MEASUREMENT OF H-ION CONCENTRATION 455 



such as described by Hurwitz, Myer and Ostenberg (1916) modi- 

 fied to suit our purpose, was found indispensable. 



The block is made by boring three rows of three holes in each 

 row. The holes are 2 cm. in diameter and 9 cm. deep. The block 

 measures 13 cm. high by 9 by 10 cm. Each row of holes is con- 

 nected by a slit that goes all the way through, 2.5 cm. high by 

 1.25 cm. wide, the upper edge of the slit being 4 cm. from top of 

 block. The block is painted with black enamel paint including 

 the slits. It is best not to paint the holes that hold the test 

 tubes (figs. 2 and 3). 



ii. pH measurement of acid or alkali production by 

 bacteria, by the use of " color standards" 



A wider range is necessary to measure the acid and alkali 

 production by bacteria than that used for the standardization of 

 culture media. "Color standards" prepared from 0.02 per cent 

 solutions of each of the following indicators: Thymol-blue-acid- 

 range; brom-phenol-blue; methyl-red; brom-cresol-purple ; cresol- 

 red; and thymol-blue-alkaline-range; in addition to brom-thymol- 

 blue. and phenol-red, used for titration of culture media, already 

 described, will give a range extending from pH 1.4 to pH 9.6 

 more than sufficient to test the acid as well as the alkali produc- 

 tion by bacteria. 



The methods for the preparation of "color standards" from 

 these additional indicators are similar to those described in part 

 I. The indicators are used in 0.02 per cent watery solutions pre- 

 pared from 0.2 per cent stock alcoholic solutions (phenol-red 0.04 

 per cent). 



The solutions necessary to bring out the extreme acid color, and 

 the extreme alkaline color of the various indicators are as follows. 



Thymol-blue acid range. The extreme acid (red) color of this 

 indicator was brought out by 0.5 per cent HC1; and the extreme 

 alkaline (yellow) of this indicator was brought out with 0.001 

 per cent HC1 (1 cc. of 0.1 per cent to 99 cc. of distilled water). 

 Various other strengths of both acid and alkaline solutions were 

 tried, but failed. 



