Place 2 mL of the periodic acid reagent in a small test tube, add 1 drop (no more) of concentrated nitric acid, and shake thoroughly. Then add 1 drop or a small crystal of the compound to be tested. Shake the mixture for 10-15 sec, and add 1 or 2 drops of 5% aqueous silver nitrate solution. Dioxane may be added to facilitate the reaction of water insoluble polyols.
Periodic Acid Reagent: Dissolve 0.5 g of paraperiodic acid (H5IO6) in 100 mL of distilled water.
The instantaneous formation of a white precipitate (silver iodate) indicates that the organic compound has been oxidized by the periodate, which is thereby reduced to iodate. This constitutes a positive test. Failure to form a precipitate, or the appearance of a brown precipitate that redissolves on shaking, constitutes a negative test.
Periodic acid has a very selective oxidizing action on 1,2-glycols, a-hydroxy aldehydes, a-hydroxy ketones, 1,2-diketones, a-hydroxy acids, and b-hydroxylamines. The rate of the reaction decreases in the order mentioned. Under the conditions specified above, a-hydroxy acids sometimes give a negative test. b-Dicarbonyl compounds and other active methylene compounds also react. It is important that the exact amounts of reagent and nitric acid be used. The test depends on the fact that silver iodate is only slightly soluble in dilute nitric acid whereas silver periodate is very soluble. If too much nitric acid is present, however, the silver iodate will fail to precipitate. Olefins, secondary alcohols, 1,3-glycols, ketones, and aldehydes are not affected by periodic acid under the above conditions. The periodic acid test is best suited for water soluble compounds.
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