Analytical Applications Distillation

J. D. Green, BP Amoco Chemicals, Hull, UK

This article is reproduced from Encyclopedia ofAnalyti-

calScience, Copyright © 1995 Academic Press.

Distillation is a widely used technique in chemical analysis for characterizing materials by establishing an index of purity and for separating selected components from a complete matrix. The technique is even more widely used in preparative chemistry and throughout manufacturing industry as a means of purifying products and chemical intermediates. Distillation operations differ enormously in size and complexity from the semi-micro scale to the 'thousands of tonnes per annum' production operations. For analytical purposes the scale employed is usually bench-level.

Numerous quoted standard specifications refer to distillation ranges as criteria of purity or suitability for use, or as indicators of performance. Published standards for analytical reagents in the AnalaR range and similar documentation by the American Chemical Society refer to distillation ranges as criteria of purity for appropriate materials.

Distillation is the process that occurs when a liquid sample is volatilized to produce a vapour that is subsequently condensed to a liquid richer in the more volatile components of the original sample. The volatilization process usually involves heating the liquid but it may also be achieved by reducing the pressure or by a combination of both. This can be demonstrated in a simple laboratory distillation apparatus comprising a flask, distillation head, condenser and sample collector (Figure 1). A thermometer is included in the apparatus as shown to monitor the progress of the operation. In its simplest form this procedure results in a separation into a volatile fraction collected in the receiver flask and a nonvolatile residue in the distillation flask. When a distillation column is incorporated in the equipment (Figure 2), the evaporation and condensation processes occur continuously. This results in a progressive fractiona-tion of the volatiles as they pass up the column. The most volatile components emerge from the top of the column initially and the less volatile components emerge later. By changing the receivers throughout the course of the distillation a separation or fractiona-tion is effected. Eventually, all the volatiles will have passed over into the sample collectors and any involatile residue present will remain in the distillation flask.

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