REACTION BY HYDROGEN-ION CONCENTRATION 117 



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 y^ NaOH 

 necessary to neutralize that solution." 1 



From the foregoing it is obviously not possible to deter- 

 mine by titration with an alkaline solution if the acidity of 

 the fluid under consideration is due to the presence of a 

 strong or a weak acid or, to put it in other words, to the 

 presence of acids readily dissociated with the liberation of 

 large amounts of H ions or to weak acids in which the reverse 

 is the case. Total acidity can, therefore, only be deter- 

 mined by estimating the H ion concentration of the fluid 

 under consideration. 



By the use of appropriate electrical devices the H.I.C. of 

 a fluid may be accurately determined and its variations, as 

 the fluid is diluted, may also be exactly detected, even 

 when reduced by dilution to such minute traces as would 

 be expressed by unwieldy decimals. To obviate the use of 

 such unwieldy decimals the symbol pH is employed, and by 

 suffixing to it a numeral representing the successive dilu- 

 tions by 10 that a normal solution may have undergone, we 

 have a brief and handy way of expressing H.I.C. Thus, 

 for instance, pH5 would symbolize the H.I.C. of a 0.00001 

 N (the fifth decimal point) solution, while pH7 (neutrality 

 as represented by distilled water) would symbolize the 

 H.I.C. of an acid the dilution of which is expressed by 

 0.0000001 N, or the seventh decimal point. We see, there- 

 fore, that in the use of the short, convenient symbol pH, we 

 have a means of noting the degree of dilution of an acid 

 which corresponds to a determined H.I.C. for that acid in that 



1 Medalia, Jour. Bacteriol., 1920, No. 5, vol. v, p. 442. 



