MEASUREMENT OF H-ION CONCENTRATION 467 



otherwise the procedure is exactly the same. It is best to have 

 two different cultures side by side, one for the acid production 

 with its respective indicator (brom-cresol-purple), and another 

 culture for the alkali production with its respective indicator 

 (cresol-red) rather than to use the two indicators in the same 

 culture. The acid and alkali production by the same organism 

 can thus be easily read off without the possibility of the color of 

 either indicator offsetting the other. 



III. pH MEASUREMENT OF THE H. I. C. OF OTHER FLUIDS AND 

 SOLUTIONS BY MEANS OF " COLOR STANDARDS" 



For measuring the H-ion concentration of other fluids and solu- 

 tions, acid or alkaline, such as urine, blood serum, etc.. the same 

 procedure described under parts I and II may be followed. The 

 color standards and the methods for their preparation will be 

 found described under parts I and II and in the accompanying 

 tables. It may be desirable to describe in detail the testing of 

 urine as a guide to the testing of other fluids. 



Procedure 



Three test tubes of the same calibre as those used for the "color 

 standards" are each filled with 10 cc. amounts of the urine to be 

 tested, previously filtered. Then 0.8 cc. of the 0.02 per cent solu- 

 tion of the proper* indicator is added to one of the three tubes with 

 the urine. The test tube is rolled between the palms of the hands 

 until well mixed. This tube of urine containing the indicator 

 is placed in the center hole of the "comparator block" with two 

 test tubes filled with distilled water behind it. The proper pairs 

 of color standards are placed, each pair on either side of the urine 

 to be tested, while in front of each pair of color standards, one 

 of the tubes of urine without indicator is placed in the comparator 

 block to offset the color of the urine. The pH represented by the 

 pair of color standard tubes that matches in color that of the 

 mine with the indicator is the pH of the urine. 



4 Some idea as to which is the proper indicator to use may be obtained by using 

 litmus paper. If highly acid to litmus, brom-phenol-blue is used; if slightly acid, 

 methyl-red or brom-cresol-purple is used. If alkaline, brom-thymol-blue or 

 cresol-red should be used. The pH is thus very easily read off. 



