PHARCAL - EX 14(SUBLIMATION )

Sublimation

certain solids can readily pass directly into the vapor phase, without going thorugh a liquid phase

Purification method

vapors can be resolidified, the overall vaporization-solidification cycle can be used as a?

Purification is successfull

only if the impurities have significantly lower vapor pressures than the material being sublimed.

Method of purification

the solid is heated(often under reduced pressure) until its vapor is high enough to collect on a cooled surface about the solid

In practice, sublimation

is loosely applies and is often used to describe the process where the solid actually melts prior to vaporization. But, not really a sublimation process

While performing sublimation, keep in mind that..

it is important to keep the temperature below the melting point of solid.

Sublimation

Although this method requires less time and less transferring operations than crystallization, its applicability is limited.

Sublimation

It cannot be used for the separation of compounds having similar vapor pressures.

Some crystals in sublimation

Camphor, iodine, naphthalene, and benzoic acid

Sublimation

is usually a property of relatively non-polar substances which also have a highly symmetrical structure.

Symmetrical compounds

have relatively high melting points and high vapor pressures.

The ease with which a substance can escape from the solid is

determined by the strength of the intermolecular forces between the molecules

Symmetrical molecular structures

will have a relatively uniform distribution of electron density and a small dipole moment

A smaller dipole moment

means a higher vapor pressure because of lower electrostatic forces in the crystal.

Materials

Benzoic Acid

Camphor

Sodium Sulfate (anhydrous)

Barium Chloride TS

Long-stemmed Funnel

Filter paper

Purified Cotton

First step

1g of benzoic acid + anhydrous sodium sulfate to evaporating dish

1g camphor and anhydrous sodium sulfate to evaporating dish

2nd step

cover tightly each with filter paper to make it air tight

Third step

prick holes in the center and cover with an inverted funnel

Fourth step

Fit a pledget of cotton at the tip of the funnel stem.

Fifth step

Fold a filter paper cone and cut the tip, so it fits the stem of the funnel

Sixth part

Partially moisten the filter paper with ice-cold water and carefully line the outer wall of the funnel. Avoid excessive water that will drip to the evaporating dish.  

Seventh step

Gently heat on a hot plate. Occasionally wet the filter paper cone to keep the funnel cool.

Eighth step

Carefully collect the sublimate that is deposited on the funne

Last step

Dissolve a small amount of the sublimate in hot water and add a few drops of BARIUM CHLORIDE TS.

clear solution

indicates that the sublimate does not contain sulfates.

Clean Up

Collect the residue and the sublimate in a PIECE OF PAPER and dispose in the SOLID WASTE BIN. Dispose the BARIUM CHLORIDE solution in the SINK with COPIOUS amount of WATER.

Ninth step

Observe the color of the RESIDUE and the SUBLIMATE.