210 BERTHA VAN H. ANTHONY AND C. V. EKROTH 



The directions are as follows: 



Phenolphthalein shall be the standard indicator in obtaining reac- 

 tion of all media. Tumeric paper possesses similar properties and its 

 use advised where phenolphthalein is not available. Titrations and 

 adjustments of reactions shall be made as follows: 



Put 5 cc. of media to be titrated in 45 cc. of distilled water. Boil 

 briskly one minute. Add 1 cc. phenolphthalein solution (5 grams of 

 commercial salt in one liter of 50 per cent alcohol.) Titrate while 

 hot (preferably while boiling) with ^ caustic soda. A faint but dis- 

 tinct pink marks the true end point. This distinct pink color may be 

 described as a combination of 25 per cent of red (wave length approxi- 

 mately 658) with 75 per cent of white as shown by the disks of the color 

 top, described under Records of Tints and Shades of Color, p. 10. 

 (The Standard^ color disks used in teaching optics may be used for 

 this purpose.) 



In practice, titration is continued until the pink color of alkaline 

 phenolphthalein matches that of the fused disks. All reactions shall 

 be expressed with reference to the phenolphthalein neutral point and 

 be expressed in percentages of normal acid or alkaline solutions re- 

 quired to neutralize them. 



One of the objects of this paper is to consider whether the 

 desired results are actually obtained by the Standard method 

 or by modifications of this method. 



In our laboratory one modification, that is, titration of broth 

 at room temperature (about 20°C.) and of agar at a temperature 

 of about 30°C. has given good results for a number of years past. 



These and other experiences have led us to investigate further 

 the following subjects: 



a. The effect of prolonged heating on meat infusions and 

 beef extract solutions as shown by the titration curves of both 

 adjusted and unadjusted portions. The results of boiling 

 samples of media in the casserole for titration. 



b. The adjustment of broth and agar, including the remelt- 

 ing of solid media. 



* A small sized top and disks costing only a few cents may be obtained from 

 Milton Bradley Educational Company, Springfield, Mass. 



