Lecture 1 - Basic Lab Techniques, Chromatography And Analytical Methods Flashcards
Definition of separation
The division of something into constituent or distinct components
The extraction or removal of a specified substance for use or rejection
In chem it relies on differences in chemical or physical properties
What is the difference between mixtures and compounds in terms of:
Composition
Connection
Properties
Separation
Mixture:
Variable composition, substances are not chemical joined, each substance in the mixture has its own properties, each substance can be separated from the mixture
Compound:
Definite composition, different elements are chemically joined together, compound has properties different from the elements it contains, it can only be separated into its elements using chemical reactions
Filtration - taking advantage of porous membranes
It seperates solids from liquids using a porous material eg filter paper or sintered glass
It is a physical separation process
Used in: gravimetric analysis, recrystallisation, solvent drying agent
Gravity filtration
Isolation of liquid from an unwanted solid
Made from:
Filter funnel (compatible with solvent)
Filter paper ( diameter =/< 2x funnel diameter, check porosity)
Clamp flask - reduce chance of glassware toppling over
Tips:
Swirl the suspension before transfer
Do not overfill the filter cone
Wash flask and solid with small amount of filtered solution then a little pure solvent
Hot filtration
It is a modification of gravity filtration to remove solvent-insoluble impurities from hot solution before allowing filtrate to cool and form crystals of purified product
Set up:
Same as gravity but place in a beaker with insulation
Use a stemless or short-stem filter funnel
Tips:
Keep glassware hot (should be in oven beforehand) and insulated
Wash filter paper through with a little hot solvent to dissolve any product that has crystallised out before the receiving flask
Suction filtration - diagram slide 15
Separation of solid from unwanted liquid
Requires:
Funnel containing perforated plate and filter paper (eg Buchner, Hirsch, sintered glass)
Clean receiver flask with side arm for attachment to a vacuum source (with tap)
Tips:
Swirl mixture to form suspension
Slowly pour suspension onto the funnel so filtration is rapid
Rate of filtration may slow as “cake” of solid becomes thicker
Wash flask and filter cake with a little filtrate. Release vacuum by removing vacuum tubing (do NOT turn off pump as there is risk of suck back)
Remove filtrate (and store) then wash filter cake with a little cold solvent to wash solid
Suck air through solid to dry. If cracks appear in fileter cake press them together by pressing with clean spatula
Do at room temp ti avoid evaporation
Vacuum filtration
Equipment:
Büchner funnel
Moistened filter paper suitable that particles of solid cannot fit through tiny holes in filter paper
Porous plate (plate with holes in) in funnel under filter paper
Rubber bung attaching funnel to flask
Büchner flask
Rubber tubing which attaches onto the aspirator to create partial vacuum in flask (suction)
Filtrate (liquid that passes through filter paper) at bottom of flask
Filter aids - diagram on slide 19
Separation of finely divided unwanted solid particle impurities from solution that would otherwise pass through or clog filter paper)
An efficient, economical filter aid must:
- have rigid intricately shaped, porous individual particles
- form a highly permeable, stable and incompressible cake
- remove even the finest solids at high rates of flow
- be chemically inert and essentially insoluble in the liquid being filtered
Tip
- wash the filter aid in the funnel with your solvent to remove any soluble impurities and discard filtrate before using with your. Material
Centrifugation - slide 20
Separation of particles in a solid-liquid mixture. Centrifugal force promotes accelerated settling of particles
Used when wanted to remove a small amount of precious material.
Will separate out in different weights (sediment (pellet) to the bottom, supernatant (liquid lying above the sediment) at top. Keep repeating by adding clear solvent until separation is complete
Setup:
Armoured casing
Angled “test tube” containing mixture
Rapidly rotating rotor
Soxhlet extraction - diagrams on slide 22 and 23
Separation of partially solvable component of a solid mixture into liquid phase using continuous extraction process. Impurities remain in paper thimble. Material has to be relatively insolvable
Requires:
Porous filter paper thimble
Soxhlet apparatus
Flask containing solvent
Heat source
Reflux condenser
Tips:
Soxhlet thimble should be higher than siphon outlet
Half fill thimble with solid to be extracted and plug with white cotton wool to prevent any solid being transferred into solvent
Recrystallisation
Separation technique which removes three different tuypes of impurities (purifying solids):
- insoluble material (hot filtration)
- small quantities (10-15% max) of unreacted starting materials and/or byproducts
- small amounts of coloured byproducts resulting from oxidation or polymerisation
Requires:
Solvent that dissolves the solid while hot
Solvent should not dissolve desired solid while cold
Cold solvent must keep impuries dissolved forever
Done by using round bottomed flask attacked to condenser with heat applied at bottom
Recrystallisation using a single solvent - slide 26
Transfer material to a suitable flask - usually a round bottomed flask
Add minimum volume of a suitable solvent eg benzoic acid in water)
Heat to reflux and ensure al material dissolves
Allow to cool as slowly as possible
How to choose a solvent in recrystallisation
Using concept of “like dissolves like”
Consider polarity and H bonding
Desirable characteristics
High dissolving power for solute at high temp and low at room temp or below
High or negligible dissolving power for solid impurities so they can be removed by filtration
Boiling point > 60 *C
Boiling point should be at least 10 *C lower than that of the compound being crystallised
Solvents in order of polarity:
Petroleum ether, toluene, acetone, ethyl acetate, ethanol, water
Recrystallisation - mixed-solvent - slide 29
Used when no single solvent can be found
Essential properties of solvent pair:
Miscible in all proportions over temp range (ie forms a homogenous mixture)
Solute must be insoluble in one solvent
Solute must be soluble in other solvent
Within 20-30 *C bp of each other
Recrystallisation tips and tricks
Rapid cooling in an ice-bath (crash crystallisation) produces small crystals occulted with mother liquor
Slowly cooling produces larger well-defined purer crystals
If crystals form in filter funnel during hot filtration wash with hot solvent
Compound does not recrystallised is caused by:
- Too much solvent (so evaporate)
- Supersaturated (seed (adding a tiny crystal of the solute (the “seed crystal”) to the supersaturated solution) or scratch with glass rod)
Compound precipitates as oil “oiling out”
- slow coooling
- scratch on cooling
- use solvent with bp less than the mp of the solid
Remeber to cover flask with watch glass to prevent solvent evaporation and stop dust getting in
Clamp flask in ice water bath
Sublimation
Used for purifying fairly volatile organic solids
Solid heated and turns directly into a gas. On cooling the gas converts back to solid
Requires
-sublimation apparatus (check slide 31)
-cold finger
-vacuum source
Tips:
After subliming be very careful when taking out the finger out of the outer tube and releasing the vacuum
Used for purifying fairly volatile organic solids
Solid heated and turns directly into a gas. On cooling the gas converts back to solid
Requires
-sublimation apparatus (check slide 3
Drying solids
Removal of solvent from solid by evaporation
Heat in an oven
(Thermally stable only, oven at least 30 *C below mp of solid)
Desiccator
(Drying agent/desiccant, solvent dependent)
Vacuum desiccator
Reduced atmospheric pressure ( scalene line, rotary evaporator) making it become more voaltile
