40 GENERAL BIOCHEMISTRY 



donor prescMii. Each step is a discrete process with a characteristic 

 equilibrium constant analogous to tliat tor water. 



[H,P04-][H30+] 7,yi._3 .,0,0p^ 



[HP0r][H30+] ,^yi._8 

 _ [P04-][H3Q+] _ 40^10-3 



pH 



Tn aqueous systems the dissociatitjii (oiisiant ol water automatically 

 fixes ihe concentration of hydrogen ion when the concentration of 

 hydroxy! ion is known. Thus for simplicity only one concentration is 

 usually stated, that of the hydrogen ion. In practice the range of 

 concentrations is so great that the use of a linear scale for plotting is 

 out of the qtiestion. The common usage of powers of 10 for expressing 

 concentration leads to the logical scale condensation of logarithms to 

 the base 10. Thus 



logiolO-^ = -4 



and to avoid the negative sign in plotting the signs are changed to 



-logiolO-^ = 4 



When the concentration involved is that of the hydrogen ion, this 

 operation defines the pH. 



pH^ -log[H30+] 



in a solution 10-^ molar with respect to H3O + , the pH is 4. 



Other correlations are shown in Table 2-2 for integral pH values. 

 Thus, unit change in pH alters both hydrogen and hydroxyl-ion con- 

 centrations by a factor of 10 and in opposite directions. Use of the 

 pH scale simplifies writing and especially the graphical representation 

 of the effect of the concentrations of these ions on chemical and 

 biological processes. 



Buffers 



Materials resisting changes in pH are called buffers. I'hey function 

 by accepting added protons (usually from hydrogen ions) or by sup- 

 plying them when a base is added. In this way changes in pH are 

 minimized. The mechanism of buffer action is the same in all aqueous 

 solutions and depends upon the presence of either a weak acid, a 

 weak base, or both and usually also upon the presence of a salt of the 



