170 



CULTURE MEDIA 



is obtained on comparison with the pair selected to give the 

 desired end point. Calculation shows that twenty-five times 

 this amount will give the quantity of N/1 NaOH per liter 

 of medium. 



This method requires no standard solutions and no special 

 apparatus. The alkali and acid should be about the strength 

 indicated, not stronger, owing to the effect of strong solutions 

 on the dyes. 



Fig. 111.— "Comparator block" for H ion determination. The three 

 tubes in the front row contain the medium. The two behind these on the 

 outside contain respectively the color pairs next above and next below the 

 end point desired. The other two tubes in the middle row contain distilled 

 water. This is a photograph of a 5-in. block. 



The following measurements are better: Block 10 cm. cube; 9 holes 



20 mm. diameter, 90 mm. deep; 3 slots 25 mm. long, 12 mm. wide; 



bottom edge of slots 30 mm. from bottom of block. 



Color comparisons are perhaps more certain when made 

 through a comparator block (Fig. Ill), though this is not 

 necessary. The tubes containing the solutions should be 

 as nearly the same size, color and clearness as possible. 



This method is the simplest yet devised for i)R deter- 



