152 NUTRIENT MEDIA 



tinige, shown in figure 98 (6). Note the amount of 

 decinormal soda solution used in the process. 



NOTE. Just before the end-point is reached, a very slight 

 opalescence may be noted in the fluid, due to the precipitation 

 of dibasic phosphates. After the true end-point is reached, the 

 further addition of about 0.5 c.c. of the decinormal soda solution 

 will produce a deep magenta colour (Fig. 98, c), which is the so- 

 called "end-point" of the American Committee of Bacteriologists. 



a & c 



FIG. 98. a, Sample of filtered meat extract or nutrient gelatine to which 

 phenolphthalein has been added. The medium is acid, as evidenced by the 

 unaltered colour of the sample, b, The same neutralised by the addition of 



NaOH. The production of this faint rose-pink colour indicates that the 

 " end-point," or neutral point to phenolphthalein, has been reached. If such 

 a sample is cooled down to say 30 or 20 C., the colour will be found to 

 become more distinct and decidedly deeper and brighter, resembling that 

 shown in c. c, Also if, after the end-point is reached, a further 0.5 c.c. or 



i.o c.c. -^~ NaOH be added to the sample, the marked alkalinity is evidenced 

 by the deep colour here shown. 



(B) Perform a "control" titration (occasionally two 

 controls may be necessary), as follows: 



1. Measure 25 c.c. of the meat extract into one of the 

 beakers, wash out the measure with boiling water, 

 and add the phenolphthalein as in the first estimation. 



2. Run in'-f NaOH from the pipette, just short of 

 the equivalent of the amount of deci-normal soda 

 solution required to neutralise the 2 5 c.c. of medium. 

 (For example, if in the first estimation 5 c.c. of ~ 

 NaOH were required to render 25 c.c. of medium 

 neutral to phenolphthalein, only add 0.48 c.c. of 

 7- NaOH.) Immerse the beaker in the water-bath. 



3. Complete the titration by the aid of the NaOH. 



