STANDARDISING THE REACTION OF MEDIA 35 



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-phthalein by the addition of NaOH, 

 the optimum degree is generally attained by the addition of 

 from 10 to 15 c.c. of normal HC1 per litre, i.e., the optimum 

 reaction is from + 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- 

 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 obviated by boiling any medium, before it is tested, 

 in the porcelain dish into which titration takes place. The soda 

 solutions are best stored in bottles such as that shown in Fig. 42, 

 having on the air inlet a little bottle filled with soda lime and 

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

 passes through is thus removed. 



Method. The following procedure includes most of the 

 improvements introduced by Eyre. The medium with all its 

 constituents dissolved is filtered and then heated for about forty- 

 five minutes in the steamer, the maximum acidity being reached 

 after this time. Of the warm medium take 25 c.c. and put in 

 a porcelain dish, add 25 c.c. distilled water, and 1 c.c. phenol- 

 phthalein solution. Run in decinormal soda till neutral point 

 is reached, indicated by the first trace of pink colour, the 

 mixture being kept hot. 1 Repeat process thrice, and take the 

 mean; this divided by 10 will give the amount (x) of normal 

 soda required to neutralise 25 c.c. of medium; then 40# = 

 amount necessary to neutralise a litre; and 40#- 10 = amount 

 of normal soda necessary to give a litre its optimum reaction. 

 Then measure the amount of medium to be dealt with, and add 

 the requisite amount of soda solution. 



Eyre uses a soda solution of ten times normal strength, which 

 is delivered out of a 1 c.c. pipette divided into hundredths ; this 

 obviates, to a large extent, the error introduced by increasing 

 the bulk of the medium if a weaker neutralising solution be 

 used. 



1 (b). Glucose Broth. To the other constituents of 1 (a) 



1 The beginner may find considerable difficulty in recognising the first 

 tint of pink in the yellow bouillon. A good way of getting over this is to 

 take two samples of the medium, adding the indicator to one only ; then 

 to run the soda into these from separate burettes ; for each few drops run 

 into the medium containing the indicator the same amount is run into the 

 other. Thus the recognition of the first permanent change in tint will be at 

 once recognised by comparing the two samples. 



