Pharma gyan Pandit

Sublimation

Sublimation is a purification technique, in which a solid is directly converted to vapor phase without passing through liquid phase. However, the compound must have a relatively high vapor pressure, and the impurities must have significantly lower vapor pressures. By heating, the solid will be vaporized and become solid again when the vapor contacts with the cold surface. Some solid compounds, such as iodine, camphor, naphthalene, acetanilide, benzoic acid, can be purified by sublimation at normal pressure. Several compounds will sublime when heating under reduced pressure. In this experiment, the impure acetanilide and impure naphthalene will be purified using a suction flask with cold finger at atmospheric pressure.

Distillation

Distillation is a widely used method for separating and purifying a mixture of liquids by heating the liquids to boiling at different temperatures to transform them into the vapor phase. The vapors are then condensed back into liquid form in a sequence from lower to higher boiling points. Distillation is used for many industrial processes, such as production of gasoline and kerosene, distilled water, organic solvents, and many other liquids. There are 4 types of distillation including simple, fractional, steam and vacuum distillations. In simple distillation, all the hot vapors produced are immediately passed into a condenser to cool and condense the vapors back to liquid. Therefore, the distillate may not be pure depending on the composition of the vapors at the given temperature and pressure. Simple distillation is usually used only to separate liquids whose boiling points differ greatly (more than 25°C), or to separate liquids from nonvolatile solids or oils. In case of very close boiling points, fractional distillation must be used in order to separate the components well by repeated vaporization-condensation cycles within a fractionating column. Steam distillation is a method for distilling compounds which are heat-sensitive by bubbling steam through a mixture. After the vapor mixture is cooled and condensed, a layer of oil and a layer of water are usually obtained. Some compounds have very high boiling points and may boil beyond their decomposition temperatures at atmospheric pressure. It is thus better to do vacuum distillation by lowering the pressure to the vapor pressure of the compound at a given temperature at which the compound is boiled, instead of increasing the temperature.