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Dissolve about 0.2 gram of rosaniline in a small amount of boiling water. Cool and add 15 cc. of a saturated solution of sulphur dioxide in water, and allow the mixture to stand several hours until it becomes colorless or pale yellow; then dilute to 200 cc. with water. The reagent should be kept in a well-stoppered bottle of dark glass.
Make an ammoniacal solution of silver nitrate by dissolving 10 grams of the salt in 100 cc. of a solution of ammonia prepared by mixing equal volumes of concentrated ammonia (sp. gr. 0.9) and water. When ready to make a test, mix a portion of this solution with an equal volume of a 10 per cent aqueous solution of sodium hydroxide. The test should be made in the cold. The sodium hydroxide should be added immediately before use; on standing or on heating, the mixture so prepared deposits a black explosive precipitate.
Dissolve 34.64 grams of pure crystalline copper sulphate in distilled water, and dilute to 500 cc. Dissolve 70 grams of sodium hydroxide and 180 grams of pure Rochelle salt in 400 cc. of water, and dilute to 500 cc. The solutions should be kept in separate bottles until used. Fehling's solution is made by mixing equal volumes of the two solutions. A solution prepared in this way is of such a strength that 1 cc. of it will oxidize 0.005 gram of dextrose.
Dissolve 10 grams of mercury in 20 cc. of hot concentrated nitric acid, and dilute the resulting solution with 30 cc. of water.
In a large flask place 10 grams of powdered magnesium, cover the latter with distilled water, and add slowly, keeping the mixture cold, 250 cc. of a cold saturated solution of oxalic acid. When the reaction is complete, filter, make the filtrate slightly acid with acetic acid, and dilute to 1000 cc. with distilled water.
Preparation of Hydrobromic Acid
The constant-boiling mixture of hydrobromic acid and water can be conveniently used to prepare alkyl bromides. The acid boils at 126°, contains about 47.5 per cent of hydrogen bromide, and has a specific gravity of about 1.48.
The acid can be prepared from potassium bromide or from bromine.
(a) From potassium bromide - In a 250 cc. distilling flask pour slowly into 85 cc: of water 98 grams of pure concentrated sulphuric acid. To the mixture add 119 grams of powdered potassium bromide and distil slowly, using a water-jacketed condenser. Collect the fraction boiling at 125°-126°. The yield is 85 to 90 per cent of the theoretical.
(b) From bromine. - In a 200 cc. distilling flask place 162 cc. of water and 10 grams of red phosphorus; cover the side-arm of the flask with a cork (Fig. 14, page 25), connect the flask with a return condenser, and place it in a water-bath so that it is completely covered with cold water. Put 40 cc. of bromine in a small separatory funnel, and support the latter in the condenser at the top. Allow the bromine to flow into the flask slowly, drop by drop. When all the bromine has been added, support the flask on a wire gauze, connect it with a condenser and distil. Collect the part which boils at 125°-126°. The yield is about 95 per cent of the theoretical.
Preparation of Hydroiodic Acid
The constant-boiling aqueous solution of hydriodic acid can be conveniently used for the preparation of alkyl iodides. The solution contains about 57 per cent of hydriodic acid, boils at 127°, and has the specific gravity of 1.68 to 1.70. The exact composition varies with the pressure at which the acid is distilled. The solution is prepared as follows: In a 100 cc. distilling flask place 65 cc. of water and 70 grams of iodine. Put the flask in a water-bath containing cold water, and force it down so that the whole bulb of the flask is submerged. Hold the flask in place by means of a clamp and ring-stand. Add 6.5 grams of red phosphorus. Let the flask stand for about 5 minutes. Support it on a wire gauze, connect with a condenser and distil. Collect what boils at 126°-127°. The yield is about 95 per cent of the theoretical.