114 BACTERIOLOGY 



is not a stable compound, as its quality and sensitiveness 

 is subject to variations, it is deemed wisest to rely, in this 

 method upon phenolphthalein, its high neutral point being 

 easily adjusted by the necessary correction. 



The method of titration as recommended by Fuller 1 is 

 essentially as follows: 5 c.c. of the culture medium are to 

 be mixed with 45 c.c. of distilled water in a porcelain evapo- 

 rating dish or casserole and boiled for three minutes, after 

 which 1 c.c. of a phenolphthalein solution (0.5 per cent, in 

 50 per cent, alcohol) is added and titration with $-$ KOH is 

 quickly made. The neutral point, slightly to the alkaline 

 side, is announced by the assumption of a permanent, pale 

 pink color, the effect of the free alkali on the indicator. 

 From the volume of ^ KOH needed to neutralize the 5 c.c. 

 of culture medium, one can readily calculate the amount 

 needed for the whole mass, the volume of which must, of 

 course, be know r n. For the neutralization of the entire mass 

 one uses not $$ KOH but y KOH, that is, an alkaline solu- 

 tion twenty times as strong as that with which the titration 

 is made and therefore of only ^V the volume. 



To illustrate : If to neutralize 5 c.c. of a nutrient gelatin 

 5.5 c.c. of ^r KOH are required, and the original mass 

 represented by the 5 c.c. is a liter in volume, manifestly 

 the whole mass would require two hundred times 5.5 c.c. 

 or 1100 c.c. of the alkaline solution, a volume much too 

 great to add to the gelatin because of the extreme dilution 

 that would result, we therefore substitute for the YTT KOH 

 in the final correction the normal KOH solution ( f KOH) ; 

 which being twenty times as strong will necessitate the 

 addition of only -^V of the volume, that is ^y|p = 55 c.c. 

 of f KOH for the liter. 



1 Jour. Am. Pub. Health Assn., Quarterly Series, 1895, p. 38L 



