28 PHYSIOLOGICAL CHEMISTRY 



Substances called buffers are of great importance in biochem- 

 istry. If one adds a small amount of hydrochloric acid to water, 

 the water becomes distinctly acid, and its pH changes from 7 

 to some smaller figure. For example 1 liter of water (or say 

 999 c.c.) (pH = 7) is acidified with 1 c.c. 0.01 HC1 (pH about 

 2). The acid is diluted 1000 times. The C H of the solution now 



is ^ 000>01 an( * the pH - = 5 - Tllis sma11 amount of dilute 



acid has caused a marked change in the pH. Suppose that acid 



N N 



were added to a solution of NaHC0 3 . If a whole liter of 



HC1 were added to a liter of the bicarbonate, the NaHC0 3 

 would be converted into H 2 C0 3 , which is a very weak acid and 

 the solution would still be somewhat near to the neutral point. 

 The NaHC0 3 , itself forming a solution which is about neutral, 

 still can neutralize strong acids, and the solution remains near 

 the neutral point until the NaHC0 3 has been all used up. 

 Na 2 HP0 4 and NaH 2 P0 4 , proteins, and various other substances 

 behave similarly, and are of the greatest importance as buffers 

 maintaining the reaction of blood and tissues near the neutral 

 point even when acids are produced or absorbed. 



Colloidal Solutions. Certain types of substances when dis- 

 solved in water form solutions which differ in many respects 

 from solutions of ordinary chemicals such as hydrochloric acid, 

 sodium chloride, sodium hydroxide, etc. The solutions often are 

 opalescent, they may set to solid jelly like masses, the dissolved 

 substance will not diffuse through a parchment membrane, and 

 they show various other distinguishing reactions. Graham gave 

 the name "colloids" to such substances to distinguish them 

 from substances which form "true solutions." These latter he 

 called crystalloids. Since many of the constituents of living 

 tissues form colloidal solutions the properties of such solutions 

 are of great importance in biology, and have been much studied. 

 It soon became evident that the colloidal state is only a condi- 

 tion in which any substance can exist, if properly prepared, 

 and is not confined to "glue-like" substances (colloidal means 



