1 8 CULTURE MEDIA 



ature of 50 C. to facilitate the solution of the peptone. This is not necessary, and 

 if the temperature is not watched closely it might go up to 65 C. or higher and we 

 should lose the clearing albuminous material from its coagulation. Of this rather 

 cloudy solution take up 10 c.c. with a pipette and let it run out into a porcelain 

 dish. Add 40 c.c. of distilled or rain water and about six drops of a 0.5% phenol- 

 phthalein solution. (Phenolphthalein, o. 5 ; dilute alcohol, TOO c. c. ) Bring the con- 

 tents of the porcelain dish to a boil and continue boiling for one or two minutes in 

 order to expel all CC>2. Now from a burette filled with decinormal sodium hydrate 

 solution, run in this solution until we have the development of a faint but distinct 

 pink in the boiling diluted bouillon which is not dissipated on further boiling. 



It is more satisfactory to take burner from beneath the porcelain dish just before 

 running in the N/io solution, again boiling so soon as a pink color is obtained. 

 Having obtained the light pink coloration we read off the number of c.c. or frac- 

 tions 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 11/2 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 correspond to the addition of 1/2 c.c. of N/i 

 NaOH to 100 c.c. of the medium at o. It is written 0.5. 



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 understood that if we had to add 

 3 1/2 c.c. of N/i 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/i NaOH solution is too corrosive for general use in a burette, and as io 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 standpoint 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 N/2O 

 NaOH. As the N/io NaOH is always at hand for titrating gastric juice, the N/io 

 is used instead. 



Should it be found difficult to carry on the titration while boiling the end reaction 

 may be fairly accurately determined in the cold. Deliver into a beaker from a 

 pipette io c.c. of the bouillon and make up to 50 c.c. with distilled water and add 

 5 drops of o. 5% phenolphthalein solution. Then run in N/io NaOH from a burette 

 and continue to add the N/io NaOH 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 +o. 7 in the cold gives a 

 delicate pink with phenolphthalein as an indicator. Titration in the cold is not very 

 satisfactory with gelatin and agar. 



