In chemistry, water(s) of crystallization or water(s) 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.
<!--Mostly, the term is limited to non-coordinated (interstitial) water. Coordinated water is directly bonded to a central atom on a lattice point. Other types of water that may be present in a crystal are anion water (with hydrogen bonds to anions),
lattice water (no direct bonding with an ion) and constitution water (water present as hydroxyl groups). Zeolite water is water that occupies vacancies (empty sites in the crystal lattice) and may be removed without changing the crystal structure. -->
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
thumb|upright=1.6|Some hydrogen-bonding contacts in . This [[metal aquo complex crystallizes with water of hydration, which interacts with the sulfate and with the centers.]]
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:
- – copper(II) sulfate pentahydrate
- – cobalt(II) chloride hexahydrate
- – tin(II) (or stannous) chloride dihydrate
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.
thumb|Structure of the polymeric center in crystalline calcium chloride hexahydrate. Three water ligands are terminal, three bridge. Two aspects of metal aquo complexes are illustrated: the high coordination number typical for and the role of water as a [[bridging ligand.]]
Crystals of hydrated copper(II) 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(II) center is pyramidal (mean 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, , 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 . 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 (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 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.
{| class="wikitable"
! Hydrated metal halides<br/>and their formulas || Coordination sphere<br/>of the metal ||Equivalents of water of crystallization<br/> that are not bound to M || Remarks
|-
| Calcium chloride<br/> || ||
|example of water as a bridging ligand
|-
| Calcium bromide<br/> ||<sup>2+</sup> || style="text-align:center;" | 1 ||the most hydrated calcium halide
|-
| Calcium iodide<br/> ||<sup>2+</sup>
|-
|Zirconium(IV) fluoride<br/>|| || || rare case where Hf and Zr differ
|-
|Hafnium tetrafluoride<br/>|| || style="text-align:center;" | 1||rare case where Hf and Zr differ || style="text-align:center;" | 2||
|-
| Vanadium(III) iodide<br/> || || ||relative to and , competes poorly<br/> with water as a ligand for V(III)
|-
| || || style="text-align:center;" |4 ||
|-
| Chromium(III) chloride<br/> || trans- || style="text-align:center;" | 2 ||dark green isomer, aka "Bjerrums's salt"
|-
| Chromium(III) chloride<br/> || || style="text-align:center;" | 1 ||blue-green isomer
|-
| Chromium(II) chloride<br/> || trans- || ||square planar/tetragonal distortion
|-
| Chromium(III) chloride<br/> || || ||violet isomer. isostructural with aluminium compound
|-
| Manganese(II) chloride<br/> || trans- || style="text-align:center;" | 2||
|-
| Manganese(II) chloride<br/> || cis- || ||cis molecular, the unstable trans isomer has also been detected
|-
| Manganese(II) bromide<br/> || cis- || ||cis, molecular
|-
| Manganese(II) iodide<br/> || trans- || ||molecular, isostructural with FeCl2(H2O)4.
|-
| Manganese(II) chloride<br/> || trans- || ||polymeric with bridging chloride
|-
| Manganese(II) bromide<br/> || trans- || ||polymeric with bridging bromide
|-
| Rhenium(III) chloride<br/> || triangulo- || ||heavy early metals form M-M bonds
|-
| Iron(II) chloride<br/> || trans- || style="text-align:center;" | two ||
|-
| Iron(II) chloride<br/> || trans- || ||molecular
|-
| Iron(II) bromide<br/> || trans- || ||molecular, isostructural with Cr analogue
|-
| Iron(III) chloride<br/> || cis- || style="text-align:center;" | two ||the dihydrate has a similar structure, both contain anions.||iodide competes poorly with water
|-
| Cobalt(II) bromide<br/> || trans- || ||molecular
|-
| Nickel(II) bromide<br/> || trans- || style="text-align:center;" | two||
|-
| Nickel(II) iodide<br/> || || || trans- || style="text-align: center" | 3 || octahedral Pt centers; rare example of non-first row chloride-aquo complex
|-
| Platinum(IV) chloride<br/> || fac- || style="text-align: center" | 0.5 || octahedral Pt centers; rare example of non-first row chloride-aquo complex
|-
| Copper(II) chloride<br/> || || ||tetragonally distorted<br/> two long Cu-Cl distances
|-
| Copper(II) bromide<br/> || || style="text-align:center;" | two ||tetragonally distorted<br/> two long Cu-Br distances || || ||coordination polymer with both tetrahedral and octahedral Zn centers
|-
| Zinc(II) chloride<br/>ZnCl<sub>2</sub>(H<sub>2</sub>O)<sub>2.5</sub> || || || tetrahedral and octahedral Zn centers
|-
| Zinc(II) chloride<br/>|| || ||water of crystallization is rare for heavy metal halides
|-
| Cadmium chloride<br/>CdCl<sub>2</sub>·2.5H<sub>2</sub>O || CdCl<sub>5</sub>(H<sub>2</sub>O) & CdCl<sub>4</sub>(H<sub>2</sub>O)<sub>2</sub> || ||
|-
| Cadmium chloride<br/>CdCl<sub>2</sub>·4H<sub>2</sub>O
|| CdCl<sub>4</sub>(H<sub>2</sub>O)<sub>4</sub> || || octahedral, doubly bridging chlorides
|-
| Cadmium bromide<br/>CdBr<sub>2</sub>(H<sub>2</sub>O)<sub>4</sub>|| || style="text-align:center;" |two || octahedral Cd centers
|-
| Aluminum trichloride<br/> || || ||isostructural with the Cr(III) compound
|-
| Aluminum triiodide<br/> || || || other hydrates are known
|-
| Aluminum triiodide<br/> || || || other hydrates are known || || kieserite || see Mn, Fe, Co, Ni, Zn analogues
|-
| MgSO<sub>4</sub>(H<sub>2</sub>O)<sub>4</sub> || [Mg(H<sub>2</sub>O)<sub>4</sub>(κ′,κ<sup>1</sup>-SO<sub>4</sub>)]<sub>2</sub> || || ||sulfate is bridging ligand, 8-membered Mg<sub>2</sub>O<sub>4</sub>S<sub>2</sub> rings
|-
| MgSO<sub>4</sub>(H<sub>2</sub>O)<sub>6</sub> || [Mg(H<sub>2</sub>O)<sub>6</sub>] || ||hexahydrate || common motif
|-
| VSO<sub>4</sub>(H<sub>2</sub>O)<sub>7</sub>|| [V(H<sub>2</sub>O)<sub>6</sub>] ||style="text-align:center;" | one|| ||hexaaquo
|-
|VOSO<sub>4</sub>(H<sub>2</sub>O)<sub>5</sub>|| [VO(H<sub>2</sub>O)<sub>4</sub>(κ<sup>1</sup>-SO<sub>4</sub>)<sub>4</sub>]|| style="text-align:center;" | one||||
|-
| Cr(SO<sub>4</sub>)(H<sub>2</sub>O)<sub>3</sub> || [Cr(H<sub>2</sub>O)<sub>3</sub>(κ<sup>1</sup>-SO<sub>4</sub>)] || || ||resembles Cu(SO<sub>4</sub>)(H<sub>2</sub>O)<sub>3</sub>
|-
| Cr(SO<sub>4</sub>)(H<sub>2</sub>O)<sub>5</sub> || [Cr(H<sub>2</sub>O)<sub>4</sub>(κ<sup>1</sup>-SO<sub>4</sub>)<sub>2</sub>] || style="text-align:center;" | one|| ||resembles Cu(SO<sub>4</sub>)(H<sub>2</sub>O)<sub>5</sub>
|-
| Cr<sub>2</sub>(SO<sub>4</sub>)<sub>3</sub>(H<sub>2</sub>O)<sub>18</sub> || [Cr(H<sub>2</sub>O)<sub>6</sub>] || style="text-align:center;" | six|| ||One of several chromium(III) sulfates
|-
| MnSO<sub>4</sub>(H<sub>2</sub>O) ||[Mn(μ-H<sub>2</sub>O)(μ<sub>4</sub>,-κ<sup>1</sup>-SO<sub>4</sub>)<sub>4</sub>] || || Ilesitepentahydrate is called jôkokuite; the hexahydrate, the most rare, is called chvaleticeite ||with 8-membered ring Mn<sub>2</sub>(SO<sub>4</sub>)<sub>2</sub> core
|-
| MnSO<sub>4</sub>(H<sub>2</sub>O)<sub>5</sub> || || || jôkokuite ||
|-
| MnSO<sub>4</sub>(H<sub>2</sub>O)<sub>6</sub> || || || Chvaleticeite ||
|-
| MnSO<sub>4</sub>(H<sub>2</sub>O)<sub>7</sub> || [Mn(H<sub>2</sub>O)<sub>6</sub>] || style="text-align:center;" | one|| mallardite
|-
| CoSO<sub>4</sub>(H<sub>2</sub>O) ||[Co(μ-H<sub>2</sub>O)(μ<sub>4</sub>-κ<sup>1</sup>-SO<sub>4</sub>)<sub>4</sub>]
|-
| NiSO<sub>4</sub>(H<sub>2</sub>O)<sub>7</sub> || [Ni(H<sub>2</sub>O)<sub>6</sub>] || one | ||morenosite ||none || ||
|-
| (NH<sub>4</sub>)<sub>2</sub>[Pt<sub>2</sub>(SO<sub>4</sub>)<sub>4</sub>(H<sub>2</sub>O)<sub>2</sub>] || [Pt<sub>2</sub>(SO<sub>4</sub>)<sub>4</sub>(H<sub>2</sub>O)<sub>2</sub>]<sup>2−</sup> || || || Pt-Pt bonded Chinese lantern structure
|-
| CuSO<sub>4</sub>(H<sub>2</sub>O)<sub>5</sub> || [Cu(H<sub>2</sub>O)<sub>4</sub>(κ<sup>1</sup>-SO<sub>4</sub>)<sub>2</sub>] || style="text-align:center;" | one|| chalcantite||sulfate is bridging ligand
|-
| CuSO<sub>4</sub>(H<sub>2</sub>O)<sub>7</sub> || [Cu(H<sub>2</sub>O)<sub>6</sub>] || style="text-align:center;" | one|| boothite
|-
| ZnSO<sub>4</sub>(H<sub>2</sub>O)<sub>6</sub> || [Zn(H<sub>2</sub>O)<sub>6</sub>] || || ||see Mg analogue
|-
| ZnSO<sub>4</sub>(H<sub>2</sub>O)<sub>7</sub> || [Zn(H<sub>2</sub>O)<sub>6</sub>] || style="text-align:center;" |one|| goslarite|| see Mg analogue
|-
|CdSO<sub>4</sub>(H<sub>2</sub>O)||[Cd(μ-H<sub>2</sub>O)<sub>2</sub>(κ<sup>1</sup>-SO<sub>4</sub>)<sub>4</sub>]|| || ||bridging water ligand
|-
|}
Hydrates of metal nitrates
Transition metal nitrates form a variety of hydrates. The nitrate anion often binds to the metal, especially for those salts with fewer than six aquo ligands. Nitrates are uncommon in nature, so few minerals are represented here. Hydrated ferrous nitrate has not been characterized crystallographically.
{| class="wikitable"
! Formula of<br/> hydrated metal ion nitrate || Coordination<br/>sphere of the metal ion ||Equivalents of water of crystallization<br/> that are not bound to M ||Remarks
|-
| Cr(NO<sub>3</sub>)<sub>3</sub>(H<sub>2</sub>O)<sub>9</sub> || [Cr(H<sub>2</sub>O)<sub>6</sub>]<sup>3+</sup> || style="text-align:center;" |three|| octahedral configuration isostructural with Fe(NO<sub>3</sub>)<sub>3</sub>(H<sub>2</sub>O)<sub>9</sub>
|-
| Mn(NO<sub>3</sub>)<sub>2</sub>(H<sub>2</sub>O)<sub>4</sub> || cis-[Mn(H<sub>2</sub>O)<sub>4</sub>(κ<sup>1</sup>-ONO<sub>2</sub>)<sub>2</sub>] || || octahedral configuration
|-
| Mn(NO<sub>3</sub>)<sub>2</sub>(H<sub>2</sub>O) || [Mn(H<sub>2</sub>O)(μ-ONO<sub>2</sub>)<sub>5</sub>] || || octahedral configuration
|-
| Mn(NO<sub>3</sub>)<sub>2</sub>(H<sub>2</sub>O)<sub>6</sub> || [Mn(H<sub>2</sub>O)<sub>6</sub>] || || octahedral configuration isomorphous with Zn analogue
|-
| Fe(NO<sub>3</sub>)<sub>3</sub>(H<sub>2</sub>O)<sub>9</sub> || [Fe(H<sub>2</sub>O)<sub>6</sub>]<sup>3+</sup> || style="text-align:center;" |three|| octahedral configuration isostructural with Cr(NO<sub>3</sub>)<sub>3</sub>(H<sub>2</sub>O)<sub>9</sub>
|-
| Fe(NO<sub>3</sub>)<sub>3</sub>)(H<sub>2</sub>O)<sub>4</sub> || [Fe(H<sub>2</sub>O)<sub>3</sub>(κ<sup>2</sup>-O<sub>2</sub>NO)<sub>2</sub>]<sup>+</sup> || style="text-align:center;" |one|| pentagonal bipyramid
|-
| Fe(NO<sub>3</sub>)<sub>3</sub>(H<sub>2</sub>O)<sub>5</sub> || [Fe(H<sub>2</sub>O)<sub>5</sub>(κ<sup>1</sup>-ONO<sub>2</sub>)]<sup>2+</sup> || || octahedral configuration
|-
| α-Ni(NO<sub>3</sub>)<sub>2</sub>(H<sub>2</sub>O)<sub>4</sub> || cis-[Ni(H<sub>2</sub>O)<sub>4</sub>(κ<sup>1</sup>-ONO<sub>2</sub>)<sub>2</sub>] || || octahedral configuration.
|-
| β-Ni(NO<sub>3</sub>)<sub>2</sub>(H<sub>2</sub>O)<sub>4</sub> || trans-[Ni(H<sub>2</sub>O)<sub>4</sub>(κ<sup>1</sup>-ONO<sub>2</sub>)<sub>2</sub>] || || octahedral configuration.
|-
|Pd(NO<sub>3</sub>)<sub>2</sub>(H<sub>2</sub>O)<sub>2</sub> || || ||square planar coordination geometry
|-
| Cu(NO<sub>3</sub>)<sub>2</sub>(H<sub>2</sub>O) || [Cu(H<sub>2</sub>O)(κ<sup>2</sup>-ONO<sub>2</sub>)<sub>2</sub>] || || octahedral configuration.
|-
| Cu(NO<sub>3</sub>)<sub>2</sub>(H<sub>2</sub>O)<sub>1.5</sub> || || ||
|-
| Cu(NO<sub>3</sub>)<sub>2</sub>(H<sub>2</sub>O)<sub>2.5</sub> || [Cu(H<sub>2</sub>O)<sub>2</sub>(κ<sup>1</sup>-ONO<sub>2</sub>)<sub>2</sub>] || style="text-align:center;" |one|| square planar
|-
| Cu(NO<sub>3</sub>)<sub>2</sub>(H<sub>2</sub>O)<sub>3</sub> || || ||
|-
| Cu(NO<sub>3</sub>)<sub>2</sub>(H<sub>2</sub>O)<sub>6</sub> || [Cu(H<sub>2</sub>O)<sub>6</sub>]<sup>2+</sup> || || octahedral configuration
|-
| Zn(NO<sub>3</sub>)<sub>2</sub>(H<sub>2</sub>O)<sub>4</sub> || cis-[Zn(H<sub>2</sub>O)<sub>4</sub>(κ<sup>1</sup>-ONO<sub>2</sub>)<sub>2</sub>] || || octahedral configuration.
|-
| Zn(NO<sub>3</sub>)<sub>2</sub>(H<sub>2</sub>O)<sub>6</sub> || [Zn(H<sub>2</sub>O)<sub>6</sub>] || || octahedral configuration isomorphous with Mn analogue
|-
|}
Gallery
<gallery>
File:Copper sulfate.jpg|Hydrated copper(II) sulfate is bright blue.
File:Copper sulfate anhydrous.jpg|Anhydrous copper(II) sulfate has a light turquoise tint.
File:ICSD CollCode71346.png|Substructure of MSO<sub>4</sub>(H<sub>2</sub>O), illustrating presence of bridging water and bridging sulfate (M = Mg, Mn, Fe, Co, Ni, Zn).
</gallery>
See also
- Hydrate
- Mineral hydration
- Hydrous oxide