Question

1. Discuss the applicatoin of distillation as a separation and purificatoin technique. Are there limits due to sample size, initial purity and final purity desired?

2. Discuss the applicatoin of gas chromatogrpahy as a separations and identification technique. Are there limits due to purity, sample size, phase of sample?

Answer 1

1. Distillation is commonly used technique for purifying liquids and separating mixtures of liquids into their individual components. In this process, a liquid is converted to its vapor state and then the vapor is condensed back to the liquid state. It is used to separate substances with different boiling points.

The vapor pressures of liquids at room temperature are less than atmospheric pressure. When the liquid is heated, its vapor pressure increases. The pure liquid starts boiling when its vapor pressure equals the atmospheric pressure. If the liquid contains an impurity with a boling point lower than the liquid, the impurity will boil off first. The liquid distills over next.

Simple distillation is effective only when separating a volatile liquid from a non- volatile substance or when separating two liquids that differ in boiling points by 50 degrees or more. If the process is repeated several times then the purity of the distillate increases but it takes long time. The technique behind Fractional distillation is same as for simple distillation but the process is repeated several times. As there are large number of vaporization-condensation cycles, a good separation of components is obtained and thus a relatively pure distillate.

2. Chromatography is a technique for separating chemical substances that relies on differences in partitioning behaviour between a flowing mobile phase and a stationary phase to separate the components in a mixture.

The sample is introduced into a stream of an inert gas, such as argon, that acts as carrier. The sample along with the carrier gas passes through the packed column, components of the sample mixture move at velocities that are influenced by the degree of interaction of each consituent with the stationary phase. As the components elute from the columnn, they can be quantified by a detector.

Gas phase equilibria are rapid and thus the resolution can be high.