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Arsenious Acid

This is the upon the surface, This part of the process arsenic or white arsenic of commerce, imrequires great attention and judgment, as, if ported chiefly from Germany, also manufacnot properly conducted, the whole batch may tured in quantity in Cornwall, England. It be carbonized and spoiled. At this point the consists of large, glassy, colorless or yellowishevaporation must be stopped, and the concen-, white, semi-transparent cakes or porcelaintrated solution emptied into warm and clean like masses, which soon become opaque on crystallizing vessels, set in a dry apartment, their exterior, and sometimes friable and where the thermometer does not fall below pulverulent. The transparent arsenic is temperate . At the end of 4 days the crystals found to be more than three times as soluble will be ready to remove from the pans, when in water at 55° Fahr. than the opaque. In they must be well drained, redissolved in as little water possible, and, after being allowed to stand for a few hours to deposit impurities, some minutes after being swallowed, hence taste it is slightly sweetish, with a slight acidity and astringency, not perceived until its dangerous character as a poison. Grude 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. The salts of again evaporated and crystallized. When the process has been well managed, the acid of the second crystallization will usually be sufficiently pure; but if this be not the case, a third, or even a fourth crystallization must be had recourse to. The mother liquors from the several pans are collected together, and, by evaporation, yield a second or third crop of crystals obtained by evaporation as before.

Citric acid crystallizes with great ease, but in some cases, where all the citrate of lime has not undergone decomposition by the sulphuric acid, a little of that salt is taken up arsenious acid are called ARSENITES. by the free citric acid, and materially ob- 3936. Self-Detecting Arsenious structs the crystallization. This is best Acid. By adding a small quantity of any of avoided by exactly apportioning the quantity the following substances to ordinary white of the sulphuric acid to that of the chalk arsenic, the mixture changes color when used, always remembering that it requires a mixed with liquids. This is proposed as a quantity of liquid sulphuric acid, containing method of preventing mistake in the use of exactly 40 parts of dry acid, to decompose 50 this poisonous article. parts of carbonate of lime. Commercial sulphuric acid is usually of the specific gravity of 1.845; it will therefore take exactly 49 pounds of this acid for 50 pounds of chalk.

In practice it is found that a very slight excess of sulphuric acid is better than leaving any citrate of lime undecomposed. The first crop of crystals is called "brown citric acid," The addition of a small quantity of a mixture of dry calomel and quicklime to the arsenic turns black when mixed with a liquid.

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

Dry sulphate of iron and dry sulphate of soda turns green. 3937. Tests for the Presence of Ar- 3941. Manganesic Acid-also called senious Acid. A weak solution of am- permanganic acid-may be obtained by mixmonio-acetate of copper added to a solution ing 8 parts of binoxide of manganese with 7 of white arsenic (arsenious acid) throws parts of chlorate of potassa, both in fine powdown a grass green precipitate of arsenite of der, adding 10 parts of hydrate of potassa, copper (Scheele's green) This precipitate, dissolved in a small quantity of water, evapafter being washed, is soluble in nitric acid, orating to dryness, powdering, exposing the and in ammonia; is turned a brownish-red powder to a low red heat in a platinum cruciby a solution of sulphuretted hydrogen, ble, dissolving the mass in a large quantity of blood-red by ferrocyanide of potassium, and water, decanting, evaporating, and crystallizyellow by nitrate of silver. ing. These crystals are permanganate of po- Arsenious acid in solution throws down a tassa, from which the acid may be obtained yellow precipitate of arsenite of silver from by conversion into permanganate of baryta, a solution of ammonio-nitrate of silver. and by careful decomposition by dilute sul- There are a number of delicate tests em-phuric acid. (Gregory.) It has a fine red ployed for detecting the presence of arsenic color, bleaches, and is rapidly decomposed by in organic matter, such as the contents of organic matter. It unites with some of the the stomach or other viscera, all more or less bases to form PERMANGANATES. involving the preparation of the matter before 3942. Benzoic Acid. This is also applying the tests, and requiring the manipu- called flowers of benzoin or benjamin. It has lation of an experienced analytical chemist. the form of white crystalline needles of a A very susceptible test, and recommended by silky lustre, possessing an agreeable odor. Cooley for its simplicity, is as follows: A so- Benzoic acid fuses at 230° Fahr., is volatile lution of the suspected matter is strongly when heated, dissolves sparingly in cold waacidulated with muriatic acid in the pro- ter, with less difficulty in boiling water, and portion of 1 part muriatic acid to from 5 to 9 very freely in alcohol. Its salts are called parts of the solution; this is boiled in a BENZOATES. Put porcelain or glass vessel containing bright 3943. Το Obtain Benzoic Acid. and clean metallic copper in the form of coarsely triturated benzoin into an iron pot sheet, gauze, or wire. In about 15 minutes, if with a flat bottom, whose diameter is from 8 the solution be weak, or less, if strong, to 9 inches; the benzoin forming therein a presence of arsenic will be noted by the layer of from 1 to 2 inches in depth. The characteristic iron-gray film of arsenic de- open end of the pot is then to be covered with posited on the surface of the copper. The a sheet of soft and loose blotting-paper, which copper, having been carefully washed and dried, may be cut into small picces 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 Lensch's test.