EXPERIMENT STATIOX REPORTS. 127 



PREPARATION OF A STANDARD SOLUTION OF LITMUS AND 

 THE MAKING OF LITMUS MEDIA. 



CHAELES W. BROWN. 



It is known to workers in bacteriology that it is extremely difficult, 

 if not impossible, to make two lots of media of the same composition, 

 and that litmus media are the most subject to variation. Milk which 

 forms a great part of our litmus media is variable in composition at 

 different times even when taken fresh from the same cow. It is the 

 general practice in making litmus media to take a solution of litmus, 

 weak or strong, and add it to the medium until the blue color is of 

 the desired intensity; the litmus solution itself, often composing ten 

 per cent or more of the medium. This method of preparing litmus 

 media is unsatisfactory, first, because it is almost impossible to 

 make two lots of media of the same intensity of blue; 

 second, because different samples of litmus do not contain 

 the same amount of the blue pigment and with solutions varying 

 in strength it is necessary to dilute one lot of media more than an- 

 other; and. third, because we do not wish to make the difference al- 

 ready existing in the media greater than can be helped by the addi- 

 tion of a stronger or weaker litmus solution, as it interferes materially 

 with the results of bacteriological experiments. In the face of this, 

 it appears to us that more uniform results could be obtained if some 

 way were found by which a standard litmus solution could be made. 

 By adding always the same percentage of the standard solution to 

 our media we would not dilute one lot more than another; again, the 

 finished media of different lots would have a uniform blue color and 

 would be more uniform in composition. 



Litmus is a mixture of a lichen dye obtained from the lichens 

 Roccella and Lecanora by allowing them to ferment after the addition 

 of ammonia and potassium carbonate. When the mass has assumed 

 a deep blue color, the liquid is pressed out, absorbed by chalk or 

 gypsum, and dried. Merck's purified litmus and the sample known 

 in our experiments as "Soluble Litmus" are made from commercial 

 litmus solution by freeing it from the red pigment orcin, which gives 

 commercial litmus the blue violet color, and drying without absorb- 

 ing it by means of chalk or gypsum. Azolitmin is a purified pigment 

 from litmus. 



With the idea that a standard solution of litmus could be prepared, 

 we carried on several experiments with samples of litmus from differ- 

 ent sources to determine : first, whether a solution of the same strength 

 could be prepared from the same sample by weighing out portions 

 separately and dissolving them in the same proportion of water ; second, 

 whether solutions of the same strength could be made from different sam- 

 ples; and, third, the relation between the solubility of different samples 

 and the relative intensity of their blue color. It was found by experi- 

 ments that anv number of solutions mav be made from the same sani- 

 pies by weighing out and dissolving in proportionate volumes of water, 



