6 14 LABORATORY EXERCISES 



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— Oxalic Acid Solution. — Weigh out exactly 6.3 grams of chemically pure oxalic 



acid (H2C2O4 plus 2H2O) and add distilled water in looo-c.c. volumetric flask. 

 After the crystals of acid have dissolved, dilute the solution until it measures exactly 

 1000 c.c. 



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 Tj— 7;tf or Normal Sodium Hydroxide. — This solution should contain 40 grams of 



NaOH in i liter. It can be made by titrating against the standard oxalic solution 

 already prepared. Weigh out 90 grams of NaOH and dissolve in 2 liters of dis- 

 tilled water. This solution is now too strong and the amount necessary to dilute it 



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 must be determined. Place exactly 50 c.c. of the oxalic acid in a beaker and add 



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a few drops of phenolphthalein solution to serve as an indicator and then add to this 

 drop by drop from a burette some of the NaOH solution, stirring with a glass rod 

 and continue until the solution is turned a faint, but permanent pink color. Read off 



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 from the burette the amount of NaOH solution used to neutralize 50 c.c. ot — 



10 



oxalic acid, which contained as much acid as 5 c.c. of normal acid. Now calculate the 

 amount of dilution necessary. Supposing 4.5 c.c. of NaOH be the amount used and 

 1950 c.c. the amount of NaOH to be diluted, the proportion would be as follows: 

 4.5 : 5 :: 1950 : x where x = 2167, and this means that 2167 c.c. of water must be 

 used. After the dilution, repeat the titration and adjust if necessary. 

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 TTpj or Normal Hydrochloric Acid. — This may be prepared by making an acid 



solution which is a little over strength, and determining the amount of dilution 



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necessary by titrating with the „ ^^ - i c.c. of c^t^ should exactly neutralize 



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ic.c.ofjj^l- 



Expressing the Reaction of Media. — Fuller's scale has been generally adopted for 

 expressing the reaction of culture media. The plus sign (+) indicates that the 

 medium is acid to phenolphthalein, while the minus sign ( — ) indicates that the me- 

 dium is alkaline to phenolphthalein, the figure following the sign indicating the 



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 degree of acidity, or alkalinity. For example, a -f 10 medium contains 10 c.c. of tt;^, 



for 1000 c.c. beyond the neutral point for phenolphthalein paper. A — 10 medium 



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 is alkaline and would require 10 c.c. of t77^\ for 1000 c.c. to bring it back to the 



neutral point. Media may then have the reaction + 5, + 10, + 15, etc., or — 5, 

 — 10, —15, etc. The neutral point for litmus is not the same as the neutral point 

 for phenolphthalein and this fact should be kept in mind when working with culture 

 media. 



25 of Fuller's scale gives approximately the neutral point for litmus, so that any 

 medium with a reaction less than + 25 is still alkaline to litmus. 



The Optimum Reaction. — For every organism there is a definite optimum reaction. 

 It lies near -f 5 for most animal pathogens, about -fio to +15 for most water and 



