The Composition of the Strong Phosphoric Acids

Abstract

The composition of the strong phosphoric acids was studied in the range 68.8 to 86.3y0 phosphorus pentoxide by weight. Improved filter-paper chromatography made possible the q~~a n t i t a t i v e determination of the nine lower members of the series, and occasio~iall~ up to the twelfth member. I t was found that when the strong phosphoric acids are prepared by heating a t 350°C., a dynamic equilibrium between the component acicls is set up which persists when the mixtures are cooled to room temperature. 0111~ linear condensed polyphosphoric acids were present in the range s t~~d i e d .The colnposition corresponding to 100% orthophosphoric acid contains about 6 mole per cent each of pyrophosphoric acid and "free water". AS the mole ratio of water to phosphorus pentoxide decreases, the number of component acids increases. Orthophosphoric acid is present to an appreciable estent in the stronger phosphoric acids. "Hexametaphosphoric acid" is 11ot a separate chemical entity, but a mixture of higher liliear polyphosphoric acids. INTIIODUCTIONThe composition of the strong phosphoric acids,* i.e. acids containing more than 72.4y0 by weight of phosphorus pentoxide, the P?05 content of pure orthophosphoric acid, is a subject of considerable interest which has been repeatedly investigated by wet analytical methods. Difficulties in these methods, however, have set a decided limitation on the qualitative and quantitative conclusions possible. A study of this subject by filter-paper chron~atography was ~~n d e r t a k e n , therefore, in order to obtain further and more specific information about compositions having a mole ratio of water to phosphorus pentoxide between 3.6 and 1.2.I t has been shown by means of paper chromatograph!. that b~, mising orthophosphoric acid with phosphorus pentoxide a t different ratios, and heating the mixture a t 350°C., we obtain a mixture containing o~ll\, linear condensed polyphosphoric acids. No cyclic ones were found. Branched acids, if present, would not be detected, since any ion-n~olecules with branching are expected to hydrolyze immediately upon dissolutio~l (40). A certain characteristic equilibrium mixture exists for ever?; given ratio of water to phosphorus pentoxide.The phosphoric acids are very viscous, so that equilibria are attained very slowly; the end result of cooling is an oil in the range 72 to 82y0 PzOj, a gum in the range 82 to 86yo PPz05, and a brittle glass a t higher P205 concentrations.These acids are merely members of a continuous series of amorphous condensed phosphoric acid mixtures which extends from orthophosphoric acid to pure phosphorus pentoxide (38). These mixtures are hygroscopic and hydrolyze upon standing unless stored in tightly closed pyrex containers. The existence of strong phosphoric acids has been known for many years. Durgin, Lum, and Malowan (14) give a list of several such acids reported libfanzrscript

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