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⭐ What are the main uses of sublimation method of separation ?

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Introduction

• Sublimation is not a procedure that is generally regarded as an analytical technique. It is a process, however, by which compounds can be purified or mixtures separated and as such can be of value as a single step or as an integral part of a more complex analytical method. It is applicable to a range of solids of inorganic or organic origin in a variety of different matrices and can be particularly useful when heat-labile materials are involved.

• As a method of sample purification sublimation has been used to produce high-purity materials as analytical standards. A specific and common example of sublimation used as a means of purification is the removal of water from heat-labile materials in the process known as freeze-drying. The technique is described more fully below.

• As a separation technique fractional sublimation has been used either to purify samples for analysis by removing undesirable components of the matrix or to remove the analyte from the matrix for subsequent analysis.

Sublimation

• Sublimation involves direct vaporization and condensation of a solid without formation of an intermediate liquid phase. Sublimation is used most extensively for separation of volatile components from non-volatile components. Hence, it is a useful preparative technique for recovery of compounds that can be sublimed at reasonable temperatures, at either atmospheric or reduced pressure. The temperature at which a sublimate first becomes noticeable in a given system is often referred to as the sublimation temperature, but it is useless for comparative purposes unless the variables such as pressure, the size and geometry of the sublimation apparatus are standardized. The effect of reduced pressure on the sublimation temperature of common substances is indicated in Table 8.4. The list of substances amenable to sublimation includes polycyclic aromatic hydrocarbons, benzoic acid, salicylic acid, camphor, saccharin, quinine, cholesterol, palmitic and stearic acids, acetylsalicylic acid and atropine. Despite its successes, sublimation has limited application and it is difficult to optimize for quantitative trace analysis because of the dependence of extraction on the vapour pressure of analytes at a given temperature and pressure, and on the nature of the matrix.

Table 8.4. Effect of reduced pressure on the sublimation temperature of some common substances [47].

• Compound Melting point (°C) Sublimation temperature (°C)
• at 101 kPa at 65 to 130 Pa
• Naphthalene 79 36–38 25
• Benzoic acid 120 43–45 25
• β-Naphthol 122 43–45 33–35
• Urea 132 59–61 49–52
• Anthracene 215 77–79 28–31

ANALYTICAL REAGENTS | Purification

Sublimation

• In sublimation, a solid substance is volatilized by heating and the vapor is condensed back to the solid at a cooled surface. The distance between the surface of the vaporized solid and the collecting surface is short compared with distances used in distillation. Sublimation may be conducted at atmospheric pressure but reduced pressure is often employed to enhance sublimation and to speed up the process. An atmosphere of inert gas at low pressure is advisable for sensitive compounds.

• Purification through sublimation is applicable to a number of organic and inorganic substances and is useful for the purification of many simple inorganic compounds used as working standards in analysis, including ammonium halides, arsenic(III) oxide, phosphorus(V) oxide, and iodine.