Phosphoric Acid

PHOSPHORIC ACID (Acidum phosphoricum), by this name are covered the acids of pentavalent phosphorus. For the convenience of classification, they can be considered as products of the interaction of phosphoric anhydride (P₂O₆) and water; the monobasic metaphosphoric acid, HPO₃( = H₂O- + P₂O₅), the tetrabasic pyrophosphoric acid, H₄P₂O₇( = 2H₂O + P₂O₅), and the tribasic orthophosphoric acid, H₃PO₄( = 3H₂O + P₂O₅, ordinary Ph. acid) are known. In aqueous solutions, the last is the most stable; the two other acids in aqueous solutions gradually turn into orthophosphoric acid. Upon the action of water on P₂O₅ at ordinary temperature, metaphosphoric acid is formed, which turns into ortho-acid upon long standing of the solution, more quickly upon heating or the action of dilute mineral acid. Ph. acid is obtained usually by oxidation of yellow phosphorus with weak nitric acid. Technical Ph. acid is obtained by the action of weak sulfuric acid on bone ash or natural phosphates (phosphorites). Orthophosphoric acid melts at 42.3°, giving a liquid of sp. gr. 1.88, is non-volatile, easily soluble in water, giving a sharply acid solution on litmus, with a pleasant sour taste. Orthophosphoric acid dissociates predominantly according to the equation: H₃PO₄^H* + H₂PO₄^-, the dissociation constant for this stage at 18° = 1.1 10² (Abbot and Bray), for the stage H₂PO₄' 5 H* + HPO₄"-2 10^-7, for the stage HPO₄" I H* + PO₄"'- 3.6 10^-13. Orthophosphoric acid is not poisonous, is used for the preparation of medicines. Ph. acid gives 3 series of salts: monobasic (monometallic) - R₂PO₄, dibasic (dimetallic) - R₂HPO₄, and tribasic (trimetallic) - R₃PO₄; salts of the first composition have an acid reaction in aqueous solution, the second and third - alkaline (due to hydrolysis). The ion PO₄"' gives a yellow precipitate with AgNO₃, soluble both in NH₃ and in HNO₃. Upon heating orthophosphoric acid above 200°, also upon heating its dimetallic salts, H₂O is split off and pyrophosphoric acid is formed (2H₃PO₄ = H₂O + H₄P₂O₇), which gives a white precipitate of composition Ag₄P₂O₇ with AgNO₃. Upon more intense heating (about 350°), orthophosphoric acid turns, losing twice as much water, into metaphosphoric acid: H₃PO₄ = H₂O-f + HPO₃; the existence of several polymeric forms of metaphosphoric acid (HPO₃)_n is recognized. Metaphosphoric acid (glassy Ph. acid, Acidum phosphoricum glaciale) represents a glassy, hygroscopic, volatilizing upon heating without decomposition, water-soluble mass (sticks or pieces); with AgNO₃ it gives a white precipitate; a freshly prepared solution of metaphosphoric acid precipitates phosphates (a distinction from other Ph. acids). Phosphates account for the greater part of the mineral salts of the organism. Especially much Ph. acid in the form of calcium and magnesium salts is contained in bones and teeth, as well as in urinary stones and other concretions; in smaller quantities in the form of salts with alkaline metals Ca and Mg, Ph. acid is found in all liquids and tissues of the organism. Ph. acid also enters into the composition of nucleic acids, phosphatides, hexophosphoric, glycerophosphoric, phosphoglyceric, creatinephosphoric, adenosinetriphosphoric, argininephosphoric (in crustaceans) acids, nucleoalbumins and other less studied compounds. In the organism of animals and plants, Ph. acid occurs only in the form of ortho- and pyrophosphoric acid. Ph. acid plays a very important role in the processes of carbohydrate and apparently fatty and lipid metabolism (see Glycogenesis, Glycolysis, Metabolism), in the processes of muscular contraction (see Muscles), in the creation of buffer properties (see) of the body's fluids and tissues, in giving hardness to the skeleton (in the form of calcium and magnesium salts) and in others. In an adult human, Ph. acid accounts for 1-1.5 kg. The exchange of Ph. acid is most closely connected with the exchange of alkaline earth metals. The daily requirement of the adult organism in Ph. acid is 2.5-3.5 g. An infant retains almost all the Ph. acid contained in breast milk, i.e. 0.07-0.1 g per 1 kg per day; the addition of Ph. acid to food increases its retention, with age this retention ability decreases. Inorganic phosphates apparently can serve as material in the synthesis of organic phosphorus-containing compounds; with insufficient delivery for such synthesis of Ph. acid from food, the body's skeletal reserves can apparently be used; a prolonged deficiency of Ph. acid in food leads to a disturbance of a number of functions (decrease in milk secretion, changes in the skeleton, etc.). For the discovery of Ph. acid, the following may serve: 1) a white crystalline precipitate of the double ammonio-magnesium salt, MgNH₄PO₄, or 2) a yellow crystalline precipitate upon heating in the presence of HNO₃ with ammonium molybdate. On methods of quantitative determination of Ph. acid - see Blood, Urine. Also applied is nephelometric determination (Kleinmann, Pinkussen), based on the obtaining of a fine suspension of strychnine-phosphomolybdate, colorimetric methods, based on the formation of a blue coloring upon reduction of phosphomolybdic acid by hydroquinone (Bell, Doisy), on the obtaining of a green coloring upon reduction of strychnine-phosphomolybdate by potassium ferrocyanide (Tisdall) and others.