents of water of crystallization
that are not bound to M
Cr(NO3)3(H2O)6 [Cr(H2O)6]3+ three octahedral configuration[31] isostructural with Fe(NO3)3(H2O)9
Mn(NO3)2(H2O)4 cis-[Mn(H2O)41-ONO2)2] none octahedral configuration
Mn(NO3)2(H2O) [Mn(H2O)(μ-ONO2)5] none octahedral configuration
Fe(NO3)3(H2O)9 [Fe(H2O)6]3+ three octahedral configuration[32] isostructural with Cr(NO3)3(H2O)9
Fe(NO3)3)(H2O)4 [Fe(H2O)32-O2NO)2]+ one pentagonal bipyramid[33]
Fe(NO3)3(H2O)5 [Fe(H2O)51-ONO2)]2+ none octahedral configuration[33]
Fe(NO3)3(H2O)6 [Fe(H2O)6]3+ none octahedral configuration[33]
Co(NO3)2(H2O)2 [Co(H2O)21-ONO2)2] none octahedral configuration
Co(NO3)2(H2O)4 [Co(H2O)41-ONO2)2 none octahedral configuration
Co(NO3)2(H2O)6 [Co(H2O)6]2+ none octahedral configuration.[34]
α-Ni(NO3)2(H2O)4 cis-[Ni(H2O)41-ONO2)2] none octahedral configuration.[35]
β-Ni(NO3)2(H2O)4 trans-[Ni(H2O)41-ONO2)2] none octahedral configuration.[36]
Pd(NO3)2(H2O)2 trans-[Pd(H2O)21-ONO2)2] none square planar coordination geometry[37]
Cu(NO3)2(H2O) [Cu(H2O)(κ2-ONO2)2] none octahedral configuration.
Cu(NO3)2(H2O)1.5 uncertain uncertain uncertain[38]
Cu(NO3)2(H2O)2.5 [Cu(H2O)21-ONO2)2] one square planar[39]
Cu(NO3)2(H2O)3 uncertain uncertain uncertain [40]
Cu(NO3)2(H2O)6 [Cu(H2O)6]2+ none octahedral configuration[41]
Zn(NO3)2(H2O)4 cis-[Zn(H2O)41-ONO2)2] none octahedral configuration.


See also


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  4. Yonghui Wang et al. "Novel Hydrogen-Bonded Three-Dimensional Networks Encapsulating One-Dimensional Covalent Chains: ..." Inorg. Chem., 2002, 41 (24), pp. 6351–6357. doi:10.1021/ic025915o
  5. Carmen R. Maldonadoa, Miguel Quirós and J.M. Salas: "Formation of 2D water morphologies in the lattice of the salt..." Inorganic Chemistry Communications Volume 13, Issue 3, March 2010, p. 399–403; doi:10.1016/j.inoche.2009.12.033
  6. Moeller, Therald (Jan 1, 1980). Chemistry: With Inorganic qualitative Analysis. Academic Press Inc (London) Ltd. pp. 909. ISBN 978-0-12-503350-3. Retrieved 15 June 2014. 
  7. K. Waizumi, H. Masuda, H. Ohtaki, "X-ray structural studies of FeBr
     · 4 H
    , CoBr
     · 4 H
    , NiCl
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    , and CuBr
     · 4 H
    . cis/trans Selectivity in transition metal(I1) dihalide Tetrahydrate" Inorganica Chimica Acta, 1992 volume 192, pages 173–181.
  8. B. Morosin "An X-ray diffraction study on nickel(II) chloride dihydrate" Acta Crystallogr. 1967. volume 23, pp. 630-634. doi:10.1107/S0365110X67003305
  9. Agron, P. A.; Busing, W. R. (1986). "Calcium and Strontium Dichloride Hexahydrates by Neutron Diffraction". Acta Crystallographica Section C 42 (2): 14. doi:10.1107/S0108270186097007. 
  10. 10.0 10.1 10.2 Donovan, William F.; Smith, Peter W. (1975). "Crystal and Molecular Structures of Aquahalogenovanadium(III) Complexes. Part I. X-Ray Crystal Structure of trans-Tetrakisaquadibromo-Vanadium(III) Bromide Dihydrate and the Isomorphous Chloro- Compound". Journal of the Chemical Society, Dalton Transactions (10): 894. doi:10.1039/DT9750000894. 
  11. Andress, K. R.; Carpenter, C. (1934). "Die Struktur von Chromchlorid- und Aluminiumchloridhexahydrat". Zeitschrift für Kristallographie, Kristallgeometrie, Kristallphysik, Kristallchemie 87: 446-p463. 
  12. Zalkin, Allan; Forrester, J. D.; Templeton, David H. (1964). "Crystal Structure of Manganese Dichloride Tetrahydrate". Inorganic Chemistry 3 (4): 529–33. doi:10.1021/ic50014a017. 
  13. Moore, J. E.; Abola, J. E.; Butera, R. A. (1985). "Structure of Manganese(II) Iodide Tetrahydrate, MnI2.4H2O". Acta Crystallographica Section C Crystal Structure Communications 41 (9): 1284–1286. doi:10.1107/S0108270185007466. 
  14. 14.0 14.1 Simon A. Cotton (2018). "Iron(III) Chloride and Its Coordination Chemistry". Journal of Coordination Chemistry 71 (21): 3415–3443. doi:10.1080/00958972.2018.1519188. 
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  17. Hennings, Erik; Schmidt, Horst; Voigt, Wolfgang (2014). "Crystal Sructures of ZnCl2·2.5H2O, ZnCl2·3H2O and ZnCl2·4.5H2O". Acta Crystallographica Section E 70 (12): 515–518. doi:10.1107/S1600536814024738. PMID 25552980. 
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