APPENDIX 369 



INDICATORS 

 Phenolphthalein, indicator for titration: 



Phenolphthalein 0.5 gm. 



50% alcohol (neutral) 100.0 c.c. 



A drop of a weak solution of alkali should produce per- 

 manent pink color when added to a small amount of this 

 solution. Phenolphthalein is colorless in the presence of 

 acid. 



Kahlbaum's azolitmin solution: Dissolve 2.5 gms. of 

 Kahlbaum's azolitmin in 100 c.c. distilled water by heating 

 in steam for half an hour. Filter (this will filter much 

 more readily if allowed to settle for some time; decant 

 upon the filter). Sterilize by heating fifteen minutes each 

 day on three successive days. Sterilization is necessary, 

 otherwise molds and other microorganisms will grow on the 

 organic material present, often changing the reaction. 



A solution of litmus or azolitmin is often added to sugar 

 and other media before sterilization for the purpose of 

 detecting microorganisms which produce a change in the 

 reaction of the media. 



Litmus is a mixture of dyes obtained from the lichens 

 Roccella and Lecanom 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, often used in bacteriological 

 work, is made from commercial litmus solution by freeing 

 it from the, red pigment orcin, and drying without absorb- 

 ing it by means of chalk or gypsum. 



Azolitmin is a purified pigment from litmus. 



