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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. 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.
In molecular formulas water of crystallization can be denoted in different ways:
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. Examples:
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. 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.
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.
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||Case of water as a bridging ligand|
|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, aka "Bjerrums's salt"|
|CrCl2(H2O)4||trans-[CrCl2(H2O)4]||none||square planar/tetragonal distortion|
|AlCl3(H2O)6||[Al(H2O)6]3+||none||isostructural with the Cr(III) compound|
|MnCl2(H2O)4||cis-[MnCl2(H2O)4]||none||cis molecular, the unstable trans isomer has also been detected|
|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)2||trans-[FeCl4(H2O)2]||none||polymeric with bridging chloride|
|FeCl3(H2O)6||trans-[FeCl2(H2O)4]||two||only hydrate of ferric chloride, isostructural with Cr analogue|
|CoI2(H2O)6||[Co(H2O)6]2+||none||iodide competes poorly with water|
|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)4||cis-[NiCl2(H2O)4]||none||note: cis molecular|
|NiI2(H2O)6||[Ni(H2O)6]2+||none||iodide competes poorly with water|
|NiCl2(H2O)2||trans-[NiCl4(H2O)2]||none||polymeric with bridging chloride|
two long Cu-Cl distances
two long Cu-Br distances
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. Cr2(SO4)3.18H2O
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]6+||six||one of several chromium(III) sulfates|
|MnSO4(H2O)||unknown||unknown||one of several manganese(II) sulfates|
|FeSO4(H2O)7||[Fe(H2O)6]2+||one||Common motif for divalent metal sulfates|
|CoSO4(H2O)7||[Co(H2O)6]2+||one||Common motif for divalent metal ion sulfates|
|NiSO4(H2O)7||[Ni(H2O)6]2+||one||Common motif for divalent metal sulfates|
|NiSO4(H2O)6||[Ni(H2O)6]2+||none||one of several nickel sulfate hydrates|
|CuSO4(H2O)5||[Cu(H2O)4(SO4)]||one||sulfate is bridging ligand|
|CdSO4(H2O)||Cd(H2O)SO4||none||bridging water ligand|
Hydrated copper(II) sulfate is bright blue.