18 



BACTERIOLOGICAL CHEMISTRY 



Usually tlic inaxiimmi effect is obtained when ec[uivalent 

 amounts of acid (or base) and its salt are present in 

 solution, at which point the ^^H is equal to the ^^K of the 

 acid or ^^OH (which is the exponential expression for 

 hydro xyl ion concentration exactly analogous to pK) is 

 equal to the ^^K of the base. Buffer action is usually 

 restricted to pJi or ^^OH ranges about one unit above and 

 below the ^^K value. By making mixtures of appropriate 

 quantities of the acid and salt, solutions having various 

 pH values within the range can be prepared, according 

 to the formula 



_ concentration of salt 



concentration of acid 



The buffer capacity will obviously depend on the 

 concentration of the mixture and must be selected 

 appropriately for the purpose required. Some of the more 

 useful buffer mixtures are shown in Table 1 . 



Table 1 



