TITRATION AND ADJUSTMENT OF CULTUEE MEDIA 223 



any further boiling of the large lot, together with the final sterili- 

 zation raises the acidity but not to just the desired point as 

 shown in the tests on meat infusions (chart 6). For example, 

 the final reaction of + 1.2 in the case of diphtheria toxin broth, 

 is found to be uniformly obtained by titrating at room temper- 

 ature and setting the reaction to + 1. This allows 0.2 rise due 

 to heating if the broth is to be sterihzed at 15 pounds pressure 

 (121. 6°C.) for one half hour. If the sterilization is to be carried 

 on at only 5 pounds pressure (108.8°C.) for one hour on three 

 successive days, or in the Arnold sterilizer, streaming steam 

 (100°C.), for the same length of time, the reaction is set at + 1.1 

 as the more moderate heating raises the broth only about 0.1 

 per cent in acidity, making the finished product + 1.2 in reac- 

 tion. The use of this method for the diphtheria and tetanus 

 toxin broths for a number of years has shown fully its value and 

 it is the method still employed in our laboratory. 



On the other hand Eyre (1915) states that the correct esti- 

 mation of acidity present can be made only by titration at the 

 boiling point. Judged from our results as shown in chart 4 

 by the /3' and <S curves, this statement is erroneous. The ^' 

 curve shows the reactions and successive adjustments to plus 

 one (+1) based upon the boiling temperature figures. The re- 

 actions at room temperature of this material are shown by the 

 /3 curve. This latter is far below the /3' curve (from 0.5 to 1 

 per cent), and gives the actual reaction of the medium at a tem- 

 perature nearer that of the incubator (37.5°C.)^^ 



To make this point clear, let us assume for example that a 

 medium is to be adjusted to a definite acidity of + 1, accord- 

 ing to the boiling titration, as stated in the Standard Method. 

 The real reaction at which the bacteria will then be grown in 

 the incubator is not that indicated by the boiUng titration 

 figure but a reaction which is lower in acidity to an extent of 

 about one per cent — in other words, almost neutral. 



On the other hand in chart 3 (a curve), there is shown a simi- 

 lar meat infusion, adjusted to plus (+ 1) at room temperature 



i""' It is only after four hours' autoclaving that the room temperature reaction 

 of plus one (+1) is reached. 



