32 PHYSIOLOGY FOR DENTAL STUDENTS. 



occurs, it has been found that paranitro-phenol turns at about 

 .000,001 (or lxlO~ 7 ), which is the H-ion concentration of pure 

 water, and is therefore the most practical point to choose as indi- 

 cating neutrality. Methyl red and rosolic acid also change color 

 about this point. Phenolphthalein, on the other hand, changes 

 color at a H-ion concentration of 1 x 10~ 9 , i. e., its is very sensitive 

 towards acids, and methyl orange, at 1 x 10 4 , i. e., it is relatively 

 insensitive towards acids. 



The indicators which change color at about the H-ion concen- 

 trations found in animal fluids are rosolic acid, paranitrophenol 

 and methyl red. By comparing the color produced by adding 

 one of these indicators to the unknown fluid with those obtained 

 by adding the same indicator to a series of solutions containing 

 varying but known H-ion concentrations, we can accurately tell 

 the H-ion concentration of the unknown solution, for the H-ion 

 concentration of the solution whose tint matches with that of the 

 unknown is the H-ion concentration of the latter. The series of 

 standard solutions is made by mixing varying proportions of acid 

 and alkaline phosphates. 



Before leaving this subject, it is important to point out that 

 the blood has an H-ion concentration which is practically the 

 same as that of water, i. e., is as nearly neutral as it could be. It 

 also has the power of maintaining this neutrality practically con- 

 stant even when large amounts of acid or alkali are added to it. 

 Although saliva and some other body fluids are not so nearly 

 neutral as blood, yet they can also lock away much acid or 

 alkali without materially changing the H-ion concentration. This 

 property is due to the fact that the body fluids contain such salts 

 as phosphates and carbonates, which exist as neutral and acid 

 salts, and can change from the one state to the other without 

 greatly altering the H-ion concentration, and yet, in so changing, 

 can lock away or liberate H- or OH-ions. This has been called 

 the ' ' buffer ' ' action, and is a most important factor in maintain- 

 ing constant the neutrality of the animal body. 



Colloids. These are substances which do not diffuse through 

 membranes when they are dissolved. Thus if blood serum be 

 placed in a dialyser which is surrounded by distilled water, all 



