Unit 2 Notes
Separating Liquids: Distillation Methods and GC Analysis
Separation Method: Distillation
Principle: Separates liquids in a mixture based on boiling points, by vaporizing and recondensing each substance
Types of distillation:
Simple -- Liquids are vaporized and condensed only once. Effective mainly for
separating substances with vast difference in boiling point
Fractional -- Mixture is vaporized and re-condensed repeatedly on a heated surface,
allowing for a number of distillation steps or “theoretical plates”
Steam -- Volatile but water-insoluble substances can co-distill with water
Vacuum – Decreased pressure allows substances to distill at lower temps.
Equipment:
Flask
Heat source
Distillation head / condenser
Fractionating column
Thermometer
Collection vessel
Fractional
distillation
Simple
distillation
Simple system allows vapor to rise and
recondense in sidearm as distillate
The temperature of vapor is measured and
can be used to monitor distillation progress
Fractionating column contains
material on which heat transfer can
take place, with a large surface area
Liquid is collected in “fractions”
Theory: Why it works
Phase diagram
showing the
liquid to vapor
transition for
mixtures of two
liquids as
composition
changes
Simple vs.
Fractional
“Theoretical Plates”
Assessing the relative effectiveness of the two methods
-- Monitor the temperature change
-- Plot temp vs.volume
What does shape of curve tell you?
-- Collect fractions
-- Determine the composition of the distillates
Things to watch out for:
Leaky connections are bad
Viewing the drops/reading thermometer
Keep fractions tightly capped & cold
Analyzing the composition of a mixture:
Gas Chromatography
Principle: Partitioning behavior is used to separate molecules
Equipment: Gow-Mac Gas Chromatograph, equipped with
heated coiled columns (nonpolar or polar packing), detector and
data readout/integration program
Process:
Liquids vaporize and then partition between
stationary and mobile phase
Stationary phase: a coated support material allows
compounds to adsorb
Mobile phase: inert gas sweeps through the column keeping vapor particles moving
Partitioning is affected by temperature and ability of compound to adsorb
Fact: The more time a compound spends on the stationary phase, the longer it takes to
emerge from the column
Detection:
Detector registers a change in current proportional to the moles of gas
whenever a compound “elutes” from the column.
(GC-MS detects the mass of the compound as well as the amount)
Data:
A plot of current vs. time is generated as compounds elute from column
= chromatogram
Gas Chromatograms
-- A peak is generated as each compound elutes
-- Area under each peak is proportional to the quantity of compound
-- % composition of the mixture can be determined
Retention Time