114 BACTERIOLOGICAL LABORATORY TECHNIC 



Weight the saucepan and contents and heat to 6oC. Supply 

 the water lost by evaporation. Neutralize either by adding 

 sufficient sodium hydrate, 10 percent solution, until red litmus 

 paper is colored a faint blue, or else titrate 10 c.c. of the mixture 

 with a decinormal solution of sodium hydrate, using phenol- 

 phthalein as an indicator, and after finding how much of a normal 

 solution is required to neutralize 990 c.c. (1,000 c.c. 10 c.c. 

 used for titration) this normal solution is added. The mixture 

 thus neutralized is then boiled for five minutes and the weight 

 restored. After boiling, from .5 percent to 1.5 percent normal 

 hydrochloric acid solution is added and the acidity thus produced 

 is spoken of as + .5 percent or + 1.5 percent as the case may be. 

 Upon boiling, the albumins are coagulated by heat, and the 

 phosphates are thrown down. The acid re-dissolves the latter. 

 The former must be removed by filtration. The filtrate is a clear 

 straw-colored fluid of an acid reaction which should not become 

 cloudy upon boiling. This is then run into flasks or test-tubes 

 and sterilized. 



The second method is much more convenient, and is prepared 

 by adding 3 grams of Liebig's beef extract to a litre of water, and 

 adding the peptone and salt, as in the previous method, and pro- 

 ceeding as before. To filter the bouillon, the filter paper must 

 be folded many times, and the funnel must be carefully cleaned. 



Newer methods for the titration of media have been constructed 

 on a physico-chemical basis, attempt being made to estimate 

 exact reaction in terms of ionic dissociation and hydrogen concen- 

 tration. Distilled water contains ionizable hydrogen according 

 to the following formula i + 10 7 = log. 7, a mathematical 

 statement abbreviated for convenience to 7., or the symbol of 

 strict neutrality and called the Ph or hydrogen concentration. 

 As the ionization of hydrogen increases, acidity becomes greater 

 and the numerical factor drops to 6.8 or lower; as the hydrogen 

 value decreases the figure rises. Substances in solution, such as 

 salts or organic matter act as buffers or agents which attempt to 



