Water of crystallization

In chemistry, water of crystallization or water 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 moist 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.

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.


In molecular formulas water of crystallization can be denoted in differential:

  • "hydrated compound  nH2O" or "hydrated compound×nH2O"
This notation is used when the compound only contains lattice water or when the crystal structure is undetermined. For example Calcium chloride: CaCl2  2H2O
  • "hydrated compound(H2O)n"
A hydrate with coordinated water. For example Zinc chloride: ZnCl2(H2O)4

Position in the crystal structure

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. 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 labilized 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.

Crystals of hydrated copper(II) sulfate consist of [Cu(H2O)4]2+ centers linked to SO42− 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.[5] 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 1/3 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.[6][7]

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+noneCase of water as a bridging ligand[8]
VBr3(H2O)6trans-[VBr2(H2O)4]+twobromides and chlorides are usually similar
VI3(H2O)6[V(H2O)6]3+noneiodide competes poorly with water
CrCl3(H2O)6trans-[CrCl2(H2O)4]+twodark green isomer, aka "Bjerrums's salt"
CrCl3(H2O)6[CrCl(H2O)5]2+oneblue-green isomer
CrCl2(H2O)4trans-[CrCl2(H2O)4]nonesquare planar/tetragonal distortion
AlCl3(H2O)6[Al(H2O)6]3+noneisostructural with the Cr(III) compound
MnCl2(H2O)4cis-[MnCl2(H2O)4]nonecis molecular, the unstable trans isomer has also been detected[10]
MnBr2(H2O)4cis-[MnBr2(H2O)4]nonecis, molecular
MnCl2(H2O)2trans-[MnCl4(H2O)2]nonepolymeric with bridging chloride
MnBr2(H2O)2trans-[MnBr4(H2O)2]nonepolymeric with bridging bromide
FeCl2(H2O)2trans-[FeCl4(H2O)2]nonepolymeric with bridging chloride
FeCl3(H2O)6trans-[FeCl2(H2O)4]+twoone of four hydrates of ferric chloride,[11] isostructural with Cr analogue
FeCl3(H2O)2.5cis-[FeCl2(H2O)4]+twothe dihydrate has a similar structure, both contain FeCl4- anions.[11]
CoI2(H2O)6[Co(H2O)6]2+none[12]iodide competes poorly with water
CoCl2(H2O)4cis-[CoCl2(H2O)4]nonenote: cis molecular
CoCl2(H2O)2trans-[CoCl4(H2O)2]nonepolymeric with bridging chloride
CoBr2(H2O)2trans-[CoBr4(H2O)2]nonepolymeric with bridging bromide
NiCl2(H2O)4cis-[NiCl2(H2O)4]nonenote: cis molecular
NiI2(H2O)6[Ni(H2O)6]2+none[12]iodide competes poorly with water
NiCl2(H2O)2trans-[NiCl4(H2O)2]nonepolymeric with bridging chloride
CuCl2(H2O)2[CuCl4(H2O)2]2nonetetragonally distorted
two long Cu-Cl distances
CuBr2(H2O)4[CuBr4(H2O)2]ntwotetragonally distorted
two long Cu-Br distances

Hydrates of transition metal sulfates

Transition metal sulfates form mono-, tetra-, and pentahydrates, each of which crystallizes in only one form. The water in these salts typically is coordinated, together with sulfate to the metal center. The sulfates of these same metals also crystallize as both tetragonal and monoclinic hexahydrates, wherein all water is coordinated and the sulfate is a counterion. 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.[13] Cr2(SO4)3  18H2O

Formula of
hydrated metal ion sulfate
sphere of the metal ion
Equivalents of water of crystallization
that are not bound to M
Cr2(SO4)3(H2O)18[Cr(H2O)6]3+sixone of several chromium(III) sulfates
MnSO4(H2O)UnknownUnknownone of several manganese(II) sulfates
FeSO4(H2O)7[Fe(H2O)6]2+oneCommon motif for divalent metal sulfates
CoSO4(H2O)7[Co(H2O)6]2+oneCommon motif for divalent metal ion sulfates
NiSO4(H2O)7[Ni(H2O)6]2+oneCommon motif for divalent metal sulfates
NiSO4(H2O)6[Ni(H2O)6]2+noneone of several nickel sulfate hydrates[14]
CuSO4(H2O)5[Cu(H2O)4(SO4)]onesulfate is bridging ligand[15]
CdSO4(H2O)Cd(H2O)SO4nonebridging water ligand[16]

See also


  1. Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8.
  2. Klewe, B.; Pedersen, B. (1974). "The crystal structure of sodium chloride dihydrate". Acta Crystallogr. B30 (10): 2363–2371. doi:10.1107/S0567740874007138.
  3. 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
  4. 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
  5. Moeller, Therald (Jan 1, 1980). Chemistry: With Inorganic qualitative Analysis. Academic Press Inc (London) Ltd. p. 909. ISBN 978-0-12-503350-3. Retrieved 15 June 2014.
  6. K. Waizumi, H. Masuda, H. Ohtaki, "X-ray structural studies of FeBr2  4H2O, CoBr2  4H2O, NiCl2  4H2O, and CuBr2  4H2O. cis/trans Selectivity in transition metal(I1) dihalide Tetrahydrate" Inorganica Chimica Acta, 1992 volume 192, pages 173–181.
  7. B. Morosin "An X-ray diffraction study on nickel(II) chloride dihydrate" Acta Crystallogr. 1967. volume 23, pp. 630-634. doi:10.1107/S0365110X67003305
  8. Agron, P.A.; Busing, W.R. "Calcium and strontium dichloride hexahydrates by neutron diffraction" Acta Crystallographica Section C 1986, volume 42, pp. 141-p1.
  9. violet isomer. isostructural with aluminium compound.Andress, K.R.; Carpenter, C. "Kristallhydrate. II.Die Struktur von Chromchlorid- und Aluminiumchloridhexahydrat" Zeitschrift für Kristallographie, Kristallgeometrie, Kristallphysik, Kristallchemie 1934, volume 87, p446-p463.
  10. 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.
  11. 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.
  12. "Structure Cristalline et Expansion Thermique de L’Iodure de Nickel Hexahydrate" (Crystal structure and thermal expansion of nickel(II) iodide hexahydrate) Louër, Michele; Grandjean, Daniel; Weigel, Dominique Journal of Solid State Chemistry (1973), 7(2), 222-8. doi: 10.1016/0022-4596(73)90157-6
  13. Baur, W.H. "On the crystal chemistry of salt hydrates. III. The determination of the crystal structure of FeSO4(H2O)7 (melanterite)" Acta Crystallographica 1964, volume 17, p1167-p1174. doi:10.1107/S0365110X64003000
  14. Stadnicka, K.; Glazer, A.M.; Koralewski, M. "Structure, absolute configuration and optical activity of alpha-nickel sulfate hexahydrate" Acta Crystallographica, Section B: Structural Science (1987) 43, p319-p325.
  15. V. P. Ting, P. F. Henry, M. Schmidtmann, C. C. Wilson, M. T. Weller "In situ neutron powder diffraction and structure determination in controlled humidities" Chem. Commun., 2009, 7527-7529. doi:10.1039/B918702B
  16. Theppitak, Chatphorn; Chainok, Kittipong "Crystal structure of CdSO4(H2O): A Redetermination" Acta Crystallographica, Section E. Structure Reports Online 2015, volume 71, pi8-pi9. doi:10.1107/S2056989015016904
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