Chapter 3: Separation and purification (C7040612)

 #toc { border: 1px solid #bba; background-color: #f7f8ff; padding: 1em; font-size: 90%; text-align: center; } #toc-header { display: inline; padding: 0; font-size: 100%; font-weight: bold; } #toc ul { list-style-type: none; margin-left: 0; padding-left: 0; text-align: left; } .toc2 { margin-left: 1em; } .toc3 { margin-left: 2em; } .toc4 { margin-left: 3em; }Table of Contents Last modified: 3256d agoWord count: 1,027 wordsLegend: Key principles // Storyline

1 Extraction

[Liquid-liquid] extraction is the separation of components in mixtures, based on differences in solubility. Extraction uses two immiscible layers, a (1) nonpolar phase; and a (2) aqueous phase. Usually, the nonpolar phase contains the mixture.

• Acid is added, and the liquid shaken. The acid protonates the bases in the nonpolar mixture, usually amines ($-NH_{2}$ , which likes to donate its lone pair electron, therefore a Lewis base, discussed ) [which is water insoluble]. The ammonium salt ($-NH_{3}^{+}$), now positively charged, is water soluble, and can be drawn off [as the aqueous layer]
• A weak base is added, and the liquid shaken. The weak base deprotonates the strong acids, like carboxylic acids [which is water insoluble]. The carboxylate salt ($-CO_{3}^{2-}$), now negatively charged, is water soluble, and can be drawn off [as the aqueous layer]
• A strong base is added, and the liquid shaken. The strong base deprotonates the weak acids, like phenol [which is water insoluble]. The phenoxide salt ($-Benzene-O^{-}$), now negatively charged, is water soluble, and can be drawn off [as the aqueous layer]

 Formative learning activity Maps to RK3.A What is resource extraction?

2 Distillation

Distillation is the separation of components in mixtures, based on differences in boiling points. The simplest example is where water is heated to boiling, and collected for drinking, such that impurities are all removed.

A mixture [in solution] is heated to boiling, which in the absence of an azeotrope, is the boiling point of the component with the lowest boiling point. Components with lower boiling points are hence boiled out earlier, into the distillation flask. Distillation should occur slowly, as increasing the temperature too quickly, can prevent the components from separating properly. Azeotrope are mixtures whose components bond with another more weakly or stronger [than to themselves], causing the mixtures to have a lower and higher boiling point [when contrasted with the constituent component, with the lowest boiling point] respectively. Azeotropes cannot be separated by simple distillation.

 Formative learning activity Maps to RK3.B What is distillation?

3 Chromatography

Chromatography is the separation of components in mixtures, by dissolving it into a solution [known as the mobile phase], and passing it over a surface [known as a stationary phase] that adsorbs the compounds in the mixture, at different rates. Adsorption [note the difference between this word and absorption] is the sticking on to a surface. Elution refers to the process by which the mobile phase moves through the stationary phase. As the mobile phase moves through the stationary phase, the compounds [in the mobile phase] with a greater affinity for the stationary phase, will travel slower. Typically, the stationary phase will be polar, and based upon like dissolves like (discussed ), polar compounds will have greater affinity.

Column chromatography involves dripping a solution [which is the mobile phase] down a column of microbeads [which is the stationary phase]. Then, the mixture is added, and is separated into its components, looking like separate horizontal lines. If the stationary phase is polar, the more polar compounds will end up near the top.

[img]column-chromatography.png[/img]

Paper chromatography involves dissolving a mixture into a nonpolar solvent [the mobile phase], which moves up the piece of paper. On a piece of paper [that is selected for attraction to polar molecules], the polar compounds will stick to the paper more strongly, and traverse less quickly. The resulting paper has a series of colored dots, each representing a different compound in the mixture. The retardation factor [or retention factor, $R_{f}$] is the ratio/fraction of the distance travelled by any component, with the distance travelled by the solvent.

[img]paper-chromatography.png[/img]

Thin layer chromatography is analogous to paper chromatography, except coated glass, plastic or aluminium foil is used.

Gas chromatography involves dissolving the mixture into a heated carrier gas [the mobile phase], and passed through a liquid [the stationary phase]. Exiting the end of the column, the compound comes out as its separate components. It is possible then to plot the abundance of each component [on the y-axis] against the time [on the x-axis].

 Formative learning activity Maps to RK3.C What is chromatography?

4 Recrystallization

Crystallization is the process of melting and then crystallizing a substance repeatedly. The crystal is pure, because since crystals are a perfect lattice, they can only be created from pure substances.

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