Iodine, test for

Fig. 17-—Iodine test for Schardinger dextrins. Microscopic appearance typical of the early stage of the decomposition of starch by B. macerans amylase. Under higher power the crystals are seen to be blue hexagons.

A solution of 0.2-0.5 gm iodine in 100 ml aqueous 0.5% potassium iodide is often used as a reagent for certain polysaccharides in lichens (the I test). The reagent is susceptible to air oxidation and should be renewed when the brownish color fades. Isolichenin, but not lichenin, will give a blue color. The chemistry of the color reaction is probably the same as that for the well-known iodine test for starch. The reaction is reversible (the color disappears upon dilution with water). 

Trichloroethanoic acid, CCI3COOH. A crystalline solid which rapidly absorbs water vapour m.p. 58°C, b.p. 196-5" C. Manufactured by the action of chlorine on ethanoic acid at 160°C in the presence of red phosphorus, sulphur or iodine. It is decomposed into chloroform and carbon dioxide by boiling water. It is a much stronger acid than either the mono- or the dichloro-acids and has been used to extract alkaloids and ascorbic acid from plant and animal tissues. It is a precipitant for proteins and may be used to test for the presence of albumin in urine. The sodium salt is used as a selective weedkiller. 

Like bromine, iodine is soluble in organic solvents, for example chloroform, which can be used to extract it from an aqueous solution. The iodine imparts a characteristic purple colour to the organic layer this is used as a test for iodine (p. 349). NB Brown solutions are formed when iodine dissolves in ether, alcohol, and acetone. In chloroform and benzene a purple solution is formed, whilst a violet solution is produced in carbon disulphide and some hydrocarbons. These colours arise due to charge transfer (p. 60) to and from the iodine and the solvent organic molecules. 

Hydrolysis by acids. Place 15 ml. of starch solution in a boiling-tube, add I ml. of cone. HCl, mix well and place in a boiling water-bath for 20 minutes. Cool and add 2 drops of iodine solution to i ml. of the solution no blue coloration is produced. On the remainder, perform tests for glucose in particular show that glucosazone can be formed. Neutralise the excess of acid before carrying out these tests. (Note that a more concentrated acid is required to hydrolyse starch than to hydrolyse the disaccharides, such as sucrose.)... 

Recovery of the wopropyl alcohol. It is not usually economical to recover the isopropyl alcohol because of its lo v cost. However, if the alcohol is to be recovered, great care must be exercised particularly if it has been allowed to stand for several days peroxides are readily formed in the impure acetone - isopropyl alcohol mixtures. Test first for peroxides by adding 0-6 ml. of the isopropyl alcohol to 1 ml. of 10 per cent, potassium iodide solution acidified with 0-6 ml. of dilute (1 5) hydrochloric acid and mixed with a few drops of starch solution if a blue (or blue-black) coloration appears in one minute, the test is positive. One convenient method of removing the peroxides is to reflux each one litre of recovered isopropyl alcohol with 10-15 g. of solid stannous chloride for half an hour. Test for peroxides with a portion of the cooled solution if iodine is liberated, add further 5 g. portions of stannous chloride followed by refluxing for half-hour periods until the test is negative. Then add about 200 g. of quicklime, reflux for 4 hours, and distil (Fig. II, 47, 2) discard the first portion of the distillate until the test for acetone is negative (Crotyl Alcohol, Note 1). Peroxides generally redevelop in tliis purified isopropyl alcohol in several days. 

This solution may also be employed in the test for bromine. If iodine has been found, add small amounts of sodium nitrite solution, warm shghtly and shake with fresh 1 ml. portions of carbon tetrachloride until the last extract is colourless boil the acid solution until no more nitrous fumes are evolved and cool. If iodine is absent, use 1 ml. of the fusion solution which has been strongly acidified with glacial acetic acid. Add a small amount of lead dioxide, place a strip of fluorescein paper across the mouth of the tube, and warm the solution. If bromine is present, it will colour the test paper rose-pink (eosin). 

The irradiation of calciferol in the presence of iodine leads to the formation of 5,6-/n7 j -vitaniin D2 [14449-19-5] (31) or [22350 1-0] (32) (67,68). 5,6-/ra j -Vitainin D as well as vitamin D (2) or (4) can be converted to isovitamin D by treatment with mineral or Lewis acids. Isocalciferol (35) [469-05-6] or (36) [42607-12-5] also forms upon heating of 5,6-/ -vitamin D. Isotachysterol (33) [469-06-7] or (34) [22350-43-2] forms from isocalciferol or vitamin D upon treatment with acid, and its production appears to be the result of sequential formation of trans- and isocalciferol from calciferol. These reactions are the basis of the antimony trichloride test for vitamin D (69—72). 

Sodium a2idodithiocarbonate decomposes with evolution of nitrogen gas on addition of iodine, thus providing a useful quaHtative test for the presence of residual carbon disulfide ia aqueous solutions (25). 

The molecular uniformity of constituting components of a nb/lcb glucan fraction of potato starch was investigated with Sepharose CL 2B (Fig. 16.16) as well as with Sephacryl S-1000 (Fig. 16.17). Therefore, each of the subsequently eluted 3-ml fractions was analyzed on their potential to form inclusion complexes with iodine, a sensitive test for the presence of nb/lcb glucans. Results are shown in Fig. 16.17 in terms of branching index, the ratio of extinction of pure iodine solution and of nb/lcb glucan/iodine complex the higher the index, the more pronounced the nb/lcb characteristics. 

Reaction.—A delicate test for ethyl alcohol is the lodofornt reaction. Pour a few drops of alcohol into a test-tube and add about 5 c.c of a solution of iodine in potassium iodide, and then dilute caustic soda solution until the iodine colour vanishes. Shake up and narm very gently to about 6o°. If no turbidity 01 precipitate appears at once, set the test-tube aside for a time. Yellow crystals of iodoform will ultimately deposit, which have a peculiar odoui, and a characteiistic star shape nhen viewed under the microscope. The same reaction is given with... 

A few- reagents react with the N—-H groups of the diaziridines. It is easy to decide w hether the resulting compounds still contain a true diaziridine ring by testing for the characteristic property of such rings to liberate from iodide solution two equivalents of iodine. 

Besides its synthetic importance, the haloform reaction is also used to test for the presence of a methylketone function or a methylcarbinol function in a molecule. Such compounds will upon treatment with iodine and an alkali... 

Examine the sample by thin-layer chromatography, using silica gel G R as the coating substance. Dissolve 10 mg of the substance to be examined in 4 mL of water R as a test solution, and dissolve 10 mg of penicillamine reference substance in 4 mL of water R as a reference solution. Apply 2 pL of each solution separately to the plate. Develop over a path of 10 cm using a mixture of 18 volumes of glacial acetic acid R, 18 volumes of water R, and 72 volumes of butanol R. Dry the plate at 100-105 °C for 5-10 min, and expose to iodine vapor for 5-10 min. The principal spot in the chromatogram obtained with the test solution is similar in position, color, and size to the principal spot in the chromatogram obtained with the reference. 

In number of labs, the NIR is a rapid, non-destructive test. It is used (everywhere) for water determination. For the petroleum industry, it is routinely used for octane and betaine values, to determine levels of additives, and as a test for unsaturation. The polymer companies, in addition to identification, use NIR for molecular weight, cross-linking, iodine value (unsaturation) block copolymer ratios, and numerous... 

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