Water of crystallization
Overview
 
In crystallography
Crystallography
Crystallography is the experimental science of the arrangement of atoms in solids. The word "crystallography" derives from the Greek words crystallon = cold drop / frozen drop, with its meaning extending to all solids with some degree of transparency, and grapho = write.Before the development of...

, water of crystallization or water of hydration or crystallization water is water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

 that occurs in crystals. Water of crystallization is necessary for the maintenance of crystalline properties, but capable of being removed by sufficient heat.
It is the total weight of water retained by certain salts at a given temperature and is mostly present in a definite (stoichiometric) ratio.
Encyclopedia
In crystallography
Crystallography
Crystallography is the experimental science of the arrangement of atoms in solids. The word "crystallography" derives from the Greek words crystallon = cold drop / frozen drop, with its meaning extending to all solids with some degree of transparency, and grapho = write.Before the development of...

, water of crystallization or water of hydration or crystallization water is water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

 that occurs in crystals. Water of crystallization is necessary for the maintenance of crystalline properties, but capable of being removed by sufficient heat.
It is the total weight of water retained by certain salts 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 but which is not directly bonded to the metal ion.
Upon crystallization
Crystallization
Crystallization is the process of formation of solid crystals precipitating from a solution, melt or more rarely deposited directly from a gas. Crystallization is also a chemical solid–liquid separation technique, in which mass transfer of a solute from the liquid solution to a pure solid...

 from water or moist solvents, many compounds
Chemical compound
A chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together...

 incorporate water molecules in their crystalline frameworks.

Compared to inorganic salts
Inorganic compound
Inorganic compounds have traditionally been considered to be of inanimate, non-biological origin. In contrast, organic compounds have an explicit biological origin. However, over the past century, the classification of inorganic vs organic compounds has become less important to scientists,...

, proteins crystallize with unusually large amounts of water in the crystal lattice. A water content of 50 % is not uncommon. The extended hydration shell is what allows the protein crystallographer to argue that the conformation in the crystal is not too far from the native conformation in solution.

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
Calcium chloride
Calcium chloride, CaCl2, is a salt of calcium and chlorine. It behaves as a typical ionic halide, and is solid at room temperature. Common applications include brine for refrigeration plants, ice and dust control on roads, and desiccation...

: CaCl2·2H2O
  • "hydrous compound(H2O)n"
A hydrate with coordinated water. For example Zinc chloride
Zinc chloride
Zinc chloride is the name of chemical compound with the formula ZnCl2 and its hydrates. Zinc chlorides, of which nine crystalline forms are known, are colorless or white, and are highly soluble in water. ZnCl2 itself is hygroscopic and even deliquescent. Samples should therefore be protected from...

: ZnCl2(H2O)4

Position in the crystal structure

A salt
Salt
In chemistry, salts are ionic compounds that result from the neutralization reaction of an acid and a base. They are composed of cations and anions so that the product is electrically neutral...

 with associated water of crystallization is known as a hydrate
Hydrate
Hydrate is a term used in inorganic chemistry and organic chemistry to indicate that a substance contains water. The chemical state of the water varies widely between hydrates, some of which were so labeled before their chemical structure was understood....

. The structure of hydrates can be quite elaborate, because of the existence of hydrogen bond
Hydrogen bond
A hydrogen bond is the attractive interaction of a hydrogen atom with an electronegative atom, such as nitrogen, oxygen or fluorine, that comes from another molecule or chemical group. The hydrogen must be covalently bonded to another electronegative atom to create the bond...

s 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:
  • 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
Equivalent weight
Equivalent weight is a term which has been used in several contexts in chemistry. In its most general usage, it is the mass of one equivalent, that is the mass of a given substance which will:...

: 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
Rhodium(III) chloride
Rhodium chloride refers to inorganic compounds with the formula RhCl3n, where n varies from 0 to 3. These are diamagnetic solids featuring octahedral Rh centres. Depending on the value of n, the material is either a dense brown solid or a soluble reddish salt...

 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 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 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
Nickel(II) chloride
Nickel chloride , is the chemical compound NiCl2. The anhydrous salt is yellow, but the more familiar hydrate NiCl2·6H2O is green. It is very rarely found in nature as mineral nickelbischofite. A dihydrate is also known. In general nickel chloride, in various forms, is the most important source of...

 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 bond
Hydrogen bond
A hydrogen bond is the attractive interaction of a hydrogen atom with an electronegative atom, such as nitrogen, oxygen or fluorine, that comes from another molecule or chemical group. The hydrogen must be covalently bonded to another electronegative atom to create the bond...

ed 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
Thermogravimetric analysis
Thermogravimetric analysis or thermal gravimetric analysis is a type of testing performed on samples that determines changes in weight in relation to change in temperature. Such analysis relies on a high degree of precision in three measurements: weight, temperature, and temperature change...

 (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
Benzene
Benzene is an organic chemical compound. It is composed of 6 carbon atoms in a ring, with 1 hydrogen atom attached to each carbon atom, with the molecular formula C6H6....

 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
NMR spectroscopy
Nuclear magnetic resonance spectroscopy, most commonly known as NMR spectroscopy, is a research technique that exploits the magnetic properties of certain atomic nuclei to determine physical and chemical properties of atoms or the molecules in which they are contained...

. Single crystal X-ray crystallography is often able to detect the presence of these solvents of crystallization as well.

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.
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
Vanadium(III) chloride
Vanadium trichloride is the inorganic compound with the formula VCl3. This purple salt is a common precursor to other vanadium complexes.-Structure:...

 
trans-[VCl2(H2O)4]+ two
VBr3(H2O)6
Vanadium(III) bromide
Vanadium bromide, also known as vanadium tribromide, is VBr3. In the solid-state, this species is a polymeric with octahedral vanadium surrounded by six bromide ligands....

 
trans-[VBr2(H2O)4]+ two bromides and
chlorides are usually
similar
VI3(H2O)6
Vanadium(III) iodide
Vanadium iodide is the inorganic compound with the formula VI3. This paramagnetic solid is generated by the reaction of vanadium powder with iodine at around 500 °C...

 
[V(H2O)6]3+ none iodide competes
poorly with water
CrCl3(H2O)6
Chromium(III) chloride
Chromium chloride is a violet coloured solid with the formula CrCl3. The most common form of CrCl3 sold commercially is a dark green hexahydrate with the formula [CrCl24]Cl.2H2O. Two other hydrates are known, pale green [CrCl5]Cl2.H2O and violet [Cr6]Cl3...

 
trans-[CrCl2(H2O)4]+ two dark green isomer
CrCl3(H2O)6
Chromium(III) chloride
Chromium chloride is a violet coloured solid with the formula CrCl3. The most common form of CrCl3 sold commercially is a dark green hexahydrate with the formula [CrCl24]Cl.2H2O. Two other hydrates are known, pale green [CrCl5]Cl2.H2O and violet [Cr6]Cl3...

 
[CrCl(H2O)5]2+ one blue-green isomer
CrCl2(H2O)4
Chromium(II) chloride
Chromium chloride is the chemical compound with the formula CrCl2. This white, crystalline solid is used for the synthesis of other chromium complexes. CrCl2 is hygroscopic. It dissolves in water to give bright blue solutions that are easily oxidized by air to give Cr-containing products...

 
trans-[CrCl2(H2O)4] none molecular
CrCl3(H2O)6
Chromium(III) chloride
Chromium chloride is a violet coloured solid with the formula CrCl3. The most common form of CrCl3 sold commercially is a dark green hexahydrate with the formula [CrCl24]Cl.2H2O. Two other hydrates are known, pale green [CrCl5]Cl2.H2O and violet [Cr6]Cl3...

 
[Cr(H2O)6]3+ none violet isomer
MnCl2(H2O)6
Manganese(II) chloride
Manganese chloride describes a series of compounds with the formula MnCl2x, where the value of x can be 0, 2, or 4. The tetrahydrate is the most common form of "manganese chloride". MnCl2·4H2O, but the anhydrous form and dihydrate MnCl2·2H2O are also known...

 
trans-[MnCl2(H2O)4] two
MnCl2(H2O)4
Manganese(II) chloride
Manganese chloride describes a series of compounds with the formula MnCl2x, where the value of x can be 0, 2, or 4. The tetrahydrate is the most common form of "manganese chloride". MnCl2·4H2O, but the anhydrous form and dihydrate MnCl2·2H2O are also known...

 
cis-[MnCl2(H2O)4] none note cis
molecular
MnBr2(H2O)4
Manganese(II) bromide
Manganese bromide is the chemical compound composed of manganese and bromine with the formula MnBr2.It can be used in place of palladium in the Stille reaction, which couples two carbon atoms using an organotin compound....

 
cis-[MnBr2(H2O)4] none note cis
molecular
MnCl2(H2O)2
Manganese(II) chloride
Manganese chloride describes a series of compounds with the formula MnCl2x, where the value of x can be 0, 2, or 4. The tetrahydrate is the most common form of "manganese chloride". MnCl2·4H2O, but the anhydrous form and dihydrate MnCl2·2H2O are also known...

 
trans-[MnCl4(H2O)2] none polymeric with
bridging chloride
MnBr2(H2O)2
Manganese(II) bromide
Manganese bromide is the chemical compound composed of manganese and bromine with the formula MnBr2.It can be used in place of palladium in the Stille reaction, which couples two carbon atoms using an organotin compound....

 
trans-[MnBr4(H2O)2] none polymeric with
bridging bromide
FeCl2(H2O)6
Iron(II) chloride
Iron chloride, also known as ferrous chloride, is the chemical compound of formula FeCl2. It is a paramagnetic solid with a high melting point, and is usually obtained as an off-white solid. FeCl2 crystallizes from water as the greenish tetrahydrate, which is the form that is most commonly...

 
trans-[FeCl2(H2O)4] two
FeCl2(H2O)4
Iron(II) chloride
Iron chloride, also known as ferrous chloride, is the chemical compound of formula FeCl2. It is a paramagnetic solid with a high melting point, and is usually obtained as an off-white solid. FeCl2 crystallizes from water as the greenish tetrahydrate, which is the form that is most commonly...

 
trans-[FeCl2(H2O)4] none molecular
FeBr2(H2O)4
Iron(II) bromide
Iron bromide is the chemical compound FeBr2. This brownish-colored solid is a useful synthetic intermediate; for example it is employed to insert Fe into porphyrins.-Structure:...

 
trans-[FeBr2(H2O)4] none molecular
FeCl2(H2O)2
Iron(II) chloride
Iron chloride, also known as ferrous chloride, is the chemical compound of formula FeCl2. It is a paramagnetic solid with a high melting point, and is usually obtained as an off-white solid. FeCl2 crystallizes from water as the greenish tetrahydrate, which is the form that is most commonly...

 
trans-[FeCl4(H2O)2] none polymeric with
bridging chloride
CoCl2(H2O)6
Cobalt(II) chloride
Cobalt chloride is an inorganic compound of cobalt and chloride, with the formula CoCl2. It is usually supplied as the hexahydrate CoCl2·6H2O, which is one of the most commonly used cobalt compounds in the laboratory. The hexahydrate is deep purple in color, whereas the anhydrous form is sky blue...

 
trans-[CoCl2(H2O)4] two
CoBr2(H2O)6
Cobalt(II) bromide
Cobalt bromide is an inorganic compound used primarily as a catalyst in some processes.-Properties:When anhydrous, cobalt bromide appears as green crystals. The hexahydrate loses four waters of crystallization molecules at 100°C forming the dihydrate:Further heating to 130°C Produces the anhydrous...

 
trans-[CoBr2(H2O)4] two
CoBr2(H2O)4
Cobalt(II) bromide
Cobalt bromide is an inorganic compound used primarily as a catalyst in some processes.-Properties:When anhydrous, cobalt bromide appears as green crystals. The hexahydrate loses four waters of crystallization molecules at 100°C forming the dihydrate:Further heating to 130°C Produces the anhydrous...

 
trans-[CoBr2(H2O)4] none molecular
CoCl2(H2O)4
Cobalt(II) chloride
Cobalt chloride is an inorganic compound of cobalt and chloride, with the formula CoCl2. It is usually supplied as the hexahydrate CoCl2·6H2O, which is one of the most commonly used cobalt compounds in the laboratory. The hexahydrate is deep purple in color, whereas the anhydrous form is sky blue...

 
cis-[CoCl2(H2O)4] none note: cis
molecular
CoCl2(H2O)2
Cobalt(II) chloride
Cobalt chloride is an inorganic compound of cobalt and chloride, with the formula CoCl2. It is usually supplied as the hexahydrate CoCl2·6H2O, which is one of the most commonly used cobalt compounds in the laboratory. The hexahydrate is deep purple in color, whereas the anhydrous form is sky blue...

 
trans-[CoCl4(H2O)2] none polymeric with
bridging chloride
CoBr2(H2O)2
Cobalt(II) chloride
Cobalt chloride is an inorganic compound of cobalt and chloride, with the formula CoCl2. It is usually supplied as the hexahydrate CoCl2·6H2O, which is one of the most commonly used cobalt compounds in the laboratory. The hexahydrate is deep purple in color, whereas the anhydrous form is sky blue...

 
trans-[CoBr4(H2O)2] none polymeric with
bridging bromide
NiCl2(H2O)6
Nickel(II) chloride
Nickel chloride , is the chemical compound NiCl2. The anhydrous salt is yellow, but the more familiar hydrate NiCl2·6H2O is green. It is very rarely found in nature as mineral nickelbischofite. A dihydrate is also known. In general nickel chloride, in various forms, is the most important source of...

 
trans-[NiCl2(H2O)4] two
NiCl2(H2O)4
Nickel(II) chloride
Nickel chloride , is the chemical compound NiCl2. The anhydrous salt is yellow, but the more familiar hydrate NiCl2·6H2O is green. It is very rarely found in nature as mineral nickelbischofite. A dihydrate is also known. In general nickel chloride, in various forms, is the most important source of...

 
cis-[NiCl2(H2O)4] none note: cis
molecular
NiBr2(H2O)6
Nickel(II) bromide
Nickel bromide, NiBr2, is the nickel salt of hydrobromic acid. It can be made by reacting nickel, nickel oxide, nickel carbonate, or nickel hydroxide with hydrobromic acid. It can also be made by reacting nickel with bromine. It is a weak reducing agent.It is yellow-brown, rhombohedral,...

 
trans-[NiBr2(H2O)4] two
NiCl2(H2O)2
Nickel(II) chloride
Nickel chloride , is the chemical compound NiCl2. The anhydrous salt is yellow, but the more familiar hydrate NiCl2·6H2O is green. It is very rarely found in nature as mineral nickelbischofite. A dihydrate is also known. In general nickel chloride, in various forms, is the most important source of...

 
trans-[NiCl2(H2O)4] none polymeric with
bridging chloride
CuCl2(H2O)2
Copper(II) chloride
Copper chloride is the chemical compound with the formula CuCl2. This is a light brown solid, which slowly absorbs moisture to form a blue-green dihydrate. The copper chlorides are some of the most common copper compounds, after copper sulfate....

 
[CuCl4(H2O)2]2 none tetragonally distorted
two long Cu-Cl distances
CuBr2(H2O)4
Copper(II) bromide
Copper bromide is a chemical compound. It is used in photographic processing as an intensifier and as a brominating agent in organic synthesis....

[CuBr4(H2O)2]n two tetragonally distorted
two long Cu-Br distances
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