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. 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 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.

Applications

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

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:

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 and anhydrous behave identically. Therefore, knowledge of the degree of hydration is important only for determining the equivalent weight: one mole of weighs more than one mole of. In some cases, the degree of hydration can be critical to the resulting chemical properties. For example, anhydrous is not soluble in water and is relatively useless in organometallic chemistry whereas is versatile. Similarly, hydrated is a poor Lewis acid and thus inactive as a catalyst for Friedel-Crafts reactions. Samples of must therefore be protected from atmospheric moisture to preclude the formation of hydrates.
Crystals of hydrated copper sulfate consist of centers linked to 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 and. In tin chloride, each Sn center is pyramidal 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,, is a white crystalline solid with greater than 50% water by weight.
Consider the case of nickel chloride hexahydrate. This species has the formula. Crystallographic analysis reveals that the solid consists of subunits that are hydrogen bonded to each other as well as two additional molecules of. Thus one third of the water molecules in the crystal are not directly bonded to, 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 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 a particularly common solvent to be found in crystals because it is small and polar. But many other solvents can be hosted in crystals, known as solvates. 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	Remarks
Calcium chloride		0	example of water as a bridging ligand
Calcium bromide	²⁺	1	the most hydrated calcium halide
Calcium iodide	²⁺	0	bridging water ligands
Calcium iodide	²⁺	0
Calcium iodide	²⁺	0	bridging water ligands
Titanium chloride	trans-	2	isomorphous with
Titanium chloride	³⁺		isomeric with ^.2H₂O
Zirconium fluoride			rare case where Hf and Zr differ
Hafnium tetrafluoride		1	rare case where Hf and Zr differ
Vanadium chloride	trans-	2
Vanadium bromide	trans-	2
Vanadium iodide			relative to and, competes poorly with water as a ligand for V
		4
Chromium chloride	trans-	2	dark green isomer, aka "Bjerrums's salt"
Chromium chloride		1	blue-green isomer
Chromium chloride	trans-		square planar/tetragonal distortion
Chromium chloride			violet isomer. isostructural with aluminium compound
Manganese chloride	trans-	2
Manganese chloride	cis-		cis molecular, the unstable trans isomer has also been detected
Manganese bromide	cis-		cis, molecular
Manganese iodide	trans-		molecular, isostructural with FeCl24.
Manganese chloride	trans-		polymeric with bridging chloride
Manganese bromide	trans-		polymeric with bridging bromide
Rhenium chloride	triangulo-		heavy early metals form M-M bonds
Iron chloride	trans-	two
Iron chloride	trans-		molecular
Iron bromide	trans-		molecular, hydrates of FeI2 are not known
Iron chloride	trans-		polymeric with bridging chloride
Iron chloride	trans-	two	one of four hydrates of ferric chloride, isostructural with Cr analogue
Iron chloride	cis-	two	the dihydrate has a similar structure, both contain anions.
Cobalt chloride	trans-	two
Cobalt bromide	trans-	two
Cobalt iodide			iodide competes poorly with water
Cobalt bromide	trans-		molecular
Cobalt chloride	cis-		note: cis molecular
Cobalt chloride	trans-		polymeric with bridging chloride
Cobalt bromide	trans-		polymeric with bridging bromide
Nickel chloride	trans-	two
Nickel chloride	cis-		note: cis molecular
Nickel bromide	trans-	two
Nickel iodide			iodide competes poorly with water
Nickel chloride	trans-		polymeric with bridging chloride
Platinum chloride	trans-	3	octahedral Pt centers; rare example of non-first row chloride-aquo complex
Platinum chloride	fac-	0.5	octahedral Pt centers; rare example of non-first row chloride-aquo complex
Copper chloride			tetragonally distorted two long Cu-Cl distances
Copper bromide		two	tetragonally distorted two long Cu-Br distances
Zinc chloride ZnCl₂_1.33			coordination polymer with both tetrahedral and octahedral Zn centers
Zinc chloride ZnCl₂_2.5			tetrahedral and octahedral Zn centers
Zinc chloride			tetrahedral and octahedral Zn centers
Zinc chloride ZnCl₂_4.5			tetrahedral and octahedral Zn centers
Cadmium chloride CdCl₂·H₂O			water of crystallization is rare for heavy metal halides
Cadmium chloride CdCl₂·2.5H₂O	CdCl₅ & CdCl₄₂
Cadmium chloride CdCl₂·4H₂O	CdCl₄₄		octahedral, doubly bridging chlorides
Cadmium bromide CdBr₂₄		two	octahedral Cd centers
Aluminum trichloride			isostructural with the Cr compound
Aluminum triiodide			other hydrates are known
Aluminum triiodide			other hydrates are known
Aluminum triiodide			the highest hydrate crystallized