138 BIOLOGY AND TECHNIQUE 



THE COLORIMETRIC METHOD OF TITRATION 



The method given above is not an accurate one because, in thje 

 titration of culture media, we are dealing with solutions which 

 contain considerable quantities of materials, such as peptone, pro- 

 teins, phosphates, etc., which have an action which is spoken of 

 technically as that of a "buffer." This term signifies the power of 

 these substances to oppose changes in reaction. The degree of 

 "buffer" action, as shown by a number of writers, but recently in 

 connection with bacteriological work particularly by Clark and 

 Lubs, 4 is proportionate to the concentration of the constituents, and 

 the consequence of the action is that volumetrically proportionate 

 amounts of acid or alkali added to such solutions do not change the 

 reaction in the same proportions. Curves found in the paper of 

 Clark and Lubs cited above, will make this sufficiently clear. In 

 consequence, when we titrate 5 cubic centimeters of the medium 

 as in the old method, to the neutral point of phenolphthalein, we 

 could bring the entire media to the neutral point of phenolphthalein 

 by adding proportionate amounts of acid or alkali, but we cannot 

 foretell the final hydrogen ion concentration attained in the medium 

 by adding fractions of this total amount. It is plain, therefore, that 

 to make an accurate adjustment of media it would be better to 

 apply either an electrode method or a colorimetric method to the 

 medium, adjusting to a standard of known hydrogen ion concen- 

 tration. The potentiometer methods have been considerably simpli- 

 fied, and have been applied by men like Clark and Lubs and others 

 who are thoroughly versed in the handling cf such instruments. 

 For routine laboratory work colorimetric methods have been in- 

 troduced which, as used by others and by us, have checked up quite 

 accurately with potentiometer measurements. 



Before describing the method at present in use, it will be best 

 to say a few words about the nomenclature used at present for the 

 expression of hydrogen ion concentrations. 



THE MEANING OF P H . The hydrogen ion concentration of pure water 

 is 0.0000001. This is more simply expressed as 1X10~ 7 . Since, 

 in pure water, hydroxyl ions are equal in concentration, to the hydro- 

 gen ions, and the hydroxyl ions are also 1 X 10~ 7 , and this is the neutral 



4 Clark and Lubs, Jour, of Bacter., 2, 1917, 1, 109, 191. 



