Water of crystallization 1249541 226013489 2008-07-16T13:10:26Z 203.116.22.138 /* Further examples */ '''Water of crystallization''' (alt. Br.E. water of crystallisation) is [[water]] that occurs in [[crystals]] but is not [[covalent bond|covalently bonded]] to a host [[molecule]] or [[ion]]. The term is archaic and predates modern structural [[inorganic chemistry]], coming from an era when the relationships between [[stoichiometry]] and structure were poorly understood. Nonetheless, the concept is pervasive and when employed precisely, the term can be useful. Upon [[crystallization]] from water or moist [[solvents]], many [[chemical compound|compounds]] incorporate water molecules in their crystalline frameworks. Often, in fact, the species of interest cannot be crystallized in the absence of water, even though no strong bonds to the "guest" water molecules may be apparent. Classically, "water of crystallization" refers to water that is found in a crystalline framework of a [[metal complex]] but that is not directly bonded to the metal ion. Obviously the "water of crystallization" is bound or interacting with ''some'' other atoms and ions or it would not be included in the crystalline framework. Consider the case of [[nickel(II) chloride]] hexahydrate. This species has the formula NiCl<sub>2</sub>(H<sub>2</sub>O)<sub>6</sub>. Examination of its molecular structure reveals that the crystal consists of [''trans''-NiCl<sub>2</sub>(H<sub>2</sub>O)<sub>4</sub>] subunits that are [[hydrogen bond]]ed to each other and two isolated molecules of H<sub>2</sub>O. Thus 1/3 of the water molecules in the crystal are not directly bonded to Ni<sup>2+</sup>, and these might be termed "water of crystallization". Compared to [[inorganic compound|inorganic salts]], [[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 crystallography|protein crystallographer]] to argue that the conformation in the crystal is not too far from the native conformation in solution. ==Further examples== 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 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: *CuSO<sub>4</sub>•5H<sub>2</sub>O - copper (II) sulfate pentahydrate *CoCI<sub>2</sub>•6H<sub>2</sub>O - cobalt (II) iodide hexahydrate *SnCl<sub>2</sub>•2H<sub>2</sub>O - stannous (tin II) chloride dihydrate Since the latter part of the 20th century, the structures of most common hydrates have been determined by [[crystallography]], so the dot formalism is increasingly obsolete. Another reason for using the dot formalism is simplicity. 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 CuSO<sub>4</sub>•5H<sub>2</sub>O and anhydrous CuSO<sub>4</sub> behave identically. Therefore, knowledge of the degree of hydration is important only for determining the [[equivalent weight]]: one mole of CuSO<sub>4</sub>•5H<sub>2</sub>O weighs more than one mole of CuSO<sub>4</sub>. In some cases, the degree of hydration can be critical to the resulting chemical properties. For example, anhydrous RhCl<sub>3</sub> is not soluble in water and is relatively useless in organometallic chemistry whereas [[Rhodium(III) chloride|RhCl<sub>3</sub>]]•3H<sub>2</sub>O is versatile. Similarly, hydrated AlCl<sub>3</sub> is a poor Lewis acid and thus inactive as a catalyst for Friedel-Crafts reactions. Samples of AlCl<sub>3</sub> must therefore be protected from atmospheric moisture to preclude the formation of hydrates. Crystals of the aforementioned hydrated copper sulfate consists of [Cu(H<sub>2</sub>O)<sub>4</sub>]<sup>2+</sup> centers linked to SO<sub>4</sub><sup>2-</sup> 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(H<sub>2</sub>O)<sub>6</sub>]<sup>2+</sup> and I<sup>-</sup>. In the 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 can arises from water. [[Glauber's salt]], a white crystalline solid Na<sub>2</sub>SO<sub>4</sub>(H<sub>2</sub>O)<sub>10</sub> is >50% water by weight. ==Desiccation== {{main article|desiccation}} {{see also|list of desiccants}} Some anhydrous compounds are hydrated so easily that they are said to be hygroscopic and are used as drying agents or [[Desiccation|desiccants]]. Common drying agents include CaCl<sub>2</sub> and Na<sub>2</sub>SO<sub>4</sub>. ==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. A serious complication to the thermal analysis for the presence of water of hydration is that compounds that contain hydrogen and oxygen will release water when heated, regardless of whether they contained water molecules. Thus, the release of water upon heating, especially to high temperatures, is insufficient criterion for the presence of water in the sample prior to heating. For example, if one heats a [[carboxylic acid]], RCO<sub>2</sub>H, one obtains H<sub>2</sub>O. No water was present in the starting carboxylic acid. ==Waters of crystallization in inorganic halides== In the table below are indicated the number of molecules of water per metal in various salts.<ref>K. Waizumi, H. Masuda, H. Ohtaki, "X-ray structural studies of FeBr<sub>2</sub>4H<sub>2</sub>O, CoBr<sub>2</sub>4H<sub>2</sub>O, NiCl<sub>2</sub> 4H<sub>2</sub>O, and CuBr<sub>2</sub>4H<sub>2</sub>O. cis/trans Selectivity in transition metal(I1) dihalide Tetrahydrate" Inorganica Chimica Acta, 1992 volume 192, pages 173-181.</ref> {| class="wikitable" ! Formula of<br/> hydrated metal halides || Coordination<br/>sphere of the metal || equivalents</br>of water of<br/>crystallization that<br/> are not bound to M || Remarks |- | [[Vanadium(III) chloride|VCl<sub>3</sub>(H<sub>2</sub>O)<sub>6</sub>]] || trans-[VCl<sub>2</sub>(H<sub>2</sub>O)<sub>4</sub>]<sup>+</sup> || two|| |- | [[Vanadium(III) bromide|VBr<sub>3</sub>(H<sub>2</sub>O)<sub>6</sub>]] || trans-[VBr<sub>2</sub>(H<sub>2</sub>O)<sub>4</sub>]<sup>+</sup> || two||bromides and<br/>chlorides are usually<br/>similar |- | VI<sub>3</sub>(H<sub>2</sub>O)<sub>6</sub> || [V(H<sub>2</sub>O)<sub>6</sub>]<sup>3+</sup> || none ||iodide competes<br/> poorly with water |- | [[Chromium(III) chloride|CrCl<sub>3</sub>(H<sub>2</sub>O)<sub>6</sub>]] || trans-[CrCl<sub>2</sub>(H<sub>2</sub>O)<sub>4</sub>]<sup>+</sup>|| two ||dark green isomer |- | [[Chromium(III) chloride|CrCl<sub>3</sub>(H<sub>2</sub>O)<sub>6</sub>]] || [CrCl(H<sub>2</sub>O)<sub>5</sub>]<sup>2+</sup>|| one ||blue-green isomer |- | [[Chromium(II) chloride|CrCl<sub>2</sub>(H<sub>2</sub>O)<sub>4</sub>]] || trans-[CrCl<sub>2</sub>(H<sub>2</sub>O)<sub>4</sub>] || none ||molecular |- | [[Chromium(III) chloride|CrCl<sub>3</sub>(H<sub>2</sub>O)<sub>6</sub>]] || [Cr(H<sub>2</sub>O)<sub>6</sub>]<sup>3+</sup> || none ||violet isomer |- | [[Chromium(III) bromide|CrBr<sub>3</sub>(H<sub>2</sub>O)<sub>6</sub>]]|| trans-[CrBr<sub>2</sub>(H<sub>2</sub>O)<sub>4</sub>]<sup>+</sup>|| two ||green isomer |- | [[Chromium(III) bromide|CrBr<sub>3</sub>(H<sub>2</sub>O)<sub>6</sub>]]|| [Cr(H<sub>2</sub>O)<sub>6</sub>]<sup>3+</sup>|| none ||violet isomer |- | [[Manganese(II) chloride|MnCl<sub>2</sub>(H<sub>2</sub>O)<sub>6</sub>]] || trans-[MnCl<sub>2</sub>(H<sub>2</sub>O)<sub>4</sub>]|| two|| |- | [[Manganese(II) chloride|MnCl<sub>2</sub>(H<sub>2</sub>O)<sub>4</sub>]] || cis-[MnCl<sub>2</sub>(H<sub>2</sub>O)<sub>4</sub>] || none ||note cis<br/>molecular |- | [[Manganese(II) bromide|MnBr<sub>2</sub>(H<sub>2</sub>O)<sub>4</sub>]] || cis-[MnBr<sub>2</sub>(H<sub>2</sub>O)<sub>4</sub>] || none ||note cis<br/>molecular |- | [[Manganese(II) chloride|MnCl<sub>2</sub>(H<sub>2</sub>O)<sub>2</sub>]] || trans-[MnCl<sub>4</sub>(H<sub>2</sub>O)<sub>2</sub>] || none ||polymeric with<br/> bridging chloride |- | [[Manganese(II) bromide|MnBr<sub>2</sub>(H<sub>2</sub>O)<sub>2</sub>]] || trans-[MnBr<sub>4</sub>(H<sub>2</sub>O)<sub>2</sub>] || none ||polymeric with<br/> bridging bromide |- | [[Iron(II) chloride|FeCl<sub>2</sub>(H<sub>2</sub>O)<sub>6</sub>]] || trans-[FeCl<sub>2</sub>(H<sub>2</sub>O)<sub>4</sub>]|| two || |- | [[Iron(II) chloride|FeCl<sub>2</sub>(H<sub>2</sub>O)<sub>4</sub>]] || trans-[FeCl<sub>2</sub>(H<sub>2</sub>O)<sub>4</sub>] || none ||molecular |- | [[Iron(II) bromide|FeBr<sub>2</sub>(H<sub>2</sub>O)<sub>4</sub>]] || trans-[FeBr<sub>2</sub>(H<sub>2</sub>O)<sub>4</sub>] || none ||molecular |- | [[Iron(II) chloride|FeCl<sub>2</sub>(H<sub>2</sub>O)<sub>2</sub>]] || trans-[FeCl<sub>4</sub>(H<sub>2</sub>O)<sub>2</sub>] || none ||polymeric with<br/> bridging chloride |- | [[Cobalt(II) chloride|CoCl<sub>2</sub>(H<sub>2</sub>O)<sub>6</sub>]] || trans-[CoCl<sub>2</sub>(H<sub>2</sub>O)<sub>4</sub>]|| two|| |- | [[Cobalt(II) bromide|CoBr<sub>2</sub>(H<sub>2</sub>O)<sub>6</sub>]] || trans-[CoBr<sub>2</sub>(H<sub>2</sub>O)<sub>4</sub>]|| two|| |- | [[Cobalt(II) bromide|CoBr<sub>2</sub>(H<sub>2</sub>O)<sub>4</sub>]] || trans-[CoBr<sub>2</sub>(H<sub>2</sub>O)<sub>4</sub>]|| none ||molecular |- | [[Cobalt(II) chloride|CoCl<sub>2</sub>(H<sub>2</sub>O)<sub>4</sub>]] || cis-[CoCl<sub>2</sub>(H<sub>2</sub>O)<sub>4</sub>] || none ||note: cis<br/>molecular |- | [[Cobalt(II) chloride|CoCl<sub>2</sub>(H<sub>2</sub>O)<sub>2</sub>]] || trans-[CoCl<sub>4</sub>(H<sub>2</sub>O)<sub>2</sub>] || none ||polymeric with<br/> bridging chloride |- | [[Cobalt(II) chloride|CoBr<sub>2</sub>(H<sub>2</sub>O)<sub>2</sub>]] || trans-[CoBr<sub>4</sub>(H<sub>2</sub>O)<sub>2</sub>] || none ||polymeric with<br/> bridging bromide |- | [[Nickel(II) chloride|NiCl<sub>2</sub>(H<sub>2</sub>O)<sub>6</sub>]] || trans-[NiCl<sub>2</sub>(H<sub>2</sub>O)<sub>4</sub>]|| two |- | [[Nickel(II) chloride|NiCl<sub>2</sub>(H<sub>2</sub>O)<sub>4</sub>]] || cis-[NiCl<sub>2</sub>(H<sub>2</sub>O)<sub>4</sub>]||none ||note: cis<br/>molecular |- | [[Nickel(II) bromide|NiBr<sub>2</sub>(H<sub>2</sub>O)<sub>6</sub>]] || trans-[NiBr<sub>2</sub>(H<sub>2</sub>O)<sub>4</sub>]|| two |- | [[Nickel(II) chloride|NiCl<sub>2</sub>(H<sub>2</sub>O)<sub>2</sub>]] || trans-[NiCl<sub>4</sub>(H<sub>2</sub>O)<sub>2</sub>] || none ||polymeric with<br/> bridging chloride |- | [[Copper(II) chloride|CuCl<sub>2</sub>(H<sub>2</sub>O)<sub>2</sub>]] || [CuCl<sub>4</sub>(H<sub>2</sub>O)<sub>2</sub>]<sub>2</sub>|| none ||tetragonally distorted<br/> two long Cu-Cl distances |- | [[Copper(II) bromide|CuBr<sub>2</sub>(H<sub>2</sub>O)<sub>4</sub>]] || [CuBr<sub>4</sub>(H<sub>2</sub>O)<sub>2</sub>]<sub>n</sub>|| two ||tetragonally distorted<br/> two long Cu-Br distances |} ==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. == References == <references/> # Wells, A.F. (1984). Structural Inorganic Chemistry, Oxford: Clarendon Press. # Chemistry, The Central Science, 5th Ed. Brown, T.L., LeMay, H.E. and Bursten, B.E., Prentice Hall, Englewood Cliffs, N.J. # Chemistry, 4th Ed. Mcmurry, Fay, Pearson Education, Patparganj, Delhi, India [[Category:Solid-state chemistry]] [[Category:Water chemistry]] [[category:desiccants]] [[de:Kristallwasser]] [[ja:結晶水]] [[nds:Kristallwater]] [[pl:Woda krystalizacyjna]] [[pt:Água de cristalização]] [[fi:Kidevesi]] [[sv:Kristallvatten]] [[zh:结晶水]]