MEASUREMENT OF H-ION CONCENTRATION 447 



writer to start with pH 6.2 instead of pH 6 as given by Clark 

 and Lubs (1917). Various difficulties had to be overcome in 

 rinding the proper acid or alkaline solution that would bring out 

 the extreme colors that would remain fairly permanent. Fi- 

 nally, however, these difficulties were overcome and the proper 

 strength of the indicators was found, that would best serve this 

 purpose. 



A method of obtaining the indicators in sterile solutions so 

 that they could be kept indefinitely, and used for testing acid 

 formation by bacteria in cultures without contaminating them 

 was also devised. 



Preparation of indicator solutions 



a. Stock alcoholic solutions. Two-tenths per cent alcoholic 

 solutions of the indicators were prepared as " stock solutions" by 

 dissolving 0.1 gram of the respective indicators in powder form in 

 50 cc. of 95 per cent ethyl alcohol (ordinary alcohol), and kept in 

 amber colored bottles, well stoppered with rubber stoppers and 

 paraffined. The following indicators of Clark and Lubs were 

 used, all of which were obtained in powder form from Hynson 

 Westcott and Dunning excepting methyl red which was obtained 

 at the Institute. Range and color changes are as given by Clark 

 and Lubs (1917). 



Indicators (Clark and Lubs) 



1. Thymol blue acid range Red-yellow pH 1.2-2.8 



2. Brom phenol blue Yellow-blue pH 3.0-4.6 



3. Methyl red Red-yellow pH 4.4-6.0 



4. Brom cresol purple Yellow-purple pH 5.2-6.8 



*5. Brom thymol blue Yellow-blue pH 6.0-7.6* 



6. Phenol red Yellow-red pH 6.8-8.4 



7. Cresol red Yellow-red pH 7.2-8.8 



8. Thymol blue alkaline range Yellow-blue pH 8.0-9.6 



* The range of color of this indicator as found by the writer should read 

 6.2-7.8. 



These "stock" alcoholic solutions are best kept in a dark place 

 — closed cupboard, or box. 



