DETERMINATION OF ACIDITY AND ALKALINITY 105 



basic nature of a solution. There is still another value which deter- 

 mines this nature and which can be best illustrated by the following 

 example. Let us take a liter of normal acetic acid. According to 

 data given on p. 40 its hydrion concentration is [H+] = 3.4 X 10~^. 

 Let us also take a Hter of 3.4 X 10~^ N HCl. The two solutions have 

 the identical [H+] and yet very different equivalent acid content. 

 But they also differ in another respect. If we add to each one cubic 

 centimeter of a 3.4 iV NaOH solution the resulting relative changes in 

 the [H+] of the two solutions will be widely different. The HCl 

 solution has become neutral in reaction, while the acetic acid has 

 remained still acid in reaction. The acetic acid solution, in spite 

 of having an identical [H+], presents a greater reserve of acid; it 

 contains, as it were, latent or potential H-ions. On the other hand 

 the HCl solution is less resistant to the addition of alkali in respect to 

 the change of its [H+]. This resistance cannot be directly expressed 

 in terms of the equivalent content of a solution. This can be seen 

 from the following example. A solution of acetic acid in pure water 

 on one hand and in a normal sodium acetate solution on the other 

 hand have the same equivalent content of free acid. But if the same 

 amount of NaOH solution be added to each, the [H+] of the pure 

 acid solution will change to a much greater extent than in the acid- 

 acetate mixture. The pure acid solution yields more readily, the 

 acid salt mixture changes its [H+] relatively much less. This resist- 

 ance to change in reaction is a property of the greatest significance in 

 physiology. The acids arising in the metabolic processes (carbonic 

 acid, lactic acid) change the reaction of the tissue fluids. For the 

 proper functioning of the organism it is necessary that the tissue 

 fluids have a great enough buffering effect to prevent the increase 

 in hydrogen ion concentration to a deleterious extent. On the other 

 hand, this resistance must permit a great enough increase in the [H+] 

 to set in motion the automatic responses of the organism for the 

 removal of these acids. This concept of resistance requires there- 

 fore a more precise definition which can be arrived at in the following 

 manner : 



If to a liter of the acid solution increasing amounts of A^ NaOH 

 solution be gradually added, then for at least a very small period of 

 this addition the change in pH will be proportional to the amount of 

 alkali added. This applies at least to infinitely small amounts of added 

 alkali. As the addition of alkali progresses, this proportionality 



