3938. Arsenic Acid

3938.    Arsenic Acid. An acid formed by the combination of metallic arsenic with oxygen. It is sour, reddens litmus, and forms salts with the bases, which are termed arseniates. By careful evaporation it may be obtained under the form of small grains, but usually has the consistence of syrup, being very deliquescent.

3939. To Obtain Arsenic Acid

3939.    To Obtain Arsenic Acid. Pour 6 parts of strong nitric acid on 1 part of white arsenic (arsenious acid) in a glass vessel, and distill until the solution acquires the consistence of a syrup, then transfer it into a platina crucible, and expose it for some time to a faint dull red heat, to expel the nitric acid. The addition of a little muriatic acid facilitates the process.

3940. Tests for the Presence of Arsenic Acid

3940.    Tests for the Presence of Arsenic Acid. Sulphuretted hydrogen gives a yellow precipitate; nitrate of silver added to the solution of an arsenite gives a precipitate of a brick red color; nitrate of lead gives a white one, and the salts of copper a bluish colored one. Pure lump sugar dissolved in an aqueous solution of arsenic acid, becomes in a few hours of a reddish color, and afterwards of a magnificent purple. For some test purposes it will be advisable to add sulphurous acid to the suspected liquor, and boil it for a short time, when the arsenic acid will be reduced to arsenious acid, in which state it will be susceptible of more delicate tests. (See No. 3937 (Testa for the Presence of Arsenious Acid).)

3941. Manganesic Acid

3941.      Manganesic Acid - also called Permanganic Acid - may be obtained by mixing 8 parts of binoxide of manganese with 7 parts of chlorate of potassa, both in fine powder, adding 10 parts of hydrate of potassa, dissolved in a small quantity of water, evaporating to dryness, powdering, exposing the powder to a low red heat in a platinum crucible, dissolving the mass in a large quantity of water, decanting, evaporating, and crystallizing. These crystals are permanganate of potassa, from which the acid may be obtained by conversion into permanganate of baryta, and by careful decomposition by dilute sulphuric acid. (Gregory.) It has a fine red color, bleaches, and is rapidly decomposed by organic matter. It unites with some of the bases to form permanganates.

3942. Benzoic Acid

3942.     Benzoic Acid. This is also called flowers of benzoin or benjamin. It has the form of white crystalline needles of a silky lustre, possessing an agreeable odor. Benzoic acid fuses at 230° Fahr., is volatile when heated, dissolves sparingly in cold water, with less difficulty in boiling water, and very freely in alcohol. Its salts are called

3943. To Obtain Benzoic Acid

3943.    To Obtain Benzoic Acid. Put coarsely triturated benzoin into an iron pot with a flat bottom, whose diameter is from 8 to 9 inches; the benzoin forming therein a layer of from 1 to 2 inches in depth. The open end of the pot is then to be covered with a sheet of soft and loose blotting-paper, which must bo attached to the rim with paste. A cone, formed with strong and thick paper, (cartridge paper), is then to be capped over the top of the pot, including the blotting paper; and this is also to be attached with paste and string. The apparatus, thus prepared, should then bo placed on the sand-bath, and exposed from 4 to 6 hours to a gentle heat. After this lapse of time, it may be removed from the sand-bath, inverted, and. the string detached, when beautiful white needles, of a silky lustre, possessing the agreeable odor of benzoic acid, will bo found in the paper cone.