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Uric Acid and Related Compounds
160. Isolation of Uric Acid from Urine (Section 333). - Add 50 cc. of concentrated hydrochloric acid to 500 cc. of urine and set aside in a cool place for 24 hours. Pour off the liquid from the crystals of uric acid which adhere to the side of the vessel. Collect the crystals, dissolve them in the smallest possible amount of boiling water, boil with a little bone-black, and filter the solution hot. On cooling, colorless crystals of uric acid are obtained. Examine them under the microscope and sketch the crystals.
161. Properties of Uric Acid (Section 333). - (a) Solubility of uric acid and its salts. - Shake up about 0.01 gram of uric acid with about 5 cc. of water. Does the acid dissolve? Add a dilute solution of sodium hydroxide, drop by drop, and shake. When all the acid has dissolved add a slight excess of dilute hydrochloric acid, heat to boiling, filter and set aside.
(b) To a dilute solution of uric acid in sodium hydroxide add a little ammonia, some magnesia mixture, and ammoniacal silver nitrate. The gelatinous precipitate formed is silver magnesium urate. The insolubility of this salt is utilized in the separation of uric acid from urine. The purine bases in urine give similar insoluble salts.
(c) Reduction of silver nitrate by uric acid. - Dissolve a trace of uric acid in a few drops of a solution of sodium carbonate, and pour the solution upon a piece of paper moistened with silver nitrate solution. The silver salt is reduced to metallic silver. This reaction is known as Schiff's test for uric acid.
(d) Murexide test for uric acid. - On a small watch-glass moisten a few crystals of uric acid with 2 or 3 drops of dilute nitric acid. Evaporate to dryness on the steam-bath. Cool and add from a glass rod a drop of ammonia. A similar color test is given by other purines.
162. Isolation of Caffeine from Tea (Section 341). - Boil gently 10 grams of tea with 500 cc. of water for 15 minutes. Filter through a folded filter, and precipitate the tannin by adding, drop by drop, to the hot filtrate a 10 per cent solution of lead acetate. When a precipitate is no longer formed, filter the solution again, and evaporate it to about 75 cc. If a precipitate has separated during the evaporation, filter again. Cool the solution, and extract it with 30 cc. of chloroform. Separate the chloroform and filter it through a dry paper. Set the solution aside to evaporate spontaneously. Observe the appearance of the crystals. Apply the murexide test to a few of the crystals. (See experiment 161d above.) Sublime the rest in watch-glasses. (See §35, page 23.) Taste a little of the sublimed caffeine.