Sublimation

Sublimation is a process used for the purification of some compounds, especially when the quantity of substance is small. The first point to determine is that the substance does actually sublime. 

This is done by heating a little of the substance in a dry test tube held in an almost horizontal position. 

The sublimate, if any, will collect on the colder parts of the tube. 

Several types of apparatus are available for sublimation. If the substance sublimes readily, the following apparatus is convenient. 

When the substance is difficult to sublime, it may be heated in a crucible placed in a round hole in a piece of asbestos board. The crucible is covered with a large clock-glass and a small flame is used so that the heat is directed only on to the crucible, as shown in Fig. 26. 

Sublimation differs from ordinary distillation because the vapour condenses to a solid instead of a liquid. Usually, the pressure in the heated system is diminished by pumping, and the vapour is condensed (after travelling a relatively short distance) on to a cold finger or some other cooled surface. This technique, which is applicable to many organic solids, can also be used with inorganic solids such as aluminium chloride, ammonium chloride, arsenious oxide and iodine. In some cases, passage of a stream of inert gas over the heated substance secures adequate vaporisation. 

Data were analyzed for congruent sublimation via EuSe(s) = Eu(g) + Se(g), neglecting the contribution of the molecular species. The experimentally determined vapor pressure of Eu in 

Much less frequently than crystallisation, sublimation is used to purify a solid compound. The principle involved is this A substance is converted by heat into the gaseous condition, and the vapours are caused to condense again on a cold surface- Under these conditions the substance frequently condenses in crystals. 

Sublimations can also be conducted in crucibles, flasks, beakers, retorts, tubes, etc. The heating may be done in an air- or oil-bath. In order to lead off the vapours rapidly, a current of an indifferent-gas is sent through the apparatus. 

Kinds and Objects of Distillation. — By distillation is meant the conversion by heat of a solid or liquid substance into a vapour and the subsequent condensation of this. When distillation is conducted at the atmospheric pressure, it is called ordinary distillation if in a partial vacuum, vacuum distillation. The object of distillation is either to test the purity of an individual substance by the determination of its boiling-point, or to separate a mixture of substances boiling at different temperatures into its constituents. 

If large quantities of a substance, are to be distilled, an ordinary flask is used. This can be converted into a fractionating flask with the aid of a cork bearing a T-tnbe, as illustrated in Fig. 14- 

For the distillation of solid substances which solidify in the condensation-tube, a fractionating flask with a wide side-tube is used. 

When a solid has a high vapor pressure, purification can occasionally be accomplished by sublimation. This is done by warming the solid to a temperature below its melting point and condensing the vapors on a cold surface. It is frequently practical to go above the melting point since the rate of evaporation is much greater than the rate of sublimation if there are only nonvolatile impurities present, purification is far more rapid. 

Sublimations can be carried out under pressure or vacuum with equal ease. The technique is useful only if impurities associated with the component being sublimed have a substantially different vapor pressure at the sublimation temperature. It is often used as a final purification step in the preparation of an analytical sample. A simple but effective apparatus is shown in Fig. 2-6 the sample is placed in the bottom of the outside tube, it is heated, and crystals of sublimate collect on the large cold-finger condenser. A simple tube such as that illustrated in Fig. 2-7 can be used for 

So far, in the discussion of industrial crystallization processes, only the crystallization of a solid phase from a supersaturated or supercooled liquid phase has been considered. However, the crystallization of a solid substance can be induced from a supersaturated vapour by the process generally known as sublimation . Strictly speaking, of course, the term sublimation refers only to the phase change solid vapour without the intervention of the liquid phase. In its industrial application, however, the term is commonly used to include the condensation (crystallization) process as well, i.e. solid vapour 

In praetiee, for heat transfer reasons, it is often desirable to vaporize the substanee from the hquid state, so the complete series of phase changes in an industrial subhmation process can be solid liquid vapour sohd. It is on the condensation side of the process that the appearance of the liquid phase is prohibited. The supersaturated vapour must condense directly to the crystalline solid state. 

Organic compounds that can be purified by sublimation include  

2-aminophenol anthracene anthranilic acid anthraquinone benzanthrone benzoic add 1,4-benzoquinone camphor 

The sublimation of ice is an important operation in the freeze-drying of foods and biological products. 

Like the vapor pressure of a liquid, the vapor pressure of a solid increases with temperature. If the vapor pressure of a solid is greater than the ambient pressure at its melting point, then the solid undergoes a direct-phase transition to the gas phase without first passing through the liquid state. This process is called sublimation. 

To be purified by sublimation, a compound must have a relatively high vapor pressure, and the impurities must have vapor pressures significantly lower than that of the compound being purified. If the impurities in a compound have similar vapor pressures, recrystallization (Sec. 3.2) or column chromatography (Sec. 6.3) may be used to purify the compound. Since few organic solids exhibit vapor pressures high enough to sublime at atmospheric pressure, most sublimations are performed at reduced pressure. 

Always protect the eyes during a vacuum distillation  

Volatile substances of which the vapours, on cooling, condense directly to crystals without passing through the liquid phase are sometimes advantageously purified by sublimation, particularly when solubility relations render recrystallisation difficult. The purification of iodine is a well-known case in point. In organic chemistry this process is particularly suitable for quinones. 

If it is desired to sublime larger amounts of substance, the upper watch-glass of the apparatus just described is replaced by a funnel, of which the diameter is somewhat less than that of the watch-glass. 

Sublimation can also be carried out in crucibles, flasks, beakers, 

Chromatography is often used with advantage for the purification of small amounts of complex organic mixtures. Chromatography techniques all rely on the differential distribution of the various components in a mixture between the mobile phase and the stationary phase. The mobile phase can either be a gas or a liquid whereas the stationary phase can either be a solid or a liquid. 

The major chromatographic techniques can also be categorised according to tbe nature of the mobile phase used -vapour phase chromatography for when a gas is the mobile phase and liquid chromatography for when a liquid is the mobile phase. 

A very useful catalog for chromatographic products and information relating to chromatography (from gas chromatography to biochromatography) is that produced by Merck, called the ChromBook and the associated compact disk, ChromCircle. 

Thz larger sublimator (Fig. 11.15a) consists of a tube with a side arm which fitted with a cold-finger condenser, and it is used as follows  

If the crude material is a solid, powder it and place it in the bottom of the outer vessel. If it is waxy or oily, wash it into the tube with a small amount of solvent, cover the side arm with a septum, and remove the solvent on a rotary evaporator. 

Put some vacuum grease on the joint of the cold-finger condenser and fit it into the sublimator (there should be a gap of approximately 1cm between the solid and the condenser). 

Evacuate the apparatus slowly to prevent any spattering of the solid. Turn on the condenser water and slowly heat the base of the sublimator. 

A fine mist of sublimed material on the condenser indicates that sublimation is beginning and the temperature should then be held fairly constant until the process is complete. 

Let us consider a system composed of a solid phase consisting of crystals of component 1, and a vapour phase consisting of a mixture of components 1 and 2. The initial state is taken as pure component 1 at a pressure p T will be the temperature at which crystals of 1 are in true equilibrium with its vapour at a pressure p.  

We now apply equation (18.21) for a displacement at constant pressure. In this example is the heat content change correspond- 

If AJi is assumed independent of T in the temperature range considered, (18.21) can be integrated to obtain the simple formula  

This equation gives the curve of co-existence at constant pressure of crystals of 1 and a vapour phase consisting of a mixture of components 1 and 2. 

The coexistence curve between crystals of 2 and the mixed vapour can be obtained in just the same way  

The suppliers of chromatography equipment for every need are too numerous to list here but can be viewed on the internet under Chromatography products . Details and orders can be obtained from the respective websites listed at the end of the section on HPLC below. 

A Dry ice is solid carbon dioxide. The solid does not melt but rather sublimes. It transforms directly from solid carbon dioxide to gaseous carbon dioxide. 

Ice also sublimes out of frozen foods. You can clearly see this in food that is frozen in an airtight plastic bag for a long time. The ice crystals that form in the bag are water that has sublimed out of the food and redeposited on the smface of the bag. For this reason, food that remains frozen for too long becomes dried out. This can be avoided to some degree by freezing foods to colder temperatures (further below 0 0, a process called deep-freezing. The colder temperature lowers the rate of sublimation and preserves the food longer. 

Which image best represents the dry ice after it has sublimed  

G Types of Intermolecular Forces Dispersion, Dipole-Dipole, 

The nature of dispersion forces was first reoognized by Fritz W. London (1900-1954), a German-Amerioan physicist. 

Researchers are interested in supercritical fluids because of their unique properties. A supercritical fluid has properties of both liquids and gases—it is in some sense intermediate between the two. Supercritical fluids can act as good solvents, selectively dissolving a number of compounds. For example, supercritical carbon dioxide is used as a solvent to extract caffeine from coffee beans. The caffeine dissolves in the supercritical carbon dioxide, but other substances—such as those responsible for the flavor of coffee—do not. Consequently, the caffeine is removed without substautially altering the coffee s flavor. The supercritical carbon dioxide is easily removed from the mixture by simply lowering the pressure below the critical pressure, at which point the carbon dioxide evaporates, leaving no residue. 

In Section 11.5, we examined a beaker of liquid water at room temperature from the molecular viewpoint. Now, let s examine a block of ice at —10 °C from the same molecular perspective, paying close attention to two common processes sublimation and fusion. 

Although both sublimation and deposition occur on the surface of an ice block open to the atmosphere at -10 °C, sublimation usually occurs at a greater rate because most of the newly sublimed molecules escape into the surrounding atmosphere and never come back. The result is a noticeable decrease in the size of the ice block over time (even though the temperature is below the melting point). 

A substance commonly associated with sublimation is solid carbon dioxide or dry ice, which does not melt under atmospheric pressure no matter what the temperature is. However, at -78 °C the CO2 molecules have enough energy to leave the surface of the dry ice and become gaseous through sublimation. 

The water molecules at the surface of an ice cube can sublime directly into the gas state. 

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The product sublimation and melting are both carried out on a noncontinuous basis. Thus time-averaged values have been taken. 

It was first described in 1608 when it was sublimed out of gum benzoin. It also occurs in many other natural resins. Benzoic acid is manufactured by the air oxidation of toluene in the liquid phase at 150°C and 4-6 atm. in the presence of a cobalt catalyst by the partial decarboxylation of phthalic anhydride in either the liquid or vapour phase in the presence of water by the hydrolysis of benzotrichloride (from the chlorination of toluene) in the presence of zinc chloride at 100°C. 

S = Heat of sublimation of sodium D = Dissociation energy of chlorine / = Ionization energy of sodium = Electron affinity of chlorine Uq = Lattice energy of sodium chloride AHf = Heat of formation of sodium chloride. 

C8H10N4O2. An alkaloid occurring in tea, coffee and guarana, from which it may be prepared by extraction, It is also manufactured by the methylation of theobromine and by the condensation of cyanoacetic acid with urea. Crystallizes with H2O or anhydrous from organic solvents. M.p. (anhydrous) 235"C, sublimes at 176 C. Odourless, and with a very bitter taste. Caffeine acts as a stimulant and diuretic, and is a constituent of cola drinks, tea and coffee. 

Carbon dioxide, COj. Sublimes — 78 5 C. A colourless gas at room temperature, occurs naturally and plays an important part in animal and plant respiration. Produced by the complete combustion of carbon-containing materials (industrially from flue gases and from synthesis gas used in ammonia production) and by heating metal carbonates or by... 

Chromium tetrafiuoride CrF4. Green, sublimes lOO C (CrFa plus Fj). 

C4H6N2O2. Sublimes 260"C sparingly soluble in water hydrolysed by alkalis or mineral acids to glycylglycine. It and substituted dike-topiperazines are formed by the condensation of amino-acids, and are obtained in small quantities on the hydrolysis of proteins. 

Colourless crystals, m.p. 345-347" C with sublimation, obtained by oxidation of m-xylene. 

Mercury(II) chloride, HgC, corrosive sublimate, m.p. 280 C, b.p. 302"C. Essentially covalent material (Hg plus CL Hg plus aqua regia). Forms complex halide ions, e.g. (HgCU) (HgCL)" in excess HCl and forms complexes. Very poisonous. 

PPha, pyridine) organic groups (olefines, aromatic derivatives) and also form other derivatives, e.g. halides, hydrides, sulphides, metal cluster compounds Compounds containing clusters of metal atoms linked together by covalent (or co-ordinate) bands, metaldehyde, (C2H40) ( = 4 or 6). A solid crystalline substance, sublimes without melting at I12 1I5" C stable when pure it is readily formed when elhanal is left in the presence of a catalyst at low temperatures, but has unpredictable stability and will revert to the monomer, ft is used for slug control and as a fuel. 

Dinilrogen pentoxide, N2O5. White solid (HNO3 plus P2OJ) readily decomposes to NO2 and O2, sublimes 32-5 "C. In solid slate (N02) (N03) gaseous molecules O2NONO2... 

Crystallizes from water in large colourless prisms containing 2H2O. It is poisonous, causing paralysis of the nervous system m.p. 101 C (hydrate), 189°C (anhydrous), sublimes 157°C. It occurs as the free acid in beet leaves, and as potassium hydrogen oxalate in wood sorrel and rhubarb. Commercially, oxalic acid is made from sodium methanoate. This is obtained from anhydrous NaOH with CO at 150-200°C and 7-10 atm. At lower pressure sodium oxalate formed from the sodium salt the acid is readily liberated by sulphuric acid. Oxalic acid is also obtained as a by-product in the manufacture of citric acid and by the oxidation of carbohydrates with nitric acid in presence of V2O5. 

The phthalocyanine molecule is remarkably stable to heat and chemical reagents. The metal-free and heavy metal compounds sublime practically unchanged at 550-580 C. 

Selenium hexafluoride, SeF m.p. — 39°C, sublimes —47°C. Formed (with Sc2Fio) from Se and F2- Chemically fairly inert. 

Crystallizes in colourless needles m.p. 300° (sublimes). Manufactured by the oxidation of p-xylene and used in the production of Terylene (see also polyesters). U.S. production 1980 2-05 megatonnes. 

Colourless needles, m.p. 199 C, sublimes easily. Solutions exhibit blue fluorescence. Occurs in coal tar but may also be obtained from the spontaneous trimerization of ben-zyne. 

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