50 PHYSIOLOGY FOR DEKTAL STUDENTS. 



without causing any considerable amount of free H- or OH-ion to 

 be set free. This has been called the "buffer action" of such 

 salts. It endows the saliva with the power of locking away con- 

 siderable quantities of acid or alkali. 



In actually measuring the neutralizing power of saliva, it is 

 best first of all to bring the saliva to a definite H-ion concentra- 

 tion by adding standard acid and then to find out how much 

 alkali is required to bring it to another definite H-ion concentra- 

 tion. 



The methods for applying the above principles are as follows : 



10 c.c. saliva is diluted in an evaporating dish with 20 c.c. 

 water which has been boiled to expel CO, and then cooled to 

 20C. About eight drops of an aqueous solution of paranitro- 

 phenol is then added and N/200 HC1 run in from a burette, with 

 constant stirring until the yellow color due to the indicator just 

 disappears. The amount of N/200 HC1 is noted. N/200 NH 4 HO 

 is then added till the yellow color just returns. 1 The difference 

 between the two readings gives the alkalinity in terms of c.c. of 

 N/200 HC1. 



The acidity may be directly measured by adding four drops 

 of an alcoholic solution of phenolphthalein to another 10 c.c. 

 sample of saliva and running in N/200 NaOH until a definite 

 pink color results. 



Addition of the acidity and alkalinity results gives the total 

 neutralizing power of the saliva, or in other words the power of 

 maintaining neutrality. This is a much more constant property 

 of saliva than the acidity or alkalinity alone, and it has conse- 

 quently been used, in the most recent work of Marshall, 2 for the 

 purpose of ascertaining whether the susceptibility to dental ca- 

 ries bears any relationship to the reaction of the saliva. It \\.is 

 found that it does not. On the other hand, this author has shown 



iThe reason for titrating back with N/200 NHHO till a yellow color again 

 reappears, when measuring the alkalinity, is to increase the accuracy of the 

 titration, it being often difficult to decide the point at which the color disap- 

 pears when N/200 acid is added, but easy to decide when it reappears when 

 N/200 NH 4 HO is used. NaOH is employed in the acidity titration because 

 phenolphthalein cannot be used with ammonia. 



2Cf. J. A. Marshall, Amer. Jour, of Physiology, 1915, XXXVI, p. 260. 



