Gas Chromatography

Gas Chromatography: Purifying alcohol

Introduction

The goal of this lab is to understand the principles of chromatography by purifying alcohol using fractional distillation. Running standards with gas chromatography we were able to see and calibrate our data to find not only how much ethanol our alcohol attained but also what a mixed unknown sample contained. Chromatography is a way of being able to separate substances in solution that can help not only identify the analytes (the studied substances) but can determine their concentration. Finding out what is present gives us a qualitative analysis and finding out how much of the analyte is in the mixture gives us the quantitative analysis. The following picture shows the structure of a chromatographer.©Ñ†

The gas or in our case a liquid is injected through the port turned into a vapor through the nebalizer. The vapor then proceeds through the column. The mobile phase is the gas and the column is the stationary phase. Every gas has different affinities at varying temperatures for the two phases. When the gas stops it is detected by the detector and recorded using the amount of time it took for the analyte to travel through the column. The time is the qualitative analysis because an analyte like ethanol will behave similar under similar conditions (the same temperature). So running standards to know what analytes are being absorbed at what times will let you know what the unknown is. The areas under your peaks will give you an estimate of what your concentration is. Again this will help find your concentration of your unknown. Concentration is directly related to the area. Here is a graph so you see what a read out would look like.©Ñ‡

The peak heights give you the time and the area below corresponds directly to the concentration. The time it takes an analyte to reach the detector is the Retention time (Rt) and it is measured by the peak heights.

Distillation

The next part of the lab was to us distillation. Distillation is the process in which a liquid is vaporized and condensed, used to separate substances of different volatilities (or vapor pressures).©Ñˆ Simple distillation is just evaporation and condensation of a solution to separate analytes. If you have a solution with substances in it with different boiling points you can heat the solution at the lowest boiling to collect the vapor of the corresponding substance. Using this method however impurities would still remain in the collected sample. Why? In RaoultЎЇs Law if you add any solute into a solvent the solutionЎЇs boiling pt will decrease. So boiling at the lower temperature you will still some of the solvent because of where itЎЇs bonded to the solute.

In this lab we used only fractional distillation. Fractional distillation is the process of purifying the sample collected by repeatedly doing simple distillations with each new product. Fractional distillation is based on RaoultЎЇs Law and HenryЎЇs Law. Henry says solubility of a substance is directly proportional to it partial pressure of its gas dissolved under equilibrium. This helps us to know at what temperature to boil the solution to evaporate and collect the new sample. By making alcohol the principles are much easier to understand and apply to other real life commodities (like fuel for our cars).

Procedures:

Making Hops

Equipment and materials:

250 mL Erlenmeyer flask with stopper/ glass tube/ Teflon tube

Hotplate

Beaker

Medium test tube/ large test tube

5mL of Ca(OH)2 solution

Mineral oil

0.5 grams of yeast in 10mL of water

Ёц cup of fruit

1.5 grams of sugar

Magnetic stir bar

Incubator

1. Obtain 1 250mL flask. Wash flask with soap and water then fill it with water and boil for 15 minutes.

2. Boil water in beaker to sterilize the stopper, hose, and medium test tube for 15 minutes.

3. Place 5mL of Ca(OH)2 in large test tube. Place a drop of mineral oil on top of the Ca(OH)2.

4. Remove beaker and turn down temp. Remove stopper, tubing and test tube.

5. Water in beaker needs to be about 30ўЄC to 60ўЄC. Put .5 grams of yeast in 10mL of water in the medium test tube. Place test tube in beaker for 5mins.

6. Empty water in flask. Mash fruit then ass the Ёц cup of fruit (Our group used Bananas, Mountain Dew, and Apple Juice) and 1.5g of sugar with 100mL of water. Stir thoroughly using magnetic bar.

7. Once mixed add yeast. Flask will have a stopper with a glass tube and Teflon tubing stretching into the large test tube of Ca(OH)2. Tape large test tube to the flask to hold in place. Then use Parafilm to seal flask around