PREPARATION OF MEDIA ii„.7 



side). The final reaction should be adjusted to pH 7.0. For precau- 

 tions in sterilization, see above, p. 1141-3. 



It is often desirable to put some indicator into such media. In select- 

 ing the proper indicator read the section below on Indicator Media. 



Sugar agar. As with sugar broth, beef-extract agar media of the 

 formula given on p. 5 may be made up with 1% of the required 

 carbohj^drate or related carbon-compound. The latter may be mixed 

 with the other ingredients only if it is known not to be appreciably 

 changed by the heat employed; otherwise it should be dissolved and 

 sterilized separately as above suggested. The reaction should be 

 adjusted to pH 7.0. An indicator may be added if desired. 



Indicator media. Carbohydrate media with some indicator to show 

 acid production are frequently of value. Litmus and Andrade's in- 

 dicator (acid fuchsin decolorized with alkali) are much used, but they 

 do not give accurate results in terms of hydrogen-ion concentration; 

 so, except for certain special purposes^, it is recommended that 

 sulphonphthalein indicators be employed. The indicators of most 

 value are: phenol red, brom thymol blue, brom cresol purple, brom 

 cresol green, and occasionally brom-chlor phenol blue. Their use is 

 governed by the following considerations:^ 



Phenol red indicates changes to the alkaline side of neutrality, as its 

 range is pH = 6.8-8.4. For use in indicator media it is best kept in a 

 1.6% alcoholic solution and 1 ml. of the solution added to 1 litre of 

 medium. 



Brgm thymol blue has a sensitive range extending slightly in either 

 direction from neutrality. It is useful in media carefully adjusted to 

 pH 7.0, but indicates such small changes in reaction as to be often 

 impractical. It is best added to media at the rate of 1 ml. of a 1.6% 

 alcoholic solution to the litre. 



Brom cresol purple indicates slightly greater changes to the acid 

 side of neutrality, as its range is pH = 5.2-6.8. For indicator media 

 1 ml. of a 1.6% alcoholic solution should be added to the litre. It 

 seems to be the most generally useful indicator for indicator media of 

 any at present available. It has, however, the defect of dichromatism. 

 If this is troublesome, it may be replaced by brom phenol red, which 

 covers the same pH-range. 



Combinations of brom cresol purple and cresol red are often 

 satisfactory when looking for changes in either direction from neu- 

 trality. When this combination is employed, the media should be 



*See next page; also Lea6et V, p. v^j-iO 

 'See also Leaflet IX. 



