118 



BIOLOGY AND TECHNIQUE 



fully washed pipette and transferred into a porcelain evaporating dish, 

 to this are added 45 c.c. of distilled water. The mixture is thoroughly 

 boiled for three minutes over a free flame. The boiling drives off CO 2 , 

 giving the true neutral point, and approximates the conditions prevailing 



during the further sterilization of 

 the medium from which the 5 c.c. 

 have been 'taken. After boiling, 1 

 c.c. of the phenolphthalein is added. 

 If the medium is acid, no color is 

 present; if alkaline, a pink or red 

 color appears. The -|^ alkali or 

 acid solution is allowed to drop 

 into the dish from a graduated 

 burette. When the neutral point 

 is approached in an acid solution, 

 each drop of sodium hydrate added 

 brings forth at first a deep red, 

 which, however, upon slight stir- 

 ring with a clean rod, completely 

 disappears. 1 The end reaction is 

 reached when a faint but clear arid 

 distinct pink color remains in the 

 fluid after stirring. 



When titrating alkaline inediaj 

 the addition of the phenolphthalein 

 produces a red color in the hot 

 medium which gradually fades upon 

 the addition of |^ HC1, becoming 



colorless at the end point of titration. Titration should be done 

 quickly and in a hot solution. From the result of the titration the 

 computation for the neutralization of the entire bulk of the medium 

 can be made by a simple arithmetical process s as illustrated in the 

 following example: 



Let us suppose that we have used : 



2.5 c.c. of -^ NaOH to neutralize 5 c.c. of the medium, 



then 2.5 c.c. of ^ NaOH will neutralize 100 c.c. " " 



and 25 c.c. of ^ NaOH will neutralize 1,000 c.c., or one liter. 



FIG. 20. TUBING' MEDIA. 



See standard textbooks on volumetric analysis. 



