E3 - Interphase Partition Analysis: Distillation and GC
Introduction
Distillation and chromatography are two important methods for the separation of chemicals. You will familiarize yourself with these two techniques in this experiment. First, A 50/50 mixture of cyclohexane and toluene will be prepared and separated via fractional distillation. Three fractions of the distillate will be collected and analyzed in the GC to determine the effectiveness of the distillation. An unknow mixture of two organic chemicals will be provided. Using the techniques of distillation and GC, you should be able to determine the composition of the mixture.
Simple or fractional distillation at atmospheric pressure or under vacuum are crucial steps in manufacturing chemicals. Although the origin of distillation is uncertain, alchemists' report its use as early as the sixth century AD.
"Aristotle's doctrine that metals and minerals had souls made sense to the esoteric alchemists, and from it they drew a logical conclusion: Distillation and sublimation purified material substances-and by extension could purify human souls as well… The word sublime originally referred to the solid material purified by evaporation and recondensation. In distillation, the residue left in the flask was termed the dead body or caput mortuum (dead head), and the condensed vapors were considered to be the soul or spirit of the material. The term "spirit" carries over even today in the names of such pharmaceutical as spirit of ammonia and sweet spirits of niter."1
Chromatography in its different versions is used either as a quality control tool or as a separation technique when distillation is not feasible. The term chromatography was coined by the Russian botanist Mikhail Tsweett around the turn of the century, he used the technique to separate chlorophylls from xanthophylls by passing solutions of the compounds through a glass column containing finely divided calcium carbonate. In this experiment you will perform the separation of two close boiling point compounds: cyclohexane and toluene by fractional distillation, and monitor the effectiveness of the separation with a gas chromatograph.
Theory
The capacity of a molecule to distribute or partition between two phases without interacting chemically with any of the phases stays at the base of most analytical separation methods. If the molecule can move through the two phases, a dynamic exchange takes place at the interface between the two phases. When the chemical potential of the molecule is the same in both phases, the partition will reach equilibrium:
Utilizing the chemical potential, µA expression of the molecule in both phases, and substituting the difference in chemical potential as the standard free energy change, G0 for the partition process:
We arrive at an expression for the Partition Constant or Distribution Coefficient expressed as the ratio between the molecular activities in each phase. Since the activity coefficients are essentially equal to one in dilute solution, the Partition Constant is finally expressed by equation (4):
Gas-Liquid Interface: Distillation
In the gas-liquid interface, the composition of the vapor in equilibrium with the liquid is richer in the more volatile components of the mixture. If the vapor phase is condensed, the process is called distillation. In distillation the molecules are partitioned between the liquid and the gaseous phases. In the gas phase the activity is expressed by the vapor pressure, PA while in the liquid phase it is expressed by the molar fraction XA at 1 atm pressure.
In the gas phase the activity coefficient will be unity if the gas is ideal. The same occurs in the liquid phase if the solution is ideal.
An ideal solution obeys Raoult's Law, which states that at a given temperature, the vapor pressure of each component is proportional to the mole
