28 PHYSICOCHEM1CAL BASIS OF PHYSIOLOGICAL PROCESSES 



Application of the Law of Mass Action in Determining the Real 

 Strength of Acids or Alkalies. We have seen that it is the degree of 

 dissociation upon which the real strength of an acid depends and that 

 this varies with dilution (page 19). The equilibrium between the un- 

 dissociated and dissociated molecules may therefore be shifted in either 

 direction by changing the concentration; in other words, the process of 

 dissociation is a reversible reaction, and may be represented as 

 AB + A' + B . The law of mass action must apply in such a case, and 

 as a matter of fact it has been found that a constant can be calculated, 

 which is known as the dissociation constant.* It is an expression of the 

 inherent ability of the acid to dissociate into ions, and is therefore the 

 best measure of the strength of the acid. This is strictly the case for all 

 of the weaker acids, but strong mineral acids (and bases) do not give 

 a satisfactory constant, so that the comparison must not be made between 

 them and weaker ones. That the dissociation constant expresses the rela- 

 tive strength of organic acids can be shown by comparing its value with 

 that of the rate at which cane sugar is inverted (see page 23), this being 

 proportional to the concentration of the H ions present. K for some or- 

 ganic acids is : Acetic, 0.000018 ; Formic, 0.000214 ; Benzoic, 0.00006 ; Sal- 

 icylic, 0.00102. 



*The equation is yr yy- = K, where it is supposed that in volume V of the solution there is 



1 gram-equivalent of electrolyte, and that the degree of dissociation is a; the quantity of undis- 

 sociated electrolyte stated in a fraction of a gram-equivalent will be 1-a, and the quantity of each 

 ion a. To illustrate, let us take acetic acid in various dilutions: 



V a K x 10 5 



0.994 0.004 1.62 



2.02 0.00614 1.88 



15.9 0.0166 1.76 



18.1 0.0178 1.78 



