Lab 5: Simple Distillation and Gas Chromatography

Two most important readings to take from a GC

retention time and percent composition

Simple distillation can separate two miscible fluids with _______

difference of 25 degrees between boiling points

How to decide between distillation and GC?

1. GC can be used with little sample 2. GC compounds can have the same boiling points but different functional groups 3. GC has flash vaporization that can cause separation below a compounds boiling point.

Factors that effected retention time

1. High temperature, lower retention time 2. High flow of carrier gas, lower retention time 3. Longer length of column, higher retention time 2. More the liquid phase interacts with the gas, higher retention time

Conditions that favor GC

1. Volatile (easily transformed to gas phase) 2. Thermally stable 3. Little fluid

Procedure for the Gas Chromatography Lab

2 mixtures: hexane/octane and heptane/octane 1. Do simple distillation of three fragments and take the GC of those fragments

3 factors that effect resolution

Boil up rate: rate of the liquid that enters the column as vapors per minute, slower the better for resolution Reflux rate: volume of liquid returned to the pot divided by the amount of liquid in the pot. Large is better number of theoretical plates: more is better

Definition of Gas Chromatography

Chromatography is defined as the separation of a mixture into its components by passing a fluid (gas or liquid containing the material to be analyzed) over a non-mobile adsorbent and separating the components by differential absorption and elution.

True or False: Rapid heating can drive heated vapor up the column, preventing the formation of an equilibrium that allows the lower boiling component to separate. As a result, this will increase the number of theoretical plates and increase the separation

False

True or False: The smaller the boiling point difference, the easier it is to separate two liquids and fewer theoretical plates are required

False

How does the separation on a packed GC column compare to other methods of separation of volatile components?

GC separation like simple distillation is based on theoretical plates. But, distillation has one or a few theoretical plates while GC has lots.

Will polar compounds have a higher or lower retention time?

Higher, the polar molecules interact with the polar liquid

A mixture of two miscible volatile liquids is distilled. Which of the following statements are true concerning the boiling point of the distillate?

If the range is within 2 °C, then the distillate can be considered pure, and the temperature range should be recorded until right before the dramatic drop in temperature, and the temperature range should start being recorded from when the liquid first starts to boil.

Why do we add boiling chips to the simple distillation?

The boiling chips will cause small bubbles in the liquid instead of just building up to one large bubble and causing the whole thing to explode

Resolution definition

The effectiveness of separation in a distillation

What error is introduced if the bulb of the thermometer, or the thermocouple, is too high in the distillation apparatus?

The temperature will read a lower temperature than what it actually is.

Which of the following statements are true concerning azeotropes?

The vapor of an azeotrope has the same composition as the unboiled mixture and Azeotropes cannot be separated by simple distillation.

True or False: Each vaporization-condensation is called a theoretical plate

True

True or False: Increasing the length of the column and packing the column with glass beads or rings are methods that produce more theoretical plates:

True

True or False: The rate of heating affects the number of theoretical plates in the column

True

Which of the following statements describe why moisture must be excluded from any sample injected into the G.C.

Water signals are very broad and make interpretation of other peaks difficult and Water contains dissolved salts that can degrade the column.

What is the stationary phase?

a very high boiling point liquid absorbed into the firebrick

Liquids with a lower vapor pressure have a ____

higher boiling point

Liquids with a higher vapor pressure have a ____

lower boiling point

Simple distillation has to be

volatile and thermally stable, no azeotropes, and have a boiling point difference of 25 degrees.

The definition of boiling point is

when vapor pressure is in equilibrium with atmospheric pressure.

How to determine the percent compostion

you will need to determine the area under each of the peaks. In a gas chromatogram, the peak areas (not just peak height) are considered to be directly proportional to the amount of sample passing the detector in the gas chromatograph.