3934. Tests for the Purity of Citric Acid

3934.    Tests for the Purity of Citric Acid. "When pure, it does not yield a crystalline precipitate when added in excess to a solution of carbonate of potassa; such a precipitate indicates the presence of tartaric acid. It is entirely soluble in water, and what is thrown down by acetate of lead from this solution, is entirely soluble in dilute nitric acid. No salt of potassa, except the tartrate, yields a precipitate with the aqueous solution. It is entirely decomposed by heat; added sparingly to cold lime water, it does not render it turbid, and when a few drops of a solution of citric acid are added to lime water, a clear liquid results, which, when heated, deposits a white powder, soluble in acids without effervescence.

3935. Arsenious Acid

3935.     Arsenious Acid. This is the arsenic or white arsenic of commerce, imported chiefly from Germany, also manufactured in quantity in Cornwall, England. It consists of large, glassy, colorless or yellowish-white, semi-transparent cakes or porcelainlike masses, which soon become opaque on their exterior, and sometimes friable and Pulverulent. The transparent arsenic is found to be more than three times as soluble in water at 55° Fahr. than tho opaque. In taste it is slightly sweetish, with a slight acidity and astringency, not perceived until some minutes after being swallowed, hence its dangerous character as a poison. Crude arsenic is obtained, as a collateral product, during the smelting of cobalt ores. Pure arsenic is obtained from the crude, by a second sublimation in cast-iron vessels. The arsenic, as imported, has usually been thus purified; and, unless otherwise adulterated,, is sufficiently pure for general purposes. It is sometimes kept in fine powder, and in this state is occasionally found adulterated with powdered lime or chalk; it is, therefore, better to purchase it in the lump. Tho salts of arsenious acid are called arsenites.

3936. Self-Detecting Arsenious Acid

3936. Self-Detecting Arsenious Acid. By adding a small quantity of any of the following substances to ordinary white arsenic, tho mixture changes color when mixed with liquids. This is proposed as a method of preventing mistake in the use of this poisonous article.

The addition of a small quantity of a mixture of dry calomel and quicklime to the arsenic turns Mack when mixed with a liquid.

A mixture of thoroughly dry sulphate of iron and ferrocyanide of potassium turns it blue.

Dry sulphate of iron and dry sulphate of soda turns green.

3937. Testa for the Presence of Arsenious Acid

3937.    Testa for the Presence of Arsenious Acid. A weak solution of am-monio-acetate of copper added to a solution of white arsenic (arsenious acid) throws down a grass green precipitate of arsenite of copper (Scheele's green) This precipitate, after being washed, is soluble in nitric acid, and in ammonia; is turned a brownish-red by a solution of sulphuretted hydrogen, blood-red by ferrocyanide of potassium, and yellow by nitrate of silver.

Arsenious acid in solution throws down a yellow precipitate of arsenite of silver from a solution of ammonia-nitrate of silver.

There are a number of delicate tests employed for detecting the presence of arsenic in organic matter, such as the contents of the stomach or other viscera, all more or less involving the preparation of the matter before applying the tests, and requiring the manipulation of an experienced analytical chemist. A very susceptible test, and recommended by Cooley for its simplicity, is as follows: A solution of the suspected matter is strongly acidulated with muriatic acid in the proportion of 1 part muriatic acid to from 5 to 9 parts of the solution; this is boiled in a porcelain or glass vessel containing bright and clean metallic copper in the form of sheet, gauze, or wire. In about 15 minutes, if the solution be weak, or less, if strong, presence of arsenic will bo noted by the characteristic iron-gray film of arsenic deposited on the surface of the copper. The copper, having been carefully washed and dried, may bo cut into small pieces and heated in a test tube over a spirit lamp, when the metallic arsenic is volatilized, and will be condensed either in metallic form or in crystals of arsenious acid. This is known as Rensch's test.