Conductivity of Hypophosphoric Acid Solutions. 117 



is a fairly strong acid having an ionization constant close 

 to that of the primary ionization orthophosphoric acid; 

 and with reference to the third it is a weak acid, approxi- 

 mating in strength to the secondary ionization of ortho- 

 phosphoric acid. This relation is further shown by the 

 behavior of these two acids toward indicators. With 

 pyrophosphoric acid as with hypophosphoric acid, two 

 equivalents of hydrogen are titrable in the presence of 

 methyl orange, with orthophosphoric acid only one. 



Finally, the close analogy between pyrophosphoric and 

 hypophosphoric acid is conspicuous in many other ways. 

 Of the six oxy-acids of phosphorus, ortho-, pyro-, and 

 meta-phosphoric acid, hypophosphoric acid, phosphorous 

 acid, and hypophosphorous acid, no two show as close a 

 mutual resemblance in general chemical behavior as do 

 pyro- and hypophosphoric acid, at least when in the 

 form of salts. In general appearance, as well as in rela- 

 tive solubility in various reagents, there is a most strik- 

 ing resemblance between those hypophosphates which 

 can be obtained as precipitates, and the corresponding 

 pyrophosphates, a similarity which is the cause of much 

 difficulty in qualitative analyses which involve these two 

 acids. In fact, in most cases, the most certain way of 

 distinguishing them is to dissolve and hydrolyze the 

 hypophosphate precipitate, best by heating with strong 

 hydrochloric acid, and to test the product for phosphor- 

 ous acid. Pyrophosphoric acid, similarly treated, yields 

 only orthophosphoric acid. 12 Thus it is only by actually 

 decomposing the hypophosphate and pyrophosphate 

 molecules that radical differences are revealed. Even 

 the hydration of pyrophosphoric acid bears a formal 

 resemblance to the hydrolysis of hypophosphoric acid, 

 since it involves the division of the molecule into two 

 smaller ones, and therefore might also be called a 

 hydrolysis in the broadest sense of the term. This reac- 

 tion, as Abbott 13 has shown, is monomolecular and is 

 catalyzed by hydrogen ions, though the effect of these 

 ions is proportional to a lower power of their concentra- 

 tion than is the case with hypophosphoric acid. 



It is evident that these various points of resemblance 



12 We are at present engaged in a study of the qualitative separation and 

 identification of the different oxy-acids of phosphorus and hope soon to 

 publish our results. 



13 J. Am. Chem. Soc, 31, 763, 1909. 



