Solubility



Solubility is a physical property referring to the ability for a given substance, the metastable.

In a solution, the solvent is often a liquid, which can be a pure substance or a 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.

Molecular view

Solubility occurs under dynamic equilibrium. This means that solubility should be viewed as a result of two simultaneous and opposing processes: dissolution and precipitation. The solubility equilibrium occurs when the two processes proceed at the same rate.

The solubility equilibrium is relatively straightforward for 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, and the value of the solubility will include all the species in the solution (in the example above, all the iron-containing complexes).

Factors affecting solubility

Solubility is defined for specific phases. For example, the solubility of calcium carbonate).

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.

Solubility may also strongly depend on the presence of other species dissolved in the solvent, for example, solubility equilibrium.

There is also a number of less common factors which may affect solubility. Solubility may depend on the crystal (or droplet) size of the solute phase (typically, solubility will increase with the decreasing crystal size for crystals much smaller than 1 μm). For highly defective crystals, solubility may increase with the increasing degree of disorder. The last two effects, although of great practical importance, are not true solubility effects because true solubility occurs at equilbrium, which requires a perfect monocrystal. For substances dissolving in an electrochemical reaction, solubility is expected to depend on the potential of the solute phase.

Temperature

The solubility of a given solute in a given solvent typically depends on temperature. For around 95% of solid solutes, the solubility increases with temperature,[2] in the temperature range from about ambient to 100 °C. In liquid water at high temperatures, (e.g., that approaching the dielectric constant, less of a polar solvent).

Gaseous solutes exhibit more complex behavior with temperature. As the temperature is raised gases usually become less soluble in water, but more soluble in organic solvents.[2]

The chart shows solubility curves for some typical inorganic anhydrous phase.

cyclodextrins.[4]

Pressure

For condensed phases (solids and liquids), the pressure dependence of solubility is typically weak and usually neglected in practice. Assuming an ideal solution, the dependence can be quantified as:

\left(\frac{\partial \ln N_i}{\partial P} \right)_T = -\frac{V_{i,aq}-V_{i,cr}} {RT}

where Ni is the mole fraction of the ith component in the solution, P is the pressure, the index T refers to constant temperature, Vi,aq is the partial molar volume of the ith component in the solution, Vi,cr is the partial molar volume of the ith component in the dissolving solid, and R is the universal gas constant[5].

partial pressure of that gas above the liquid, which may be written as:

p = kc \,

where k is a temperature-dependent constant (for example, 769.2 L•atm/mol for dioxygen (O2) in water at 298 K), p is the partial pressure (atm), and c is the concentration of the dissolved gas in the liquid (mol/L).

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 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.

Rate of dissolution

drug delivery. Critically, the dissolution rate depends on the presence of mixing and other factors that determine the degree of undersaturation in the liquid solvent film immediately adjacent to the solid solute crystal. In some cases, solublity equlibria can take a long time to establish (hours, days, months, or many years; depending on the nature of the solute and other factors). In practise, it means that the amount of solute in a solution is not always determined by its thermodynamic solubility, but may depend on kinetics of dissolution (or precipitation).

The rate of dissolution and solubility should not be confused--they are different concepts (kinetic and thermodynamic, respectively).

Quantification of solubility

Solubility is commonly expressed as a concentration, either mass concentration (g of solute per kg of solvent, or g per 100 mL (dL) of solvent) or molarity, molality, or mole fraction or similar. The maximum equilibrium amount of solute that can dissolve per amount of solvent is the solubility of that solute in that solvent under the specified conditions. The advantage of expressing solubility in this manner is its simplicity, while the disadvantage is that it can strongly depend on the presence of other species in the solvent (for example, the common ion effect).

Solubility constants are used to describe saturated solutions of ionic compounds of relatively low solubility (see temperature can affect the numerical value of solubility constant. The solubility constant is more complicated than solubility. However, the value of this constant is generally independent of the presence of other species in the solvent.

partial pressure. It is a special case of a solubility equilibrium.

The enthalpy of fusion.

The octanol) and a hydrophilic solvent (water). The logarithm of these two values enables compounds to be ranked in terms of hydrophilicity (or hydrophobicity).

Applications

Solubility is of fundamental importance in a large number of scientific disciplines and practical applications, the most obvious ones being in chemical engineering, material science and geology.

For example, solubility of a substance is useful when separating mixtures. 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 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.)

Solubility of ionic compounds in water

The solubility of a salt which ionizes in water is determined by the 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 le Chatelier's principle, and silver chloride will precipitate from the solution.

Main article: Solubility chart
SolubleInsoluble
uranyl compounds)
NH4+ compounds)
NH4+ compounds)
Tl+)
NH4+ compounds)

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, petroleum jelly 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 not, on the other hand, 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.

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 place into the substrate.

See also

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

References

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