22 GENERAL MICROBIOLOGY 



Acidity of the medium 



(+34) 3.4 c.c. per 100 c.c. of the medium 



Desired acidity (+15) . . 1.5 c.c. per 100 c.c. of the medium 

 Amount of normal alkali - 



to be added 1.9 c.c. per 100 c.c. of the medium 



or 10X1.9 c.c. = 19 c.c. N/l NaOH per 1000 c.c. of medium 



Since normal solutions are of equal strength by volume, 

 that is, 1 c.c. of N/l acid will just neutralize 1 c.c. of N/l 

 alkali, it will readily be seen that if 15 c.c. N/l NaOH are 

 required to neutralize the acid present in 1 liter of medium, 

 then there must be present in that liter exactly 15 c.c. of 

 N/l acid, or we should say the reaction is ( + 15) fifteen 

 degrees acid. For any other degree of acidity add enough 

 normal alkali to reduce the acidity to the point desired. 



The reaction of a medium changes somewhat after its 

 neutralization, especially during sterilization, but also upon 

 standing afterward at ordinary temperature. - This change 

 is toward an increased acidity, and is most marked in 

 media rich in dextrose. Consequently it is necessary to 

 determine the titre of a medium at the time it is used rather 

 than to quote figures obtained before sterilization. 



MILK, CIDER, VINEGAR, WORT, AND FRUIT JUICES 



Procedure. 1. Into an evaporating dish measure 5 c.c. 

 of the medium to be tested, by means of suitable pipette. 

 Make up to 50 c.c. with distilled water. 



Do not heat. The above media should not be heated 

 before titration, as they contain volatile acids or other organic 

 substances which may register as acid and which may be 

 driven off by boiling, 



2. Add 1 c.c. phenolphthalein solution. 



3. Add, gradually, from an accurate burette, IN/20 

 NaOH until the first permanent pink appears. 



4. Note the amount of NaOH required for the titration. 



5. Always run duplicates. 



