442 LEON S. MEDALIA 



strictly avoided in order not to confuse the average laboratory 

 worker. Those interested in the subject, will find the articles 

 by Sorensen (1912), and Clark and Lubs (1917), of special value. 



Principles underlying the H-ion concentration method of titration 



This method is based upon the electrolytic dissociation theory 

 or ionic-theory developed by Arrhenius in 1887, which assumes 

 that acids, bases, or salts, in aqueous solutions are dissociated 

 to a greater or less extent into ions, i.e., electrically charged atoms, 

 or groups of atoms. The unit charge is that which is associated 

 with one hydrogen-ion. The strength of an acid in solution, ac- 

 cording to this theory, depends upon the free or dissociated hydro- 

 gen-tons present in that solution. The comparative strength of 

 two equivalent or " normal" solutions will vary according to the 

 ability of the components of each to dissociate and allow free 

 hydrogen-ions to accumulate in the solution. The relative 

 amount of free hydrogen-tons (percentage dissociation) in 

 ''normal" solutions has been found to show wide variations. 

 Thus, the percentage dissociation or the amount of H-ions set 

 free in •& HC1 solution was found (Talbot 1908) to be 90 per 

 cent, while that of & acetic acid is 1.4 per cent. Hydrochloric 

 acid according to these findings is therefore 64 times stronger in 

 acidity than acetic acid, although 10 cc. of ■£> of either acid will re- 

 quire a similar 10 cc. portion of •& NaOH to neutralize it. Ac- 

 cordingly the only correct method of measuring the acid strength 

 of a solution is to determine the amount of free H-ions or the 

 hydrogen-ion concentration (H.I.C.), of that solution, and not to 

 determine the amount of ttt NaOH necessary to neutralize that 

 solution. 



Another grave source of error in the titration method in common 

 vogue is the fact that organic substances known as "buffers" when 

 present in solutions (such as culture media) are capable of com- 

 bining with the t^ NaOH added during the titration, deviating 

 it from and preventing its neutralizing, the acid with which it 

 was meant to react. 



From the foregoing it is evident that the NaOH method of 

 titration with phenolphthalein is erroneous and highly misleading. 



