Water of crystallization

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. 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.

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.

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. 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. 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. 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.

Hydrated metal halides
and their formulas
Coordination sphere
of the metal
Equivalents of water of crystallization
that are not bound to M
Calcium chloride
[Ca(μ-H2O)6(H2O)3]2+ none example of water as a bridging ligand
Titanium(III) chloride
trans-[TiCl2(H2O)4]+ two isomorphous with VCl3(H2O)6
Titanium(III) chloride
[Ti(H2O)6]3+ none isomeric with [TiCl2(H2O)4]Cl.2H2O
Zirconium(IV) fluoride
(μ−F)2[ZrF3(H2O)3]2 none rare case where Hf and Zr differ
Hafnium tetrafluoride
(μ−F)2[HfF2(H2O)2]n(H2O)n one rare case where Hf and Zr differ
Vanadium(III) chloride
trans-[VCl2(H2O)4]+ two
Vanadium(III) bromide
trans-[VBr2(H2O)4]+ two
Vanadium(III) iodide
[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
trans-[CrCl2(H2O)4]+ two dark green isomer, aka "Bjerrums's salt"
Chromium(III) chloride
[CrCl(H2O)5]2+ one blue-green isomer
Chromium(II) chloride
trans-[CrCl2(H2O)4] none square planar/tetragonal distortion
Chromium(III) chloride
[Cr(H2O)6]3+ none violet isomer. isostructural with aluminium compound
Aluminum trichloride
[Al(H2O)6]3+ none isostructural with the Cr(III) compound
Manganese(II) chloride
trans-[MnCl2(H2O)4] two
Manganese(II) chloride
cis-[MnCl2(H2O)4] none cis molecular, the unstable trans isomer has also been detected
Manganese(II) bromide
cis-[MnBr2(H2O)4] none cis, molecular
Manganese(II) iodide
trans-[MnI2(H2O)4] none molecular, isostructural with FeCl2(H2O)4.
Manganese(II) chloride
trans-[MnCl4(H2O)2] none polymeric with bridging chloride
Manganese(II) bromide
trans-[MnBr4(H2O)2] none polymeric with bridging bromide
Iron(II) chloride
trans-[FeCl2(H2O)4] two
Iron(II) chloride
trans-[FeCl2(H2O)4] none molecular
Iron(II) bromide
trans-[FeBr2(H2O)4] none molecular, hydrates of FeI2 are not known
Iron(II) chloride
trans-[FeCl4(H2O)2] none polymeric with bridging chloride
Iron(III) chloride
trans-[FeCl2(H2O)4]+ two one of four hydrates of ferric chloride, isostructural with Cr analogue
Iron(III) chloride
cis-[FeCl2(H2O)4]+ two the dihydrate has a similar structure, both contain FeCl4 anions.
Cobalt(II) chloride
trans-[CoCl2(H2O)4] two
Cobalt(II) bromide
trans-[CoBr2(H2O)4] two
Cobalt(II) iodide
[Co(H2O)6]2+ none iodide competes poorly with water
Cobalt(II) bromide
trans-[CoBr2(H2O)4] none molecular
Cobalt(II) chloride
cis-[CoCl2(H2O)4] none note: cis molecular
Cobalt(II) chloride
trans-[CoCl4(H2O)2] none polymeric with bridging chloride
Cobalt(II) chloride
trans-[CoBr4(H2O)2] none polymeric with bridging bromide
Nickel(II) chloride
trans-[NiCl2(H2O)4] two
Nickel(II) chloride
cis-[NiCl2(H2O)4] none note: cis molecular
Nickel(II) bromide
trans-[NiBr2(H2O)4] two
Nickel(II) iodide
[Ni(H2O)6]2+ none iodide competes poorly with water
Nickel(II) chloride
trans-[NiCl4(H2O)2] none polymeric with bridging chloride
Platinum(IV) chloride
trans-[PtCl4(H2O)2] 3 octahedral Pt centers; rare example of non-first row chloride-aquo complex
Platinum(IV) chloride
fac-[PtCl3(H2O)3]+ 0.5 octahedral Pt centers; rare example of non-first row chloride-aquo complex
Copper(II) chloride
[CuCl4(H2O)2]2 none tetragonally distorted
two long Cu-Cl distances
Copper(II) bromide
[CuBr4(H2O)2]n two tetragonally distorted
two long Cu-Br distances
Zinc(II) chloride
2 ZnCl2 + ZnCl2(H2O)4 none coordination polymer with both tetrahedral and octahedral Zn centers
Zinc(II) chloride
Cl3Zn(μ-Cl)Zn(H2O)5 none tetrahedral and octahedral Zn centers
Zinc(II) chloride
[ZnCl4]2− + Zn(H2O)6]2+ none tetrahedral and octahedral Zn centers
Zinc(II) chloride
[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. Many of the metal sulfates occur in nature, being the result of weathering of mineral sulfides. Many monohydrates are known.

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] 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
MgSO4(H2O)6 [Mg(H2O)6] none hexahydrate common motif
MgSO4(H2O)7 [Mg(H2O)6] one epsomite common motif
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
VOSO4(H2O)5 [VO(H2O)41-SO4)4] one
Cr(SO4)(H2O)3 [Cr(H2O)31-SO4)] none resembles Cu(SO4)(H2O)3
Cr(SO4)(H2O)5 [CR(H2O)41-SO4)2] one resembles Cu(SO4)(H2O)5
Cr2(SO4)3(H2O)18 [Cr(H2O)6] six One of several chromium(III) sulfates
MnSO4(H2O) [Mn(μ-H2O)(μ4,-κ1-SO4)4] none szmikite see Fe, Co, Ni, Zn analogues
MnSO4(H2O)4 [Mn(μ-SO4)2(H2O)4] 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 see Mg analogue
FeSO4(H2O) [Fe(μ-H2O)(μ41-SO4)4] none see Mn, Co, Ni, Zn analogues
FeSO4(H2O)7 [Fe(H2O)6] one melanterite see Mg analogue
FeSO4(H2O)4 [Fe(H2O)4(κ′,κ1-SO4)]2 none sulfate is bridging ligand, 8-membered Fe2O4S2 rings
FeII(FeIII)2(SO4)4(H2O)14 [FeII(H2O)6]2+[FeIII(H2O)41-SO4)2]
none sulfates are terminal ligands on Fe(III)
CoSO4(H2O) [Co(μ-H2O)(μ41-SO4)4] 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 see Fe, Mg analogues
NiSO4(H2O) [Ni(μ-H2O)(μ41-SO4)4] none see Mn, Fe, Co, Zn analogues
NiSO4(H2O)6 [Ni(H2O)6] none retgersite One of several nickel sulfate hydrates
NiSO4(H2O)7 [Ni(H2O)6] morenosite
(NH4)2[Pt2(SO4)4(H2O)2] [Pt2(SO4)4(H2O)2]2- none Pt-Pt bonded Chinese lantern structure
CuSO4(H2O)5 [Cu(H2O)41-SO4)2] one chalcantite sulfate is bridging ligand
CuSO4(H2O)7 [Cu(H2O)6] one boothite
ZnSO4(H2O) [Zn(μ-H2O)(μ41-SO4)4] 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
ZnSO4(H2O)6 [Zn(H2O)6] none see Mg analogue
ZnSO4(H2O)7 [Zn(H2O)6] one goslarite see Mg analogue
CdSO4(H2O) [Cd(μ-H2O)21-SO4)4] none bridging water ligand

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)9 [Cr(H2O)6]3+ three octahedral configuration 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
Mn(NO3)2(H2O)6 [Mn(H2O)6] none octahedral configuration
Fe(NO3)3(H2O)9 [Fe(H2O)6]3+ three octahedral configuration isostructural with Cr(NO3)3(H2O)9
Fe(NO3)3)(H2O)4 [Fe(H2O)32-O2NO)2]+ one pentagonal bipyramid
Fe(NO3)3(H2O)5 [Fe(H2O)51-ONO2)]2+ none octahedral configuration
Fe(NO3)3(H2O)6 [Fe(H2O)6]3+ none octahedral configuration
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.
α-Ni(NO3)2(H2O)4 cis-[Ni(H2O)41-ONO2)2] none octahedral configuration.
β-Ni(NO3)2(H2O)4 trans-[Ni(H2O)41-ONO2)2] none octahedral configuration.
Pd(NO3)2(H2O)2 trans-[Pd(H2O)21-ONO2)2] none square planar coordination geometry
Cu(NO3)2(H2O) [Cu(H2O)(κ2-ONO2)2] none octahedral configuration.
Cu(NO3)2(H2O)1.5 uncertain uncertain uncertain
Cu(NO3)2(H2O)2.5 [Cu(H2O)21-ONO2)2] one square planar
Cu(NO3)2(H2O)3 uncertain uncertain uncertain
Cu(NO3)2(H2O)6 [Cu(H2O)6]2+ none octahedral configuration
Zn(NO3)2(H2O)4 cis-[Zn(H2O)41-ONO2)2] none octahedral configuration.
Hg2(NO3)2(H2O)2 [H2O–Hg–Hg–OH2]2+ linear


See also

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