Affinities of Poly basic Acids. 



21 



Methyl-orange . . . 



Litmus 



Phenol - phthalei'n 



Oxalic. 



Acetic. 



Tartaric. 



Citric. 



low results 



sharp 



sharp 



very low 

 results 

 indefinite 



sharp 



very low 

 results 

 indefinite 



sharp 



very low 

 results 

 indefinite 



sharp 



Engel 1 , too, has shewn that when a solution of phosphoric 

 acid is titrated it appears monobasic if methyl-orange is 

 used as the indicator, but dibasic with phenol-phthalei'n, and 

 tribasic with Poirier's soluble blue. He also showed that 

 boric acid is neutral to methyl-orange, feebly acid to litmus 

 or phenol-phthalei'n, and acid to soluble blue. 



From these instances it appears that litmus is liable to 

 give lower results than phenol-phthalei'n, and in the case of 

 litmus, the final colour-change is usually very indefinite. It 

 also is evident that any knowledge of the basicity of an acid 

 obtained by titration is only relative to the nature of the 

 indicator employed. And, further, when a polybasic acid 

 has been only partially neutralised its acid character may be 

 enfeebled altogether out of proportion to the amount of 

 a base which has been added to it. 



This raises the wider question of the basicity of a poly- 

 basic acid, and of its affinity as a function of the affinity of 

 any one of the replaceable hydrogen atoms which it contains. 



On the Affinities of Polybasic Acids. 



It will be in place, at the outset, to point out the essen- 

 tial character of an acid. The evolution of an all-embracing 

 yet sufficient definition of an acid has been a gradual one, 

 and it is of no small interest to trace the views of Paracelsus, 

 Boyle, Stahl, and Becker, the oxygen-acid theory of 

 Lavoisier, and Berzelius, and the hydrogen-acid views of 



1 Comptes Rendus, 102, 262 (1886). 



