INDICATORS AND THEIR EMPLOYMENT 123 



ing alkali tubes, 1, 2, 3, 4, 5, 6 drops of the indicator solution 

 are placed. (If the dropper be held vertically all drops will 

 be of practically the same volume.) 



Each pair of tubes thus contain 10 drops of the indicator 

 solution between them, and the series of six pairs, when 

 viewed by transmitted light, will correspond to pH values 

 shown in the foregoing table. 



In order to determine the hydrogen-ion concentration of 

 an unknown solution whose reaction lies within the ranges 

 pH6.9 and pHS.l, 5 c.c. of it are placed in a test-tube, 

 10 drops of the indicator solution are added, and its color 

 is compared with those of the six pairs of tubes; its H.I.C. 

 being the same as that pair with which its color corresponds. 



When this unknown solution is a bacterial culture medium 

 the procedure is as follows: 1 c.c. of the medium to be 

 titrated is added to 4 c.c. of distilled water in a test-tube. 

 Ten drops of the indicator solution are then added and 

 the initial reaction is determined by comparing the color of 

 the mixture with each of the six pairs of standard colors 

 already prepared. From this initial reaction, usually too 

 acid, the desired reaction is obtained by titration with the 

 |^ NaOH solution. When the desired reaction for 1 c.c. of 

 the medium is thus obtained, fifty times that figure in y 

 NaOH (not <nr NaOH) will be needed for a liter of the 

 medium. 



If in making the titrations the volume of fluid in the 

 unknown solution is greatly increased by the titrate, then 

 the volumes in the standard solutions must be correspond- 

 ingly increased by the addition of distilled water. 1 



1 For details see, Clark and Lubs, Jour. Bacteriol., 1917, No. 2, vol. ii, p. 

 109. Barnett and Chapman, Jour. Am. Med. Assn., 1918, No. 15, vol. Ixx, p. 

 1062. Report of Committee on Descriptive Chart, Soc. Am. Bacteriologists, 

 Jour. Bacteriol., 1919, No. 2, vol. iv, p. 119. Medalia, Jour. Bacteriol., 

 1920, No. 5, vol. v, p. 441. 



