3920. Phosphoric Acid

3920.    Phosphoric Acid. This acid, in its pure or anhydrous state, can only be obtained by the direct combination of its elements, phosphorus and oxygen, 1 equivalent of phosphorus combining with 5 of oxygen. It consists of a white, flaky, extremely deliquescent powder, which, when fused and cooled, assumes a vitreous appearance. It is capable of assuming three separate conditions in combination with water as a base; the union of 1 equivalent of anhydrous acid with 1 equivalent of water produces monobasic or glacial phosphoric acid, called also metaphos-phoric acid; 1 equivalent of anhydrous acid, with 2 of water, gives bibasic or pyrophos-phoric acid; 1 of anhydrous acid with 3 of water forms tribasic, or commercial phosphoric acid. This last is the common form of the acid. These three forms of the acid are not pure phosphoric acid in different degrees of dilution, as they have distinguishing characteristics. Monobasic phosphoric acid coagulates albumen, and gives white gelatinous uncrystallizable precipitates with the soluble salts of baryta, lime, and silver; the bibasic does not coagulate albumen, and makes, when neutralized only, a white precipitate with nitrate of silver; the tribasic does not affect albumen, and, when neutralized, throws down a yellow precipitate (phosphate of silver) from nitrate of silver. Tribasic phosphoric acid is the usual form under which phosphoric acid combines with the bases to form phosphates.

3921. To Obtain Phosphoric Acid

3921.    To Obtain Phosphoric Acid. This is obtained by heating nitric acid in a tubulated retort connected with a receiver; small fragments of phosphorus are dropped into the acid, singly and at intervals. As soon as tho phosphorus is dissolved, the heat is increased, and the undecomposed acid distilled off. The residuum is then evaporated to a syrupy consistence, and forms the phosphoric acid of commerce.

3922. To Obtain Hydrated or Glacial Phosphoric Acid

3922.    To Obtain Hydrated or Glacial Phosphoric Acid. Phosphoric acid {see last receipt) is gradually heated to redness in a platinum crucible, and the glacial acid obtained by evaporation. Solid hydrated or glacial phosphoric acid contains 89 per cent, of real acid, and 11 per cent, of water. It is a highly deliquescent, glassy-looking substance, very soluble in water, yielding a solution exhibiting powerful acid properties. Its concentrated solution has nearly the same properties as the solid acid; its dilute solution is not poisonous, and does not precipitate albumen. (Cooley.)

3923. Anhydrous Phosphoric Acid

3923.    Anhydrous Phosphoric Acid. This is evolved by burning phosphorus in a stream of dry air, or under a bell-jar, copiously supplied with dry air. The product is anhydrous phosphoric acid in snow-like flakes. These must be collected immediately, and put into a warm, dry, well-stoppered bottle. A few seconds' exposure to the air causes tho anhydrous acid to deliquesce into a syrupy liquid, its attraction for water being intense. Its anhydrous state cannot be restored after i deliquescence or solution.