Principles

The underlying principles are conveniently illustrated by reference to a vapour-liquid equilibrium diagram (Figure 3). The diagram relates to a binary mixture containing components P and Q. The lower curve gives the composition of the liquid boiling at various temperatures whilst the upper curve gives the composition of the vapour in equilibrium with the boiling liquid. Points x and y therefore give the boiling points of the individual components P and Q respectively. For example, point A shows that at X degrees the vapour has a composition of approximately 90% P, whilst point B shows that the boiling liquid with which it is in equilibrium, has a composition of approximately 80% P. In a continuous distillation process, such as occurs in a distillation column, liquid of composition C (90% Q, 10% P) vaporizes to vapour of composition D which condenses to liquid of composition E. Subsequently liquid E becomes vapour F and liquid G (composition: 50% Q, 50% P). This continuous process of vaporization and condensation occurs in the distillation column until a volatile fraction leaves the top of the column and is removed from the process by being collected in the collection flask. At the same time the liquid in the distillation flask becomes progressively more concentrated in the involatile component.

Distillation techniques may be classified into several different types including:

• Distillation at atmospheric pressure

• Distillation under reduced pressure

• Steam distillation

• Molecular distillation (short-path distillation)

• Azeotropic distillation

• Isopiestic distillation

Distillation at atmospheric or reduced pressure produces a separation according to the general principles discussed in the introduction.

Steam distillation is a means of distilling that part of a sample that is volatile in steam at a lower temperature than would otherwise be the case. This method is typically used for removing phenols from an aqueous sample. A means of introducing steam into the distillation flask must be provided.

Figure 1 Simple distillation apparatus comprising distillation flask (DF), distillation head (DH), thermometer (T), condenser (C) and receiver(or collection) flask (RF). (Reproduced by permission of Longman Scientific & Technical from Furniss etal., 1989.)

Molecular distillation, sometimes termed short-path distillation, is used principally for compounds normally having high boiling points. In such cases, very low pressures are needed to achieve the desired low boiling points. The apparatus is constructed such that the condensing surface is located only a short distance from the distilling liquid and the pressure is reduced so that the process is governed to a large extent by the mean free path of the molecules involved. Hence the terms short-path distillation and molecular distillation.

Azeotropic distillation occurs when a mixture of two materials distils at constant composition. This technique is commonly used to remove water from samples. As an example, toluene may be added to a complex sample containing water, the distillation

Figure 2 Distillation apparatus including distillation column (DC). (Reproduced by permission of Longman Scientific & Technical from Furniss etal., 1989.)

Composition

Figure 3 Vapour-liquid diagram for a binary mixture of components 'P' and 'Q', illustrating the principles of distillation (see text for details).

Composition

Figure 3 Vapour-liquid diagram for a binary mixture of components 'P' and 'Q', illustrating the principles of distillation (see text for details).

process results in the toluene-water azeotrope distilling. The distillate can then be examined to determine the water content of the original sample.

Isopiestic distillation is a convenient way of producing metal-free aqueous samples of volatile acids. The 'crude' acid is placed in an open container, such as a beaker, in a desiccator containing also an open beaker of pure water. The acid vaporizes and subsequent condensation in the pure water produces an aqueous sample of the volatile acid without any of the involatile contaminants such as metals.

The alternative terms 'flash' distillation and 'fractional' distillation are sometimes used to describe some of the above procedures carried out in a particular way. Flash distillation effects a crude separation into volatiles and residue, whilst fractional distillation produces a series of 'cuts' of different volatility (or boiling point) ranges.

Additionally, there are other forms of sample purification and separation that are either a type of distillation or are related to a distillation process:

• Simultaneous distillation/extraction (see application section)

• Dean and Stark distillation (see application section)

• Simulated distillation (gas chromatographic technique)

Analytically, distillation is used for two principal purposes, firstly as a criterion of purity and secondly as a means of preparing a sample for analysis. Many specification tests include reference to a distillation range within the limits of which a stated percentage of the material of interest distils. Alternative distillation may be used to separate volatiles from a sample prior to a suitable analytical technique being employed on the distillate or on the residue. Standard tests are documented that involve distillation as a sample pretreatment method prior to titrimetry, potentiometry and spectrophotometry.

It is of course essential, if meaningful comparative results are to be obtained, that the design and use of the apparatus are standardized for such determinations.

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