Hydration number

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A sodium cation is solvated by water molecules with their partially negative charged lone pairs pointing inwards towards the positively charged sodium ion Na+H2O.svg
A sodium cation is solvated by water molecules with their partially negative charged lone pairs pointing inwards towards the positively charged sodium ion

The hydration number of a compound is defined as the number of molecules of water bonded to a central ion, often a metal cation. The hydration number is related to the broader concept of solvation number, the number of solvent molecules bonded to a central atom. The hydration number varies with the atom or ion of interest.

Contents

In aqueous solution, solutes interact with water molecules to varying degrees. Metal cations form aquo complexes, wherein the oxygen of water bind to the cation. This first coordination sphere is encased in further solvation shells, whereby water bonds to the coordinated water via hydrogen bonding. For charged species, the orientation of water molecules around the solute dependent on its radius and charge, [1] with cations attracting water’s electronegative oxygen and anions attracting the hydrogens. Uncharged compounds such as methane can also be solvated by water and also have a hydration number. Although solvation shells can contain inner and outer shell solvent-solute interactions, the hydration number generally focuses on the inner shell solvent molecules that directly interact with the solute. [2]

A variety of definitions exist for hydration number. One such approach counts the number of water molecules bound to the compound more strongly (by 13.3 kcal/mol or more) than they are bound to other water molecules. [3] Hydration number estimates are not limited to integer values (for instance, estimates for sodium include 4, 4.6, 5.3, 5.5, 5.6, 6, 6.5, and 8), with some of the spread of estimated values being due to differing detection methods. [4]

Determination of hydration number

Hydration numbers can be determined by a variety of experimental methods. These include Raman spectroscopy, [5] neutron and X-ray scattering, [6] luminescence, [7] and NMR. [8] Hydration numbers can change depending on whether the species is locked into a crystall or in solution. The apparent hydration number of a species can vary depending on which experimental method was used. [4] The hydration number of large alkali metal cations are difficult to characterize. [9]

Using NMR methods

NMR is the most informative technique for determining hydration numbers in solution. 1H and 17O NMR signals, even for paramagnetic species, can be interpreted to give information on hydration number. Aside from paramagnetism, another complication with NMR is the rate of exchange between bound and unbound water. The second coordination sphere is another aspect to be considered. Finally, ion pairing where the anion enters the solvation shell of the cation must be assessed. [10]

X-ray crystallography

X-ray crystallography provides definitive information on hydration numbers, especially for cations. Most salts crystallize from water with aquo ligands bonded to the cation. Typical hydration numbers are six for first row transition metal ions and nine for lanthanides. Anions compete with aquo ligands for coordination to the cation. A major question concerns the relationship between structure of such hydrates in the crystal and in aqueous solution. X-ray crystallography provides little insight about the hydration numbers for anions and monocations, much less neutral solutes. In such cases, water is bonded so weakly that crystallization is a major perturbation on stoichiometry.

Using Ion movement methods

Ion movement methods involve assessing the resistance to motion hence estimating an effective volume for a solvated ion and from that volume the solvation number. The motion may be from diffusion, mechanically engineered by changes to viscosity or caused by electrical means. Many of these methods give the sum of anion and cation contributions but some can work out values for independent ions. For monoatomic ions, decreasing ionic radius shows decreasing conductivity suggesting that the effective radius of the hydrated ion increases as ionic radius decreases (larger ions are less mobile so their ability to move charge is decreased). Once the mobility of the ions is determined it is possible to estimate diffusion coefficients and from those hydrodynamic radii. The hydrodynamic radii may be used to calculate the number of solvent molecules. [11]

Other hydrates

Even nonpolar entities hydrate and thus can in principle be assigned hydration numbers. For example even methane (CH4) forms a hydrate called methane clathrate, which are stable under pressure.

Related Research Articles

<span class="mw-page-title-main">Alkali metal</span> Group of highly reactive chemical elements

The alkali metals consist of the chemical elements lithium (Li), sodium (Na), potassium (K), rubidium (Rb), caesium (Cs), and francium (Fr). Together with hydrogen they constitute group 1, which lies in the s-block of the periodic table. All alkali metals have their outermost electron in an s-orbital: this shared electron configuration results in their having very similar characteristic properties. Indeed, the alkali metals provide the best example of group trends in properties in the periodic table, with elements exhibiting well-characterised homologous behaviour. This family of elements is also known as the lithium family after its leading element.

<span class="mw-page-title-main">Acid–base reaction</span> Chemical reaction

An acid–base reaction is a chemical reaction that occurs between an acid and a base. It can be used to determine pH via titration. Several theoretical frameworks provide alternative conceptions of the reaction mechanisms and their application in solving related problems; these are called the acid–base theories, for example, Brønsted–Lowry acid–base theory.

<span class="mw-page-title-main">Ionic bonding</span> Chemical bonding involving attraction between ions

Ionic bonding is a type of chemical bonding that involves the electrostatic attraction between oppositely charged ions, or between two atoms with sharply different electronegativities, and is the primary interaction occurring in ionic compounds. It is one of the main types of bonding, along with covalent bonding and metallic bonding. Ions are atoms with an electrostatic charge. Atoms that gain electrons make negatively charged ions. Atoms that lose electrons make positively charged ions. This transfer of electrons is known as electrovalence in contrast to covalence. In the simplest case, the cation is a metal atom and the anion is a nonmetal atom, but these ions can be of a more complex nature, e.g. molecular ions like NH+
4 or SO2−
4. In simpler words, an ionic bond results from the transfer of electrons from a metal to a non-metal in order to obtain a full valence shell for both atoms.

<span class="mw-page-title-main">Solvation</span> Association of molecules of a solvent with molecules or ions of a solute

Solvation describes the interaction of a solvent with dissolved molecules. Both ionized and uncharged molecules interact strongly with a solvent, and the strength and nature of this interaction influence many properties of the solute, including solubility, reactivity, and color, as well as influencing the properties of the solvent such as its viscosity and density. If the attractive forces between the solvent and solute particles are greater than the attractive forces holding the solute particles together, the solvent particles pull the solute particles apart and surround them. The surrounded solute particles then move away from the solid solute and out into the solution. Ions are surrounded by a concentric shell of solvent. Solvation is the process of reorganizing solvent and solute molecules into solvation complexes and involves bond formation, hydrogen bonding, and van der Waals forces. Solvation of a solute by water is called hydration.

In chemistry, hydronium (hydroxonium in traditional British English) is the common name for the aqueous cation H3O+, the type of oxonium ion produced by protonation of water. It is often viewed as the positive ion present when an Arrhenius acid is dissolved in water, as Arrhenius acid molecules in solution give up a proton (a positive hydrogen ion, H+) to the surrounding water molecules (H2O). In fact, acids must be surrounded by more than a single water molecule in order to ionize, yielding aqueous H+ and conjugate base. Three main structures for the aqueous proton have garnered experimental support: The Eigen cation, which is a tetrahydrate, H3O+(H2O)3; the Zundel cation, which is a symmetric dihydrate, H+(H2O)2; and the Stoyanov cation, an expanded Zundel cation, which is a hexahydrate: H+(H2O)2(H2O)4. Spectroscopic evidence from well-defined IR spectra overwhelmingly supports the Stoyanov cation as the predominant form. For this reason, it has been suggested that wherever possible, the symbol H+(aq) should be used instead of the hydronium ion.

<span class="mw-page-title-main">Solubility</span> Capacity of a substance to dissolve in a solvent in a homogeneous way

In chemistry, solubility is the ability of a substance, the solute, to form a solution with another substance, the solvent. Insolubility is the opposite property, the inability of the solute to form such a solution.

<span class="mw-page-title-main">Aqueous solution</span> Solution in which the solvent is water

An aqueous solution is a solution in which the solvent is water. It is mostly shown in chemical equations by appending (aq) to the relevant chemical formula. For example, a solution of table salt, or sodium chloride (NaCl), in water would be represented as Na+(aq) + Cl−(aq). The word aqueous means pertaining to, related to, similar to, or dissolved in, water. As water is an excellent solvent and is also naturally abundant, it is a ubiquitous solvent in chemistry. Since water is frequently used as the solvent in experiments, the word solution refers to an aqueous solution, unless the solvent is specified.

<span class="mw-page-title-main">Solvation shell</span> Solvent interface of a solute

A solvation shell or solvation sheath is the solvent interface of any chemical compound or biomolecule that constitutes the solute in a solution. When the solvent is water it is called a hydration shell or hydration sphere. The number of solvent molecules surrounding each unit of solute is called the hydration number of the solute.

Salting out is a purification technique that utilizes the reduced solubility of certain molecules in a solution of very high ionic strength. Salting out is typically used to precipitate large biomolecules, such as proteins or DNA. Because the salt concentration needed for a given protein to precipitate out of the solution differs from protein to protein, a specific salt concentration can be used to precipitate a target protein. This process is also used to concentrate dilute solutions of proteins. Dialysis can be used to remove the salt if needed.

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.

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Ion chromatography separates ions and polar molecules based on their affinity to the ion exchanger. It works on almost any kind of charged molecule—including large proteins, small nucleotides, and amino acids. However, ion chromatography must be done in conditions that are one unit away from the isoelectric point of a protein.

<span class="mw-page-title-main">Cryptand</span> Cyclic, multidentate ligands adept at encapsulating cations

In chemistry, cryptands are a family of synthetic, bicyclic and polycyclic, multidentate ligands for a variety of cations. The Nobel Prize for Chemistry in 1987 was given to Donald J. Cram, Jean-Marie Lehn, and Charles J. Pedersen for their efforts in discovering and determining uses of cryptands and crown ethers, thus launching the now flourishing field of supramolecular chemistry. The term cryptand implies that this ligand binds substrates in a crypt, interring the guest as in a burial. These molecules are three-dimensional analogues of crown ethers but are more selective and strong as complexes for the guest ions. The resulting complexes are lipophilic.

<span class="mw-page-title-main">Radical anion</span> Free radical species

In organic chemistry, a radical anion is a free radical species that carries a negative charge. Radical anions are encountered in organic chemistry as reduced derivatives of polycyclic aromatic compounds, e.g. sodium naphthenide. An example of a non-carbon radical anion is the superoxide anion, formed by transfer of one electron to an oxygen molecule. Radical anions are typically indicated by .

Liquid–liquid extraction (LLE), also known as solvent extraction and partitioning, is a method to separate compounds or metal complexes, based on their relative solubilities in two different immiscible liquids, usually water (polar) and an organic solvent (non-polar). There is a net transfer of one or more species from one liquid into another liquid phase, generally from aqueous to organic. The transfer is driven by chemical potential, i.e. once the transfer is complete, the overall system of chemical components that make up the solutes and the solvents are in a more stable configuration. The solvent that is enriched in solute(s) is called extract. The feed solution that is depleted in solute(s) is called the raffinate. LLE is a basic technique in chemical laboratories, where it is performed using a variety of apparatus, from separatory funnels to countercurrent distribution equipment called as mixer settlers. This type of process is commonly performed after a chemical reaction as part of the work-up, often including an acidic work-up.

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In chemistry, ion association is a chemical reaction whereby ions of opposite electric charge come together in solution to form a distinct chemical entity. Ion associates are classified, according to the number of ions that associate with each other, as ion pairs, ion triplets, etc. Ion pairs are also classified according to the nature of the interaction as contact, solvent-shared or solvent-separated. The most important factor to determine the extent of ion association is the dielectric constant of the solvent. Ion associates have been characterized by means of vibrational spectroscopy, as introduced by Niels Bjerrum, and dielectric-loss spectroscopy.

In chemistry, metal aquo complexes are coordination compounds containing metal ions with only water as a ligand. These complexes are the predominant species in aqueous solutions of many metal salts, such as metal nitrates, sulfates, and perchlorates. They have the general stoichiometry [M(H2O)n]z+. Their behavior underpins many aspects of environmental, biological, and industrial chemistry. This article focuses on complexes where water is the only ligand, but of course many complexes are known to consist of a mix of aquo and other ligands.

A metal ion in aqueous solution or aqua ion is a cation, dissolved in water, of chemical formula [M(H2O)n]z+. The solvation number, n, determined by a variety of experimental methods is 4 for Li+ and Be2+ and 6 for most elements in periods 3 and 4 of the periodic table. Lanthanide and actinide aqua ions have higher solvation numbers (often 8 to 9), with the highest known being 11 for Ac3+. The strength of the bonds between the metal ion and water molecules in the primary solvation shell increases with the electrical charge, z, on the metal ion and decreases as its ionic radius, r, increases. Aqua ions are subject to hydrolysis. The logarithm of the first hydrolysis constant is proportional to z2/r for most aqua ions.

References

  1. ↑ Vaslow, Fred (1963). "The Orientation of Water Molecules in the Field of an Alkali Ion". The Journal of Physical Chemistry. 67 (12): 2773–2776. doi:10.1021/j100806a063.
  2. ↑ Rempe, Susan B.; Pratt, Lawrence R. (2001). "The hydration number of Na+ in liquid water". Fluid Phase Equilibria . 183–184: 121–132. arXiv: physics/0006026 . doi:10.1016/s0378-3812(01)00426-5. S2CID   1282292.
  3. ↑ Zavitsas, Andreas A. (2016). "Some opinions of an innocent bystander regarding the Hofmeister series". Current Opinion in Colloid & Interface Science. 23: 72–81. doi:10.1016/j.cocis.2016.06.012.
  4. 1 2 Mähler, Johan; Persson, Ingmar (2 January 2012). "A Study of the Hydration of the Alkali Metal Ions in Aqueous Solution". Inorganic Chemistry. 51 (1): 425–438. doi:10.1021/ic2018693. PMC   3250073 . PMID   22168370.
  5. ↑ Uchida, Tsutomu; Hirano, Takashi; Ebinuma, Takao; Narita, Hideo; Gohara, Kazutoshi; Mae, Shinji; Matsumoto, Ryo (1 December 1999). "Raman spectroscopic determination of hydration number of methane hydrates". AIChE Journal. 45 (12): 2641–2645. doi:10.1002/aic.690451220.
  6. ↑ Rempe, Susan B.; Pratt, Lawrence R.; Hummer, Gerhard; Kress, Joel D.; Martin, Richard L.; Redondo, Antonio (1 February 2000). "The Hydration Number of Li+ in Liquid Water". Journal of the American Chemical Society. 122 (5): 966–967. arXiv: physics/0001011 . doi:10.1021/ja9924750.
  7. ↑ Werner, Eric J.; Avedano, Stefano; Botta, Mauro; Hay, Benjamin P.; Moore, Evan G.; Aime, Silvio; Raymond, Kenneth N. (1 February 2007). "Highly Soluble Tris-hydroxypyridonate Gd(III) Complexes with Increased Hydration Number, Fast Water Exchange, Slow Electronic Relaxation, and High Relaxivity". Journal of the American Chemical Society. 129 (7): 1870–1871. doi:10.1021/ja068026z. PMC   3188311 . PMID   17260995.
  8. ↑ Dec, Steven F.; Bowler, Kristin E.; Stadterman, Laura L.; Koh, Carolyn A.; Sloan, E. Dendy (1 January 2006). "Direct Measure of the Hydration Number of Aqueous Methane". Journal of the American Chemical Society. 128 (2): 414–415. doi:10.1021/ja055283f. PMID   16402820.
  9. ↑ Smirnov, P. R.; Trostin, V. N. (1 December 2007). "Structures of the nearest surroundings of the K+, Rb+, and Cs+ ions in aqueous solutions of their salts". Russian Journal of General Chemistry. 77 (12): 2101–2107. doi:10.1134/S1070363207120043. S2CID   95796483.
  10. ↑ Burgess, John (1999-01-01), Burgess, John (ed.), "2 - Solvation numbers", Ions in Solution, Woodhead Publishing, pp. 28–30, ISBN   978-1-898563-50-1 , retrieved 2023-01-14
  11. ↑ Burgess, John (1999-01-01), Burgess, John (ed.), "2 - Solvation numbers", Ions in Solution, Woodhead Publishing, pp. 32–33, ISBN   978-1-898563-50-1 , retrieved 2023-01-14
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