APPENDIX 159 



cent: using 4- and - to indicate acid and alkalinity respectively. 

 Thus a + ro medium indicates that the medium is acid to the 

 extent of ro c.c. of normal acid per 100 c.c. of the medium. 



The reaction of + ro was adopted by the English Committee 

 on Standard Methods and should be employed as the standard 

 reaction for all media, the addition of alkali being stopped at 

 the first appearance of a pink colour. 



The actual standardization may be conveniently done as 

 follows : 



Ten c.c. of the medium (gelatine, agar, broth) are pipetted 

 into an evaporating basin or small beaker containing about 

 50 c.c. of hot distilled water. Half a c.c. of phenolphthalein 

 solution (o'5 per cent, solution in 50 per cent, alcohol) is added 



N 

 and the mixture is boiled for several minutes. sodium hydrate 



solution is cautiously run into the beaker from a burette until 

 the first tinge of pink permanently remains. The amount of 

 alkali added is then accurately read off. 



This is repeated with a fresh 10 c.c. of the medium. Results 

 not differing by more than O'l c.c. should be obtained. The 



N 

 mean is taken, and from this the amount of alkali required to 



neutralise the whole litre of the medium is calculated. 



The neutral point to phenolphthalein gives a medium too 

 alkaline and a + I per cent, reaction is required. From the 

 calculated amount of normal alkali required, ro c.c. is deducted 

 for each 100 c.c. of medium. The calculated amount of alkali, 

 less this deduction, is then added, and a+ ro per cent, reaction 

 is obtained. In other words alkali is added insufficient to quite 

 neutralize the medium to phenolphthalein, and the addition of 



N 



ro per cent, of alkali would still be required to make it com- 

 pletely neutral. 



As an example of an actual standardization, the following is 

 given : 



Two separate 10 c.c. of a litre of nutrient agar each required 



N 

 the addition of 1*2 c.c. sodium hydrate solution to make them 



