Separation and Purification Flashcards
Separation relies on differences in which physical or chemical properties
Melting point, boiling point, solubility, polarity and acidity/basicity
What does filtration do
Separates two phases, one passes through filter and other does not
What is filter
A porous material such as paper, sintered glass or metal, porous polymer
How does filtration work
Pressure difference forces the filtrate through
Describe gravity filtration
Paper filter, for collecting filtrate
Describe Buchner filtration
Paper or glass filer, vaccum applied. For collecting solid
What does crystallisation require
Difference in solubility between desired compound and impurities
Describe the procedure of crystallisation
Find a solvent that dissolves compound and when hot but not cold. Add a minimum quantity of hot solvent to dissolve. Allow to cool. Wait for crystals. Filter
What does evaporation rely on
A large difference in boiling point.
Describe evaporation
One component is essentially non-volatile. It is usually for the removal of a solvent. Accelerated by heat and/or vacuum
What does distillation rely on
Boiling point differences
Describe distillation
Liquid is in equilibrium with vapour. The more volatile substance has a higher vapour pressure. Liquid boils when vapour pressure is equal to atmospheric pressure, Vapour becomes enriched in more volatile compound. Cool the vapour to condense it
What is simple distillation apparatus used for
Used for compounds with large differences in boiling point (at least 25 degrees).
What is fractional distillation used for
Liquids with similar boiling points
What does fractional distillation require
A fractionating column
Describe sublimation
Some solids do not melt and convert straight to gas (sublime). This allows easy purification from non-volatile material
What does liquid-liquid extraction rely on
Differences in solubility between two solvents
What does liquid-liquid extraction require
Usually water/ organic solvent. Solvent must be immiscible with H2O. Solvent should be easy to evaporate (so low bp)
What are the solubility trends for hydrocarbons in water
Decrease solubilty
What are the solubility trends for polar groups in water
Increase solubility
What are the solubility trends for hydrogen bonding groups in water
Increase solubility
What are the solubility trends for charges in water
Often increase solubility
What do you use acid/base chemistry for
To control solubility (salt formation increases aqueous solubility)
What does chromatography rely on
Equilibrium of a substance between two phases. Stationary phase- immobilise solid/ liquid (in column or as 2 dimensional sheet). Mobile phase (elution)- a flowing liquid or gas (passes through column or up the sheet)
What is elution
The motion of molecules along the column (molecules with higher affinity for stationary phase elute more slowly, increasing affinity for mobile phase speeds up elution).
Why does relative affinity for mobile vs stationary phase depend on molecular structure
Hyydrogen bonding, polarity, van der Waals. Different compounds move at different speeds- emerge from the column at different times
Describe planar chromatography
Paper stationary phase- cellulose stationary phase, water or water + organic solvent mobilise phase, useful for sugars, plants pigments and amino acids. Thin layer (TLC)- silica gel or alumina particles coated on plastic/ glass/ aluminium sheets, organic solvent mobile phase, widely used by organic chemists
Describe planar chromatography
Apply spots of mixture to base-line. Stand in solvent to elute. Mark solvent front- calculate Rf vales (Rf= distance moved by compound/ distance moved by solvent)
Describe planar chromatography
Coloured compounds visualised directly. Visualise colourless compounds by UV light or staining.
How do you visualise colourless compounds by UV light in planar chromatography
Compounds may fluoresce (bright spots). Stationary phase often has fluorescent modifier added (dark spots on fluorescent green background)
How do you visualise colourless compounds by staining in planar chromatography
Best stain depends on compounds being analysed. Iodine- stains many compounds brown. KMnO4- charring/ brown staining. Ninhydrin- staines amines blue
Describe column chromatography
Gravity/ flash chromatography. For mg-grams. Glass column packed with silica gel/ alumina. Organic solvent mobilise phase flows through. Can be automated
Describe how organic solvent mobile phase flows through
Flow rate increased using gas pressure (“flash”).
Collect fractions in separate tubes.
Check purity by TLC
Describe how gravity/ flash chromatography can by automated
Disposable silica cartridges.
Automatic fraction collector.
UV monitoring.
What is HPLC
High performance (or pressure) liquid chromatography
Describe HPLC
From analytical scale (100 bar, stainless steel column and tubing). Detector (UV, refractive index, fluorescence, mass spectrometer etc.). Optional autosampler, fraction collector
Describe gas chromotography
Mobile phase is inert gas (Ar, He, N2). Two types of column- packed and open tubular capillary. Contained in an oven. Many stationary phase materials
