Solubility

From Wikipedia, the free encyclopedia

Solubility refers to the ability for a given substance, the solute, to dissolve in a solvent.^[1] It is measured in terms of the maximum amount of solute dissolved in a solvent at equilibrium. The resulting solution is called a saturated solution. Certain substances are soluble in all proportions with a given solvent. An example for this is ethanol in water. This property is more correctly described as miscible.

Under various conditions, the equilibrium solubility can be exceeded to give a so-called supersaturated solution, which is metastable. The solvent is often a liquid, which can be a pure substance or a mixture. The species that dissolves, the solute, can be a gas, another liquid, or a solid. Solubilities range widely, from infinitely soluble such as ethanol in water, to poorly soluble, such as silver chloride in water. The term insoluble is often applied to poorly soluble compounds, though strictly speaking there are very few cases where there is absolutely no material dissolved.

1 Molecular view
2 Factors affecting solubility
3 Rate of dissolution
4 Quantification of solubility
5 Applications
6 Solubility of ionic compounds in water
7 Solubility of organic compounds
8 Solid solubility
9 See also
10 External links
11 References

[edit] Molecular view

The process of dissolving, called dissolution, is relatively straightforward for covalent substances such as ethanol. When ethanol dissolves in water, the ethanol molecules remain intact but form new hydrogen bonds with the water. When, however, an ionic compounds such as sodium chloride (NaCl) dissolves in water, the sodium chloride lattice dissociates into separate ions which are solvated (wrapped) with a coating of water molecules. Nonetheless, NaCl is said to dissolve in water, because evaporation of the solvent returns crystalline NaCl.

Sometimes the term "dissolving" is applied to an irreversible chemical reaction, as with iron in nitric acid, but in such a case the thermodynamic concept of solubility does not apply.

When it dissolves, a solute may form several species in the solution. For example, water above the crystals of ferrous hydroxide, Fe(OH)₂, will, at equilibrium, contain Fe²⁺, Fe(OH)⁺, Fe(OH)₂, Fe(OH)₃^- and possibly other complexes. Therefore, the solubility of ferrous hydroxide depends on pH. In general, solubility in the solvent phase can be given only for a specific solute which is thermodynamically stable.

[edit] Factors affecting solubility

The solubility of one substance dissolving in another is determined by the balance of intermolecular forces between the solvent and solute and the entropy change that accompanies the solvation. Factors such as temperature and pressure will alter this balance, thus changing the solubility.

[edit] Temperature

Solubility of various salts as function of temperature

The solubility of a given solute in a given solvent often depends on temperature. For around 95% of solid solutes, the solubility increases with temperature^[2], but gaseous solutes exhibit more complex behaviour. As the temperature is raised gases usually become less soluble in water, but more soluble in organic solvents.^[3]

The chart shows solubility curves for some typical inorganic salts (all solids).^[4] Many salts behave like barium nitrate and disodium hydrogen arsenate, and show a large increase in solubility with temperature. Some solutes (e.g. NaCl in water) are fairly independent of temperature. A few, such as cerium(III) sulfate, become less soluble in hot water. Occasionally, a more complex pattern is observed, as with sodium sulfate, where the less soluble decahydrate crystal loses water of crystallization at 32 °C to form a more soluble anhydrous phase.

Organic compounds nearly always become soluble as the temperature is raised, in most solvents. The technique of recrystallization, used for purification of solids, depends on this differences in solubility in hot and cold solvent. There are a few exceptions, such as certain cyclodextrins.^[5]

[edit] Pressure

Henry's law states that the solubility of a gas is directly proportional to the pressure of that gas, which may be written as:

$p = kc \,$

where k is a constant (for example, 769.2 L•atm/mol for dioxygen (O₂) at 298 K).

[edit] Polarity

A popular aphorism used for predicting solubility is "Like dissolves like"^[6] This indicates that a solute will dissolve best in a solvent that has a similar polarity to itself. This is a rather simplistic view, since it ignores many solvent-solute interactions, but it is a useful rule-of-thumb. For example, a very polar (hydrophilic) solute such as urea is highly soluble in highly polar water, less soluble in fairly polar methanol, and practically insoluble in non-polar solvents such as benzene. In contrast, a non-polar or lipophilic solute such as naphthalene is insoluble in water, fairly soluble in methanol, and highly soluble in non-polar benzene.^[7]

Liquid solubilities also generally follow this rule. Lipophilic plant oils, such as olive oil and palm oil, dissolve in non-polar gasoline (petrol), but polar liquids like water will not mix with gasoline.

Synthetic chemists often use the different solubilities of compounds to separate and purify compounds from reaction mixtures.

[edit] Rate of dissolution

Dissolution is not always an instantaneous process. It is fast when salt and sugar dissolve in water but much slower for a tablet aspirin or a large crystal of hydrated copper(II) sulfate. The speed at which a solid dissolves in a liquid may depend on its crystalline properties (crystalline vs amorphous, crystal size) and the presence of polymorphism. This is especially important in designing methods for controlled drug delivery.

[edit] Quantification of solubility

Solubility is commonly expressed as a concentration, either molarity or molality, but also as a mole fraction. The maximum equilibrium amount of solute that can normally dissolve per amount of solvent is the solubility of that solute in that solvent. It is often expressed as a maximum concentration of a saturated solution. These maximum concentrations are often expressed as grams of solute per 100 ml of solvent.

Solubility constants are used to describe saturated solutions of ionic compounds of relatively low solubility (see solubility equilibrium). For salts, solubility in aqueous solutions or the maximum amount of salt that can be dissolved is the solubility constant. The solubility constant is a special case of an equilibrium constant. It describes the balance between dissolved salt and undissolved salt. The solubility constant is also "applicable" (i.e. useful) to precipitation, the reverse of the dissolving reaction. As with other equilibrium constants, temperature can affect the numerical value of solubility constant.

Henry's law is used to quantify the solubility of gases in liquids as a function of the gas's partial pressure.

The Flory-Huggins solution theory is a theoretical model describing the solubility of polymers. The Hansen Solubility Parameters and the Hildebrand solubility parameters are empirical methods for the prediction of solubility. it is also possible to predict solubility from other physical constants such as the enthalpy of fusion.

The partition coefficient (Log P) is a measure of differential solubility of a compound in a hydrophobic solvent (octanol) and a hydrophilic solvent (water). The logarithm of these two values enables compounds to be ranked in terms of hydrophilicity (or hydrophobicity).

[edit] Applications

The solubility of a substance is very useful when separating mixtures of compounds. For example, a mixture of salt (sodium chloride) and silica may be separated by dissolving the salt in water, and filtering off the undissolved silica. The synthesis of chemical compounds, by the milligram in a laboratory, or by the ton in industry, both make use of the relative solubilities of the desired product, as well as unreacted starting materials, byproducts, and side products to achieve separation.

Another example of this would be the synthesis of benzoic acid from phenylmagnesium bromide and dry ice. Benzoic acid is more soluble in an organic solvent such as dichloromethane or diethyl ether, and when shaken with this organic solvent in an separatory funnel, will preferentially dissolve in the organic layer. The other reaction products, i.e. the magnesium bromide will remain in the aqueous layer, clearly showing that separation based on solubility is achieved. (On a practical note, the benzoic acid obtained after evaporating the organic solvent should ideally be purified by recrystallizing from hot water.)

[edit] Solubility of ionic compounds in water

The solubility of a salt which ionizes in water is determined by the solubility product (K_sp) which is a constant at a given temperature. Silver chloride is a relatively insoluble salt in water. It ionizes:

Ag⁺ + Cl^- ↔ AgCl (s)

The solubility product of AgCl, 1.8E-10 is also the equilibrium constant of this reaction which is calculated by multiplying the concentrations of silver and chloride ions in a saturated solution, i.e. [Ag+][Cl-]. Thus the maximum concentration of a pure solution of silver chloride possible is sqrt(1.8E-10) = 1.34 E-5 M. However, if there chloride ions were added, perhaps as a sodium chloride solution, the equilibrium will shift according to le Chatelier's principle, and silver chloride will precipitate from the solution.

Main article: Solubility chart

Soluble	Insoluble
Group I and NH₄⁺ compounds	carbonates (except Group I, NH₄⁺ and uranyl compounds)
nitrates	sulfites (except Group I and NH₄⁺ compounds)
acetates (ethanoates)	phosphates (except Group I and NH₄⁺ compounds)
chlorides, bromides and iodides (except Ag⁺, Pb²⁺, Cu⁺ and Hg₂²⁺)	hydroxides and oxides (except Group I, NH₄⁺, Ba²⁺, Sr²⁺, Ca²⁺ and Tl⁺)
sulfates (except Ag⁺, Pb²⁺, Ba²⁺, Sr²⁺ and Ca²⁺)	sulfides (except Group I, Group II and NH₄⁺ compounds)

[edit] Solubility of organic compounds

The principle outlined above under polarity, that like dissolves like, is the usual guide to solubility with organic systems. For example, vaseline will dissolve in gasoline; both of which are lipophilic. This is because vaseline jelly consists of long carbon chains, as does the gasoline. It will, on the other hand, not dissolve in alcohol or water, since the polarity of these solvents is too high. Sugar will not dissolve in gasoline, since sugar is too polar in comparison with gasoline. A mixture of gasoline and sugar can therefore be separated by filtration, or extraction with water.

[edit] Solid solubility

The term is often used in the field of metallurgy to refer to extent that an alloying element will dissolve into the base metal without forming a separate phase. The Solubility Line (or curve) is the line (or lines) on a phase diagram which give the limits of solute addition. That is, the lines show the maximum amount of a component that can be added to another component and still be in solid solution. In microelectronic fabrication, solid solubility refers to the maximum concentration of impurities one can can place into the substrate.

[edit] See also

Look up soluble, solubility in Wiktionary, the free dictionary.

[edit] External links

ALOGPS interactive calculation of aqueous solubility of compounds at Virtual Computational Chemistry Laboratory using several algorithms
QUANTUM web based calculation of aqueous and DMSO solubility of compounds QUANTUM web based prediction of aqueous and DMSO solubility of compounds

[edit] References

^ Atkins' Physical Chemistry, 7th Ed. by Julio De Paula, P.W. Atkins ISBN 0198792859
^ John W. Hill, Ralph H. Petrucci, General Chemistry, 2nd edition, p524, Prentice Hall, 1999.
^ John W. Hill, Ralph H. Petrucci, General Chemistry, 2nd edition, p527, Prentice Hall, 1999.
^ Data taken from the Handbook of Chemistry and Physics, 27th edition, Chemical Rubber Publishing Co., Cleveland, Ohio, 1943.
^ A highly water-soluble 2+1 b-cyclodextrin–fullerene conjugate Salvatore Filippone Frank Heimanna and André Rassat Chem. Commun., 2002, 1508 - 1509, DOI:10.1039/b202410a
^ Kenneth J. Williamson, Macroscale and Microscale Organic Experiments, p40, 2nd edition, D. C, Heath, Lexington, Mass., 1994.
^ Data taken from the Merck Index, 7th edition, Merck & Co., 1960.