EXAMINATION OF WATER FOE MICRO-ORGANISMS 65 



These experiments show how important it is that 

 in comparative examinations of water by different 

 •observers, the alkalinity of the gelatine-peptone em- 

 ployed should be most carefully determined. Reinsch 

 points out how unsatisfactory for such purposes is the 

 estimation of the degree of alkalinity of the culture 

 material by litmus-paper, as the sensitiveness of the 

 latter is so exceedingly variable. In order to accu- 

 rately determine the alkalinity of a,ny given culture 

 material, this author suggests the adoption of Schultz's ^ 

 method. Schultz recommends the addition of a 0-4 

 per cent, solution of caustic soda, which is carefully 

 measured out in drops by means of a burette. Thus 

 1 c.c. of the culture medium is placed in a test-tube, 

 to which a drop of phenolphthalein is added as indi- 

 -cator ; caustic soda is then run in until the liquid be- 

 comes of a faint pink colour.- (The vessel containing 

 the culture liquid should stand on white paper so that 

 the colour may be more easily recognised.) For the 

 sake of accuracy this should be repeated three times, 

 and the mean taken of the three observations, in order 

 to ascertain the amount of caustic soda which should 

 be added to the main body of the culture material 

 in order to secure the particular degree of alkalinity 

 required. 



To calculate the amount for a larger quantity of 

 the culture medium it is only necessary to multiply the 

 quantity of the alkali required for 1 c.c. by the number 

 of cubic centimetres of. culture material to be neutral- 



^ ' Zur Frage von der Bereitung einiger Niihrsubstrate,' Centralhlatt 

 filr BaMeriologie, vol. x., 1891, p. 52. 



^ This pink colour, v^^hich is produced in the presence of the phenol- 

 phthalein, indicates that the liquid contains free alkali, that in fact its 

 reaction is now alkaline. Of course the phenolphthalein is not added 

 when the neutralisation of the larger quantity of material is effected, but 

 only in the test-titration. 



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