ACIDITY l'^3 



Note that the original pH of the buffer has dropped only 0.22 unit. This 

 answer shows very clearly the effect of the buffer because if 10 ml. of 0.1 A'' HCl 

 are added to 40 ml. of plain water, without any buffer present, the resulting H + 

 concentration is 1 m.e.q. in 50 ml., or 0.02M . Therefore the pH is — log 0.02, 

 or 1.7, a very much greater drop. 



The salt-acid combination is most effective when the salt and acid arc 

 present in equal molecular proportions. Of course, there is a limit to the 

 capacity of the buffer to take up hydrochloric acid or sodium hydroxide. 

 For example, when about 85 per cent of the sodium acetate has been 

 converted into acetic acid, or vice versa, the limit is close at hand. On 

 adding more acid or alkali, the pH of the solution changes rapidly, and, 

 hence, there is little buffer action. 



The most widely used buffers are mixtures of sodium or potassium salts 

 of relatively weak acids and the corresponding free acid. For most 

 purposes, acids such as phosphoric, carbonic, acetic, and other organic 

 acids are used. Carbonates, bicarbonates, and phosphates, together 

 with proteins, form the most important buffers in the body. These main- 

 tain the pH within very narrow limits even though considerable acid or 

 base is added. 



Measurement of pH 



Each hydrogen ion bears an electric charge, and the concentration of 

 these ions can be measured most accurately by electrometric means. 

 This method, however, requires the use of relatively expensive apparatus 

 and an experienced operator. For many dyes there is a particular degree 

 of acidity at which there is a very definite change in color, and fortu- 

 nately the various dyes, or indicators, change color at different .hydrogen- 

 ion concentrations. This fact serves as a basis for a colorimetric method 

 that is quite simple, as well as fairly accurate. The method consists of 

 matching the color produced by an appropriate indicator in the unknown 

 solution with the color of a standard solution of known hydrogen-ion 

 concentration to which the same indicator has been added. If the color 

 of the unknown is the same as that of the standard, the hydrogen-ion 

 concentration likewise must be the same. Standard color charts and 

 glass discs of appropriate colors that may be substituted for the standard 

 solutions possess the added advantage of being more permanent than 

 the solutions. 



In Table 7-3 are given the approximate pH values of a number of 

 biological materials. 



