Fractional Distillation and Gas Chromatography

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CHEM 3152-003

Experiment 2: Fractional Distillation and Gas Chromatography


The purpose of this experiment was to separate a mixture of ethyl acetate (EtOAc) and toluene by using the method of fractional distillation and to analyze the fractions using gas chromatography (GC).

ethyl acetate toluene

Distillation is a method known as separating or purifying a liquid or mixture by vaporization and condensation. In a simple distillation, the liquids being separated boil below 150 0C at 1 atmosphere from nonvolatile impurities and another liquid that boils at least 25 0C higher than the first. A solution is heated to its boiling point. The vapor of the more volatile component of the solution is set apart from the boiling mixture and is condensed and collected.

In fractional distillation, the liquid mixtures being separated are soluble in each other and boil at less than 25 0C from each other at 1 atmosphere. Each component is called a fraction. As the mixture is heated, it boils, and the vapor that comes off this liquid is richer in the lower boiling component. The composition of the liquid still in the flask has changed a little?it is richer in the higher boiling component. As more of this liquid boils, hotter vapor comes up, mixes with the first fraction, and produces a new vapor. This vapor is richer in the lower boiling component. In a nutshell, fractional distillation is revaporizing the condensate. Simple distillation is only one cycle of the vaporization/condensation.

Another technique used for this experiment is gas chromatography (GC). GC is the separation of compounds between a stationary and a mobile phase. GC can also be referred to as vapor-phase chromatography (VPC) and gas-liquid chromatography (GLC). The sample enters the GC at the injection port. Two components A and B are different, they will stick on the absorbent and there is separation (VPC). Some of the absorbents are coated with a liquid phase. Different components of the mixture injected will spend a longer time in the liquid phase than the other will. Again, there will be separation. This technique is known as (GLC). The sample travels from the column to the detector. Here, the presence of a sample is detected and is essentially converted into a GC peak. There are usually two detectors in the instrument. When there is only gas flowing over both detectors, the circuit is said to be balanced and there is no signal, therefore no pen movement. If the sample in the carrier gas goes by one detector, the sample has a conductivity different from that of pure carrier gas and the sample detector loses heat at a different rate from the reference detector causing the circuit to become unbalanced. A signal then goes to the recorder giving a GC peak. Figure 2 on page 14 in the OCLM shows a diagram of this instrument. "Chromatography is the separation of compounds by repeated partitioning between a stationary and a mobile phase" (OCLM p. 13). This means there is a separation of compounds by repeated division between a liquid and a gas phase. A mixtures ability to be resolved depends on its constituents' difference in vapor pressure. Another factor is the polarity of the compounds being separated.

The mobile phase is often referred to as the carrier fluid. In gas chromatography, this phase is usually a gas. The mobile phase runs through the column and is absorbed onto the stationary phase. A compound will be swept along faster in the mobile phase rather the stationary phase. The column contains the stationary phase. The stationary phase is usually an absorbent or liquid coated on the surface of solid particles. Partition is the repeated division between these two phases. The partition coefficient is the amounts in the stationary and mobile phases.

As the sample flows through the column, its different components will adsorb to the stationary phase in variation. Those with strong attraction to the support move more slowly than those...
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