150 NUTRIENT MEDIA 



Although meat extract always reacts acid to phenol- 

 phthalein, it occasionally reacts neutral or even alka- 

 line to litmus; and again, meat extract that has been 

 rendered exactly neutral to litmus still reacts acid to 

 phenolphthalein. This peculiar behaviour depends 

 upon two factors : 



1. Litmus is insensitive to many weak organic acids 

 the presence of which is readily indicated by phenol- 

 phthalein. 



2 . Dibasic sodium phosphate which is formed during 

 the process of neutralisation is a salt which reacts 

 alkaline to litmus, but neutral to phenglphthalein. 

 In order, therefore, to obtain an accurate estimation 

 of the reaction of any given sample of meat extract, 

 it is essential that 



1 . The meat extract be previously exposed to a tem- 

 perature of 100 C. for forty-five minutes. 



2. The estimation be performed at the boiling-point. 



3 . Phenolpthalein be used as the indicator. 



The estimation is carried out by means of titration 

 experiments against standard solutions of caustic soda, 

 in the following manner : 



Method of Estimating the Reaction. 



Apparatus Required: Solutions Required: 



1. 25 c.c. burette graduated i. icN NaOH, accurately 

 in tenths of a centimetre. standardised. 



2. i c.c. pipette graduated in 2 . 2 NaOH, accurately stand- 

 hundredths, and provided ardised 



with rubber tube, pinch- 

 cock, and delivery nozzle. n ^ T 



, ,. j 3. NaOH, accurately stand- 



3. 2 5 c.c. measure (cylinder or 3 * 10 



pipette, calibrated for 98 ardised. 



C. not 15 C.). 4> Q 5 per cent so i ut i on o f 



4. Several 60 c.c. conical phenolphthalein in 50 per 

 beakers or Erlenmeyer cent> alcohoL 



flasks. 



5. White porcelain evaporat- 

 ing basin, filled with boil- 

 ing water and arranged 



