TITRATION OF MEDIA. IJ 



color of the same intensity as our standard, we read off the number of 

 c.c. or fractions of a c.c. of N/io sodium hydrate solution added to 

 produce the color. This number gives the acidity of the bouillon in 

 percentage of N/i acid solution.* 



Percent acid means that so many c.c. of N/i acid added to 100 c.c. 

 of the medium at the neutral point would give that percentage reaction. 

 Thus i 1/2 c.c. of N/i HC1 solution added to 100 c.c. of medium at o, 

 would give us i 1/2% of acidity or +1.5. 



Percent alkaline means so many c.c. of N/i sodium hydrate 

 solution added to 100 c.c. of the medium at the neutral point. Thus 

 a 1/2% alkaline medium would be one whose alkalinity would cor- 

 respond to the addition of 1/2 c.c. of N/i NaOH to 100 c.c. of the 

 medium at o. It is written .5. 



If we took 100 c.c. of the medium and put it in a beaker and then 

 ran in N/i NaOH solution from a burette, it will be readily under- 

 stood that if we had to add 31/2 c.c. of N/ 1 NaOH to obtain the pink 

 color, it would show that the acidity of the 100 c.c. of medium, being 

 tested, corresponded to 3.5 c.c. of N/i acid solution, and that its 

 acidity was equal to 3 1/2% of N/i acid solution, or that its reaction 

 was +3.5. 



As N/ 1 NaOH solution is too corrosive for general use in a burette, 

 and as 10 c.c. of medium is more convenient to work with than 100 c.c., 

 we use a solution one-tenth the strength of the N/i NaOH and we take 

 only one-tenth of the 100 c.c. of medium. In this way it is the same 

 from a stand-point of directly reading off our percentage reaction as if 

 we had 100 c.c. of medium and used N/i NaOH solution. The 

 A. P. H. Association recommends 5 c.c. of the medium and the use of 

 X/20 NaOH. As the N/io NaOH is always at hand for titrating 

 gastric juice, the N/io is used instead. 



* A more satisfactory method for one with experience in titrating is to continue 

 to add the N/io XaOH solution from the burette, drop by drop, until the addition 

 of a drop fails to show any intensifying of the purplish violet color at the spot 

 where it came in contact with the diluted bouillon in the beaker. This marks the 

 end reaction. A reaction of about +.7 in the cold gives a delicate pink. In titrating 

 at the boiling temperature (probably preferable for gelatine and agar) use a por- 

 celain dish. and boil for i or 2 minutes. Then add the sodium hydrate solution 

 from the burette until a faint pink develops. Then add about 4 additional drops 

 from the burette and the reading will be fairly accurate. 



