STANDARDISING REACTION OF MEDIA. 37 



to make a litre of the medium neutral to phenol-phthaleine. 

 Thus, for example, " reaction = —15," will mean that the me- 

 dium is alkaline, and requires 15 c.c. of normal HCl to make a 

 litre neutral. It has been found that when a medium such as 

 bouillon reacts neutral to litmus, its reaction to phenol-phtha- 

 leine, according to the above standard, is on the average +25. 

 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. It is probably 

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

 neutral to phenol-phthaleine by the addition of NaOH, the 

 optimum degree is attained by the addition of from 10 to 15 c.c. 

 of normal HCl per litre, i.e. the optimum reaction is -H 10 to +15. 

 In other words, the optimum reaction for bacterial growth lies, 

 as Fuller has pointed out, about midway between the neutral 

 point indicated by phenol-phthaleine and the neutral point in- 

 dicated by litmus. 1 



The only objection to the use of phenol-phthaleine is that its. 

 action is somewhat vitiated if free CO2 be present. This can 

 be completely obviated as follows. Before testing 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. 52, having on the air inlet a little bottle filled with soda 

 lime with tubes fitted as in the large one. The COg 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 45 c.c. of dis- 

 tilled water and i c.c. phenol-phthaleine, and boil. Run in deci- 

 normal soda till neutral point is reached. Repeat process thrice 

 and take the mean to obtain amount of soda required. From 

 this calculate acidity of medium per litre, and neutralise with 

 normal soda solution. Check calculation by a fresh titration of 

 5 c.c. of the neutralised medium. Steam for half an hour, and 

 take reaction again to see that it is constant. Now add normal 

 HCl in the ratio of 1.5 c.c. per cent. 



1 For the majority of pathogenic bacteria, Fuller's optimum reaction is still 

 too acid. It will be found that an acid reaction of .5-. 3 is much more satis- 

 factory. 



