MEASUREMENT OF H-ION CONCENTRATION 449 



set up in a rack, the tubes containing the acid solution in a row, 

 behind which is another row of seven tubes containing the alka- 

 line solution (fig. 1). Brom thymol blue, 0.02 per cent watery 

 solution, is added to the tubes with NaOH in amounts of 0.1 

 cc. increasing by 0.1 cc. up to 0.7 cc. from left to right, while in 

 the acid tubes the indicator is added beginning with 0.7 cc. de- 

 creasing by 0.1 cc. to 0.1 cc. from left to right, respectively: so 

 that each pair of tubes contains together 0.8 cc. of the indicator, 

 the amount increasing by 0.1 cc. in the tubes with the alkaline 

 solution, while it decreases by 0.1 cc. in the acid solution. The 

 tubes are then labelled in pairs from 1 to 7, the label also bearing 

 the name of the indicator and the pH. The pH of the first pair 

 is always 0.2 higher (more alkaline) than the initial range of the 

 particular indicator. Brom thymol blue, according to my findings, 

 begins to show a change of color at pH 6.2 (no difference in color 

 was found with this indicator between pH 6, and 6.2, but there 

 is a difference between 6.2 and 6.4). The first pair of this indi- 

 cator will accordingly be pH 6.4, each succeeding pair will in- 

 crease by pH 0.2 up to the seventh pair which will be pH 7.6. 

 Therefore the seven pairs with brom thymol blue yield a range 

 of "standard colors" of pH 6.4 to pH 7.6, at an interval of pH 

 0.2, most valuable for titration of culture media since neutrality 

 is at pH 7, which with this indicator is at pair no. 4 yielding a 

 green color — not yellowish green (pair no. 3) nor bluish green 

 (pair no. 5). If a wider range is desired (rarely necessary) a 

 set of "standard colors" with the phenol red can be prepared in 

 the same way as given for brom thymol blue, i.e., the same strengt h 

 of NaOH and HC1 is used, the only difference being the change 

 of the indicator to phenol red. The seven pairs of tubes of this 

 indicator will yield a range from pH 7 to pH 8.2 overlapping at 

 pH 7, 7.2, 7.4, and 7.6. The phenol red does not make as clear 

 cut a range, as easily differentiated at pH 0.2 intervals, as does 

 brom thymol blue and is not really necessary for the titration of 

 culture media which as a rule are neutral, or thereabouts. Figure 

 1 shows the way the test tubes are set up for the preparation of 

 the " color standards." Tables 3 and 4 give the composition of 

 each pair for the two indicators, the color change of each, as 

 nearly as it can be described, and the pH value of each pair. 



JOURNAL OF UACTEIUOLOUY. VOL. V, NO. 5 



