Iodine evaporation

Note The iodine evaporates relatively quickly from the layer after a chromatogram has been removed from the iodine chamber. This evaporation of the iodine can be considerably delayed by covering the chromatogram with a glass plate the edge can also be sealed with adhesive tape, if necessary. Iodine solution can also be used for detection instead of iodine vapor. 

Another reversal location method is to expose the TLC plate to iodine vapour in a closed chamber that contains some iodine crystals. Iodine is lipophilic and accumulates in hpophihc sample spots, showing a brown color on a pale yellow-brown background. The same result is obtained by spraying with an iodine solution (250 mg iodine dissolved in 100 ml of heptane). In nearly all cases, this iodine accumulation is totally reversible without altering the sample, because outside the closed chamber iodine evaporates quickly from the plate. Caution should be taken with this iodine treatment in the case of unsaturated compounds because iodine vapor can react with double bonds [16]. 

The plate is put into a tank containing iodine crystals. This treatment will produce brown spots with many organic compounds the staining is reversible, so that if it is necessary to recover the compound once it has been located, the iodine may be allowed to evaporate by exposing the plate to air and then the marked spot containing the compound of interest may be scraped off the plate. If a permanent record of the plate is required it has to be covered to prevent the iodine evaporating or the iodine spots may be sprayed with starch solution in order to stain them permanently. Iodine is used as a location agent in pharmacopoeial TLC tests of fixed oils and of cetrimide. 

It should be noted that iodine evaporation from the strongly acidic solutions was observed by Rybkin and Seredenko in molten caesium iodide at 650 °C (POJ solutions) [63] and by Rybkin and Banik [64] who studied solutions of PO, H2PO4, VO, etc. in molten Nal at 700 °C. 

In the first place, they found [62] that the adsorption of iodine on a (111) surface disordered by ion bombardment or by electrochemical oxidation would order the Pt substrate so that, after desorption of the iodine, a well-ordered (111) surface was produced. Adsorption of iodine from I2 vapour under atmospheric pressure yielded the structure shown as (c) in Fig. 15. On heating this, the series of structures indicated in Fig. 15 ensued after which, at 900 K, the iodine evaporated, leaving the well-ordered Pt substrate. (Later work [63] has shown that a similar ordering can be obtained electrochemic-... 

The analytical procedure employed is shown in Figure 122.3. Approximately 0.5 g of a powdered sample was mixed with V2O5 and placed in a quartz boat. The boat was then placed in a quartz tube and the sample was heated to 1100°C under a wet oxygen flow (pyrohydrolysis). Iodine evaporated from the sample was collected in a receiver TMAH solution. The iodine in the receiver solution was determined by inductively coupled plasma mass spectrometry (ICP-MS Hewlett Packard 4500 Model). The operating conditions of ICP-MS were as described in the earlier report (Muramatsu and Wedepohl, 1998). Its detection limit is 0.3 ng ml (ppb). The analytical procedure was checked by using standard materials, e.g., NIST SRM 1537a (tomato leaves), 1549 (milk powder), 1572 (citrus leaves) and so on (Schnetger and Muramastu, 1996). 

The plate is placed in a dense atmosphere of iodine vapour for 5 min or sprayed with an iodine solution (e. g., 0.6% in chloroform). Excess iodine evaporates on standing in the air. The spots turn blue on spraying with a starch solution (1% in water). The background also turns blue if there is too much iodine still on the layer (test on a comer or part of the covered layer). 

The best chemical detection method is exposure to iodine vapor in a closed chamber, which will visualize compounds of many chemical classes as light or dark brownish zones on a tan background. In most cases, the vapor can be subsequently evaporated, leaving compounds unchanged. The zones must, of course, be marked, usually by outlining with a sharply pointed object, before the iodine evaporates, making the materials invisible. 

Visualization of sugars by iodine vapor was popular in paper chromatography. This method is not very sensitive, but it is nondestructive because of the short exposure time required. The adsorbed iodine evaporates when the plate is exposed to the air (19). 

About 0.5 g of iodine is placed in a small flask fitted with a long reflux air condenser and 15 cm of fuming nitric acid (b.p. 380 K) are added. The mixture is then heated on a water bath at 385-390 K in a fume cupboard until the reaction seems to be complete. This takes about an hour. The solution is then transferred to an evaporating basin and evaporated to dryness on a steam bath. The iodic acid... 

The plate is removed from the tank, the position of the solvent front marked, and the solvent allowed to evaporate from the plate. If the components of the mixture are coloured, the separation is obvious if colourless, they must be located either by viewing under U.. or by standing the plate in a closed dry tank containing crystals of iodine, whose vapour makes brown spots show i p. 

Industrial Hquid chlorine is routinely analy2ed for moisture, chlorine, other gaseous components, NCl, and mercury foUowing estabHshed procedures (10,79). Moisture and residue content in Hquid chlorine is determined by evaporation at 20°C foUowed by gravimetric measurement of the residue. Eree chlorine levels are estimated quantitatively by thiosulfate titration of iodine Hberated from addition of excess acidified potassium iodide to the gas mixture. 

Seaweeds. The eadiest successful manufacture of iodine started in 1817 using certain varieties of seaweeds. The seaweed was dried, burned, and the ash lixiviated to obtain iodine and potassium and sodium salts. The first process used was known as the kelp, or native, process. The name kelp, initially apphed to the ash of the seaweed, has been extended to include the seaweed itself. About 20 t of fresh seaweed was used to produce 5 t of air-dried product containing a mean of 0.38 wt % iodine in the form of iodides of alkah metals. The ash obtained after burning the dried seaweed contains about 1.5 wt % iodine. Chemical separation of the iodine was performed by lixiviation of the burned kelp, followed by soHd-Hquid separation and water evaporation. After separating sodium and potassium chloride, and sodium carbonate, the mother Hquor containing iodine as iodide was treated with sulfuric acid and manganese dioxide to oxidize the iodide to free iodine, which was sublimed and condensed in earthenware pipes (57). 

The Japan Nuclear Fuel Service Company reprocesses LWR fuel in faciUties which take advantage of French shear and dissolver designs, German iodine removal technology, and British reduced-pressure evaporation. 

Rhenium Halides and Halide Complexes. Rhenium reacts with chlorine at ca 600°C to produce rheniumpentachloride [39368-69-9], Re2Cl2Q, a volatile species that is dimeric via bridging hahde groups. Rhenium reacts with elemental bromine in a similar fashion, but the metal is unreactive toward iodine. The compounds ReCl, ReBr [36753-03-4], and Rel [59301-47-2] can be prepared by careful evaporation of a solution of HReO and HX. Substantiation in a modem laboratory would be desirable. Lower oxidation state hahdes (Re X ) are also prepared from the pentavalent or tetravalent compounds by thermal decomposition or chemical reduction. 

A good technical grade of carbon tetrachloride contains not more than the following amounts of impurities 1 ppm acidity as HCl, 1 ppm carbon disulfide if manufactured by carbon disulfide chlorination, 20 ppm bromine, 200 ppm water, and 150 ppm chloroform. The residue should not exceed 10 ppm on total evaporation. The product should give no acid reaction with bromophenol blue, and the starch iodine test should indicate the absence of free chlorine. 

The chromatogram is observed and documented as soon as the spots are readily visible. The iodine can then be allowed to evaporate from the chromatogram (fume cupboard ). The chromatogram can then be subjected to further reactions or processes after this reversible reaction. 

The mixture is cooled to room temperature, then filtered. The solvent is removed under reduced pressure, leaving the tribromide (47) as a foam. The foam is mixed with sodium iodide (9.55 g, 0.064 mole) and acetone (74 ml) and heated under reflux in a nitrogen atmosphere for 3.5 hr. The acetone is removed under reduced pressure and the residue is treated with chloroform and aqueous sodium thiosulfate solution. The chloroform layer is separated and washed with sodium thiosulfate solution until it is free from iodine, then dried over magnesium sulfate, filtered and evaporated to dryness under reduced pressure. The crude product (48) is obtained as a brown sohd (4.85 g) which is chromatographed over alumina (122 g, Merck acid-washed). The column is developed with hexane, benzene and ethyl acetate mixtures. The product (3.43 g) is eluted by benzene and benzene-ethyl acetate (10 1). Recrystallization from acetone yields purified 3jS-acetoxy-pregna-5,14,16-trien-20-one (48), 3.25 g, mp 158-159° 309 m/ (e 10,700). 

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