! ' APPENDIX E 



THE TITRATION OF BOUILLON 



After cooking the ingredients, the amount of water lost by evapo- 

 ration should be restored. Stir for a moment, then pipette out 5 cc. 

 into a porcelain evaporating dish. Add 20 cc. distilled water and boil 

 over a flame for three minutes. Add a few drops of phenolphthalein. 



Fill a burette with a one-twentieth normal sodium hydrate solution 

 and place the porcelain dish beneath the pinchcock of the burette. 

 Run in the alkaline solution, stirring the bouillon with a glass rod, 

 until a permanent, faint pink color is obtained. Read off the amount 

 of alkali used. Titrate two other 6-cc. samples in the same way. If the 

 amounts of alkali used for each of the three samples are reasonably 

 close together, average them and record this figure as the amount of 

 N/20 alkali necessary to neutralize 5 cc. of bouillon. Compute the 

 amount necessary to neutralize the bouillon remaining in the flask. 



The balance of the bouillon is then to be neutralized, but the addi- 

 tion of so much weak alkali would dilute the bouillon too much ; there- 

 fore it is better to use a normal solution of sodium hydrate, of which 

 only one twentieth of the original computation is necessary. 



Test the accuracy of the work by pipetting out a small sample 

 and adding a drop or two of phenolphthalein. If a faint pink color 

 does not appear, the titration should be repeated. 



For most bacteriological work it is best to have the medium defi- 

 nitely acid. This condition is obtained by adding a definite volume 

 of normal hydrochloric acid, usually 0.5, 1.0, or 1.5 cc. per liter of 

 medium. This is expressed as + 0.5, -f 1.0, or + 1-5. 



The reaction of the medium is usually expressed as plus or minus. 

 A reaction expressed as + 1 means that 1 cc. of normal alkali must 

 be added to a liter of the culture medium in order to make it exactly 

 neutral to phenolphthalein. In the same way, reaction expressed 

 as — 1 means that 1 co. of normal acid must be added to bring 

 1 liter to the neutral point. It is essential that phenolphthalein 

 be used as the indicator for this titration. 



A reaction of + 1.0 is used for work in water and soil bacteriology ; 

 + 0.5 is better for many pathogenic organisms. 



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