water of crystallization

Water of crystallization is water that occurs in crystals but is not 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 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 NiCl₂(H₂O)₆. Examination of its molecular structure reveals that the crystal consists of [trans-NiCl₂(H₂O)₄] subunits that are hydrogen bonded to each other and two isolated molecules of H₂O. Thus 1/3 of the water molecules in the crystal are not directly bonded to Ni²⁺, and these might be termed "water of crystallization".

Compared to 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 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 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. Examples:

• CuSO₄•5H₂O - copper (II) sulfate pentahydrate
• CoCl₂•6H₂O - cobalt (II) chloride hexahydrate
• SnCl₂•2H₂O - stannous (or 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 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 the aforementioned hydrated copper sulfate consist of [Cu(H₂O)₄]²⁺ centers linked to SO₄^2- 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(H₂O)₆]²⁺ and I^-. 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.

Desiccation

Some anhydrous compounds are hydrated so easily that they are said to be hygroscopic and are used as drying agents or desiccants. Common drying agents include CaCl₂ and Na₂SO₄.

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.

Waters of crystallization in inorganic halides

In the table below are indicated the number of molecules of water per metal in various salts.^[1]

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

1. ^ K. Waizumi, H. Masuda, H. Ohtaki, "X-ray structural studies of FeBr₂4H₂O, CoBr₂4H₂O, NiCl₂ 4H₂O, and CuBr₂4H₂O. cis/trans Selectivity in transition metal(I1) dihalide Tetrahydrate" Inorganica Chimica Acta, 1992 volume 192, pages 173-181.

1. Wells, A.F. (1984). Structural Inorganic Chemistry, Oxford: Clarendon Press.
2. Chemistry, The Central Science, 5th Ed. Brown, T.L., LeMay, H.E. and Bursten, B.E., Prentice Hall, Englewood Cliffs, N.J.
3. Chemistry, 4th Ed. Mcmurry, Fay, Pearson Education, Patparganj, Delhi, India