10 BACTERIOLOGICAL CHEMISTRY 



only on the amount of acid present ; thus, for example, 

 all tenth -normal acid solutions will have the same titratable 

 acidity whatever the particular acid present may be, 

 whether it is a strong acid like hydrochloric acid or a 

 weak acid like acetic acid. But the ^^H values of different 

 tenth-normal acid solutions will vary widely, depending 

 on the acid concerned. This variation in pK value of 

 acid solutions containing the same equivalent weight of 

 acid results from the different degrees of dissociation 

 of the acids with the consequent production of different 

 concentrations of hydrogen ions. Thus we saw that 0-lN- 

 hydrochloric acid is 91 per cent, dissociated and has a pK 

 value of approximately 1, whilst O'lN-acetic acid, which 

 is 1-3 per cent, dissociated, has pH 2*9 ; that is hydro- 

 chloric acid is nearly one hundred times as strong an 

 acid as acetic acid, although the two solutions have the 

 same titratable acidity. In other words, merely knowing 

 the amount of titratable acid in a solution does not tell 

 us enough about its properties and probable effect on a 

 culture of bacteria or on a fermentation reaction. We 

 must know also the ^^H value of the solution, which will 

 give us a much better idea of the magnitude of effect 

 to expect. 



The logarithmic or exponential mode of expressiofi is 

 also used for the statement of dissociation constants, 

 which are referred to as ^^K values where 



pK = — log K 



K being the dissociation constant under consideration. 

 Thus the dissociation constant of acetic acid at 25°C. is 

 1-8 X 10"^ and its pK. value is, therefore, 



2iK = - log (1-8 X 10-5) = 5 ~ log 1-8 = 5 - 0-255 = 4-745 



An acid like phosphoric acid, H3PO4, which has three 

 ionisable hydrogen atoms, has three pK. values, one 

 corresponding to the dissociation of each liydrogen ion. 

 The three dissociation constants and pK values of 



