STANDARDISING BOUILLON 155 



Briefly, the method of standardising a litre of media 

 to +10 consists in subtracting 10 from the initial litre 

 of the medium mass; the remainder indicates the 

 number of cubic centimetres of normal soda solution 

 that must be added to the medium, per litre, to render 

 the reaction +10. 



Standardising Nutrient Bouillon. For example, 1000 

 c.c. bouillon are prepared; at the first titration it is 

 found 



1. 25 c.c. require the addition of 5.50 c.c. NaOH 

 to neutralise. 



Two controls give the following results: 



2. 25 c.c. require the addition of 5.70 c.c. ~ NaOH 

 to neutralise. 



3. 25 c.c. require the addition of 5.60 c.c. ~ NaOH 

 to neutralise. 



Averaging these two controls, 25 c.c. require the 

 addition of 5.65 c.c. NaOH to neutralise, and there- 

 fore 1000 c.c. require the addition of 226 c.c. ~ NaOH, 

 or 22.60 c.c. Y NaOH, or 2.26 c.c. 10 or NaOH. 



Initial litre of the bouillon = +22.6, and as such 

 requires the addition of (22.6 c.c. 10 c.c.) = 12. 6 c.c. 

 of ~ NaOH per litre to leave its finished reaction + 10. 



But the three titrations, each on 25 c.c. of medium, 

 have reduced the original bulk of bouillon to (1000 

 75 c.c.) = 925 c.c. The amount of -f- NaOH required 

 to render the reaction of this quantity of medium +10 

 may be deduced thus : 



1000 c.c. : 925 c.c. :: 12.6 c.c. :x. 



Then* =11.65 c - c - f NaOH. 



Whenever possible, however, the required reaction 

 is produced by the addition of dekanormal soda solu- 

 tion, on account of the minute increase it causes in the 

 bulk, and the consequent insignificant disturbance of 

 the percentage composition of the medium. By means 

 of a pipette graduated to o.oi c.c. it is possible to de- 



