Water of crystallization

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.^[1] 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 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.^[2]

Position in the crystal structure

Some hydrogen-bonding contacts in FeSO₄·7H₂O. This metal aquo complex crystallizes with water of hydration, which interacts with the sulfate and with the [Fe(H₂O)₆]²⁺ 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.^{[3] [4]} 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.^[5] 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 made labile 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.

Structure of the polymeric [Ca(H₂O)₆]²⁺ 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 Ca²⁺ and the role of water as a bridging ligand.

Crystals of hydrated copper(II) sulfate consist of [Cu(H₂O)₄]²⁺ centers linked to SO2−4 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.^[6] 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 one third 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.^[7][8]

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 CaCl2(H2O)6	[Ca(μ-H2O)6(H2O)3]2+	none	example of water as a bridging ligand[9]
Titanium(III) chloride TiCl3(H2O)6	trans-[TiCl2(H2O)4]+[10]	two	isomorphous with VCl3(H2O)6
Titanium(III) chloride TiCl3(H2O)6	[Ti(H2O)6]3+[10]	none	isomeric with [TiCl2(H2O)4]Cl.2H2O[11]
Zirconium(IV) fluoride ZrF4(H2O)3	(μ−F)2[ZrF3(H2O)3]2	none	rare case where Hf and Zr differ[12]
Hafnium tetrafluoride HfF4(H2O)3	(μ−F)2[HfF2(H2O)2]n(H2O)n	one	rare case where Hf and Zr differ[12]
Vanadium(III) chloride VCl3(H2O)6	trans-[VCl2(H2O)4]+[10]	two
Vanadium(III) bromide VBr3(H2O)6	trans-[VBr2(H2O)4]+[10]	two
Vanadium(III) iodide VI3(H2O)6	[V(H2O)6]3+	none	relative to Cl− and Br−, I− competes poorly with water as a ligand for V(III)
Nb6Cl14(H2O)8	[Nb6Cl14(H2O)2]	four
Chromium(III) chloride CrCl3(H2O)6	trans-[CrCl2(H2O)4]+	two	dark green isomer, aka "Bjerrums's salt"
Chromium(III) chloride CrCl3(H2O)6	[CrCl(H2O)5]2+	one	blue-green isomer
Chromium(II) chloride CrCl2(H2O)4	trans-[CrCl2(H2O)4]	none	square planar/tetragonal distortion
Chromium(III) chloride CrCl3(H2O)6	[Cr(H2O)6]3+	none	violet isomer. isostructural with aluminium compound[13]
Manganese(II) chloride MnCl2(H2O)6	trans-[MnCl2(H2O)4]	two
Manganese(II) chloride MnCl2(H2O)4	cis-[MnCl2(H2O)4]	none	cis molecular, the unstable trans isomer has also been detected[14]
Manganese(II) bromide MnBr2(H2O)4	cis-[MnBr2(H2O)4]	none	cis, molecular
Manganese(II) iodide MnI2(H2O)4	trans-[MnI2(H2O)4]	none	molecular, isostructural with FeCl2(H2O)4.[15]
Manganese(II) chloride MnCl2(H2O)2	trans-[MnCl4(H2O)2]	none	polymeric with bridging chloride
Manganese(II) bromide MnBr2(H2O)2	trans-[MnBr4(H2O)2]	none	polymeric with bridging bromide
Rhenium(III) chloride Re3Cl9(H2O)4	triangulo-[Re3Cl9(H2O)3]	none	heavy early metals form M-M bonds[16]
Iron(II) chloride FeCl2(H2O)6	trans-[FeCl2(H2O)4]	two
Iron(II) chloride FeCl2(H2O)4	trans-[FeCl2(H2O)4]	none	molecular
Iron(II) bromide FeBr2(H2O)4	trans-[FeBr2(H2O)4]	none	molecular,[17] hydrates of FeI2 are not known
Iron(II) chloride FeCl2(H2O)2	trans-[FeCl4(H2O)2]	none	polymeric with bridging chloride
Iron(III) chloride FeCl3(H2O)6	trans-[FeCl2(H2O)4]+	two	one of four hydrates of ferric chloride,[18] isostructural with Cr analogue
Iron(III) chloride FeCl3(H2O)2.5	cis-[FeCl2(H2O)4]+	two	the dihydrate has a similar structure, both contain FeCl−4 anions.[18]
Cobalt(II) chloride CoCl2(H2O)6	trans-[CoCl2(H2O)4]	two
Cobalt(II) bromide CoBr2(H2O)6	trans-[CoBr2(H2O)4]	two
Cobalt(II) iodide CoI2(H2O)6	[Co(H2O)6]2+	none[19]	iodide competes poorly with water
Cobalt(II) bromide CoBr2(H2O)4	trans-[CoBr2(H2O)4]	none	molecular[17]
Cobalt(II) chloride CoCl2(H2O)4	cis-[CoCl2(H2O)4]	none	note: cis molecular
Cobalt(II) chloride CoCl2(H2O)2	trans-[CoCl4(H2O)2]	none	polymeric with bridging chloride
Cobalt(II) bromide CoBr2(H2O)2	trans-[CoBr4(H2O)2]	none	polymeric with bridging bromide
Nickel(II) chloride NiCl2(H2O)6	trans-[NiCl2(H2O)4]	two
Nickel(II) chloride NiCl2(H2O)4	cis-[NiCl2(H2O)4]	none	note: cis molecular[17]
Nickel(II) bromide NiBr2(H2O)6	trans-[NiBr2(H2O)4]	two
Nickel(II) iodide NiI2(H2O)6	[Ni(H2O)6]2+	none[19]	iodide competes poorly with water
Nickel(II) chloride NiCl2(H2O)2	trans-[NiCl4(H2O)2]	none	polymeric with bridging chloride
Platinum(IV) chloride [Pt(H2O)2Cl4](H2O)3[20]	trans-[PtCl4(H2O)2]	3	octahedral Pt centers; rare example of non-first row chloride-aquo complex
Platinum(IV) chloride [Pt(H2O)3Cl3]Cl(H2O)0.5[21]	fac-[PtCl3(H2O)3]+	0.5	octahedral Pt centers; rare example of non-first row chloride-aquo complex
Copper(II) chloride CuCl2(H2O)2	[CuCl4(H2O)2]2	none	tetragonally distorted two long Cu-Cl distances
Copper(II) bromide CuBr2(H2O)4	[CuBr4(H2O)2]n	two	tetragonally distorted two long Cu-Br distances[17]
Zinc(II) chloride ZnCl2(H2O)1.33[22]	2 ZnCl2 + ZnCl2(H2O)4	none	coordination polymer with both tetrahedral and octahedral Zn centers
Zinc(II) chloride ZnCl2(H2O)2.5[23]	Cl3Zn(μ-Cl)Zn(H2O)5	none	tetrahedral and octahedral Zn centers
Zinc(II) chloride ZnCl2(H2O)3[22]	[ZnCl4]2− & [Zn(H2O)6]2+	none	tetrahedral and octahedral Zn centers
Zinc(II) chloride ZnCl2(H2O)4.5[22]	[ZnCl4]2− & [Zn(H2O)6]2+	three	tetrahedral and octahedral Zn centers
Cadmium chloride CdCl2·H2O[24]		none	water of crystallization is rare for heavy metal halides
Cadmium chloride CdCl2·2.5H2O[25]	CdCl5(H2O) & CdCl4(H2O)2	none
Cadmium chloride CdCl2·4H2O[26]		none	octahedral
Cadmium bromide CdBr2(H2O)4[27]	[CdBr4(H2O)2	two	octahedral Cd centers
Aluminum trichloride AlCl3(H2O)6	[Al(H2O)6]3+	none	isostructural with the Cr(III) compound

Examples are rare for second and third row metals. No entries exist for Mo, W, Tc, Ru, Os, Rh, Ir, Pd, Hg, Au. AuCl₃(H₂O) has been invoked but its crystal structure has not been reported.

Hydrates of metal sulfates

Transition metal sulfates form a variety of hydrates, each of which crystallizes in only one form. The sulfate group often binds to the metal, especially for those salts with fewer than six aquo ligands. 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.^[28] Many of the metal sulfates occur in nature, being the result of weathering of mineral sulfides.^[29][30] Many monohydrates are known.^[31]

Formula of hydrated metal ion sulfate	Coordination sphere of the metal ion	Equivalents of water of crystallization that are not bound to M	mineral name	Remarks
MgSO4(H2O)	[Mn(μ-H2O)(μ4,-κ1-SO4)4][31]	none	kieserite	see Mn, Fe, Co, Ni, Zn analogues Substructure of MSO4(H2O), illustrating presence of bridging water and bridging sulfate (M = Mg, Mn, Fe, Co, Ni, Zn).
MgSO4(H2O)4	[Mg(H2O)4(κ′,κ1-SO4)]2	none		sulfate is bridging ligand, 8-membered Mg2O4S2 rings[32]
MgSO4(H2O)6	[Mg(H2O)6]	none	hexahydrate	common motif[29]
MgSO4(H2O)7	[Mg(H2O)6]	one	epsomite	common motif[29]
TiOSO4(H2O)	[Ti(μ-O)2(H2O)(κ1-SO4)3]	none		further hydration gives gels
VSO4(H2O)6	[V(H2O)6]	none		Adopts the hexahydrite motif[33]
VSO4(H2O)7	[V(H2O)6]	one		hexaaquo[34]
VOSO4(H2O)5	[VO(H2O)4(κ1-SO4)4]	one
Cr(SO4)(H2O)3	[Cr(H2O)3(κ1-SO4)]	none		resembles Cu(SO4)(H2O)3[35]
Cr(SO4)(H2O)5	[Cr(H2O)4(κ1-SO4)2]	one		resembles Cu(SO4)(H2O)5[36]
Cr2(SO4)3(H2O)18	[Cr(H2O)6]	six		One of several chromium(III) sulfates
MnSO4(H2O)	[Mn(μ-H2O)(μ4,-κ1-SO4)4][31]	none	szmikite	see Fe, Co, Ni, Zn analogues
MnSO4(H2O)4	[Mn(μ-SO4)2(H2O)4][37]	none	Ilesitepentahydrate is called jôkokuite; the hexahydrate, the most rare, is called chvaleticeite	with 8-membered ring Mn2(SO4)2 core
MnSO4(H2O)5		?	jôkokuite
MnSO4(H2O)6		?	Chvaleticeite
MnSO4(H2O)7	[Mn(H2O)6]	one	mallardite[30]	see Mg analogue
FeSO4(H2O)	[Fe(μ-H2O)(μ4-κ1-SO4)4][31]	none		see Mn, Co, Ni, Zn analogues
FeSO4(H2O)7	[Fe(H2O)6]	one	melanterite[30]	see Mg analogue
FeSO4(H2O)4	[Fe(H2O)4(κ′,κ1-SO4)]2	none		sulfate is bridging ligand, 8-membered Fe2O4S2 rings[32]
FeII(FeIII)2(SO4)4(H2O)14	[FeII(H2O)6]2+[FeIII(H2O)4(κ1-SO4)2]− 2	none		sulfates are terminal ligands on Fe(III)[38]
CoSO4(H2O)	[Co(μ-H2O)(μ4-κ1-SO4)4][31]	none		see Mn, Fe, Ni, Zn analogues
CoSO4(H2O)6	[Co(H2O)6]	none	moorhouseite	see Mg analogue
CoSO4(H2O)7	[Co(H2O)6]	one	bieberite[30]	see Fe, Mg analogues
NiSO4(H2O)	[Ni(μ-H2O)(μ4-κ1-SO4)4][31]	none		see Mn, Fe, Co, Zn analogues
NiSO4(H2O)6	[Ni(H2O)6]	none	retgersite	One of several nickel sulfate hydrates[39]
NiSO4(H2O)7	[Ni(H2O)6]		morenosite[30]
(NH4)2[Pt2(SO4)4(H2O)2]	[Pt2(SO4)4(H2O)2]2−	none		Pt-Pt bonded Chinese lantern structure[40]
CuSO4(H2O)5	[Cu(H2O)4(κ1-SO4)2]	one	chalcantite	sulfate is bridging ligand[41]
CuSO4(H2O)7	[Cu(H2O)6]	one	boothite[30]
ZnSO4(H2O)	[Zn(μ-H2O)(μ4-κ1-SO4)4][31]	none		see Mn, Fe, Co, Ni analogues
ZnSO4(H2O)4	[Zn(H2O)4(κ′,κ1-SO4)]2	none		sulfate is bridging ligand, 8-membered Zn2O4S2 rings[32][42]
ZnSO4(H2O)6	[Zn(H2O)6]	none		see Mg analogue[43]
ZnSO4(H2O)7	[Zn(H2O)6]	one	goslarite[30]	see Mg analogue
CdSO4(H2O)	[Cd(μ-H2O)2(κ1-SO4)4]	none		bridging water ligand[44]

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.

Formula of hydrated metal ion nitrate	Coordination sphere of the metal ion	Equivalents of water of crystallization that are not bound to M	Remarks
Cr(NO3)3(H2O)9	[Cr(H2O)6]3+	three	octahedral configuration[45] isostructural with Fe(NO3)3(H2O)9
Mn(NO3)2(H2O)4	cis-[Mn(H2O)4(κ1-ONO2)2]	none	octahedral configuration
Mn(NO3)2(H2O)	[Mn(H2O)(μ-ONO2)5]	none	octahedral configuration
Mn(NO3)2(H2O)6	[Mn(H2O)6]	none	octahedral configuration[46]
Fe(NO3)3(H2O)9	[Fe(H2O)6]3+	three	octahedral configuration[47] isostructural with Cr(NO3)3(H2O)9
Fe(NO3)3)(H2O)4	[Fe(H2O)3(κ2-O2NO)2]+	one	pentagonal bipyramid[48]
Fe(NO3)3(H2O)5	[Fe(H2O)5(κ1-ONO2)]2+	none	octahedral configuration[48]
Fe(NO3)3(H2O)6	[Fe(H2O)6]3+	none	octahedral configuration[48]
Co(NO3)2(H2O)2	[Co(H2O)2(κ1-ONO2)2]	none	octahedral configuration
Co(NO3)2(H2O)4	[Co(H2O)4(κ1-ONO2)2	none	octahedral configuration
Co(NO3)2(H2O)6	[Co(H2O)6]2+	none	octahedral configuration.[49]
α-Ni(NO3)2(H2O)4	cis-[Ni(H2O)4(κ1-ONO2)2]	none	octahedral configuration.[50]
β-Ni(NO3)2(H2O)4	trans-[Ni(H2O)4(κ1-ONO2)2]	none	octahedral configuration.[51]
Pd(NO3)2(H2O)2	trans-[Pd(H2O)2(κ1-ONO2)2]	none	square planar coordination geometry[52]
Cu(NO3)2(H2O)	[Cu(H2O)(κ2-ONO2)2]	none	octahedral configuration.
Cu(NO3)2(H2O)1.5	uncertain	uncertain	uncertain[53]
Cu(NO3)2(H2O)2.5	[Cu(H2O)2(κ1-ONO2)2]	one	square planar[54]
Cu(NO3)2(H2O)3	uncertain	uncertain	uncertain[55]
Cu(NO3)2(H2O)6	[Cu(H2O)6]2+	none	octahedral configuration[56]
Zn(NO3)2(H2O)4	cis-[Zn(H2O)4(κ1-ONO2)2]	none	octahedral configuration.
Hg2(NO3)2(H2O)2	[H2O–Hg–Hg–OH2]2+	linear[57]

Photos

• 
  Hydrated copper(II) sulfate is bright blue.
• 
  Anhydrous copper(II) sulfate has a light turquoise tint.

See also

• Hydrate
• Mineral hydration
• Hydrous oxide