160 CULTURE MEDIA 



deprive the medium of valuable nitrogenous food material 

 which is replaced by adding about 1 per cent by weight of 

 commercial peptone. It is usual also (though not always 

 necessary) to add about 0.5 per cent by weight of common 

 salt which helps to restore the proper concentration of min- 

 eral ingredients lost by the boiling. The chlorine is also 

 an essential element. The reaction is now determined and 

 adjusted to the desired end point, "standardized," as it is 

 called. The medium is again thoroughly boiled and filtered 

 boiling hot. The adjusting of the reaction and the boiling 

 ordinarily cause a precipitate to form which is largely phos- 

 phates of the alkaline earths with some protein. The 

 filtered medium is collected in suitable containers, flasks or 

 tubes, which are plugged with well-fitting non-absorbent 

 cotton plugs and sterilized, best in the autoclave for twenty 

 minutes at 15 pounds pressure, or discontinuously in stream- 

 ing steam at 100°. If careful attention is paid to standardi- 

 zation and to sufficient boiling where indicated, the meat 

 broth prepared as above should be clear, only faintly yellowish 

 in color and show no precipitate on cooling. 



The conventional method for standardizing an acid me- 

 dium is as follows: Take 5 cc of the medium, add 45 cc of 

 distilled water and 1 cc of phenolphthalein as indicator. 

 Boil the solution and while still hot run in from a burette 

 N/20 NaOH solution until a faint pink color appears. 

 From the number of cc of N/20 NaOH used to ''neutral- 

 ize" the 5 cc of medium it is calculated how many cc of 

 N/1 NaOH are necessary to give the desired end reaction 

 to the volume of medium which is to be standardized. The 

 resulting reaction is expressed as per cent acid or alkaline to 

 phenolphthalein. If it is necessary to add to each 100 cc of 

 the medium 1 cc of N/1 NaOH to make it neutral to 

 phenolphthalein the reaction is called 1 per cent acid; if to 

 each 100 cc of medium there is added 1 cc of N/1 alkali in 

 addition to the quantity necessary to neutralize the reaction 

 is called 1 per cent alkaline. Objections to this method of 

 standardization will be given later. 



