CHAPTEE XLIX. 



SIMPLE CHEMICAL MANIPULATIONS NORMAL SOLUTIONS AND 



INDICATORS REQUIRED IN LABORATORY WORK 



IN BACTERIOLOGY. 



REFERENCE has been made in the preceding pages to certain 

 simple chemical manipulations and tests employed in standardizing 

 the reaction of culture media, estimating the change in their reaction 

 or that in milk in consequence of bacterial growth, and determining 

 the exact amount of acid or alkali formed. These manipulations 

 require a set of simple chemical apparatus and a few chemical reagents, 

 standard solutions, indicators, etc. 



Apparatus. The apparatus required is the following: 



1. A moderate-sized balance of medium delicacy, carrying about 

 50 grams, sensitive to 1 milligram or less, and a moderately good set 

 of weights from 20 grams to 1 milligram. 



2. A number of volumetric flasks holding 1000 c.c., 500 c.c., 250 c.c., 

 100 c.c., and 50 c.c. 



3. A number of pipettes holding 10, 5, 2 and 1 c.c.; one 10 c.c. 

 pipette graduated in y 1 ^ c.c. and one 1 c.c. pipette graduated in y^ c.c. 



4. A number of graduates, beakers, porcelain evaporating dishes, 

 mortars, flasks, funnels, test-tubes, fermentation tubes, glass and 

 rubber tubing, and glass stirring rods, and a Kipp gas generator. 



5. Several burettes. These are used for the delivery of an accurately 

 measured quantity of standard or normal solutions. A burette is 

 made from a long, glass tube of even bore throughout, holding 50 or 

 100 c.c. On the glass tube lines of division are engraved, correspond- 

 ing generally to y^ c.c. The outlet of the burette is either a rubber tube 

 with clip or burette clamp and a glass tip or a ground-glass stopcock. 

 Burettes are used in an upright position, held in a burette stand, 

 and the fluids from them are generally delivered slowly, drop by drop, 

 by manipulating the clip, the clamp, or the stopcock. 



Gravimetric and Volumetric Analysis. The method of determining 

 quantitatively a chemical substance by obtaining it first in a pure 

 state or in a compound of known composition and then weighing it 

 on a delicate chemical balance is called the gravimetric method. This 

 method, for instance, is used in the laboratory in determining exactly 

 the amount of butter fat in milk, but it is not often used in bacteriology. 

 The method of determining quantitatively the amount of an acid 

 alkali or other chemical compound by the aid of volumetric or standard 

 solutions is called volumetric analysis, and this is the method generally 

 employed in bacteriological work. 



