STANDARD MEDIA 65 



amount of normal or deka- normal soda solution required 

 to neutralise a given volume of the acid beef-broth (e.g. a 

 litre) can be calculated, and this amount is then added. 

 Although neutral to phenolphthalein, the medium is now 

 strongly alkaline to litmus too alkaline for the optimum 

 growth of most organisms. The reason for this is that 

 the di-sodium hydrogen phosphate (Na 2 HP0 4 ) present in 

 the medium is alkaline to litmus but neutral to phenol- 

 phthalein. To reduce the alkalinity (to litmus) normal 

 hydrochloric acid is then added. The American Com- 

 mittee recommended an acidity of -f- 1-5 that is, to 

 every 100 c.c. of the medium neutral to phenolphthalein 

 1-5 c.c. of the normal hydrochloric acid are added. Eyre 

 advises a reaction of -{- 1-0 (i.e. 1 c.c. of normal hydro- 

 chloric to every 100 c.c.), while Chester considers that the 

 acidity should not exceed + 0-5. Whatever the reaction 

 adopted, it should be stated. Similarly, if a medium is 

 used which is alkaline to phenolphthalein, this is expressed 

 by the minus sign ; e.g. a reaction of 1-5 indicates that 

 to every 100 c.c. 1-5 c.c. of normal hydrochloric acid must 

 be added to render it neutral to phenolphthalein, or, what 

 is almost (but not quite) the same thing, that to the 

 neutral medium 1-5 c.c. of normal caustic soda solution 

 have been added to every 100 c.c. Various methods are 

 adopted to obtain the final reaction ; the American 

 Committee recommend first neutralising and then adding 

 sufficient acid (or alkali) ; Eyre, having calculated the 

 acidity, adds only sufficient alkali to reduce the reaction 

 to the required point. Eyre describes the reaction as that 

 represented by the number of c.c.s of normal alkali or 

 acid per litre, e.g. -f 10 on Eyre's scale is equivalent to 

 the American -f 1-0. In making nutrient broth, agar 

 and gelatin, the salt and peptone and agar or gelatin are 

 added and dissolved, and the titration and neutralisation 

 are carried out as described, on the fluid medium itself, 



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