CULTURE MEDIA 87 



added, namely 14.7 to 4.9 c.c., according to the purpose for which 

 the broth is to be used. 



The amount of normal alkali finally decided upon is added to 

 the broth, which is then weighed in its pan. It is then cooked 

 by boiling over the direct flame for half an hour or by heating 

 in the autoclave at 110 C. for 15 to 20 minutes. It is now 

 cooled to about 50 C., filtered through paper, filled into tubes 

 and sterilized, either in the autoclave at 110 C. for 15 minutes 

 or by fractional sterilization in streaming steam at 100 C. for 

 15 minutes on three consecutive days. 



Broth may be prepared from meat extract instead of meat. 

 Meat extract 3 grams, peptone 10 grams and salt 5 grams are 

 dissolved in 1000 c.c. of water, boiled, filtered and titrated against 



N 



sodium hydroxide. The subsequent steps are the same as in 



preparation of broth from fresh meat. 



Remarks upon Titration. The titration of bacteriological 

 media made from meat or meat extract is an important step in 

 their preparation. There is some confusion on this point because 

 of the use of different indicators in ascertaining the reaction. 

 The neutral point indicated 1 by litmus is very nearly the actual 

 neutral point in respect to acidity and alkalinity, and this point 

 is not appreciably displaced in either direction by the addition 

 of a neutral mixture of a feebly dissociated acid and its salts to 

 the solution. The end reaction indicated by phenolphthalein 

 when it turns pink is actually a point at which there is a slight 

 excess of alkali. This is so nearly the actual neutral point in 

 inorganic solutions, when electrolytic dissociation is marked, 

 that the error is not appreciable. In solutions of organic sub- 

 stances, especially when considerable amounts of feebly dissoci- 

 ated substances, such as are contained in peptone or gelatin, are 

 present, this error becomes very appreciable. The discrepancy 

 between the end point for litmus and for phenolphthalein will 



1 Washburn, E. W., The significance of the term alkalinity in water analysis and 

 the determination of alkalinity by means of indicators. Report Illinois Waterworks 

 Association, 191 1. 



