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Water of hydration

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Water of hydration

In crystallography, water of crystallization or water of hydration or crystallization water is water that occurs inside crystals. Water is often necessary for the formation of crystals.[1] In some contexts, water of crystallization is the total weight 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.

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

Nomenclature

In molecular formulas water of crystallization can be denoted in different ways:

  • "hydrous compoundnH2O" or "hydrous 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
  • "hydrous 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.[2] [3] 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 the aforementioned hydrated copper(II) sulfate consist of [Cu(H2O)5]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. The cobalt iodide mentioned above occurs as [Co(H2O)6]2+ and I. 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".

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 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.[4]

Formula of
hydrated metal halides
Coordination
sphere of the metal
equivalents
of water of
crystallization that
are not bound to M
Remarks
VCl3(H2O)6 trans-[VCl2(H2O)4]+ two
VBr3(H2O)6 trans-[VBr2(H2O)4]+ two bromides and
chlorides are usually
similar
VI3(H2O)6 [V(H2O)6]3+ none iodide competes
poorly with water
CrCl3(H2O)6 trans-[CrCl2(H2O)4]+ two dark green isomer
CrCl3(H2O)6 [CrCl(H2O)5]2+ one blue-green isomer
CrCl2(H2O)4 trans-[CrCl2(H2O)4] none molecular
CrCl3(H2O)6 [Cr(H2O)6]3+ none violet isomer
MnCl2(H2O)6 trans-[MnCl2(H2O)4] two
MnCl2(H2O)4 cis-[MnCl2(H2O)4] none note cis
molecular
MnBr2(H2O)4 cis-[MnBr2(H2O)4] none note cis
molecular
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
FeCl2(H2O)2 trans-[FeCl4(H2O)2] none polymeric with
bridging chloride
CoCl2(H2O)6 trans-[CoCl2(H2O)4] two
CoBr2(H2O)6 trans-[CoBr2(H2O)4] two
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
NiCl2(H2O)2 trans-[NiCl2(H2O)4] 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

See also

References

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