142 



BIOLOGY AND TECHNIQUE 



A series of tubes so prepared, each of which contains 10 c.c. of 

 each of the graded mixtures, with indicator added, represents a 

 colorimetric scale against which the media can be standardized. 



This standardization is now carried out as follows, and can be 

 utilized for any media which are not too highly colored and not turbid: 

 Into a thoroughly cleaned test tube 2 c.c. of the medium are measured 

 and this diluted with 8 c.c. of redistilled water. 10 drops of indicator 

 are then added, and after thoroughly mixing, a color reading is taken 

 against the scale. If the reaction is too acid, as is usually the case, 



N 

 add =77 NaOH from a burette or graduated pipette, a drop at a time, 



until the color matches that of the standard tube. By calculating 

 from the amount of weak alkali added, the total quantity of media 



N 

 is then brought to the desired P# with NaOH. In the titration of 



agar, Clark and Lubs recommend that the broth be titrated and 

 adjusted before the addition of the agar, in order to avoid possible 

 colloidal changes between the agar and the indicator, and that the 

 agar be added after the adjustment is made. It is possible, however, 

 without excessive error, to carry out the titration of media containing 

 agar in the same way as outlined above, adding cold water to the 

 hot agar, and making the comparison at once at a temperature of 

 35 to 40 before the mixture has jelled. In solutions like bacterial 

 media, which haw a certain amount of color or some turbidity, a 

 so-called " comparator " may be used in the form of a wooden box 

 painted black with four holes for test tubes and a slit in front and 

 behind, so that it can be looked through against a source of light. 

 The arrangement of this is given in the following cut: 



LIGHT SOURCE 



EYE 



Arrangement for Reading Titration. 



