CHEM 43A E5

Distillation is

a separation and purification technique that relies on differences in boiling points (and vapor pressure) to separate components of a liquid mixture.

more volatile

lower boiling point or higher vapor pressure

vapors contain a greater percentage of the

more volatile component

resultant liquid now contains an increased percentage of the

more volatile compound and a decreased percentage of the less volatile compound.

Simple Distillation - Method

1) The solution is heated. The part of the solution that has the lowest boiling point evaporates first

2) The vapour is then cooled, condensed & is collected

3) The rest of the solution is left behind in the flask

To complete separate and purify (highly enrich) liquids via simple distillation,

the distillation must be repeated numerous times, each time increasing the purity ofthe collected liquid.

Fractional distillation

is a technique for accomplishing a large number of simple distillations in a single continuous operation.

A fractionating column (fractional distillation column) has

extensive surface area that induces the vapors to condense before the collection flask

How does fractional distillation work?

The cooled condensate falls down the fractionating column where it encounters hot vapors rising up. Thehot vapors transfer heat to the condensate causing the liquid components to return to the gas phase according to their boiling points andthe vapor becomes enriched in the more volatile component. As the vapors heat thecondensate, the vapors cool and condense according to their boiling point such that the percentage of the less volatile componentin the condensate increases. Eventually, the vapor that reachesthe top of the column is highly enrichedin the more volatile component.Whereas the condensate that has returned to the distillation flask contains primarilyless volatile component.

Position the thermometer so that the thermometer tip (bulb)is positioned

just below the side arm of the distillation head

thermometer position is important so that it measures

an accurate temperature of vapors just before they enter the chilled condenser and begin cooling.

why insulate the fractionating column with cotton wool.

This will ensure a steady distillation.

ideal constant rate of distillation;

approximately one drop per second from the tip of the bent adapter into the receiving flask.

As the distillation progresses you will need to

slowly raise the temperature as the liquid in the round bottom contains more of the less volatile component.

The temperature of the more volatile vapors will be ____ than the less volatile component.

lower

Gas chromatography (GC)

is ananalytical techniquethat uses a gas as the mobile phasein a chromatographic separation.

solution containing the compounds to be separated and analyzed is injected into the GC instrument where

the sample is rapidly heated under reduced pressureto vaporize everything

Steps of gas chromatography?

1. the sample is rapidly heated under reduced pressureto vaporize everything

2. Next an inert gas, (typically helium or nitrogen) carries the vaporized compounds through the stationary phase

3. . A detector at the exit reports the time it takes for each component to pass through the column

gas chromatography retention time

time for it to cross

the retention times can be compared against

a standard to identify each component

we can use the peak area to

uantify the relative amountof each component.

relative response factors,

where one of the components is set to have a value of one (1) and the other is relative to that number.

The first step is to inject a mixture of standards where the ratio between the components is known to be

1:1 by volume

choose thereference peak

typically we choose the smaller value and normalize the area to 1. This ensures all correction factors are equal to or greater than 1

a: 2500 2500/2300=1.1

b:2300 2300/2300=1.0

Control of heating is essential for a successful distillation. If the heat is too high, the vapors____through the fractionating column and maximum separation _____ If the heat is too low, the vapors____through the fractionating column and the observed thermometer reading will_____. Thus rate of distillation is always a compromise between speed and efficiency. Too fast and separation will be____too slow and_____. The heating rate is approximated by observing the drop rate of the distillate into the receiver. A good drop rate is around_____. If the drop rate is too fast, the best solution is to quickly___

will quickly pass

cannot occur

may not pass

decrease

poor

you will run out of time

1-2 drop per sec

lower the lab jack and heating mantle

gas chrom mobile phase

gas

gas chrom stationary phase

polymer, liquid phase

gas chrom requires

everything in the gas phase= vapors!

small solids with some vapor pressure, or heat to get them into gas phase

How does gas chromatography work?

- different substances are carried by gas (mobile phase) and travel through a column packed with a solid material (stationary phase) they become separated because they travel at different speeds, compounds attracted to polymers, then go back into gas phase, then stick back (condensation and evaporation cycle)

gas chrom depends on

attraction of compounds to stationary phase, and volatility

more volatile in gas chrom comes off

first!

higher vapor pressure

lower boiling point, more voltalie

steam distillation

For temperature sensitive materials like natural aromatic compounds. Many organic compounds tend to decompose at high sustained temperatures. simple distillation setup, organic oil is immisicble and extract