126 LABORATORY BACTERIOLOGY 



addition of alkali- is a very faint darkening of the fluid, which on 

 the addition of more alkali becomes a more evident color, and 

 develops into what may be described as an Italian pink. A':stiU 

 further addition of alkali suddenly develops a clear and bright pink 

 color, and this is the reaction always to be obtained. 



All titrations should be made quickly and in hot solutions, to 

 avoid complications arising from the presence of carbon dioxid. 



When this manipulation is carried out uniformly, as here sug- 

 gested, and the end point having the same intensity of color is 

 always reached, very satisfactory and closely-agreeing results may 

 be obtained. 



Neutralization of Media. — The next step in the process is to add 

 to the bulk of the medium the calculated amount of reagent, either 

 alkali or acid, as may be determined. . For the purpose of neutraliza- 

 tion a normal solution of sodium hydroxid or of hydric chlorid is 

 used, and after being thoroughly stirred the fluid thus neutralized is 

 again tested in the same manner as at first to insure the proper 

 reaction of the medium being attained. When neutralization is to 

 be effected by the addition of alkali, it not infrequently happens that 

 after the calculated amount of normal solution of sodium hydroxid 

 has been added the second test by titration will show that the 

 medium is still acid to phenolphthalein, to the extent sometimes of 

 from o.j to I %. This discrepancy is perhaps due to side reactions, 

 which are not understood; the reaction of the medium, however, 

 must be brought to the desired point by the further addition of 

 sodium hydroxid, and the titrations and additions of alkali must be 

 repeated until the medium has the desired reaction {i.e. o.o \ — 

 0.005 %) see below). 



After the prescribed period of heating it is frequently found that 

 the medium is again slightly acid, usually about c; %. Without 

 correcting this the fluid is to be filtered and the calculated amount 

 of acid or alkali is to be added to change the reaction to the one 

 desired. ' 



A still fiirther change in reaction is not infrequently to be observed 

 after sterilization, the degree of acidity varying apparently with the 

 composition of the media and the degree and continuance of the 

 heat. 



Manner of expressing the Degree of Reaction of Culture Media. — 

 Since at the time the reaction is first determined culture media 



