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 CO2, 

 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 h 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 sHght stir- 

 ring with a clean rod, completely 

 disappears.' The end reaction is 

 reached when a faint but clear and 

 distinct pink color remains in the 

 fluid after stirring. 



When titrating alkahne media, 

 the addition of the phenolphthalein 

 produces a red color in the hot 

 medium which gradually fades upon 

 the addition of -^ HCl, becoming 

 point of titration. Titration should be done 

 From the result of the titration the 



Fig. 20. — Tubing Media. 



colorless at the end 



quickly and in a hot solution. 



computation for the neutralization of the entire bulk of the medium 



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



following example: 



Let us suppose that we have used: 



2.5 c.c. of J^ NaOH to neutralize 

 then 2.5 c.c. of y NaOH will neutralize 



5 c.c. of the medium, 

 100 c.c. " 



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



> See standard textbooks on volumetric analysis. 



