Gas Chromatography:
See:
http://teaching.shu.ac.uk/hwb/chemistry/tutorials/chrom/gaschrm.htm
http://video.google.com/videoplay?docid=-3410718631045613107
Distillation:
Distillation is the process of heating a liquid to a temperature at which
it boils, cooling the hot vapors, and collecting
the condensed vapors.
Mankind has used distillation for thousands of years.
Distillation was probably first used by ancient Arab chemists to isolate
perfumes. Vessels with a trough on the rim to collect distillate, called
diqarus, date back to 3500 BC. Distillation was mentioned by Aristotle
(384-322 B.C.) as a method of purifying water, and Pliny the Elder (23-79
A.D.) recorded one of the earliest references
to a rudimentary still, an apparatus used to perform alcohol distillation. Illegal
distillers of alcoholic beverages,
"moonshiners", use it to produce concentrated solutions of ethanol.
In the organic chemistry laboratory, distillation is still an important
and powerful tool, particularly for the separation and purification of
organic compounds that are normally liquids at room temperature.
The
boiling point of a compound—determined by distillation—is well-defined
and therefore one of the physical properties of a compound by which it can
be identified just as melting point. More commonly, distillation is used
to purify a compound by separating
it from more or less-volatile materials. When different compounds
in
a mixture
have different
boiling points, they separate into individual components when the mixture
is carefully distilled.
------------------------------------------------------------------------
Boiling Point Determination:
The boiling point is the temperature at which the vapor pressure of the liquid
phase of a compound equals the external pressure acting on the surface of
the liquid. The external pressure is usually the atmospheric pressure. For
instance, consider a liquid heated in an open flask. The vapor pressure of
the liquid will increase as the temperature of the liquid increases, and
when the vapor pressure equals the atmospheric pressure, the liquid will
boil. Different compounds boil at different temperatures because each has
a different, characteristic vapor pressure: compounds with higher vapor pressures
will boil at lower temperatures.
Boiling points are usually measured by recording the boiling point (or range)
on a thermometer while performing a distillation. This method is used whenever
there is enough of the compound to perform a distillation. The distillation
method of boiling point determination measures the temperature of the vapors
above the liquid. Since these vapors are in equilibrium with the boiling
liquid, they are the same temperature as the boiling liquid. The vapor temperature
rather than the pot temperature is measured because if you put a thermometer
actually in the boiling liquid mixture, the temperature reading would likely
be higher than that of the vapors. This is because the liquid can be superheated
or contaminated with other substances, and therefore its temperature is not
an accurate measurement of the boiling temperature.
Purification Methods:
Simple Distillation:
Simple distillations are used frequently in the organic chemistry. They
are useful in the following circumstances:
- the liquid is relatively pure to begin with (e.g., no more than 10% liquid contaminants)
- the liquid is contaminated by a liquid with a boiling point that differs by at least 70°C
“
Simple” distillation may be a misleading term, since it takes a lot
of practice in simple distillation to become proficient in this technique. You
will use this technique in a later experiment.
Fractional Distillation:
Read Lab Text/Guide pp. 136-138
Fractional distillation is used.for mixtures of liquids whose boiling
points are similar (separated by less than 70°C) and cannot be
separated by a single simple distillation.
|
 |
What happens in the fractionating column?
Consider a mixture with composition C1 (~80%
B and ~20% A) that begins to boil; the dotted line on the Phase Diagram below.
The vapor over the top of the boiling liquid C1 will
be richer in the more volatile component A, and will have the composition
C2.
That vapor now starts to travel up the fractionating column. Eventually
it will reach a height in the column where the temperature is low enough
that it will condense to a liquid. The composition of that liquid will
still be C2.The liquid trickles down the column
where it meets new hot vapor rising, which causes the already condensed
vapor to reboil.
Some of the liquid of composition C2 will boil to give a vapor of composition
C3. This new vapor will again move further up the fractionating column
until it gets to a temperature where it can condense which has the composition
C4.
Each time the vapour condenses to a liquid, this liquid will start to trickle
back down the column where it will be reboiled by up-coming hot vapor.
Each time this happens the new vapor will be richer in the more volatile
component. The aim is to balance the temperature of the column so that
by the time vapor reaches the top after huge numbers of condensing and
reboiling
operations,
it consists only of the more volatile component - in this case, B.
Whether or not this is possible depends on the difference between the boiling
points
of the two liquids. The closer they are together, the
longer the column
has to be.
Since the vapor is richer in the more volatile component, the liquid left behind must be richer in the other, higher boiling one. As the condensed liquid trickles down the column constantly being reboiled by up-coming vapor, each reboiling makes it richer and richer in the less volatile component - in this case, A. By the time the liquid drips back into the flask, it will be very rich in A. So, over time, as B passes out of the top of the column into the condenser, the liquid in the flask will become richer in A. If you are very, very careful over temperature control, eventually you will have separated the mixture into B in the collecting flask and A in the original flask.
Each step in the Phase Diagram represents a "Plate" or one simple distillation,
where you would recover a fraction at a certain temperature range and then
use it alone in a second simple distillation which would constitue a second
"Plate".
What makes the column efficient?
To make the boiling-condensing-reboiling process as effective as possible,
it has to happen over and over again. By having a lot of surface area
inside the column, you aim to have the maximum possible contact between
the liquid
trickling down and the hot vapor rising. If this didn't occur, the
liquid would all be on the sides of the condenser, while most of the vapor
would be going up the middle
and never
come into contact with it.The longer the column and the higher the surface
area the more efficient the separation and the higher the number of "Plates".
Calculating the number of theoretical plates in the column and HETP (Height
Equivalent Theoretical Plate)
The very first fraction of distillate determines the efficiency
of the column. The efficiency can be quantified as the column's number of
"theoretical plates" by analyzing the mole fraction distribution
of the components in this fraction using the following equation.
# of theoretical plates = log (ncyclohexane/ ntoluene) / log α
for this mixture α = 2.33
HETP = column height in centimeters / (# of theoretical plates -1)