topic 6: separation tehniques Flashcards
what are the types of separation techniques
- Filtration = gravity/vacuum
- Distillation
- Recrystallisation
- Chromatography
how does filtration work
(separates a solid from a liquid)
how does distillation work
(separates mixtures of liquids)
how does recrystallisation work
(removes impurities from solids)
which techniques have one thing in common
Filtration = components of the mixture are in different physical states e.g. solid and liquid
and
distillation = liquid in the mixture have different boiling points
(they use physical or
chemical differences to separate compounds)
compare the boiling points of water alcohol and oils
alcohol has a lower bp than water
oils have a higher bp than water
how does filtration work ? (gravity)
-> Filtration is the physical separation of a solid from a liquid
->by passing the mixture through a porous material (e.g., filter paper or sintered glass). The solid is trapped on the porous material, while the liquid passes through.
-> The most common type is gravity filtration with fluted filter paper.
which filter paper style is better
fluted filter paper is better than conical filter paper as it has more surface area
when do we use vacuum filtration and gravity filtration
gravity filtration is used is the particle size is big as its quicker and less cost effective
vacuum filtration is used if the particle size in the solution are very small (into the micrometres)
why is the Buchner funnel made of ceramic
it doesn’t react with anything
why is a stopper used in vacuum filtration
need a rubber stopper to make the vaccum
why do we typically use the filtrate types
gravity filtration is used when we typically want the filtrate not the residue
vacuum filtration is typically used if we want to collect the solid
how does a vacuum filtration work
- filter paper is placed inside the Buchner
- the funnel is secured onto the filter flask using a rubber stopper to create and airtight seal
- the solution containing crystals (solid) is poured into the funnel
- vacuum is applied which the causes the filtrate (liquid) to pass through into the conical flask and the solid crystals on top
why do we use distillation
- 2 liquids with different bp
- a solid that is already dissolved into the liquid
how does recrystallisation work
1) dissolve the impure crystals just enough hot water to do the job
2) hot filtration with suction takes out non soluble impurities
3) crystals allowed to reform recrystallisation
4) cold filtration under gravity takes out soluble impurities
5) dry the pure impurities
The desired product must be soluble in the solvent when it is hot and
insoluble in the solvent when it is cold
how does distillation work
- the mixture is heated in a distillation flask using a Bunsen burner
- the component with the lower bp starts to evaporate leaving the substance with the higher bp behind
- the steam enters a condenser (cooling tube) surrounded by cold water
- the steam cools down turning it back into liquid (condenses)
- the purified liquid is free from impurities.
why is chromatography used
to separate components of a mixture on the
basis of differences in their physical characteristics, e.g. molecular size,
shape, charge, solubility etc
what is the stationary phase
stays there doesnt go anywhere solid (a porous, finely divided solid, e.g. paper, silica)
what is the mobile phase
a solvent or mixture of solvents which moves over the stationary
phase)
interactions with the stationary phase
- chemicals that travel further from the stationary phase = interacts weekly with stationary phase less retention times
- chemicals that are near the stationary phase = interact strongly with the stationary phase more
why is silica polar in TLC
Thin layer chromatography (TLC) uses silica bonded to an aluminium sheet as the stationary phase. Silica is polar due to its –OH groups, allowing it to interact with the components of the mixture through hydrogen bonding.
how do we do chromatography
A small amount of the sample solution is spotted onto the plate
The plate is then put into a developing tank with a small amount of solvent
The solvent travels up the plate and takes the sample with it, separating
the components as it goes
what is the Rf value
Rf=distance travelled by the compound/
distance travelled by the solvent
and it can help to identify unknown compounds
gas chromatography
cylinder contains gas either helium/nitrogen is the mobile phase
column is the stationary phase is inside an over because in dc separation occurs due to bp lighter out quicker than less light
column over separated by boiling points
sample injection = inject something with low bp
detector = detect the things going out
how does GC work
Samples are injected as solutions
* Components of the sample are vaporised and pass into the column
* They are separated according to their affinity for the stationary phase
(usually correlates with boiling point)
* A signal is recorded by the detector and a chromatogram is produced
* Retention times are measured for each component of the mixture
Detectors commonly used in GC include:
Flame ionisation detector – burns all organic compounds coming out of the
column and detects current
Mass spectrometer – “weighs” molecules and gives structural information
Chromatography - LC
- Samples are injected as solutions
- Components of the sample are washed into the column
- They are separated according to their affinity for the stationary phase
- A signal is recorded by the detector and a chromatogram is produced
- Detectors commonly used in HPLC include:
- UV detector – measures absorbance of ultraviolet light
- Mass spectrometer
