6 EXPERIMENTAL DAIRY BACTERIOLOGY 



is stated in terms of the quantity of normal acid or alkali 

 required to neutralize the medium. A medium having an 

 acidity which would require the addition of 10 cc. of normal 

 alkali to neutralize 1000 cc. is regarded as 1 per cent acid, 

 and this condition is expressed as + 1 per cent. If alkaline, 

 so that 5 cc. of normal acid is required to render 1000 cc. 

 neutral to phenolphthalein, the reaction is expressed as 0.5 

 per cent. In milk and also in the usual culture media the 

 neutral point of phenolphthalein is not identical with that of 

 litmus, due to the different behavior of these indicators with 

 acid salts. The standard reaction of culture media for general 

 use is + 1 per cent. 



Neutralization of media. The following reagents will be 

 required: twentieth normal sodium hydrate (N/20 NaOH), 

 normal sodium hydrate (N/l NaOH), normal hydrochloric 

 acid (N/l HC1), and an alcoholic solution of phenolphthalein 

 (5 grams of the compound in 1000 cc. of 50 per cent alcohol) 

 to be used as the indicator. The phenolphthalein should be 

 dissolved in the alcohol and dilute (N/20) sodium hydrate 

 solution added until a very faint pink color is noted. 



With a pipette place 5 cc. of the medium to be tested in a 

 small flask or evaporating dish, add 45 cc. of distilled water, 

 and boil one minute to drive off C0 2 , which affects the indi- 

 cator. Add 5 drops of the indicator and titrate while hot with 

 the N/20 NaOH. A faint but distinct pink color marks the 

 true end point. Each 0.1 cc. of N/20 NaOH used for 5 cc. 

 of medium is equal to 0.1 per cent acid; thus a medium 

 having an acidity of + 1 per cent should require 1 cc. of 

 N/20 NaOH for 5 cc. of medium. The final reaction should 

 not differ from that desired by more than 0.2 per cent, i.e. 

 the reaction should be between -f 0.8 and -f 1.2 per cent in 

 case + 1 per cent is desired. 



