Staining iodine-sulfur

After a plate has been exposed to the mobile-phase solvent for the required time, the compounds present can be viewed by several methods. Polynuclear aromatic hydrocarbons, other compounds with conjugated systems, and compounds containing heteroatoms (nitrogen, oxygen, or sulfur) can be viewed with long-and short-wave ultraviolet light. The unaided eye can see other material, or the plates can be developed in iodine. Iodine has an affinity for most petroleum compounds, including the saturated hydrocarbons, and stains the compounds a reddish-brown color. 

Figure 9.1 Longitudinal 10-pm bisection ofa steeped dent com kernel (magnified X6, iodine stained). Note how starch has been lost from floury endosperm cells as a result of sulfur dioxide action.

Amyloid is a substance of uncertain composition (111) sometimes found in diseased connective tissues. The name is derived from its resemblance to starch in turning blue when exposed to iodine and dilute sulfuric acid. This method of staining is so poor that it is now seldom used. A... 

Procedure. A small amount of the solid test material or a drop of its solution is treated in a micro test tube with several drops of chromic-sulfuric acid (preparation see Vol. II page 65). The open end of the test tube is covered with a disk of filter paper moistened with 2 N sodium hydroxide. The test tube is kept for 5 minutes in boiling water. The disk of reagent paper is then placed on a glass plate, spotted with several drops of permolybdic acid solution (for preparation see Vol. II page 68) and then dried with a current of cold air. If iodine is present, a brown fleck will appear but it fades within a few minutes. The stain is then treated with a drop of 0.1 % alcoholic solution of fluorescein. A positive response is indicated by the development of a red spot. 

Procedure. A small amount of the solid test material or a drop of its solution is placed in a micro test tube and evaporated if need be. A few mg of potassium permanganate and 2 drops of 6 iST sulfuric acid are introduced and the mouth of the tube is covered with a disk of filter paper moistened with freshly prepared reagent solution. The tube is then immersed in boiling water. The appearance of a blue-green stain within 2-3 minutes shows the presence of chlorine. If the quantity of chlorine is small, the fleck is grey-violet. The test is clearly distinct even in the presence of a one hundred-fold amount of bromine or iodine since the latter yield a yellowish stain that is characteristic of these two halogens. The latter fleck, in contrast to the chlorine stain, is easily reduced (made colorless) with a drop of 0.1 iV sodium thiosulfate. 

The most frequently used method of detection is based on the insolubility of chitin in a concentrated solution of alkali at 160°. Partial deacetylation occurs during this treatment, resulting in the formation of chitosan. This degradation product reacts with iodine-potassium iodide to give a brown color, which becomes violet on addition of sulfuric acid. When chitosan is treated directly with sulfuric acid, spherocrystals of chitosan sulfate are formed. These crystals are characteristically stained with fuchsin and picric acid. X-ray diffraction diagrams have also been used for the identification... 

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