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.
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]
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:
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.
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".
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.
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] | nne | 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) chloride 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.
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 |
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] | |
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.
Magnesium chloride is an inorganic compound with the formula MgCl2. It forms hydrates MgCl2·nH2O, where n can range from 1 to 12. These salts are colorless or white solids that are highly soluble in water. These compounds and their solutions, both of which occur in nature, have a variety of practical uses. Anhydrous magnesium chloride is the principal precursor to magnesium metal, which is produced on a large scale. Hydrated magnesium chloride is the form most readily available.
Barium chloride is an inorganic compound with the formula BaCl2. It is one of the most common water-soluble salts of barium. Like most other water-soluble barium salts, it is a white powder, highly toxic, and imparts a yellow-green coloration to a flame. It is also hygroscopic, converting to the dihydrate BaCl2·2H2O, which are colourless crystals with a bitter salty taste. It has limited use in the laboratory and industry.
Nickel(II) chloride (or just nickel chloride) is the chemical compound NiCl2. The anhydrous salt is yellow, but the more familiar hydrate NiCl2·6H2O is green. Nickel(II) chloride, in various forms, is the most important source of nickel for chemical synthesis. The nickel chlorides are deliquescent, absorbing moisture from the air to form a solution. Nickel salts have been shown to be carcinogenic to the lungs and nasal passages in cases of long-term inhalation exposure.
Cadmium chloride is a white crystalline compound of cadmium and chloride, with the formula CdCl2. This salt is a hygroscopic solid that is highly soluble in water and slightly soluble in alcohol. The crystal structure of cadmium chloride (described below), is a reference for describing other crystal structures. Also known are CdCl2•H2O and the hemipentahydrate CdCl2•2.5H2O.
Uranyl chloride refers to inorganic compounds with the formula UO2Cl2(H2O)n where n = 0, 1, or 3. These are yellow-colored salts.
Nickel(II) carbonate describes one or a mixture of inorganic compounds containing nickel and carbonate. From the industrial perspective, an important nickel carbonate is basic nickel carbonate with the formula Ni4CO3(OH)6(H2O)4. Simpler carbonates, ones more likely encountered in the laboratory, are NiCO3 and its hexahydrate. All are paramagnetic green solids containing Ni2+ cations. The basic carbonate is an intermediate in the hydrometallurgical purification of nickel from its ores and is used in electroplating of nickel.
Indium(III) sulfate (In2(SO4)3) is a sulfate salt of the metal indium. It is a sesquisulfate, meaning that the sulfate group occurs 11/2 times as much as the metal. It may be formed by the reaction of indium, its oxide, or its carbonate with sulfuric acid. An excess of strong acid is required, otherwise insoluble basic salts are formed. As a solid indium sulfate can be anhydrous, or take the form of a pentahydrate with five water molecules or a nonahydrate with nine molecules of water. Indium sulfate is used in the production of indium or indium containing substances. Indium sulfate also can be found in basic salts, acidic salts or double salts including indium alum.
Nickel nitrate is the inorganic compound Ni(NO3)2 or any hydrate thereof. In the hexahydrate, the nitrate anions are not bonded to nickel. Other hydrates have also been reported: Ni(NO3)2.9H2O, Ni(NO3)2.4H2O, and Ni(NO3)2.2H2O.
Manganese(II) sulfate usually refers to the inorganic compound with the formula MnSO4·H2O. This pale pink deliquescent solid is a commercially significant manganese(II) salt. Approximately 260,000 tonnes of manganese(II) sulfate were produced worldwide in 2005. It is the precursor to manganese metal and many other chemical compounds. Manganese-deficient soil is remediated with this salt.
Magnesium bromide are inorganic compounds with the chemical formula MgBr2(H2O)x, where x can range from 0 to 9. They are all white deliquescent solids. Some magnesium bromides have been found naturally as rare minerals such as: bischofite and carnallite.
Cobalt(II) sulfate is any of the inorganic compounds with the formula CoSO4(H2O)x. Usually cobalt sulfate refers to the hexa- or heptahydrates CoSO4.6H2O or CoSO4.7H2O, respectively. The heptahydrate is a red solid that is soluble in water and methanol. Since cobalt(II) has an odd number of electrons, its salts are paramagnetic.
Cobalt(II) bromide (CoBr2) is an inorganic compound. In its anhydrous form, it is a green solid that is soluble in water, used primarily as a catalyst in some processes.
Beryllium sulfate normally encountered as the tetrahydrate, [Be(H2O)4]SO4 is a white crystalline solid. It was first isolated in 1815 by Jons Jakob Berzelius. Beryllium sulfate may be prepared by treating an aqueous solution of many beryllium salts with sulfuric acid, followed by evaporation of the solution and crystallization. The hydrated product may be converted to anhydrous salt by heating at 400 °C.
Manganese(II) nitrate refers to the inorganic compounds with formula Mn(NO3)2·(H2O)n. These compounds are nitrate salts containing varying amounts of water. A common derivative is the tetrahydrate, Mn(NO3)2·4H2O, but mono- and hexahydrates are also known as well as the anhydrous compound. Some of these compounds are useful precursors to the oxides of manganese. Typical of a manganese(II) compound, it is a paramagnetic pale pink solid.
Thorium(IV) nitrate is a chemical compound, a salt of thorium and nitric acid with the formula Th(NO3)4. A white solid in its anhydrous form, it can form tetra- and pentahydrates. As a salt of thorium it is weakly radioactive.
Sodium magnesium sulfate is a double sulfate of sodium and magnesium. There are a number of different stoichiometries and degrees of hydration with different crystal structures, and many are minerals. Members include:
Vanadium(II) sulfate describes a family of inorganic compounds with the formula VSO4(H2O)x where 0 ≤ x ≤ 7. The hexahydrate is most commonly encountered. It is a violet solid that dissolves in water to give air-sensitive solutions of the aquo complex. The salt is isomorphous with [Mg(H2O)6]SO4. Compared to the V–O bond length of 191 pm in [V(H2O)6]3+, the V–O distance is 212 pm in the [V(H2O)6]SO4. This nearly 10% elongation reflects the effect of the lower charge, hence weakened electrostatic attraction.
Nickel is one of the metals that can form Tutton's salts. The singly charged ion can be any of the full range of potassium, rubidium, cesium, ammonium (), or thallium. As a mineral the ammonium nickel salt, (NH4)2Ni(SO4)2 · 6 H2O, can be called nickelboussingaultite. With sodium, the double sulfate is nickelblödite Na2Ni(SO4)2 · 4 H2O from the blödite family. Nickel can be substituted by other divalent metals of similar sized to make mixtures that crystallise in the same form.
A transition metal nitrate complex is a coordination compound containing one or more nitrate ligands. Such complexes are common starting reagents for the preparation of other compounds.
Nickel(II) perchlorate is a inorganic compound with the chemical formula of Ni(ClO4)2, and it is a strong oxidizing agent. Its colors are different depending on water. For example, the hydrate forms cyan crystals, the pentahydrate forms green crystals, but the hexahydrate (Ni(ClO4)2·6H2O) forms blue crystals.
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