ADJUSTMENT OF THE REACTION OF MEDIA 35 



degree of alkalinity is the optimum for bacterial growth. It is 

 probable 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. According to Fuller, the optimum reaction for bacterial 

 growth lies about midway between the neutral point indicated 

 by phenol-phthalein and the neutral point indicated by litmus. 



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. Eun 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 40a; = 

 amount necessary to neutralise a litre; and 40a- 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. In using these strong solutions care must be taken to 

 remove any fluid adhering to the outside of the pipette. When 

 the acid or alkali has been added the reaction of the medium 

 must be again taken before sterilisation. 



The present state of our knowledge of the principles involved in the 

 proper adjustment of the reaction of media is not satisfactory. 2 The 

 reaction of a medium depends on the hydrogen-ion concentration. The 

 precise hydrogen-ion content of media as they are ordinarily prepared, 

 either by the colorimetric or titration methods, has, however, not been 



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 iudicator 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. 



2 See Clark, Journ. Infect. Dis., Chicago, vol. xvii. (1915), p. 109. For 

 information regarding indicators, see Walpole, Biochemical Journal, vol. vii. 

 (1913), p. 260 ; vol. viii. (1914), p. 628. 



