THE PREPARATION OF CULTURE MEDIA 147 



with tautomeric rearrangements of the original substances, and the 

 state of these substances in a dissociated or undissociated form 

 determines the color. The degree of dissociation as determined by 

 the hydrogen ion concentration brings about a predominance of one 

 or the other tautomeric compound, and the color depends upon 

 which one of these compounds is associated with the color. Chapter 

 three of Clark's book goes into this matter with sufficient thorough- 

 ness for bacteriological work. 



In cultural work not many indicators are needed. For the actual 

 titration of media, the necessary indicators are given in another 

 section. For actual addition to culture media, we may restrict our- 

 selves to a limited number of useful indicators. 



Litmus is the indicator most commonly used in former years, is 

 a product obtained from a species of lichen, and is obtained by 

 oxidation of the orcin contained in this plant. Asolitmin is the 

 indicator substance in the litmus, chemically complex and not com- 

 pletely analyzed. 



The litmus solutions used in the preparation of media are best 

 made up as follows : Litmus in substance Merck 's purified, or Kaul- 

 baum's is dissolved in water to the extent of 5 per cent. The 

 solution is made by heating in an Arnold sterilizer for about one 

 to two hours, shaking occasionally. The solution is then filtered 

 through paper and sterilized. It should be kept sterile, as molds 

 will grow in it otherwise. 



A standard litmus solution, which is marketed for laboratory 

 purposes, known as "Kubel and Tiemann's" solution, may be used. 



Andrade indicator is made up as follows: 



0.5 per cent aqueous acid fuchsin 100 c.c. 



Normal NaCH 16 c.c. 



The red color fades out gradually ; the indicator should be yellow 

 after standing two or three hours. If it remains red or reddish, 

 1 c.c. more of normal NaOH may be added. 



Acid production turns this indicator red, due to the neutralization 

 of the alkali present and the liberation of the acid color base. The 

 medium in which Andrade indicator is used, must be adjusted to 

 the neutral point of this indicator which is equivalent to P^ 7.2. 

 The reaction of the medium is right for this indicator, when the 

 medium containing 1 per cent of the indicator is red when hot, and 

 colorless when cold. 



