STANDARDISATION OF MEDIA 65 



very alkaline, or if an acid medium is required, dilute 

 hydrochloric or lactic acid may be employed for neutrali- 

 sation. 



Standardisation. Slight variations in the composition 

 and reaction of the nutrient media have a marked influence 

 upon the characters of the growths of micro-organisms 

 developing upon them. In order to obtain more uni- 

 formity in reaction, chemical titration with standard 

 alkali or acid should be employed. Media so " neu- 

 tralised " are termed " standard nutrient media." 



Standardisation may be most simply described in the 

 case of nutrient broth. A 100 c.c. Erlenmeyer flask is 

 rinsed out with boiling distilled water, 25 c.c. of the 

 nutrient broth are introduced into it, and 0-5 c.c. of 

 phenolphthalein solution is added (0-5 per cent, phenol- 

 phthalein in 50 per cent, alcohol). This is kept boiling, 

 and decinormal caustic soda solution l is run in from a 

 25 c.c. burette until a faint pink tinge appears in the 

 boiling fluid. From the amount of soda solution used 

 the amount of normal or deka-normal soda solution 

 required to neutralise a given volume of the broth (e.g. a 

 litre) can be calculated, and this amount is then added. 

 Although neutral to phenolphthalein, the medium is now 

 strongly alkaline to litmus too alkaline for the optimum 

 growth of most organisms. The reason for this is that 

 the di-sodium hydrogen phosphate (Na 2 HP0 4 ) present 

 in the medium is alkaline to litmus but not to phenol- 

 phthalein. To reduce the alkalinity (to litmus) normal 



(N\ 

 - j is meant the equivalent weight in grammes 



of a substance dissolved in (i.e. made up to) a litre of water ; a " deci- 



(N \ 

 j ) contains one-tenth of, a deka-normal ten times, this 



amount. A normal solution of caustic soda contains 40 grm. of pure 

 NaOH (NaOH = 40), of sulphuric acid 49 grm. of pure H 2 S0 4 ( H ^ 4 = 

 per litre. 



M.B. 5 



