STANDARDISING THE REACTION OF MEDIA. 45 



stated above, the deci- normal soda solution will be em- 

 ployed. The indicator must always be phenol-phthalein, 

 (.5 per cent in 50 per cent alcohol). Commencing 

 alkalinity the point to be aimed at is shown by the 

 appearance of a pink colour. 



It has been found that when a medium such as bouillon 

 reacts neutral to litmus, the addition on the average of 

 2.5 per cent of normal soda solution is necessary before it 

 reacts neutral to phenol-phthalein. Now as litmus was 

 originally introduced by Koch, and as nearly all bacterial 

 research has been done with media tested by litmus, it is 

 evidently difficult to say exactly what precise degree of 

 alkalinity is the optimum for bacterial growth, for it is 

 evident from what has been said that such precision is 

 only attained by the use of phenol-phthalein. It is prob- 

 ably safe to say, however, that when a medium has been 

 rendered neutral to this indicator by the addition of 

 NaOH, the optimum degree is attained by the addition 

 of 1.5 per cent normal HC1, the medium being then 

 slightly alkaline to litmus. In other words, the optimum 

 reaction for bacterial growth lies, as Fuller has pointed 

 out, about midway between the neutral point indicated 

 by phenol-phthalein and the neutral point indicated by 

 litmus. 



The only objection to the use of phenol-phthalein is 

 that its action is somewhat vitiated if free CO 2 be present. 

 This can be completely obviated as follows. Before test- 

 ing any medium it is boiled in the porcelain dish into 

 which titration takes place. The soda solutions are best 

 stored in bottles such as that shown in Fig. 40, having on 

 the air inlet a little bottle rilled with soda lime with tubes 

 fitted as in the large one. The CO 2 of the air which passes 

 through is thus removed. 



Method. The practical application of these principles 

 is as follows. Take the medium with all its constituents 

 dissolved and filter it. Place 5 c.c. in a porcelain dish, 

 add 50 c.c. of distilled water and i c.c. phenol-phthalein, 

 and boil. Run in deci- normal soda till neutral point is 



