20 GENEKAL MICROBIOLOGY 



A. Liquefiable, e.g., nutrient agar, nutrient gelatin. 



B. Non-liquefiable, including: 1. Media liquid in a 

 natural state but which, once solidified, cannot be liquefied 

 by physical means, e.g., media prepared from albuminous 

 fluids and tissues such as egg, blood serum, etc., or synthetic 

 media solidified with sodium silicate. 



2. Media which are solid in the natural state, e.g., 

 vegetable media such as potato, carrot, banana, etc. 



EXERCISE 3. TITRATION OF MEDIA 



The titration of bacteriological media made from meat 

 is an important step in their preparation, as microorganisms 

 are sensitive to the reaction of the nutrient substrate. 



Procedure. The following method is used for laboratory 

 media, with the exception of milk, wort, cider, vinegar, fruit 

 juices, etc. See p. 22. 



1. Put 5 c.c. of the medium to be tested and 45 c.c. of 

 distilled water in an evaporating dish. 



2. Boil briskly one minute with constant stirring (to 

 drive off all dissolved CO2 which registers as acidity). 



$. Add 1 c.c. phenolphthalein solution for indicator. 



4. Titrate while hot, preferably while boiling, with N/20 

 sodium hydroxide, or N/20 hydrochloric acid as the case de- 

 mands. ' A faint but distinct permanent rose color marks the 

 end point. This color should remain permanent for five minutes. 



5. Compute and record the reaction of the medium in 

 degrees of Fuller's scale, which is the number of cubic centi- 

 meters of normal* acid or alkali present in 1000 cubic 



* A solution is said to be normal when it contains 1 gram equiv- 

 alent of an acid or base in 1 liter. 



A gram equivalent of an acid or a base is that quantity which is 

 equivalent to or will neutralize 1 gram molecule of a mono-basic acid 

 or of a mon-acid base. 



The advantage of the system is that 1 c.c. of any normal solution 

 will exactly neutralize or be exactly equivalent to 1 milligram equiva- 

 lent of any acid or base. (Noyes, Wm. A., Textbook of Chemistry, 

 1913, p. 184.) 



