TITRATION OF MEDIA 21 



centimeters of the medium, using phenolphthalein as indi- 

 cator. 



6. Alkaline media are denoted by placing a minus ( ) 

 sign before the number of degrees of alkalinity; thus, 15 

 would indicate that the medium was 15 alkaline, or that 

 15 c.c. normal acid must be added per liter to neutral- 

 ize it. 



Acid media are denoted by placing a plus. (+) sign 

 before the number of degrees of acidity; thus, +15 

 would indicate that the medium was 15 acid or that 

 15 c.c. of normal alkali must be added per liter to neu- 

 tralize it. 



Example. 



Burette reading after titrating 5.4 c.c. 



Burette reading before titrating 2.0 c.c. 



Number of c.c. N/20 NaOH required 



to neutralize the acid in 5 c.c. of 



the medium .4 c.c. 



If 5 c.c. of the medium (which is 1/20 of 100 c.c.) require 

 3.4 c.c. of 1/20 normal NaOH to neutralize the acid pres- 

 ent, 100 c.c. of the medium would require 20X3.4 c.c. or 

 68 c.c. of 1/20 normal NaOH. 



As a normal solution is 20 times the strength of a 1/20 

 normal solution, 100 c.c. of the medium would require 1/20 

 of 68 c.c. or 3.4 c.c. of normal NaOH for neutralization; and 

 one liter or 1000 c.c. of medium would require 10X3.4 

 c.c. or 34 c.c. N/l NaOH for neutralization; i.e., the 

 medium is 34 acid. Fuller's scale. This is the litre of the 

 medium. 



When N/20 acid or alkali and a 5 c.c. portion of medium 

 (in 45 c.c. of distilled water) are used, each 1/10 of 1 c.c. 

 corresponds to 1 Fuller's scale. 



Adjustment of Reaction. If it is desired to leave the 

 medium with a, e.g., +15 reaction, we have: 



