MEASUREMENT OF H-ION CONCENTRATION 459 



The detailed composition of the pairs of tubes comprising the 

 "color standards" of each indicator, as well as its equivalent in 

 pH as found by calibration with standard solutions of known H. I. 

 C. prepared according to Clark and Lubs (1917) will be found on 

 tables 5, 6, 7, 8, 9, and 10; and part I, tables 3 and 4. The color 

 of each pair of tubes corresponding to the particular pH is de- 

 scribed as nearly as possible. 



Procedure for measuring acid production by bacteria 



In order to establish the best method to determine the acid 

 production by bacteria quantitatively it was thought desirable 

 to test out the various indicators as to their ability fa) to stand 

 heating while being sterilized; (b) as to the action produced upon 

 such indicators by bacteria. For that purpose the following 

 experiments were undertaken: 



Test tubes similar in size to those used for the "color standards" 

 were filled each with 10 cc. amounts of glucose and of lactose 

 bouillon (pH 7) using a 10 cc. automatic filler (fig. 4). These 

 were divided into three sets of sixteen tubes each. To one set 

 of the glucose and one set of the lactose bouillon 0.8 cc. amounts 

 of the respective indicators were added to each tube before ster- 

 ilizing. To another set, the same amount of the respective indi- 

 cators was added after sterilizing, and the third set was used as 

 a control — the indicators being added after inoculation with Bad. 

 coli and incubation for twenty-four hours. Sterilization of the 

 three sets was done at the same time by the fractional method in 

 the Arnold sterilizer, on three successive days at 100°C. for three- 

 quarters of an hour at a time. 



Thus the first set was used to test the ability of the different 

 indicators to stand heat. The second set was used to test the 

 ability of the different indicators to resist the action of the 

 bacteria; and the third set as a control of the first two sets. 

 Table 11 gives the results of these tests in detail. 



Action of heat upon the indicators used 



A study of table 1 1 will show that after sterilization, seven of 

 the indicators : thymol-blue-acid range ; brom-phenol-blue ; brom- 

 cresol purple; brom-thymol-blue; phenol-red; cresol-red; and 

 thymol-blue-alkaline range; did not change by sterilization in the 



