Purification by Sublimation

Segregation via vaporisation and re-sublimation relies on the fact that, in general, impurities have sublimation temperatures different to that of the purified compound [22]. Depending on the sublimation temperature of the host, impurities are either more volatile or remain as residue. Purification by sublimation can be further distinguished by two modifications  

Step sublimation In this arrangement the pristine material is placed inside a quartz tube and heated slightly (less than —10 K) above its sublimation point under HV conditions at -10 Torr. Low-weight residues from the synthesis or pre-purification evaporate upon heating and re-sublime far apart from the pristine and the purified material. If their molecular weight is higher than that of the host (here we neglect effects by permanent dipoles on the sublimation temperature), the impurities will remain in the pristine material. 

If re-contamination of the aheady purified fraction can be avoided, step sublimation offers the advantages of purifying several grams of material on the time scale of hours at defined thermal condition. The latter is of particular importance for newly synthesised compounds with unknown sublimation temperatures. 

Gradient sublimation To ensure a high material yield in combination with a high selectivity, sublimation over an extended temperature gradient ( 500 K/m) is the preferred method if the sublimation temperature of the host material is known. Inside a glass tube separation across the temperature gradient takes place and, in most cases, the purified fraction occurs spatially well-separated from the contaminants. The efficiency can be improved in terms of yield and stability in the presence of an additional inert carrier gas, e.g. Ar or N2, which at low pressure (10 Torr) reduces the molecular mean free path and equilibrates temperature fluctuations across the glass tube. 

---

Many mercury compounds are labile and easily decomposed by light, heat, and reducing agents. In the presence of organic compounds of weak reducing activity, such as amines (qv), aldehydes (qv), and ketones (qv), compounds of lower oxidation state and mercury metal are often formed. Only a few mercury compounds, eg, mercuric bromide/77< 5 7-/7, mercurous chloride, mercuric s A ide[1344-48-5] and mercurous iodide [15385-57-6] are volatile and capable of purification by sublimation. This innate lack of stabiUty in mercury compounds makes the recovery of mercury from various wastes that accumulate with the production of compounds of economic and commercial importance relatively easy (see Recycling). 

The endothermic nitride is susceptible to explosive decomposition on friction, shock or heating above 100°C [1], Explosion is violent if initiated by a detonator [2], Sensitivity toward heat and shock increases with purity. Preparative precautions have been detailed [3], and further improvements in safety procedures and handling described [4], An improved plasma pyrolysis procedure to produce poly (sulfur nitride) films has been described [5], Light crushing of a small sample of impure material (m.p. below 160°C, supposedly of relatively low sensitivity) prior to purification by sublimation led to a violent explosion [6] and a restatement of the need [4] for adequate precautions. Explosive sensitivity tests have shown it to be more sensitive to impact and friction than is lead azide, used in detonators. Spark-sensitivity is, however, relatively low [7],... 

A purified grade of 1-naphthol should be used. Material available from Eastman Organic Chemicals, Aldrich Chemical Company, Inc., and Matheson Coleman and Bell is satisfactory. Experiments with technical grade 1-naphthol have indicated that this material requires purification by sublimation in order to give satisfactory results. 

The relatively high volatility of complexes of this type makes their purification by sublimation routine and also lends itself to mass spectrometric characterization. To date a complex of stoichiometry M[N(SiMe3)2]4 has not been isolated, even the large metals Th4+ and U4+ giving only partially substituted derivatives, e.g. QTh[N(SiMe3)2]3.98... 

Selenium dioxide, unlike sulfur dioxide, is not readily prepared by direct combination of the elements. The best methods depend upon combustion of selenium in the presence of nitrogen dioxide1 or oxidation of selenium by nitric acid.2 The first method includes purification by sublimation the second gives a pure product only if the selenium is pure or if the product is sublimed. 

By the first of these methods blue crystalline scales are obtained which admit of purification by sublimation in a current of chlorine or carbon dioxide.5 Density, 2-937.6... 

Method of purification By sublimation with superheated steam or by crystallization from benzene followed by sublimation for very pure crystals, zone melting of solid anthracene. 

The most common procedure for preparing bismuth alkoxides is the substitution reaction of bismuth halides with alkali metal alkoxides. The alternative is treatment of bismuth amides with alcohols. Bismuth alkoxides such as trimethoxide, triethoxide and triisopropoxide were first prepared in 80-93% yields by the reaction between BiCl3 and alkali metal alkoxides by Mehrotra et al. in 1966. Bismuth alkoxides are less soluble in organic solvents and the yields after purification by sublimation are generally very low (10-20%) [66IJC537]. Known aliphatic alkoxides include Bi(OEt)3... 

Various types of sublimation apparatus are in use. Purification by sublimation is relatively a slow process and depends upon the rate of diffusion of the vapour to the cold condensing surface. In order to make the method sufficiently efficient, as large a cooled condensing surface as possible should be used, and condensing and evaporating surfaces should be close together. A simple laboratory sublimation set-up which incorporates these features is shown in Figure 2.8. 

Orange-red, monoclinic needles from benzene, mp 178. Additional purification by sublimation in high vacuum, (bath temp 100"). mp 180. bp ca. 185°, Further heating results in deflagration and explosion. Practically insol in cold water, hydrolyzed by boiling water. Slightly sol in benzene, abs ethanol, carbon disulfide. Handle with caution May dec explosively on striking or at temps much above 100. ... 

NaOCl (Clorox) and 30% H2 02 reacts with (1) at -5 to -15 to form the cndo-peroxide (2) in 37% yield. This substance readily rearranges at 45° to trans-henzene trioxide (3). Singlet oxygen generated from triphenyl phosphite ozonide (3, 323-324) reacts with (1) to form (2), but purification by sublimation results in conversion to (3). This trioxide also results from photolysis of (2) (27% yield). 

In a quartz tube at about 700°C, BegC is allowed to react with carefully purified and dried HI or with a hydrogen stream containing 12 vapor. The subsequent purification by sublimation is the same as that described above for BeClg. 

II. A method described by Hardt avoids contamination of the product by ammonium salts and solvent occlusions and makes purification by sublimation unnecessary. 

Fig. 309. Purification by sublimation and filling of a vessel with niobium (V) chloride, a, b) bomb tube c,d) openings for introducing spatulas and long-handle hooks g) storage vessel.

---